NERVOUS SYSTEM Flashcards
(44 cards)
Stimulus
Change in an organism’s environment that can be detected by receptor cells
Receptor
Specialised cell that detects a stimulus and initiates a nerve impulse by creating a generator potential
Kinesis
Change in the speed of random movement in response to environmental stimulus (non-directional response)
Taxes
Directed movement toward or away from a stimulus (directional response)
Positive Taxes: Organism moves towards stimulus
Negative Taxis: Organism moves away from stimulus
Tropism
Response to stimulus by growing in a certain direction
Central Nervous System
The brain and the spinal cord
Peripheral Nervous System
Pairs of nerves that go to and from the brain and spinal cord (the CNS)
Sensory Neurones
Carries nerve impulses from receptors TO the CNS
Motor Neurones
Carries nerve impulses FROM CNS to the effectors, it is of two types:
Voluntary Nervous System: Caries nerve impulses to body muscles and is voluntary
Autonomic Nervous System: Carries nerve impulses to glands, smooth muscles and cardiac muscle and is involuntary
The Autonomic Nervous System is of two types
Sympathetic Nervous System: Stimulates effectors and therefore speeds up any activity
Parasympathetic Nervous System: Inhibits effectors and therefore slows down any activity
THEY ARE BOTH ANATGONISTIC
The Medulla Oblongata changes the heart rate in response to two receptors:
Chemoreceptors
Chemoreceptors in the Carotid and Aortic arteries which are sensitive to a change in CO2 level in blood
When CO2 levels high (e.g. during exercise):
There is a change in pH which is detected by chemoreceptor
Chemoreceptor sends impulses to Medulla Oblongata
Medulla Oblongata increases frequency of impulses down Sympathetic nerve to SAN
Heart beats faster
Increased blood flow removes CO2 faster
The Medulla Oblongata changes the heart rate in response to two receptors:
Pressure receptors
Pressure receptors in the Carotid Sinus which are sensitive to a change in blood pressure
If blood pressure is high, pressure receptors cause Medulla Oblongata to increase frequency of impulses down the Parasympathetic nerve to slow down the heart rate
Pacinian Corpuscle
it is a pressure receptor found on the skin and it detects SPECIFIC pressures and vibrations
Pressure on skin changes shape of pacinian corpuscle and Lamella layers are distorted
Distortion of lamella layers causes Stretch Mediated Sodium Channels to open which are on the sensory neurone
Sodium Channel allow positive Na+ ions to enter the negative sensory neurone causing depolarisation
This is known as a generator potential, if it reaches the threshold it triggers an action potential
The amount of sodium channels that open depends on the pressure applied by the stimulus
Therefore objects with small surface area cause a large stimulus as they cause a lot of pressure, like a pin
Rod Cells
they are photoreceptors that are found on the retina and they detect light, they then send impulses to the Optic Nerve which takes it to the brain
Many rods converge to one neurone (RETINAL CONVERGENCE) Low giving unclear image As brain cannot distinguish between the separate rods that generated the impulse Rhodopsin Found evenly all over retina Very sensitive to light due to RETINAL CONVERGENCE Black and white vision in poor light
Cone Cells
Only single cone per neurone
High giving sharp image
As brain can distinguish where the impulse came from as impulse was sent by single cone
Iodopsin
All over retina but more concentrated at fovea, therefore we move our heads to see stuff properly
Only functions in bright light, i.e not as sensitive to light
Seeing colour and detail in bright light
RETINAL CONVERGENCE:
Stimulation of several rods results in enough Neurotransmitters being released to reach the threshold value for an action potential in the bipolar neurone in low light intensities (also called spatial summation)
Why we have a high degree of visual sensitivity in low light levels
Several rod cells connected to each bipolar cell
Additive effect of small amount of light striking several rod cells
This creates a large enough depolarisation to generate an action potential
Why we have a high degree of visual acuity
Each cone cell connected to an individual neurone
Light strikes individual cone cell to generate a separate action potential
Very small area of retina stimulated, so very accurate vision
Reflex Arc/Spinal Reflex (this is a special pathway of neurones)
Rapid automatic response to a stimulus
Involves just 3 neurones
Does not involve the brain
One neurone is inside the Spinal Cord which is called the Intermediate/Relay Neurone, the other two neurones are the Sensory and Motor neurone
They are under genetic control and are effective from birth
The sequence is:
Stimulus → Receptor → Sensory neurone → Intermediate neurone → Motor neurone → Effector → Response
Importance of the Reflex Arc/Spinal Reflex
Brain is not overloaded with situations in which the response would be the same
Brain is free to carry out more complex responses
Protects from harmful stimuli as it is fast
Along the axon, there is
Active Transport: Na+/K+ pump; 3Na+ leaves axon and 2K+ enters. Therefore the p.d. of the axon is -70mV.
The splitting of ATP controls this active transport.
Facilitated Diffusion: There are Sodium and Potassium ion channels (INTRINSIC PROTEINS) in the membrane. The Na+ ions come into axon through their channel and K+ ions come out of axon through their channel. There are more K+ channels than Na+ channels. At resting, both channels are closed, but they still ‘leak’.
Therefore, the resting potential is -70mV
The Myelin Sheath
provides protection and electrical insulation
Action Potential
Depolarisation:
Neurone stimulated causing a few chemical-gated Na+ channels to open allowing Na+ ions to diffuse into axon
If stimulus large enough to make enough Na+ move in that the potential changes from -70 to -50 (threshold value), then all the other Na+ channels open that are VOLTAGE gated
More Na+ enter (positive feedback – change creating more change)
Repolarisation:
When potential reaches +40mV, then K+ channels open causing K+ to rush out making potential negative again
Hyperpolarisation:
The K+ channels remain open a little longer causing a little extra K+ to leave, making the potential -80mV
At -80mV, the K+ channels close and the Na+/K+ pump restores the potential to -70mV using ATP
All action potentials are the same size
Speed of transmission
Axon Diameter: the larger, the quicker the impulse
Less resistance to flow of ions in/out of axon, therefore depolarisation reaches other parts of neurone cell membrane quicker. Also this allows less leakage of ions.
Temperature: the higher, the quicker the impulse
Ions diffuse faster in facilitated diffusion, also in Na+/K+ pump is controlled by active transport which needs ATP from respiration, respiration depends on enzymes which rely on temperature
Saltatory Conduction; action potential goes from one Node of Ranvier to another due to insulating myelin. Therefore action potentials can only occur at Nodes of Ranvier
Therefore is something has a very low temperature, to compensate, it may have a large axon diameter.
