Topic 6 Flashcards
(85 cards)
1
Q
What is a stimulus
A
A detectable change in the internal or external environment of an organism that leads to a response in the organism.
2
Q
How does the ability to respond to stimuli increase the chances of survival for an organism
A
To be able to detect and move away from harmful stimuli, such as predators and extremes of temperature or to detect and move towards a source of food clearly aid survival.
3
Q
How are stimuli detected
A
Receptors
4
Q
Outline receptors
A
-They are specific to one type of stimulus
-A coordinator formulates a suitable response to stimulus.
5
Q
Outline a coordinator
A
Formulates a suitable response to a stimulus. It may be at molecular level or involve a large organ such as the brain
6
Q
Outline effectors
A
A response is produced by an effector which may be at a molecular or involve the behaviour of a whole organism.
7
Q
What are the types of communication and their advantages
A
-Hormonal communication which occurs via chemicals and is a relatively slow process
-The nervous system has many different receptors and effectors linked to a central coordinator
8
Q
What is the simplest nervous response to a stimulus
A
A reflex arc
9
Q
What are the two major divisions of the nervous system
A
The central nervous system, made up of the brain and spinal cord
The peripheral nervous system, made up of pairs of nerves that originate from the brain or spinal cord
10
Q
Outline the subdivisions of the peripheral nervous system
A
-Sensory neurones: carry nerve impulses from receptors towards the central nervous system
-Motor neurones: carry nerve impulses away from the central nervous system to effectors
11
Q
Outline the motor nervous system divisions
A
-The voluntary nervous system: carries nerve impulses to body muscles and is under voluntary control
-The autonomic nervous system, which carries nerve impulses to glands, smooth muscle and cardiac muscle and is not under voluntary control
12
Q
Outline the spinal cord
A
A column of nervous tissue that runs along the back and lies inside the vertebral column for protection. It emerges at intervals along the spinal cord in pairs of nerves
13
Q
What is a reflex
A
An involuntary response to a sensory stimulus
14
Q
What is a reflex arc
A
The pathway of neurones involved in a reflex
15
Q
What is a spinal reflex
A
A reflex with one of the neurones in the spinal cord
16
Q
What is a spinal reflex
A
A reflex with one of the neurones in the spinal cord
17
Q
The main stages of a spinal reflex arc:
A
- The stimulus
- A receptor
- A sensory neurone
- A coordinator
- A motor neurone
- An effector
- The response
18
Q
Give an example of a spinal reflex arc (heat)
A
- heat from the hot object
- temperature receptors generate nerve impulses in the sensory neurone
- The sensory neurone passes nerve impulses to the spinal cord
- A coordinator links the sensory neurone to the motor neurone in the spinal cord
- Motor neurone carries nerve impulses from the spinal cord to a muscle in the upper arm
- an effector the muscle is stimulated to contract
- The hand is pulled away from the hot object
19
Q
Why are reflex arcs important
A
-Unconscious so the brain can carry out more complex responses. Some impulses are sent to the brain so it is informed of what is happening and sometimes override the reflex
-Protects the body from harm. They are effective from birth and do not have to be learnt.
-They are fast. because the neurone pathway is short with very few synapses
-The absence of any decision making process also means the action is rapid
20
Q
Outline features of all receptors
A
-Specific to a single type of stimulus
-Produces a generator potential by acting as a transducer: all stimuli involve a change in some form of energy. It is the role to convert the change in form of energy by the stimulus into a form (action potentials) that can be understood by the body
-They convert the energy of the stimulus into a nervous impulse known as a generator potential.
21
Q
Outline features of the pacinian corpuscle
A
-It responds only to mechanical pressure and no other stimuli
-Transduces the mechanical energy of the stimulus into a generator potential
22
Q
Where are pacinian corpuscles
A
-Occur deep in the skin and are most abundant on the fingers, soles of feet and external genitalia as well as joint, ligament and tendons
-In the centre of layers of tissue, each separated by a gel
23
Q
Outline the structure of the pacinian corpuscle
A
-The sensory neurone ending at the centre of the Pacinian corpuscle has a special type of sodium channel in its plasma membrane. This is called a stretch-mediated sodium channel
24
Q
Outline stretch-mediated sodium channels
A
Their permeability to sodium changes when they are deformed (e.g. by stretching)
25
How do pacinian corpuscles function in resting state
The stretch-mediated sodium channels of the membrane around the neurone of a Pacinian Corpuscle are too narrow to allow sodium ions to pass along them and the neurone has a resting potential.
