6.2 - Nervous coordination Flashcards
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 Schwamm cells where there is now myelin sheath
Name 3 processes Schwann cells are involved in.
- electrical insulation
- phagocytosis
- nerve regeneration
How does an action 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 begind 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 membrance 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 the cell and 2K+ into the cell.
Establishes electrochemical gradient: cell contents more negative than extracellular environment.
Name the stages in generating an action potential.
- Depolarisation
- Repolarisation
- Hyperpolarisation
- Return to the 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 diffusionb 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 membrance 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 the speed of conductance?
Higher temperature = faster
- faster rate of diffusion
- faster rate of respiration = more ATP for active transport to re-establish resting potential
- Temp too high = membrane proteins denature
Suggest an appropriate statisical test to determine whether a factor has significant effect on the speed of conductance.
Students 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.
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-gated Ca2+ channels to open.
- Vesicles move towards & fuse with presynaptic membrane.
- Exocytosis of neurotransmitter into synaptic cleft.
How do neurotransmitters cross the synaptic cleft?
Via 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.
Define summation.
Neurotransmitters from several sub-threshold impulses accumalates to generate action potential.
Name the 2 types of summation.
- Temporal summation
- Spatial summation
What is the difference between temporal and spatial summation.
- Temporal: one presynaptic neuron releases neurotransmitter several times in quick succession.
- Spatial multiple presynaptic neurons release neurotransmitter
What are colinergic synapses?
They use acetylcholine (ACh) as a primary neurotransmitter. They can either be ecitory or inhibitory and are located at:
- motor end plate (muscle contraction)
- preganglionic neurons (exciitation)
- parasympathetic postganglionic neurons
What happend to ACh from the synaptic cleft?
- Hydrolysis into actyle and choline by acetylcholinesterase.
- Acetyl & choline diffuse back into presynaptic membrane.
- ATP is used to reform acetylcholine for storage in vesicles.
Explain the importance of acetylcholinesterase.
- Prevents the overstimulation of skeletal muscle cells.
- Enables acetyl and choline to be recycled.
What happens in the inhibitory synapse?
- Neurotransmitter binds to and opens Cl- channels on postsynaptic membrane & triggers K+ channels to open.
- Cl- moves in and K+ moves out via facilitated diffusion.
- Potential difference becomes negative: hyperpolarisation.
Describe the structure of a neuromuscular junction
Synaptic cleft between a presynaptic neuron and a skeletal muscle cell.
How might drugs increase synaptic transmission?
- Inhibit AchE
- Mimic the shape of a neurotransmitter
How might drugs decrease synaptic transmission?
- Inhibit release of neurotransmitter
- Decrease permeability of postsynaptic membrane to ions
- Hyperpolarise postsynaptic membrane
