3.6.2 Nervous Coordination Flashcards
Describe the general structure of a motor neurone
Cell body: contains organelles and high proportion of rough endoplasmic reticulum
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 neurone
Schwan 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 3 processss Schwann cells are involved in
- electrical insulation
- phagocytosis
- nerve regeneration
How does an action potential pass along an unmyelinated neuron?
1) stimulus leads to influx of Na+ ions. First section of membrane depolarises.
2) local electrical currents cause sodium voltage-gated channels further along membrane to open. The section behind begins to repolarises.
3) 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 (-70mV)
How is resting potential established?
1) membrane is more permeable to K+ than Na+
2) sodium-potassium pump actively transports 3Na+ out of cell and 2K+ into cell.
Establishes electrochemical gradient. Inside the cell is more negative than the exterior environment
Name the stages in generating an action potential
1) depolarisation
2) repolarisation
3) hyperpolarisation
4) return to resting potential
What happens during depolarisation?
1) stimulus: facilitated diffusion of Na+ ions into cell down an electrochemical gradient
2) potential difference across membrane becomes more positive
3) if membrane reaches threshold potential (-50mV), voltage gated Na+ channels open
4) significant influx of Na+ ions reversed potential difference back to +40mV
What happens during repolarisation?
1) voltage gated Na+ channels close and voltage gated K+ channels open
2) facilitated diffusion of K+ ions out of cell down their electrochemical gradient.
3) potential difference across membrane becomes more negative
What happens during hyperpolarisation?
1) overshoot when k+ ions diffuse out/ PD becomes more negative than resting potential
2) refractory period: no stimulus is large enough to raise membrane potential to threshold
3) 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 undirectional impulse
- ensures discrete impulses
- limits frequency of impulse transmission
What is the ‘all or 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 speed of conductance
- myelin sheath
- axon diameter
- temperature
How does axon diameter affect the speed of conductance?
Greater=faster
- less resistance to flow of ions (depolarisation and repolarisation)
- less ‘leakage’ of ions (easier to maintain membrane potential)
How does temperature affect speed of conductance?
Higher temp= faster
- faster rate of diffusion
- 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= raises membrane to threshold potential more quickly after hyperpolarisation = greater frequency of impulses
What is the function of synapses ?
- electrical impulse cannot travel over a junction between neurones
- neurotransmitters send impulses between neutrons/ 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, ER and vesicles of neurotransmitter
Synaptic cleft: 20-30nm gap between neurons
Postsynaptic neuron: has complementary receptors to neurotransmitter (ligand-gated Na+ channels)
Outline what happens in the presynaptic neuron when an action potential is transmitted from one neuron to another
1) wave of depolarisation travels down presynaptic neuron, causing voltage gated Ca2+ channels to open
2) vesicles move towards and fuse with presynaptic membrane
3) exocytosis of neurotransmitter into synaptic cleft
How do neurotransmitters cross the synaptic cleft
Via simple diffusion