26
What happens when pressure is applied to the Pacinian Corpuscle (4)
-It is deformed and the membrane around its neurone becomes stretched
-This stretching widens the sodium channels in the membrane and Na+ diffuses into the neurone
-The influx of Na+ changes the membrane potential producing a generator potential (depolarised)
-The generator potential in turn creates an action potential that passes along the neurone and then via other neurones, to the central nervous system
27
How do rod and cone cells work in the eyes
They are light receptors and are transducers by conserving light energy into the electrical energy of a nerve impulse
28
Outline rod cells
-Cannot distinguish different wavelengths of light and less to images only being seen in black and white
-There are more rod cells than cone cells
29
How do rod cells function
-Many rod cells are connected to a single sensory neurone in the optic nerve. They are used to detect light of low intensity. A certain threshold has to be exceeded before a generator potential is created in the bipolar cells to which they are connected.
30
Why is multiple rod cells being connected to a single bipolar cell an advantage
-There is a greater chance the threshold value will be exceeded than it only a single rod cell were connected to each bipolar cell.
-This is due to summation (rapid build-up of neurotransmitter in the synapse)
-As a result, rod cells allow us to see in low intensity, although only in black and white
31
How do rod cells respond to low-intensity light
-In order to create a generator potential, the pigment in the rod cells (rhodopsin) must be broken down. There is enough energy from low-intensity light to cause this breakdown.
32
Why do rod cells give low visual acuity
-Many rod cells link to a single bipolar cell so light received by rod cells sharing the same neurone will only generate a single action potential travelling to the brain regardless of how many of the neurones are stimulated.
-In perception the brain cannot distinguish between the separate sources of light that stimulated them, so two dots close together cannot be resolved and will appear as a single blob.
33
Outline cone cells
-Three different types, each responding to a different range of wavelengths of light. Depending on the proportion of each type that is stimulated we can perceive images in full colour.
-Less cone cells than rod cells
34
Why do cone cells only respond to high light intensity
-Often each cone cell has its own separate bipolar cell connected to a sensory neurone in the optic nerve
-The stimulation of a number of cone cells cannot be combined to help exceed the threshold value and so create a generator potential
35
How are cone cells sensitive to difference ranges of wavelengths
-They contain a different pigment than found in rod cells- iodopsin which requires a higher light intensity for its breakdown to achieve the energy needed to create a generator potential. Each type of cone cells contains a specific type of iodopsin
36
How do cone cells give good visual acuity
-It has its own connection to a single bipolar cell, meaning if two adjacent cone cells are stimulated the brain receives two impulses to it can distinguish the two separate sources of light that stimulated the two cone cells and two dots close together can be resolved and will appear as two dots
37
What is the fovea
Light is focused by the lens on the part of the retina opposite the pupil, which receives the highest intensity of light. Therefore rod cells aren’t found here but cone cells are.
38
Outline the distribution of rod and cone cells
-Cone cells are found at the fovea and the concentration diminishes further away from the fovea
-At the peripheries of the retina where light intensity is at its lowest only rod cells are found
39
What is the autonomic nervous system
Controls the involuntary activities of muscles and glands
40
What are the two divisions of the autonomic nervous system
-Sympathetic nervous system
-Parasympathetic nervous system
41
Outline the sympathetic nervous system
-In general, this stimulates effectors and so speeds up any activity.
-It acts like an emergency controller and controls effectors during exercise or powerful emotions.
-It helps us cope with stressful situations by heightening our awareness and preparing us for activity.
42
Outline the parasympathetic nervous system
-This inhibits effectors and so slows down any activity. It controls activities under normal resting conditions. It is concerned with conserving energy and replenishing the body’s reserves.
