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 schwann cells where there is no myelin sheath
Name 3 processes schwann cells are involved in
- electrical insulation
- phagocytosis
- nerve regeneration
How does an AP pass along an unmyelinated neuron?
- Stimulus leads to influx of Na+ ions - first section of membrane depolarises
- Local electrical currents cause sodium vgc further along membrane to open - meanwhile section 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 (approx -70 mV)
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 4 stages in generating an AP
- Depolarisation
- Repolarisation
- Hyperpolarisation
- Return to resting potential
What happens during depolarisation?
- Stimulus—> faciliated 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?
- Na+ vgc close and K+ vgc open
- Facilitated diffusion of K+ ions out of cell down their electrochemical gradient
- p.d. across membrane become 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
- K+ vgc close & sodium-potassium pump re-establishes resting potential
Explain the importance of the refractory period
No AP 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 AP
All AP 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?
- 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
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 receptor to neurotransmitter (ligand-gated Na+ channels)
Outline what happens in the presynaptic neuron when an AP is transmitted from one neuron to another
- Wave of depolarisation travels down presynaptic neuron, causing Ca2+ vgc 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 AP 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, AP 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 and name the 2 types
Neurotransmitter from several sub-threshold impulses accumulates to generate AP
- temporal summation
- spatial summation
No summation at neuromuscular junctions
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 cholinergic synapses?
Use acetylcholine as primary neurotransmitter - excitatory or inhibitory found at:
- motor end plate (muscle contraction)
- preganglionic neurons (excitation)
- parasympathetic postganglionic neurons (inhibition)
What happens to acetylcholine from the synaptic cleft?
- Hydrolysis into acetyl and choline by acetylcholinesterase (AChE)
- Acetyl & choline diffuse back into presynaptic membrane
- ATP is used to reform acetylcholine for storage in vesicles
Explain the importance of AChE
- prevents overstimulation of skeletal muscle cells
- enables acetyl and choline to be recycled
What happens in an inhibitory synapse?
- Neurotransmitter binds to and 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
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 shape of neurotransmitter
How might drugs decrease synaptic transmission?
- inhibit release of neurotransmitter
- decrease permeability of postsynaptic membrane to ions
- hyperpolarise postsynaptic membrane
