The Nervous System Flashcards
1
Q
What is a receptor?
A
These are the organs or tissue in an organism that detect stimuli to which the animals respond
. E.g. sensory endings in the skin, nose, tongue, eyes and ears.
2
Q
What is an impulse?
A
This is an electrical transmission sent from the receptors to the coordinating center.
3
Q
What is a nerve impulse?
A
A wave of depolarization that moves along the surface of a nerve cell
4
Q
Define an effector
A
This is an organ that respond to the stimuli and carry out the response.
Eg muscles and glands
5
Q
What is the use of the central nervous system (CNS)?
A
It interprets and determines the nature of the response.
The CNS consists of the brain and spinal cord.
6
Q
What are the functions of the nervous system?
A
- It receives impulses from all sensory organs of the body.
- It stores information.
- It correlates various stimuli from different sensory organs.
- It sends messages to all parts of the body making them function accordingly.
- It’s involved in temperature regulation.
7
Q
What are sensory neurones?
A
These are cells that transmit impulses from the receptor cells to the central nervous system.
8
Q
What are motor neurones?
A
This is a neurone that transmits impulses from the central nervous system to the effector organs such as muscles and glands, where a response is made.
The muscles respond by contracting while glands respond by secreting substances.
9
Q
What is a Relay neurone?
A
This is a neuron located in the central nervous system and transmits impulses from the sensory neurone to the motor neurone.
The axon extends towards the motor neurone. It is also referred to as an intermediate neurone.
A relay neurone is either unipolar, bipolar or multipolar.
10
Q
Give the functions of the parts of a neuron
Cell body
Axon
Myelin sheath
Dendrites
Node of Ranvier
Dendron
Schwann cell
Nissl’s granules
A
Cell body: The nucleus in the cell body controls all activities of the neuron.
Axon: Transmits impulses over long distances in the body. Each axon is filled with cytoplasm called axoplasm.
Myelin sheath: This is a fatty material that covers the axon. It insulates the axon and speeds up the transmission of impulses.
Dendrites: These are hair-like structures surrounding the cell body. They conduct incoming signals.
Node of Ranvier: This is the space on the axon between two adjacent myelin sheaths. It speeds up nervous transmission.
Dendron: It is a branch through which impulses are transmitted to the body.
Schwann cell; this is a cell which secretes the myelin sheath.
Nissl’s granules; these are groups of ribosomes responsible for protein synthesis.
11
Q
What are the differences between motor and sensory neurons?
A
Motor neuron:
Has a long axon.
It has a cell body at the terminal end of the axon.
It has a short dendrons.
It carries impulses from the central nervous system to the effector organ.
It has several dendrons.
Terminal dendrites connect with effector organ.
Sensory neuron:
Has a short axon.
Has a cell body located on the axon branch.
It has a long dendron.
It carries impulses from the receptor cells to the central nervous system.
It has one dendron.
Terminal dendrites connect to relay neurones.
12
Q
How is the motor neurone suited for functioning?
A
- The nucleus is relatively large to coordinate the metabolic activities all over the large cytoplasm of the cell.
- There are very many rows of rough endoplasmic reticula (Nissl’s granules) for massive production of proteins and neurotransmitters.
- The dendrites are numerously branched to increase the surface area for synapting with several other neurones.
- Axon is long to carry impulses to the target parts.
- The axon membrane is wrapped with a myelin sheath for electrical insulation.
- The axon membrane is wrapped with a thick myelin sheath for protection against damage.
- The axon membrane is wrapped with a myelin sheath at intervals around the axon which increases speed of impulse transmission through salutatory conduction.
13
Q
Define a resting potential
A
A resting potential is a negative potential difference existing across a membrane of an axon when the outside of the neuron is more positive while the axoplasm is more negative and no impulse is being transmitted.
14
Q
Describe polarization/ resting state of the neuron
A
- When no impulse is being transmitted, the axoplasm is negatively charged relative to the outside of the neuron
- Na+/K+ pump is on; it pumps out 3 Na+ and 2 K+ ions by active transport
- The axon membrane is 20 times more permeable to K+ than Na+
- The axoplasm has a higher concentration of K+ than Na+ in contrast to the outside of the axon which has a higher Na+ concentration than K+ concentration
- Both the Na+ gated channels and potassium gated channels are closed
- Due to high concentration of K+ inside the cell and greater permeability of the membrane, K+ leaks out of the cell and K+ ion loss from the axon is greater than Na+ ion gain
- Axoplasm gains an overall more negative charge relative to the outside due to outward movement of positive ions and the cell is polarized.
15
Q
What is an action potential?
A
This is a change in membrane voltage that occurs in any excitable tissue in presence of an impulse in transit/ an excitation.
16
Q
Describe depolarization/ action potential of the neuron
A
- Upon arrival of an impulse, the Na+/ K+ pump switches off
- The sodium gated channel opens and the permeability of the axon membrane to sodium ions increases (or is reversed)
- Opening of one sodium gated channel initiates opening of others and this is a positive feedback mechanism
- There is an influx of Na+ which causes the inside of the neuron to become positively charged
- The neuron is depolarized
17
Q
Describe repolarization of the neuron
A
- Na+/K+ pump switches on
- Na+ gated channels and K+ gated channels start to close
- Na+ gated channels close quickly
- K+ gated channels are still open and K+ leak out
18
Q
Describe hyperpolarization of the neuron
A
- K+ gated channels slightly delay in closing compared to Na+ gated channels
- K+ still leaks out creating a more negative charge inside the axoplasm relative to the outside
19
Q
Define refractory period
A
This is a time during which the membrane cannot be depolarised again.
- Occurs during repolarization and hyperpolarization.
