Nervous coordination Flashcards
Topic 6.2
General structure of motor neuron
Cell body: contains organelles & high proportion of RER.
Dendrons: branch into dendrites which carry impulses towards cell body.
Axon: long, unbranched fibre carries nerve impulses away from cell body.
Describe additional features of myelinated motor neuron
- Schwann cells: wrap around axon many times
- Myelin sheath: made from myelin-rich membranes of Schwann cells.
Nodes of Ranvier: very short gaps between neighbouring Schwann cells where there is no myelin sheath
How does action potential pass along an unmyelinated neuron?
- Stimulus lead to influx of Na+ ions. First section of membrane depolarises.
- Local electrical currents cause sodium voltage-gated channels further along membrane to open. Meanwhile, the section behind begins to repolarise.
- Sequential wave of depolarisation
Explain why myelinated axons conduct impulses faster than myelinated axons
Saltatory conduction: Impulse ‘jumps’ from one node of Ranvier to another. Deplarisation cannot occur where myelin sheath acts as electrical insulator
So impulse does not travel along whole axon length.
How is resting potential established?
- Membrane is more permeable to K+ than Na+.
- Sodium-potassium pump actively transports 3Na+ out of cell & 2K+ into cell
Establishes electrochemical gradient: cell contents more negative than extracellular environment
What happens during depolarisation? (1st stage of action potential)
- Stimulus -> facilitated diffusion of Na+ ions into cell down electrochemical gradient.
- p.d. across membrane becomes more positive.
- If membrane reaches threshold potential (-50mV),
voltage-gated Na+ channels open. - Significant influx pf Na+ reverses pd.d to +40mV.
What happens during repolarisation? (2nd stage of action potential)
- Volatge-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? (3rd stage of action potential)
- ‘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 & sodium-potassium pump re-establishes resting potential.
Importance of refractory period
No action potential can be generated in hyper-polarised sections of membrane:
- Ensures unidirectional impulse
- Ensures discrete impulses
- Limits frequency of impulse transmission
How does axon diameter affect speed of conductance?
Great diameter= faster
- Less resistance to flow of ions (depolarisation & repolarisation)
- Less ‘leakage’ of ions (easier to maintain membrane potential)
How does temperature affect speed of conductance?
Higher temperature= faster
-Faster rate of diffusion (depolarisation & repolarisation)
- Faster rate of respiration (enzyme-controlled) = ATP for active transport to re-establish resting potential.
Temperature too high = membrane proteins denature
Function of synapses
- Electrical impulse cannot travel over junction between neurons
- Neurotransmitters send impulses between neurons/ from neurons to effectors
- New impulses can be initiated in several different neurons for multiple simultaneous responses
Describe structure of a synapse
Presynaptic neuron end in synaptic knob: contains lots of mitochondria, endoplasmic reticulum & vesicles of neurotransmitter.
Synaptic cleft: 20-3- nm gap between neurons
Postsynaptic neuron: has complementary receptors to neurotransmitter (ligand-gated Na+ channels)
What happens in presynaptic neuron when action potential is transmitted from one neuron to another?
- Wave of depolarisation travels down presynaptic neuron, causing voltage-gated Ca= channels to open.
- Vesicles move towards & fuse with presynaptic membrane
- Exocytosis of neurotransmitter into synpatic cleft
What happens in postsynaptic neuron when action potential is transmitted from one neuron to another
- Neurotransmitter binds to specific receptor on postsynaptic membrane.
- Ligand-gated Na+ channels open.
- If influx of Na+ ions raises membrane to threshold potential, action potential is generated.
Define summation & name 2 types
Neurotransmitter from several sub-threshold impulses accumulates to generate action potential:
- Temporal summation
- Spatial summation
NB no summation at neuromuscular junctions.
Differences between temporal and spatial summation
Temporal: one presynaptic neuron releases neurotransmitter several time in quick succession.
Spatial: multiple presynaptic neurons release neurotransmitter
What are cholinergic synapses?
Use acetylecholine as primary neurotransmitter. Excitatory or inhibitory. Located at:
- Motor end plate (muscle contraction)
- Preganglionic neurons (excitation)
- Parasympathetic postganglionic neurons (inhibition e.g. of heart or breathing rate)
What happens to acetylcholine from synaptic cleft?
- Hydrolysis into acetyl and choline by acetylcholinerase (AChE)
- Acetyle & choline diffuse back into presynaptic membrane
- ATP is used to reform acetylcholine for storage in vesicles
What happens in inhibitory synapse?
- Neurontransmitter binds to & opens Cl- channels on postsynaptic membrane & triggers K+ channels to open.
- Cl- moves in & K+ moves out via facilitated diffusion.
- p.d. becomes more negative: hyperpolarisation
How might drugs DECREASE synaptic transmission
- Inhibits release of neurotransmitter
- Decreases permeability of postsynaptic membrane to ions
- Hyperpolarise postsynaptic membrane
What is the ‘all or nothing’ principle?
Any stimulus that causes membrane to reach threshold potential will generate action potential
All action potentials have same magnitude
Explain how synaptic transmission is unidirectional
Only presynaptic neuron contains vesicles of neurotransmitter & only postsynaptic membrane has complementary receptors.
So impulse always travels presynaptic -> postsynaptic
Explain importance of AChE
- Prevents overstimulation of skeletal muscle cells
- Enables acetyl & choline to be recycled
How might drugs INCREASE synaptic transmission?
- Inhibit AChE
- Mimic shape of neurotransmitter