43
How do the sympathetic and parasympathetic nervous systems work together
They are antagonistic and oppose one another, if one system contracts a muscle the other relaxes it
44
Outline how heart rate is controlled
-It is myogenic so its contraction is initiated from within the muscle itself. Within the wall of the right atrium of the heart is a distinct group of cells known as the sinoatrial node (SAN). Here the initial stimulus for contraction originals and has a basic rhythm of stimulation that determines heartbeat so it is referred to as the pacemaker,
45
The sequence of events that controls basic heart rate:
1. wave of electrical excitation spreads out from the SA node across both atria, causing them to contract
2. A layer of non-conductive tissue (AV septum) prevents the wave crossing to the ventricles
3. The wave of excitation enters a second group of cells called the AV node which lies between the atria
4. The AV node after a short delay conveys a wave of electrical excitation between the ventricles along a series of specialised muscle fibres called Purkyne tissue which collectively make up the bundle of His
5. The bundle of his conducts the wave through the AV septum to the base of the ventricles, where the bundle branches into smaller fibres of Purkyne tissue
6. The wave of excitation is released from the Purkyne tissue, causing the ventricles to contract quickly at the same time, from the bottom of the heart upwards
46
Summarised version of the sequence of control of basic heart rate:
-A wave of electrical activity spreads out from the SA node
-Wave spreads across both atria causing them to contract and reaches the AV node
-AV node conveys wave of electrical activity between the ventricles along the bundle of His and releases it at the apex causing the ventricles to contract
47
What is the bundle of His
A series of specialised muscle fibres called Purkyne tissue which collectively make up a structure called the bundle of His
48
Where are changes to the heart rate controlled
-The medulla oblongata
49
What centres does the medulla oblongata have that allows it to control change in heart rate
-A centre that increases heart rate, linked to the SA nod by the sympathetic nervous system
-A centre that decreases heart rate, linked to the SA node by the parasympathetic nervous system
50
What are chemoreceptors
Found in the wall of the carotid arteries and are sensitive to changes in the pH of the blood that results from changes in CO2 concentration. (In solution CO2 forms an acid and lowers pH)
51
How do chemoreceptors control change of heart rate in response to pH change:
1. When the blood has a higher CO2 conc. it’s pH is lowered
2. The chemoreceptors in the aorta and carotid artery detect this and increase the frequency of nervous impulses to the centre in the medulla oblongata that increases heart rate
3. This centre increases the frequency of impulses via the sympathies nervous system to the SA node. This increases the rate of production of electrical waves by the SA node and increases the heart rate
4. The increased blood flow that this causes leads to more CO2 being removed by the lungs and so the CO2 conc. of the blood returns to normal
5. As a consequence the pH of blood rises to normal and chemoreceptors reduce impulse frequency to medulla oblongata
6. The medulla oblongata reduces the frequency of impulses to SA node leading to a reduction in the heart rate
52
Summarised version of how chemoreceptors control heart rate:
-Increased muscle activity
-More CO2 produced by respiration so blood pH is lowered
-Chemical receptors increase frequency of impulses to the medulla oblongata
-centre in MO increases frequency of impulses to SA node via sympathetic nervous system
-SA node increases heart rate
-Increased blow flow removes CO2 faster
-CO2 conc. returns to normal
53
How do pressure receptors control change in heart rate when blood pressure is higher than normal
-Pressure receptors in the walls of the carotid artery and aorta transmit more nervous impulses to the centre of the MO that decreases heart rate.
-This centre sends impulses via the parasympathetic nervous system to the SA node leading to a decrease in heart rate
54
How do pressure receptors control change in heart rate when blood pressure is lower than normal
-Pressure receptors in the walls of the carotid artery and aorta transmit more nervous impulses to the centre of the MO that increases heart rate.
-This centre sends impulses via the sympathetic nervous system to the SA node leading to an increase in heart rate
55
Outline the nervous system
-Use nerve cells to pass electrical impulses along their length and stimulate target cells by secreting neurotransmitters on to them, resulting in rapid communication between specific parts of an organism. The responses produced are often short-lived and restricted to a localised region of the body.