20
Q
Differentiate between absolute refractory period and relative refractory period
A
Absolute refractory period is when it is not possible to elicit another action potential despite the size of the stimulus.
While
Relative refractory period is when it is more difficult to elicit an action potential, but still possible if a greater stimulus is used than is needed at rest.
21
Q
What is the importance of refractory period?
A
- Determines the maximum frequency at which an axon can transmit an impulse.
- Ensures separation of action potential and specify the stimulus causing the excitation.
- Prevents spreading of action potential and makes it flow in one direction.
22
Q
What factors affect nerve conduction speed?
A
1) Axon diameter: Impulses are faster in an axon with larger diameter because longitudinal resistance of axoplasm decreases with increasing diameter of axon.
Small cells or cells with large surface area to volume ratio or ion leakage weakens membrane.
Myelin sheath stops ion leakage; therefore large diameter only important for unmyelinated neurons.
2) Temperature:
Homoiotherms with steady body temperature have faster impulse propagation than poikilotherms which have fluctuating body
temperature. Temperature has a direct effect on diffusion of ions
3) Myelination and saltatory conduction:
Myelination speeds up conduction. In a myelinated neuron, the conduction velocity is directly proportional to the fiber diameter. Schwann cells prevent diffusion of ions; flow of current occurs only between adjacent nodes of Ranvier. Therefore, depolarization only at nodes of Ranvier because action potential ‘jumps’ from node to node.
23
Q
What causes generation of an action potential?
A
Influx of Na+ to create a positively charged axoplasm in relation to the outside
24
Q
State the ‘All-or-nothing law’ in the transmission of nerve impulses.
A
It states that if the strength of the stimulus is below certain threshold intensity, no action potential is evoked.
If however the stimulus is above the threshold, a full sized potential is evoked and remains the same no matter the intensity of the stimulus.
25
Define a synapse
This is the point where the axon of one neuron meets and joins with the dendrite or cell body of another neuron.
26
When a synapse is inhibitory, which neurotransmitter is released, what occurs?
Neurotransmitter is noradrenaline
Opening of Cl- ion or K+ ion-gated channels in the post-synaptic membrane, causing hyperpolarization which makes it difficult to generate an action potential
27
When a synapse is excitatory, which neurotransmitter is released, what occurs?
Neurotransmitter is acetylcholine
Opening of the Na+ gated channels to cause depolarization in the post-synaptic membrane.
28
Describe the process of transmission of an impulse across a synapse
- When an impulse arrives on the presynaptic knob, the presynaptic membrane becomes more permeable to Calcium ions.
- Calcium ions from the synaptic cleft enter the knob and induce the vesicles to fuse with the presynaptic membrane.
- The vesicles fuse with the membrane and discharge/release the neurotransmitter substances into the synaptic cleft.
- The released neurotransmitter substances then diffuse across the synaptic cleft and attach to specific receptor sites on the postsynaptic membrane.
What follows depends on whether the synapse is either excitatory or inhibitory.
- At excitatory synapse, the reception of neurotransmitter substance (acetyl choline) on the receptor sites makes the post synaptic membrane more permeable to sodium ions which diffuse into the postsynaptic membrane.
- The potential difference of the membrane therefore changes, the membrane is depolarized and an excitatory postsynaptic potential (EPSP) results.
- This fills up until the threshold is reached which results into an action potential being fired in the post synaptic neuron. At that point the impulse has crossed the synapse.
- At an inhibitory synapse, release of transmitter substances (noradrenaline) into the synaptic cleft leads to the opening up of chloride ion channels in the post synaptic membrane resulting into chloride ions entering and potassium ions leaving.
- The interior of the post synaptic membrane becomes more negative relative to the outside. This is known as inhibitory postsynaptic potential (IPSP) and makes it difficult to generate an action potential in the post-synaptic cell.
29
What happens after a neurotransmitter substance has performed its function?
- Acetyl choline is normally hydrolyzed from to choline and acetyl; the reaction is catalyzed by an enzyme, acetyl cholinesterase
- These two products then re-enter the presynaptic knob and combine back to form/ re-synthesize the transmitter substance which is packed into vesicles ready for reuse.
30
Define summation in nervous coordination
This is a phenomenon used to describe how the depolarizing effect of several excitatory post synaptic potentials (EPSPs) is additive.
31
What is spatial summation?
Two or more EPSPs from different neurons strike the postsynaptic neurons simultaneously and produce sufficient depolarization to generate an action potential
32
What is temporal summation?
Rapid repeated release of transmitter substances resulting in successive weak impulses adding up to generate an action potential in the post synaptic membrane
The effect of the second impulse adds to the first
33
What is accommodation in relation to nervous coordination?
This means supply of neurotransmitter substance is exhausted and its re-synthesis can’t keep pace with the rate at which the impulse are reaching the synapse
34
Give the functions of the synapse
- They transmit information between neurones.
- They filter out low frequency impulses.
- They act as valves to ensure that impulses pass across them in one direction only.
- They also act as junctions allowing impulses to be divided up along many neurones or merge into one.
- To protect effectors from damage by overstimulation.
- Synapses may be involved in memory and the learning process.
35
Give the disadvantages of synapses
- Slows down the speed of transmission.
- Are highly prone to drugs and fatigue which may inhibit impulse transmission.
36
What is a neuromuscular junction?
This is a single synapse or junction made between one motor neuron and one muscle fiber
37
Describe the functioning of a neuromuscular junction
- Arrival of an action potential at the synaptic terminal of motor neuron causes the influx of Ca2+ ions into the presynaptic neuron's cytosol
- There is exocytosis of synaptic vesicles containing acetylcholine.
- Acetylcholine diffuses across the synaptic cleft of neuromuscular junction to depolarize the sarcolemma and trigger an action potential that brings about contraction and relaxation of the muscle.
