5.3 Neuronal Communication Flashcards
1
Q
what is a myelinated nerve cell?
A
- their axon is insulated by a myelin sheath with small uninsulated sections called nodes of Ranvier
- the myelin sheath is formed by Schwann Cells which wrap around the axon
- means that electrical impulses do not travel down the whole axon but jump from one node to another = quicker impulse conduction
2
Q
what is a non myelinated neurone?
A
- the axon is uninsulated
- impulse travels down the entire length of the axon = slower impulse
3
Q
what is the structure of a motor neurone?
A
- Large cell body that lies within the spinal cord or brain
- a nucleus that is in the cell body
- lots of highly branched short dendrites extending from the cell body = large surface area for axon terminals of other neurones
- they have a single long axon
- carry impulses from the CNS to effectors
- ends at motor end plate
- dendron absent
4
Q
what is the structure of a sensory neurone?
A
- cell body branches off in the middle of the cell
- single long dendron carries impulses to cell body
- single short axon carries impulses away from cell body
- outside CNS
- connected to a sensory receptor
5
Q
what is the structure of a relay neurone?
A
- located in the CNS
- short dendrites
- axons are varying lengths
6
Q
what is the general structure of a neurone?
A
- dendrons/dendrites = carry impulses towards the cell
- axons = carry impulses away from the cell
- cell body contains lots of mitochondria and endoplasmic reticulum
- plasma membrane has specific channel proteins for Na +/ K+ ions
7
Q
What is the name for the receptors for: light, chemicals, mechanical strain, blood pressure, body fluids?
A
- light = photoreceptors
- chemicals = chemoreceptors
- mechanical strain = mechanoreceptors
- blood pressure = baroreceptors
- bodily fluids = osmoreceptors
8
Q
A
9
Q
What is the function of pacinian corpuscles?
A
- type of mechanoreceptor found deep within the skin - present in soles of the feet, joints, fingertips, tendons and ligaments
- respond to changes in pressure
- receptors are stimulated by pressure on the skin = establishes a generator potential
- connect to sensory neurones
10
Q
What is the structure of the pacinian corpuscles?
A
- consist of many layers of membrane around the end of the neurone - neuron has stretch mediated sodium ions channels which open when sufficient pressure is applied
- layers separated by a gel which contains sodium ions
11
Q
What is the process of converting mechanical pressure into a nerve impulse in a pacinian corpuscle?
A
- No pressure = stretch mediated sodium ion channels are too narrow, sodium remains outside the membrane = resting potential maintained
- Pressure is applied = layers are distorted causing stretch mediated sodium channels to open = sodium ions enter the neurone
- Generator potential is established = an influx of sodium ions enter the= depolarization of membrane = if enough generator potential produced, action potential produced = nerve impulse moves along the axon
12
Q
How does the active transport of sodium and potassium ions help establish/ maintain the resting potential?
A
- carrier proteins (sodium-potassium pumps) are present in the membranes of neurones = use ATP to actively transport 3 sodium ions out of the axon for every 2 potassium ions they transport in = larger concentration of +ve ions outside axon then inside = establishes electrochemical gradient = resting potential
13
Q
What is resting potential?
A
- in a resting axon, inside of axon always has a negative electrical potetial compared to the outside = resting potential
- usually -70 mV
14
Q
How does differential membrane permeability help establish and maintain resting potential?
A
- cell surface membrane has selective protein channels = allow sodium+potassium ions to move acroos mebrane by facilitated diffusion
- protein channels are less permeable to sodium ions that potassium ions = potassium ions diffuse back down their concentration gradient - out of axon - faster
- inside = low Na +, high K+
- outside = high Na+, low K+
15
Q
How is an action potential generated?
A
- Stimulus (an electrical impulse from another neurone or a chemial change) triggers sodium ion channels to open = Na+ diffuse into neurone down electrochemical gradient
- If threshold potential reached (about -55mV) action potential generated - voltage gated ion channels in axon membrane open = Na+ pass into axon down electrochemical gradient = reduces potential difference across the membrane as inside axon is less negative = depolarization = more channels open = more Na+ enter = more depolarization = positive feedback = action potential generated = +30mV
- Very soon after +30mV reached, sdium ion voltage gated channels in this section close = potassium ions diffuse back voltage gated channels open = diffusion of K+ out of axon down conc gradient = returns potential difference to normal = negative feedback
- Potassium ions diffuse back channels slow to close = too many K+ions out = hyperpolarisation = too negative potential difference = refractory period
- Once potassium ions diffuse back voltage gated channels are shut, sodium-potassium pump restores resting potential
16
Q
How is an action potential transmitted along an axon?