56
Outline the hormonal system
-Produces hormones that are transported in the blood plasma to their target cells, which have specific receptors on their cell surface membranes and the change in the concentration of hormones stimulates them. This results in a slower less specific form of communication between parts of an organism.
-The responses are often long lasting and widespread
57
What is a neurone
Specialised cells adapted to rapidly carry electrochemical changes called nerve impulses from one part of the body to another.
58
What are mammalian motor neurones made up of
-A cell body
-Dendrons
-An axon
-Schwann cells
-A myelin sheath
-nodes of Ranvier
59
Outline a cell body in a motor neurone
Contains all the usual cell organelles, including a nucleus and large amounts of rough endoplasmic reticulum. This is associated with the production of proteins and neurotransmitters.
60
Outline dendrons in a neurone
-Extensions of the cell body which subdivide into smaller branched fibres, called dendrites that carry nerve impulses towards the cell body
61
Outline an axon in a neurone
-A single long fibre that carries nerve impulses away from the cell body
62
Outline Schwann cells
-Surround the axon protecting it and providing electrical insulation.
-They also carry out phagocytosis (the removal of cell debris) and play a part in nerve regeneration.
-Schwann cells wrap themselves around the axon many times, so that layers of their membranes build up around it.
63
Outline the myelin sheath
-Forms a covering to the axon and is made up of the membranes of the Schwann cells. These membranes are rich in a lipid called myelin. Neurones with a myelin sheath are called myelinated neurones.
64
Outline nodes of Ranvier
Constrictions between adjacent Schwann cells where there is no myelin sheath. These are 2-3um long and occur every 1-3mm in humans
65
What are sensory neurones
-Transmit nerve impulses from a receptor to an intermediate or motor neurone
-They have one dendron that is often very long and carries the impulse towards the cell body and one axon that carries it away from the cell body
66
What are motor neurones
Transmit nerve impulses from an intermediate or relay neurone to an effector, such as a gland or a muscle, Motor neurones have a long axon and many short dendritic
67
What are relay neurones
Transmit impulses between neurones. They have numerous short processes
68
How are sodium ions and potassium ions controlled across the membrane by the phospholipid bilayer
-Prevents ions diffusing across it
-Channel proteins span the bilayer and they have channels called ion channels which pass through them. Some have ‘gates’ which can be opened or closed so the ions can moved through them by facilitated diffusion.
-Some gates remain open at the time
-Some carrier proteins actively transport K+ into the axon and Na+ out (sodium potassium pump)
69
Summarise resting potential
As a result of controls of ions, the inside of an axon is negatively charged relative to the outside. This ranges from -50 to -90 mVs but is usually -65 in humans
-The axon is said to be polarised
70
How is the potential difference inside and outside the axon established at resting potential
1. Na+ is actively transported out by pumps
2. K+ is actively transported in by pumps
3. The active transport of Na+ is greater than that of L+; three Na+ move out for every two K+ that moves in
4. Outward movement or Na+ is more than inward of K+ so there is more Na+ in surrounding fluid than the axon, creating an electrochemical gradient
5. Na+ diffuses back into the axon while K+ diffuses out
6. Gates that allow K+ to move out are open, whilst gates allowing Na+ to move in are closed
71
Summarise action potential
When a stimulus of sufficient size is detected by a receptor in the nervous system it causes a temporary reversal of the charges either side of a particular point on the axon membrane. If it is great enough the negative charged of -65mV inside becomes a otitis charge of around +40mV. This is the action potential and the axon is said to be depolarised
72
Summarise why depolarisation occurs
The channels in the axon membrane change shape and open or close depending on the voltage across the membrane
73
Full process of an action potential:
1. At rest some K+ channels are open but the Na+ channels are closed
2. Energy from stimulus causes Na+ gated channels to open and Na+ diffuses into the axon through channels and their electrochemical gradient, they cause a reversal in the pD across the membrane
3. As Na+ diffuses into axon more Na+ channels open causing a greater influx
4. Once potential of around +40mV is established Na+ gates close and the voltage gates on K+ begin to open
5. With some K+ open the gradient that was preventing further outward movement of K+ ions is now reversed, causing more K+ channels to open so more K+ diffuses out, starting repolarisation
6. The outward diffusion of K+ causes an overshoot of the electrical gradient with the inside of the axon being more negative than usual (hyper polarisation). The K+ closable gates close and pump pumps again causing Na+ to be pumped out and K+ in. The resting potential of -65mV is re-established and the axon is repolarised
74
Summarise how action potentials move across an axon
-Rapidly
-As one region of axon produces an action potential and becomes depolarised it acts as a stimulus for the depolarisation of the next region of the atom. Action potentials are generated along each small region of the axon membrane.