38
What makes up the central nervous system?
This is made up of the brain and spinal cord
39
What are the three distinct areas of the brain?
- forebrain
- midbrain
- hindbrain.
40
What is the function of the cerebrum (cerebral hemisphere)?
- It coordinates learning, memory, reasoning, conscience and personality.
- It is responsible for intelligence.
41
What is the function of the thalamus?
It transmits impulses of sensations received from sense organs to the cerebral cortex.
42
What is the function of the hypothalamus?
- It controls activities of the pituitary gland.
- It also coordinates and controls the autonomous nervous system.
43
What is the function of the midbrain?
- It relays audio and visual information.
- It is also responsible for movement of the head and the trunk.
44
What is the function of the cerebellum?
- It is responsible for balance and muscular coordination.
45
What is the function of the Medulla oblongata?
- It controls heartbeat, blood pressure, breathing rate, coughing and sneezing.
46
Give the functions of the brain
- It receives impulses from all receptors and sends back impulses to the effectors.
- It integrates and coordinates all activities in the body such that the body works efficiently.
- It stores information
- It is involved in cranial reflex actions but it does not initiate them.
47
What protects the spinal cord from mechanical damage?
The vertebral column.
48
Give the functions of the spinal cord
- It connects the peripheral nervous system to the brain.
- It is a center for simple spinal reflex actions.
- Receives impulses from receptors.
- Interprets messages especially in reflex arc.
- Sends impulses to the effectors.
49
What two nervous systems make up the peripheral nervous system?
- It is divided into autonomic nervous system and somatic nervous system.
50
Of what importance is the autonomic nervous system?
- It is responsible for the involuntary control of internal organs, blood vessels, smooth muscles and cardiac muscles.
51
Of what importance is the somatic nervous system?
- The somatic nervous system is responsible for the voluntary control of skin, bones, joints and skeletal muscles.
52
Define a Voluntary action
A voluntary action is one initiated *consciously* under the direct control of the brain
i.e. they are actions one does at will e.g. dancing, laughing, stealing, etc.
53
What are Involuntary actions?
These are the ones that occur without conscious thoughts e.g. breathing, etc.
54
What is a reflex action?
This is an automatic (involuntary) response to a particular stimuli.
A reflex action occurs as a result of impulses travelling along neurons in a path called a reflex arc.
55
What is a simple reflex action?
This is an involuntary quick response to a stimulus without conscious thought.
It is also known as an instinctive reflex which does not have to be learnt.
They include sneezing, coughing, salivating, the knee jerk and removal of a hand from a hot flame.
56
Describe the reflex arc of a hand being withdrawn from a hot flame
- The stimulus is perceived by the receptors, which change it into nervous impulse by *transduction*.
- The impulse travels along the sensory neurone to the spinal cord.
- In the grey matter of the spinal cord, the sensory neurone makes synaptic connections to the relay neurone and impulses move from the sensory neurone to the relay neurone across synapses.
- The relay neurone in turn transmits the impulse to the motor neurone across a synapse.
- The impulse then moves from the spinal cord to the effector muscles through the motor neurone.
- The impulse causes the muscles to contract or relax depending on the stimulus.
57
Give the characteristics of a simple reflex action
- It occurs rapidly i.e. the action occurs very fast.
- It is inborn (innate) but not learnt.
- It is coordinated by either the brain or spinal cord but usually initiated by spinal cord
- It occurs without one’s will.
- It is a repeated response to a similar stimulus.
- Three neurons are involved.
58
What examples of simple reflex actions can be given?
1) Blinking when a foreign body falls on the eye.
2) Withdraw of the arm when someone accidentally touches a hot body.
3) Sneezing.
4) Knee jerk i.e. a relaxed leg gives a forward kick when tapped slightly below the patella.
5) Withdraw of the foot from a sharp object.
59
Describe how a hand is withdrawn from a hot object (an example of a simple reflex action)?
- When one accidentally touches a hot body using a finger, the receptors in the finger receive the stimulus and change it into nervous impulses that travel along the sensory neurone to the spinal cord and then cross the synapse.
- The impulse is then handed over to the relay neurone in the spinal cord (grey matter) and then cross another synapse.
- The relay neurone in turn hands over the impulse to the motor neuron.
- The motor neuron then carries the impulse from the spinal cord to the effector muscles of the hand.
- This causes the muscles to contract and the hand is removed from the hot body.
- At the same time, the original message is sent to the brain which then interprets it as pain or heat.
60
Give the importance of simple reflex actions to animals
- They help animals to avoid danger.
- They control activities in the body, which we do not have conscious control over.
- They form a basis of some animals’ behaviour, e.g. amoeba.
61
What is a conditioned reflex action?
This is the type of reflex action which involves learning in which organisms learn to respond to strange or meaningless stimuli by associating it with other meaningful/familiar stimuli, e.g. the Ivan Pavlov’s experiment.
62
What are the characteristics of a conditioned reflex action?
- It is a temporary reflex
- It involves learning
- It is coordinated in the brain
- It involves more than one stimuli
- It involves association of stimuli
- It is reinforced by repetition
- Responses are involuntary
63
What are the similarities between simple and conditioned reflex actions?
- They both involve the central nervous system particularly the brain.
- Both are autonomic responses
- Both are associated with a stimulus.
- Both involve neurons for the transmission of impulses
64
What are the differences between simple and conditioned reflex actions?
Conditioned reflex actions
- Stimulus and responses are not directly related
- More than one stimulus is required to cause a response It involves learning
- Takes time
- It is coordinated in the brain only
- Responses occur as a result of repetition and practice.
- Is a learned automatic response
- Can be reinforced through rewards or punishment.
