Nervous coordination Flashcards
(37 cards)
Describe the general composition of a motor neuron.
Cell body, dendron, axon
Describe the structure of a cell body
contains organelles & high proportion of RER.
Describe the structure of a dendron.
branch into dendrites which carry impulses towards cell body
Describe the structure of an 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 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, usually about -70 mV in humans).
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.
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.
- p.d. 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 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 the speed of conductance.
● Myelin sheath
● Axon diameter
● 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 an appropriate statistical test to determine
whether a factor has a significant effect on the speed of conductance.
Student’s t-test (comparing means of continuous data)
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 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 the structure of a synapse.
Presynaptic neuron ends in synaptic knob: contains lots of mitochondria, endoplasmic reticulum vesicles of neurotransmitter.
synaptic cleft: 20-30 nm gap between neurons.
Postsynaptic neuron: has complementary receptors to neurotransmitter (ligand-gated Na+ channels).
