Nervous Coordination Flashcards
Describe the general structure of a motor neurone
Cell Body: Contains organelles and a high amount of RER
Dendrons: Branch into dendrites which carry impulses towards the cell body
Axon: Long unbranched fibre that carries nerve impulses away from the cell body
Describe the additional features of a myelinated motor neurone
Schwann cells: wrap around axon
Myelin sheath: Made of schwann cells
Nodes of Ranvier: gaps between schwann cells where there is no myelin sheath
How does an action potential pass along an un-myelinated neurone?
- stimulus leads to an influx of Na+ ions, the first section of the membrane depolarises
- Local electrical currents cause sodium voltage gated channels further along the membrane to open
- A wave of depolarisation occurs
Explain why myelinated axons conduct impulses faster than un-myelinated axons
Saltatory conduction: the impulse ‘jumps’ between each node of Ranvir. Depolarisation cannot occur where myelin sheath act as electrical insulators.
Impulses don’t travel along the whole length of the axon
What is resting potential?
The potential difference (voltage) across a neurone when it is not stimulated. Usually about -70mV.
How is resting potential established?
- Membrane is more permeable to K+ than Na+
2. Sodium potassium pump actively transports 3 Na+ out of the cell and 2K+ into the cell
What stages are involved in generating an action potential?
- Depolarisation
- Repolarisation
- Hyperpolarisation
- Return to resting potential
What happens during depolarisation?
- Stimulus- facilitated diffusion of Na+ ions into cell down electrochemical gradient
- P.d across membrane becomes positive
- If membrane reaches threshold potential, voltage gated Na+ channels open
- Significant influx of Na+ ions reversers the p.d to +40mV
What happens during repolarisation?
- Voltage gated Na+ channels close and voltage gated K+ channels open
- Facilitated diffusion of K+ ions out of cell down their electrochemical gradient
- P.d across membrane becomes more negative
What happens during hyperpolarisation?
- ‘Overshoot’ when K+ ions diffuse out = p.d becomes more negative than resting potential
- Refractory period: no stimulus is large enough to raise membrane potential to threshold
- voltage gated K+ channels close and sodium potassium pump re establishes resting potential
Explain the importance of the refractory period.
No action potential can be generated in hyperpolarised sections of membrane.
Ensures a unidirectional impulse
Ensures discrete impulses
Limits frequeny of impulse transmission
What is the all or nothing principle?
An action potential will only occur if the threshold is reached
What 3 factors affect the speed of conductance?
Myelin sheath
Axon diameter
Temperature
How can organisms detect the strength of a stimulus?
Larger stimulus raises membrane to threshold potential more quickly meaning a greater frequency of impulses
What is the function of a synapse?
Electrical impulses cannot travel over the junction between neurones
Neurotransmitters send impulses between neurones
New impulses can be initiated in several neurones at the same time
What happens in the presynaptic neurone when an action potential is transmitted from one neurone to another?
- Wave of depolarisation travels down causing voltage gated Ca2+ channels to open
- Causes vesticles to move down and fuse with the presynaptic membrane
- Exocytosis of neurotransmitter into synaptic cleft
How do neurotransmitters cross the synaptic cleft?
Via simple diffusion
What happens in the postsynaptic neurone when an action potential is transmitted from one neurone to another?
- Neurotransmitter binds to specific receptor on postsynaptic membrane
- Na+ channels open
- If influx of Na+ ions raises membrane to the threshold an action potential is generated
What is summation?
A neurotransmitter from several sub threshold impulses accumulates to generate an action potential
What is the difference between temporal and spatial summation?
Temporal= One presynaptic neurone releases neurotransmitter several times in a short period Spatial= Multiple neurones release a neurotransmitter
What are cholinergic synapses?
Use acetylcholine as their primary neurotransmitter, excitatory of inhibitory
What happens to acetylcholine from the synaptic cleft?
- Hydrolysis into acetyl and choline by acetylcholinesterase
- Acetyl and choline diffuse back into presynaptic membrane
- ATP is used to reform acetylcholine
What happens in an inhibitory synapse?
- Neurotransmitter binds to and open Cl- channels on postsynaptic membrane and triggers K+ channels to open
- Cl- moves in and K+ moves out via facilitated diffusion
- p.d becomes more negative
What is a neuromuscular junction?
Synaptic cleft between a presynaptic neurone and a skeletal muscle cell
Give 3 differences between cholinergic synapses and a neuromuscular junction
Cholinergic- To another neurone, excitatory or inhibitory, motor, sensory and relay neurones
Neuromuscular junction- Skeletal muscle, always excitatory, only motor neurones
How might drugs increase synaptic transmission?
Mimic shape of neurotransmitter
How might drugs decrease synaptic transmission?
Inhibit release of neurotransmitter
Decrease permeability of postsynaptic membrane to ions
Hyperpolarise postsynaptic membrane