Simple reflex action
- Stimulus and response are related
- Only one stimulus is needed to cause a response
- No learning but inborn
- Takes a very short time
- Co-ordinated in either the brain or spinal cord
- Responses occur instantly after a stimulus.
- Is an inborn, automatic response
- It is always constant
65
What are the similarities between the nervous and endocrine system?
- Both are affected by change in stimulus.
- Both cause a response.
- They provide a means of co-ordination in the body.
- Both systems transmit messages.
66
Give differences between the nervous system and endocrine system
Nervous system
- Nerve impulses are electrical
- Responses are fast as the impulses are carried fast.
- Impulses go along nerve fibres.
- This effect is more localized (specific).
- Stimulus arises from any part of the body where sensory receptors are located.
Endocrine system
- Impulses are chemical
- Responses are slow but long lasting.
- Hormones are carried in blood.
- Effect is wide spread in the whole body.
- Stimulus arises from specific places only e.g. endocrine glands.
67
Which two parts is the autonomic nervous system divided into?
The sympathetic nervous system and parasympathetic nervous system.
68
What are the functions of the sympathetic nervous system?
- Dilates the pupil of the eye
- Inhibits salivary gland secretion
- Relaxes bronchi in lungs
- Accelerates heart
- Inhibits activity of stomach and intestines
- Inhibits activity of pancreas
- Stimulates glucose release from liver
- Inhibits gall bladder
- Stimulates adrenal medulla
- Inhibits emptying of the bladder
- Promotes ejaculation and vaginal contractions
69
What are the functions of the parasympathetic nervous system?
- Contraction the pupil of the eye
- Stimulates salivary gland secretion
- Constricts bronchi in lungs
- Slows heart
- Stimulates activity of stomach and intestines
- Stimulates activity of pancreas
- Stimulates gall bladder
- Stimulates adrenal medulla
- Promotes emptying of the bladder
- Promotes erection of the genitals
70
State the characteristics (properties) of receptor cells
- Transduction
- Sensitivity
- Adaptation
- Inhibition
- Precision
- Receptors are sensitive to low intensity stimulation
- They are specialized in structure and position.
71
What is transduction as a characteristic of receptor cells?
Process by which receptor cells change physical stimuli into an electrical impulse.
72
Define Sensitivity as a characteristic of receptor cells
Ability of receptor cells to detect the slightest change in their environment (stimulus).
73
What is Adaptation as a characteristic of receptor cells?
If a stimulus is maintained, receptor cells are able to adapt to it so that the stimulus no longer causes an impulse, however strong it is.
For example, the inability to hear a clock’s ticking in a room after prolonged exposure to its ticking.
74
What are phasic receptors?
Receptors which adapt
rapidly
75
What are tonic receptors?
Receptors which adapt slowly
76
Give ways how adaptation as a characteristic of receptors is advantageous
- It provides animals with precise information about changes in the environment.
- It enables the nervous system ignore unchanging environmental conditions and concentrate on monitoring those of survival value.
- It prevents overloading of the nervous system with irrelevant information which reduces energy wastage.
77
What is inhibition as a characteristic of receptor cells?
Receptor cells can be stopped from firing impulses by special synaptic connections. As a result, certain impulses are transmitted only when required.
78
What is precision as a characteristic of receptor cells?
Receptors are able to transmit the information precisely without alteration.
79
True or false
Receptors are characteristically sensitive to low intensity stimulation.
True
E.g. in some insects, tactile receptors can respond to airborne sounds
when stimulated just 3.6 nm. In rod cells of human eyes, high sensitivity results from retinal convergence.
80
Describe the sclera
It is a tough white outer layer of connective tissue of the eyeball
81
Describe the choroid
A thin, pigmented inner layer of the eyeball
82
What is the function of the Sclera?
It maintains the shape of the eyeball and protects the inner layer of the eye.
83
What is the function of the Conjunctiva?
It protects the cornea.
84
What is the function of the Choroid layer?
It’s a pigment layer present beneath the sclera
The pigmentation prevents unnecessary *reflection* within the eye.
85
What is the use of the Ciliary muscles?
They alter the shape of the lens.
Their contraction results into the spherical shape of the lens and the relaxation results in the flattening of the lens.
86
What is the function of the Suspensory ligaments?
They hold the lens in position.
87
What is the function of the aqueous humour?
- It maintains the shape of the eye.
- It also *refracts* light into the pupil and the lens.
88
What is the use of the vitreous humour?
- It is transparent and maintains the shape of the eye.
- It refracts light to the retina.
89
What is the use of the ciliary body?
This contains ciliary muscles, which control the size of the lens during viewing nearby or distant objects.
90
What is the use of the lens?
It refracts light to make an image on the retina.
91
Of what use is the iris?
It is responsible for controlling the amount of light entering the eye.
92
What is the use of the retina?
It is where the image is formed in the eye.
93
What is the blind spot?
This is a region where the nerve fibers leave the eye to enter the optic nerve.
It has no light sensitive cells. When an image falls on this point, it is not taken to the brain thus blind spot.
94
What is the fovea?
This is a small depression in the center of the retina.
It has only cones in a high concentration. It is therefore a region on the retina that contains the largest number of sensory cells. Due to this, it produces the most accurate images in the eye.
95
What do the eyelids do?
These protect the eye and remove any foreign bodies that enter it. Regular blinking enables the spread of the fluid all over the exposed surface of the eye.
96
What is the use of Eye lashes?
They prevent dust particles and other objects from entering the eye.
97
Describe the process of control of light entering the eye
98
Describe the process of accommodation of the eye
99
What are Rods?
Photoreceptors which are more sensitive to light but do not distinguish colours
100
Of what use are rods in vision?
