6.2 - Nervous Communication Flashcards
(35 cards)
Describe the general structure of a 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 the additional features of a 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
Name the 3 processes Schwann cells are involved in.
Electrical insulation
Phagocytosis
Nerve regeneration
How does an action potential pass along an unmyelinated neuron?
- Stimulus leads 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 unmyelinated axons.
Saltatory conduction - impulse ‘jumps’ from one node of Ranvier to another. Depolarisation cannot occur where myelin sheath acts as electrical insulator.
So impulse does not travel along whole axon length.
What is resting potential?
Potential difference (voltage) across neuron membrane when not stimulated
-50 to -90 mV
About -70mV in humans
How is resting potential established?
- Membrane is more permeable to K+ than Na+
- Sodium-potassium pump actively transports 3 Na+ out of cell & 2 K+ into cell.
Establishes electrochemical gradient - cell contents more negative than extracellular environment.
Name the stages 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.
- Potential difference across membrane becomes more positive.
- If membrane reaches threshold potential (-50mV), voltage-gated Na+ channels open.
- Significant influx of Na+ ions reverses 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 & 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 unidirectional impulse
- ensures discrete impulses
- limits frequency of impulse transmission
What is the ’all of nothing’ principle?
Any stimulus that causes the membrane to reach threshold potential will generate an action potential.
All action potentials have same magnitude.
Name the factors that affect the speed of conductance.
Myelin sheath
Axon diamete
Temperature
How does axon diameter affect the speed of conductance?
Greater 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) = more ATP for active transport to re-establish resting potential.
Temperature too high = membrane proteins denature
Suggest appropriate units for the maximum frequency of impulse conduction.
Hz
How can an organism detect the strength of a stimulus?
Larger stimulus raises membrane to threshold potential more quickly after hyperpolarisation = greater frequency of impulses.
What is the function of synapses?
Electrical impulses 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 the structure of a synapse.
Presynaptic neuron ends in synaptic knob: contains lots of mitochondria, endoplasmic reticulum & vesicles of neurotransmitter.
Synaptic cleft = 20-30nm gap between neurons
Postsynaptic neuron has complementary receptors to neurotransmitter
Outline what happens in the presynaptic neuron when an action potential is transmitted from one neuron to another**.
- Wave of depolarisation travels down presynaptic neuron, causing voltage-gates Ca2+ channels to open.
- Vesicles move towards & fuse with presynaptic membrane.
- Exocytosis of neurotransmitter into synaptic cleft.
How do neurotransmitters cross the synaptic cleft?
Simple diffusion
Outline what happens in the postsynaptic neuron when an 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.
Explain why synaptic transmission is unidirectional.
Only presynaptic neuron contains vesicles of neurotransmitter & only postsynaptic membrane has complementary receptors.
So impulse always travels presynaptic —> postsynaptic.