A
- acion potential = depolarisation of that section of axon = current causes opening of sodium ion channels a little up the axon = influx of sodium ions in this section = paction potential in this direction
- previous section is in the repolarisation stage = unresponsive
- wave of depolarization ensures action potential continue only in one direction towards axon terminal
17
Q
How does generation of action potential differ in myelinated and non myelinated nuerones?
A
- in myelinated depolaristion only occurs at the nodes
In non myelinated occurs down the whole axon
= both in one direction
18
Q
What is the importance of the refractory period?
A
- refractory period = when sodium ion channels (in repolarisation) and potassium ion channels (in hyper-polarization) are closed = that section is in a period of recovery and unresponsive
- ensures that acton potentials are discrete events
- ensures new action potential generated ahead
- impulse only travels in one direction = successful and efficient transmission of nerve impulses
- minimum time between action potentials
- length of refractory period determines max frequency o transmission of impulses
19
Q
How does myelinated affect the speed of conduction of impulses?
A
- myelin increases the speed of conduction as depolarization can only occurs at at nodes of Ranvier
- presence of Schwann cells = action potential jumps from one node to the next = saltatory conduction = faster
20
Q
How does the diameter of the nerve cell impact the speed of conduction?
A
- impulse will be conducted faster at neurones with a thicker axon as axon membrane has a greater surface area over which diffusion of ions can occur = increases rate of diffusion of Na+ and K+ = increases rate of depolarisaton
- axon with greater diameter possess a greater volume of cytoplasm = reduces their electrical resistance
21
Q
How does temperature affect the speed of conduction?
A
- colder conditions slow down conduction of nerve pulses as less kinetic energy for facilitated diffusion of Na+ and K+ ions
22
Q
What is the process of synaptic transmission?
A
- An impulse arrives at end of the presynaptic neruone
- Vesicles move towards and fuse with presynaptic membrane = releases neurotransmitters into synaptic cleft
- Neurotransmitters diffuse across synaptic cleft down conc gradient and attach to receptors on the postsynaptic membrane
- This triggers an impulse which travels down the postsynaptic neruone
- Neurotransmitters are recycled or destroyed
23
Q
What is a cholinergic synapse?
A
- a synapse that uses acetylcholine (ACh) as a neurotransmitter
24
Q
What is the process of transmission across a cholinergic synapse?
A
- Action potential arrives, depolarizing presynaptic membrane
- Ca 2+ ion channels open and calcium ions diffuse into the presynaptic neruone
- Presynaptic vesicles with ACh fuse with membrane and ACh released
- ACh diffuses acroos synaptic cleft and binds to receptors on the proteins
- Receptors open and sodium ions diffuse down electrochemical gradient into cytoplasm of postsynaptic neuron
- Post synaptic nuerone is depolarised
- ACh is broken down into acetate and choline by acetylcholinesterase
- Choline recycled into ACh
25
What are the key benefits of synapses?
- unidirectionality - synapses ensure the one way transmission of impulses as neurotransmitter is released on one side and receptors on the other
- summation - the effect of multiple impulses added together when small individual impulses do not reach threshold = allows for the effect of a stimulus to be magnified and prevents overwhelming the nervous system
- temporal summation - multiple impulses arrive with quick succession are added together to generate an action potential
- spatial summation - multiple impulses arrive simultaneously at different synaptic knobs
26
What are inhibitory synapses?
- inhibitory neurotransmitters can prevent the generation of an action potential by opening potassium ions channels into synaptic cleft the postsynaptic membrane which causes hyperpolarisation of the membrane
27
What is an excitatory synapse?
- excitatory neurotransmitters stimulate the generation of an action potential in the post synaptic membrane = opens sodium ions channels into channels in the postsynaptic membrane which causes depolarization of threshold reached
28
29
What happens if a neurone is subject to both excitatory and inhibitory synapses?
- sodium ions enter cell body following stimulation by excitatory synapse
- potassium ions diffuse out of cell body y stimulation fromm inhibitory synapse
- these cancel t the effect of each other and therefore threshold potential is not reached