They enable one to see at night (at low light intensities), but only in black and white.
101
What are cones?
Cones are photoreceptors that provide colour vision
102
True or false
Cones are less sensitive
True
They contribute very little to night vision.
103
True or false
The fovea, the centre of the visual field, has rods, but has a very low density of cones
False
The fovea, the centre of the visual field, has no rods, but has a very high density of cones
104
True or false
The ratio of rods to cones increases with distance from the fovea
True
105
Which photoreceptors are present in the peripheral regions on the eye?
Only rods.
106
In day light, how may you achieve your sharpest vision?
By looking directly at an object, such light shines on the tightly packed cones in your fovea.
107
At night, why is looking directly at a dimly lit object ineffective?
Rods are the more sensitive light receptors which are found outside the fovea and more in the periphery of the eye.
108
Describe the structure of the human retina
- The retina is composed of 3 layers of cells containing a characteristic type of cells, i.e.
i) Photoreceptor layer (outermost layer):
- This contains the photosensitive cells, the rods and cones partially embedded in the choroid.
ii) Intermediate layer:
- This contains bipolar neuron with synapses connecting the photoreceptor layer to the third layer.
- Horizontal and Amacrine cells are found in this layer and enable lateral inhibition to occur.
iii) Inter surface layer:
- This contains ganglion cells with dendrites in contact with bipolar neuron and axons of the optic nerves.
109
What function do Horizontal cells and amacrine cells provide?
They function in neural pathways that integrate visual information before it is sent to the brain.
110
Describe the structure of rods and cones
- Rods and cones have an essentially similar structure and their photosensitive pigments are attached to the outer surface of the membrane in the outer segment.
- They have four similar regions where structure and function are shown below;
i) Outer segment:
- This is the photosensitive region where light energy is converted into a generator potential.
- The entire outer segment is composed of flattened membranous vesicles containing the photosensitive pigments.
ii) Constriction:
- The outer segment is almost separated from the inner segment by an in folding of the outer membrane.
- The two regions remain in contact by cytoplasm and pair of cilia which pass between the two.
iii) Internal/innersegment:
- This is an actively metabolic region.
- It’s packed with mitochondria producing energy for visual processes and polysomes for synthesis of proteins involved in the production of membranous vesicles and visual pigments.
- The nucleus is located in this region.
iv) Synaptic region:
- Here the cells form synapses with bipolar cells. Different bipolar cells may have synapses with several rods; this is called synaptic convergence and it increases visual sensitivity.
- Mono synaptic bipolar cells link one cone to one ganglion cell and this gives the cone greater visual acuity than the rods.
111
Why do cones have high visual acuity?
It is because they are highly packed at the fovea with direct connection to the optical nerves.
One cone cell synapses to one bipolar cell which in turn synapses onto one ganglion cell as the information is relayed to the visual cortex.
112
Define Visual acuity
This is the amount of detail that can be seen (image sharpness).
113
Give differences between rods and cones
Rods:
- Photochemical pigment is readily regenerated when bleached.
- Poor colour vision
- Have retinal convergence.
- The photosensitive pigment is rhodopsin.
- Outer segment is rod shaped
- Greater numbers (about 109 cells per eye)
- Distributed more at the periphery of the retina (absent at fovea), so used for peripheral vision.
- Sensitive to low light intensity – can detect a single photon of light, so are used for night vision.
- Only 1 type, so only give monochromatic vision.
- Many rods usually connected to one bipolar cell, so poor acuity (i.e. rods are not good at resolving fine detail).
Cones:
- Pigment take long to be regenerated once bleached.
- High ability of recognizing colours.
- Lack retinal convergence.
- The photosensitive pigment is iodopsin.
- Outer segment is cone shaped
- Fewer numbers (about 106 cells per eye).
- Concentrated in the fovea, so can only detect images in centre of retina.
- Not sensitive to low light intensity – need bright light, so only work in the day.
- 3 types (red green and blue), so are responsible for colour vision.
- Each cone usually connected to one bipolar cell, so good acuity (i.e. cones are used for resolving fine detail such as reading).
114
What is the significance of many rod cells connecting to one bipolar neuron?
- Retinal convergence increases the eye’s sensitivity.
- Light which activates one rod cell may be insufficient to generate an action potential in the bipolar neuron connected to it
- yet several rods when hit by light simultaneously may activate action potential the bipolar neuron due to summation of generator potentials.
- Impulse propagation to the CNS results in image formation.
115
What is the significance of one cone cell connecting to one bipolar neuron?
It improves visual acuity / precision / resolving power of the eye because light from two objects close together stimulate two cones, the brain receives two impulses and interprets this as two separate objects rather than when different impulses summate into a single message received by the brain
116
Describe light reception in the rods
- Each rod or cone in the vertebrate retina contains visual pigments that consist of a light absorbing molecule called retinal
- This is a derivative of vitamin A bound to a membrane protein called an opsin.
- The opsin present in rods, when combined with retinal, makes up the visual pigment rhodopsin.
- Absorption of light by rhodopsin shifts one bond in a retinal from a cis to a trans arrangement, converting the molecule from an angled shape to a straight shape.
- This change in configuration destabilizes and activates rhodopsin.
- Because it changes the color of rhodopsin from purple to yellow, light activation of rhodopsin is called “bleaching.”
- When this potential difference is large enough, it results into an impulse being generated into an optic nerve leading to the brain.
- Rhodopsin returns to its inactive state when enzymes convert retinal back to the cis form for it to be stimulated again (dark adaptation).
- In very bright light, however, rhodopsin remains bleached, and the response in the rods becomes saturated.
- If the amount of light entering eyes decreases abruptly, the bleached rods do not regain full responsiveness for some time.