-The previous region of the membrane undergoes repolarisation
75
Outline the passage of an action potential along an unmyelinated axon
1. At resting potential Na+ outside axon is high, whereas K+ inside is high. The overall concentration of + ions is greater on the outside compared to inside so the axon is polarised
2. Stimulus causes influx of Na+ and reversal of charge on axon membrane. This is action potential and membrane is depolarised
3.Localised electrical currents established by influx of Na+ causes opening of Na channels along the axon so Na+ influx in this region causing depolarisation. Behind this region Na+ channels close and K+ open so K+ leave the axon so depolarisation moves along the membrane
4. The action potential is propagated along the axon. The outward movement of the K+ has continued to the extent that the axon membrane behind the action potential has returned to its original charged state, it has been re polarised
5. Repolarisation of the axon allows Na+ to be actively transported out, again retuning axon to its resting potential in readiness for a new stimulus if it comes
76
Outline passage of an action potential along a myelinated axon
-The fatty sheath of myelin around the axon acts as an electrical insulator, preventing action potentials from forming
-At intervals of 1-3mm there are breaks in this myelin insulation, called Nodes of Ranvier, action potentials occur at this point.
-Localised circuits arise between adjacent nodes and action potentials jump from node to node in a process called saltatory conduction. This means the passage is faster compared to an unmyelinated axon
77
Why does an axon take longer to pass along an unmyelinated axon
The events of depolarisation in an unmyelinated neurone take place all the way along an axon
-Whereas in a myelinated axon action potentials can jump from node of ranvier to node in saltatory conduction
-Increases speed of conductance from 30ms-1 to 90ms-1
78
How does the diameter of the axon affect the speed at which an action potential travels
-The greater the diameter, the faster the speed. This is due to less leakage of ions from a large axon (makes membrane potentials harder to maintain)
79
How does the temperature affect the speed at which an action potentials travels along the axon
-Affects the rate of diffusion of ions and therefore the higher the temperature the faster the nerve impulse. The energy for active transport comes from respiration. Enzymes function more rapidly at higher temperatures which controls respiration
80
What is the all-or-nothing principle
When a certain level of stimulus, called the threshold value triggers an action potential a. Below the threshold no action potential and therefore no impulse is generated.
81
How can an organism perceive the size of a stimulus
-The number of impulses passing in a given time, the larger the stimulus the more impulses generated in a given time.
-By having different neurones with different threshold values, the brain interprets the number and type of neurones that pass impulses as a result of a given stimulus and thereby determines its size
82
What is the refractory period
After an action potential in any region or an axon there is a period when inward move,ent of sodium ions is prevented as the sodium voltage gated channels are closed. During this time it is impossible for a further action potential to be generated.
83
Why does the refractory period ensure action potential are propagated in one direction only
AP can only pass from active to a resting region. Because AP cannot be propagated in a region that is refractory, which meant that they can only move in a forward direction preventing AP from spreading in both direction with they otherwise would do.
84
How does the refractory period produce discrete impulses
Anew action potential cannot be formed immediately behind the first one, Ensuring that action potential are separated from one another
85
How does the refractory period limit the number of action potentials
As APs are separated from one another this limits the number of APs that can pass along an axon in a given time and thus limits the strength of stimulus that can be detected