- This is why you are temporarily blinded if you pass abruptly from the bright sunshine into a dark place.
117
Why are rods are more sensitive than the cones?
i) Their pigment is readily broken down and regenerate faster than that of the cones. That’s why they are mostly used for vision during conditions of low illumination or darkness.
ii) They show retinal convergence where separate rods add up or summate to build a generator potential upto a threshold.
118
Suggest why convergence of rod cells in the retina of human eyes reduces the ability to read small print in dim light.
Light falling on groups of rods is transduced into an impulse in only one receptor neurone so the power of resolution is low
119
Suggest a chemical explanation of convergence in the eye
- Only when many synaptic knobs release their neural transmitter simultaneously does its concentration reach a threshold at which depolarization of the post-synaptic membrane occurs.
120
What is the significance of one cone cell connecting to one bipolar neuron?
It improves visual acuity / precision / resolving power of the eye because light from two objects close together stimulate two cones, the brain receives two impulses and interprets this as two separate objects rather than when different impulses summate into a single message received by the brain
121
Why should pilots waiting to fly at night not be exposed to bright lights?
Even a short exposure to bright light would cause the rhodopsin in the rods to break down so destroying the pilot’s night vision.
122
Many nocturnal mammals like cats have a reflective layer – the tapetum lining the back of the eye rather than a pigmented layer. Suggest the value of this arrangement
Light will be reflected back off the tapetum so that it passes twice through the light-sensitive cells, thereby increasing the size of the generator potential developed in the cells
123
Explain why when sitting in a dimly lit room, , objects often appear slightly fuzzy and colours are more difficult to distinguish
Seeing objects using rod cells leads to lower visual acuity; rod cells are insensitive to colour
124
Explain why brightly coloured objects often appear grey in dim light
Only rods are stimulated by low-intensity (dim) light.
Rods cannot distinguish between wavelengths / colours of light, therefore the object is perceived only in a mixture of black and white i.e. grey.
125
At night, it is easier to see a star in the sky by looking slightly to the side of it rather than directly at it. Suggest why
this is so.
- Light reaching the earth from a star is of low intensity.
- Looking directly at a star focuses light on to the fovea, where there are only cone cells.
- Cone cells respond only to bright light, so they are not stimulated by dim light from the star and it cannot be seen.
- Looking to one side of the star ensures that light from the star is focused to the periphery of the retina, where there are mostly rod cells.
- These are stimulated by low light intensity and therefore the star is seen.
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What is the role of Horizontal cells?
They cause lateral inhibition which increases sensitivity and visual acuity i.e. they inhibit (cancel out)
equal intensity stimuli if received from two adjacent rods thereby increasing contrast between weakly stimulated and strongly stimulated areas. E.g. edges of objects stand out more clearly.
127
What is the role of Amacrine cells?
After partial processing, they transmit information about changes in the level of illumination
128
Explain why when a person moves from bright sunlight into a dimly-lit room objects in the room cannot at first be seen but they gradually become visible.
- In bright light, the circular muscles contract to narrow the pupil; and reduce on over-stimulation of the retinal cells by entry of light into the eye.
- In dim light, radial muscles contract to dilate the pupil slowly to allow entry of light; whose threshold at first is low to stimulate the rods for objects to be seen; but later improves to enable vision as the pupil dilates fully;
129
Explain why in the dimly-lit room, objects are only visible in black and white.
- Rods which are sensitive to light of low intensity do not respond to light of various wavelengths; causing images to appear black and white;
130
Explain why some nocturnal animals like cats close their pupils to a vertical slit and also squint in bright light
- The retina of nocturnal animals is almost entirely composed of rods; with rhodopsin which is particularly sensitive to low levels of light and breaks down so rapidly in bright light;
- The slit pupil and squinted eyes reduce the amount of light entering the eye to enable rhodopsin form faster than it breaks down for vision to occur;
131
Describe colour perception in the cones
- There are three different kinds of cone cell, each with a different form of opsin (they have the same retinal).
- These three forms of iodopsin are sensitive to different parts of the spectrum, so there are red cones (10%), green cones (45%) and blue cones (45%).
- Trichromatic theory of colour vision suggests that different colours are produced by the degree of stimulation of the different types of cone.
- When viewing an object, the three types of cone are stimulated to various degrees depending on the wavelength of light the object reflects.
- By comparing the nerve impulses from the three kinds of cone, the brain can interpret any colour.
- For example, when the red/blue/green receptors are stimulated in the ratio of 10:86:15, the color of an object is interpreted by the brain to be blue. When the ratio is 13:14:86, the color is interpreted as green, and when it is 100:20:99 the color is yellow.
132
What is Colour blindness?
It is the inability to distinguish between colours.
133
Explain what causes colour blindness
- The red, green and blue opsin proteins are made by three different genes.
- The green and red genes are on the X chromosome, which means that males have only one copy of these genes (i.e. they’re haploid for these genes).
- About 8% of males have a defect in one or other of these genes, leading to red-green colour blindness.
134
What is Binocular vision?
Binocular vision occurs when the visual fields of both eyes overlap so that the fovea of both eyes are focused on the same object.
135
What are the advantages of binocular vision over monocular vision?
- Larger visual fields.
- Damage to one eye is compesated for by the other e.g. it cancels the effect of the blind spot and provides the basis of stereoscopic vision.
136
What is an Eye defect?
An eye defect is a condition where the eye fails to focus an object well unless aided by external lenses.
137
What causes Myopia?
It results from eyeball being too long, so that the image is brought to a focus in front of the retina
138
Which lens corrects myopia?
It is corrected by a concave lens.
139
What causes hypermetropia/ Hyperopia?
It results from eyeball being too short, so that the image is brought to a focus behind the retina
140
What lens corrects hypermetropia?
It is corrected by a convex lens.
141
What is Astigmatism?
This is the condition in which asymmetry of the cornea and or lens causes uneven refraction of light around 360 degrees of a circle, resulting in an image that is not sharply focused on the retina.
142
What type of eyes do arthropods have?
Arthropods have compound eyes and some have simple eyes.
143
What are the characteristics of simple eyes in arthropods?
Simple eyes consist of a single lens that can distinguish between light and dark but cannot produce an image.
144
What are the characteristics of compound eyes in arthropods?
A compound eye consists of up to a thousand light detectors called ommatidia, each with its own light-focusing lens. Each ommatidium detects light from a tiny portion of the visual field.
145
What makes compound eyes effective for arthropods?
Compound eyes are very effective at detecting movement, which is an important adaptation for flying insects and small animals constantly threatened with predation. They can detect flickering at a rate six times faster than the human eye, allowing insects to resolve each frame of a film as a separate still image (flicker fusion).
146
Do insects have color vision?
Yes, insects have excellent color vision, and some, including bees, can see into the ultraviolet (UV) range of the electromagnetic spectrum. This allows them to detect environmental differences that are invisible to humans.
147
What are the functions of the parts of a compound eye?
The lens converges light rays onto the tip of the rhabdom. Pigment cells regulate the amount of light reaching the retinal cells and separate the ommatidium from its neighbor. The rhabdom is the light-sensitive part of the ommatidium where photochemical stimulation occurs, leading to depolarization of the membrane of the retinal cells. The ommatidia serve the same function as the rods and cones of the vertebrate eye but are much larger, resulting in reduced visual acuity in arthropods.
148
What are the three basic functions of the mammalian ear?
The mammalian ear performs three basic functions: detection of sound (hearing), head movements, and changes in gravity (balance or posture).
149
What are the three areas of the ear?
The ear is made up of three areas: the outer ear, middle ear, and inner ear.
150
What are the components and functions of the outer ear?
The outer ear consists of the pinna, auditory canal, and eardrum. The pinna (auricle) receives and concentrates sound waves. The auditory canal, which has hairs and wax to trap foreign bodies, transmits sound waves to the eardrum (tympanum). The eardrum is a thin membrane that transmits sound waves to the middle ear.
151
What are the components and functions of the middle ear?
The middle ear is an air-filled cavity in the skull comprised of three small bones called ossicles: the hammer (malleus), anvil (incus), and stapes (stirrup). These ossicles transmit sound vibrations from the eardrum to the oval window (fenestra ovalis), which then transmits the vibrations to the inner ear. The middle ear communicates with the mouth cavity through the Eustachian tube, which equalizes air pressure on both sides of the eardrum.
152
What are the components and functions of the inner ear?
The inner ear is filled with fluid and consists mainly of the cochlea, which contains the sensory auditory nerve that transmits impulses to the brain.
The inner ear also includes the semi-circular canals, utriculus, and sacculus, which form the vestibular apparatus responsible for controlling body balance and orientation.
The round window (fenestra rotunda) equalizes pressure in the cochlea.
153
Can you describe the process of hearing in mammals?
The process of hearing in mammals involves several steps:
- The pinna receives and concentrates sound waves.
- Sound waves are transmitted to the eardrum, which vibrates.
- Vibrations from the eardrum are transmitted to the ossicles in the middle ear.
- The ossicles transmit vibrations to the oval window at the entrance of the vestibular canal of the cochlea.
- The perilymph (fluid in the vestibular canal) vibrates, causing Reissner’s membrane to be displaced.
- Displacement of Reissner’s membrane causes the endolymph in the median canal to vibrate, which in turn causes the basilar membrane to vibrate.
- The vibration of the basilar membrane stimulates sensory cells in the organ of Corti, generating impulses.
- The auditory nerve transmits these impulses to the brain, which interprets them as sounds.
- The vibrations of the basilar membrane disturb the perilymph in the tympanic canal, and the round window absorbs these vibrations.
154
What is deafness?
Deafness is the inability to hear and can be caused by defects in the nerves or the conduction of sound waves.
155
What are the causes of deafness?
1) Accumulation and hardening of wax in the outer auditory canal, which presses against the eardrum. This can be controlled by using cotton buds to remove excess wax after softening it with warm water.
2) Blocking of the Eustachian tube due to accidents or infections such as the common cold. This can be treated with antibiotics to kill the bacteria causing the infection.
3) Some individuals are born with thick eardrums that do not easily vibrate. Hearing aids can solve this issue.
4) Ruptured eardrum due to accidents or infections. Sometimes the eardrum heals on its own, or a hearing aid can be used.
5) Damage to the cochlea from prolonged exposure to loud noise. This can be prevented by keeping sound volume low, as the cells of the organ of Corti in the cochlea cannot be repaired once damaged.
6) Fused ossicles due to infections causing inflammation in the middle ear or congenital conditions. This can be treated with medication to kill the microorganisms or through surgical operations to replace the ossicles.
7) Damage to the hearing center of the brain also causes deafness.
156
How does the human brain discriminate sound intensity and pitch?
The human brain discriminates sound quality in terms of pitch and intensity. The pitch of a sound depends on its wavelength, with high tones resulting from high-frequency (short wavelength) sounds and low tones from low-frequency (long wavelength) sounds.
157
How does the basilar membrane contribute to sound discrimination?
The basilar membrane is about 2-5 times wider at the apex than at its base between the oval and round windows. High-frequency sounds vibrate a short portion of the basilar membrane near the oval window, stimulating hair cells in that area, which the brain interprets as high-pitched sounds. Low-frequency sounds cause a larger portion of the basilar membrane to vibrate, stimulating hair cells further along the membrane. The brain interprets the impulses from these cells as low-pitched sounds. This allows the brain to determine the pitch of each sound based on the source of the impulse from the cochlea.
158
What is the role of the semi-circular canals in maintaining body balance?
The semi-circular canals are important for dynamic equilibrium.
Each canal terminates in a swelling known as the ampulla, which contains the cupula, a dome-shaped gelatinous structure in contact with sensory hairs.
There are three canals filled with fluid, arranged in three planes: vertical canals detect upward movement, horizontal canals detect backward and forward motion, and lateral canals detect side-to-side movements of the head.
When the head moves, the endolymph in the ampulla moves in the opposite direction, deflecting the cupula and stimulating the cristae (sensory cells).
These cells generate impulses that are transmitted by the vestibular neurons to the brain.
The brain interprets the pattern of impulses, detecting the direction and speed of movement, and sends instructions to relevant organs to maintain dynamic balance.
159
What is the role of the utriculus and sacculus in maintaining body balance?
The utriculus and sacculus contain structures called maculae, which maintain body posture (static equilibrium).
160
What is echolocation in bats?
Echolocation in bats allows them to fly swiftly at night without colliding with obstacles by using sound orientation in the environment.
Bats produce high-frequency sounds (short wavelengths) that are beyond human hearing.
They detect objects by using the echoes of these sounds, a phenomenon known as echolocation.
161
What is the structure and function of the macula in detecting head position?
Each macula consists of a patch of sensory cells with free ends embedded in the otolith, a gelatinous granule of calcium carbonate.
The otolith detects the position of the head with respect to gravity.
By varying the head position, the pull of gravity on the otolith tilts the sensory hairs, generating a pattern of impulses to the brain.
The brain interprets these impulses to provide information about the head's position and sends instructions to relevant muscles to restore balance.
When the head is upright, the otolith is positioned on top of the sensory cells, and no stimulation occurs.
162
What are the major advantages of sound echolocation in bats?
1. The sound vibrations produced by bats do not spread widely, and their echoes are refined enough to pinpoint obstacles accurately.
2. The short wavelengths of the sounds used in echolocation allow bats to locate even small objects.
163
How do bats and humans differ in their dependence on sound?
Bats depend on sound they produce and its deflection by objects for navigation and obstacle detection. Humans primarily depend on sounds produced by vibrating objects in the environment. Humans can discriminate between sounds and perceive sounds of low frequency.
164
What is the role of the skin as a sense organ?
The skin is responsible for the senses of pain, touch, pressure, and temperature. It contains mechanoreceptors, pain receptors, and thermoreceptors. Mechanoreceptors sense physical deformation caused by mechanical energy such as pressure, touch, stretch, and motion. Touch receptors, which are dendrites of sensory neurons, are embedded in layers of the skin. Pain receptors detect harmful conditions like extreme pressure and temperature, triggering defensive reactions. Thermoreceptors in the skin and hypothalamus detect heat and cold.
165
How do mechanoreceptors function in the skin?
Mechanoreceptors in the skin sense physical deformation caused by pressure, touch, stretch, and motion. They are often embedded in layers of the skin or at the base of hairs. For example, cats and rodents have sensitive mechanoreceptors at the base of their whiskers to detect tunnel size and nearby objects.
166
What triggers pain receptors in the skin?
Pain receptors detect stimuli that reflect harmful conditions, such as extreme pressure and temperature, and trigger defensive reactions like withdrawal from danger.
167
How do thermoreceptors in the skin work?
Thermoreceptors in the skin and hypothalamus detect heat and cold. They activate sensory receptors in response to temperature changes, explaining why we describe spicy foods as "hot" because they activate the same sensory receptors as hot things.
168
What is the role of the nose in sensing smell?
The nose is the receptor organ for smell and contains chemoreceptor cells stimulated by chemicals in the air. When air containing a chemical enters the nose, it dissolves in the moisture (mucus) in the nasal cavity, stimulating the chemoreceptor cells. These cells send nervous impulses through a sensory neuron to the olfactory lobe of the brain for interpretation.
169
What is the function of the tongue as a chemoreceptor organ?
The tongue changes chemical stimuli in the mouth into nervous impulses.
170
What are taste buds responsible for?
Taste buds contain chemo-receptor cells that distinguish between different tastes.
171
What are the four different kinds of taste distinguished by the tongue?
The tongue distinguishes between sweet, sour, salty, and bitter tastes.
172
How does the tongue process chemical stimuli in the mouth?
Chemicals dissolve in the saliva in the buccal cavity, stimulating taste buds in different parts of the tongue, which send impulses through sensory neurons to the brain for interpretation.
173
Give the differences between compound eye and mammalian eye
Compound eye
- No rods and cones.
- Consists of many repeated units able to function on their own - ommatidia.
- Lens is crystalline and very elastic.
- Has a rhabdom.
- No muscles attached to it i.e. it’s immovable.
- Has no eye lids and isn’t protected at all.
- Has a fixed focus (no accommodation).
- Overlap image is greater.
- Detect light parallel to its longitudinal access.
- Has poor resolving ability and poor visual acuity. Shows near sightedness.
Mammalian eye
- Rods and cones are present.
- The whole eye functions as a single unit.
- Lens is membranous and elastic.
- No rhabdom.
- Has muscles attached to it and is very movable. Has eyelids for external protection.
- Has adjustable focus (accommodation is possible). Overlap image is small.
- Detects light reaching it at all angles.
- Good resolving ability and greater visual acuity. Can see both near and far objects.
174
Give the similarities between compound eye and mammalian eye
- Both contain pigmented cells.
- Both have the cornea.
- Both possess convex lens.
- Both have nerve fibres to the brain.
- There is overlap of image in both.
