6.2: Nervous Coordination Flashcards
Describe general structure of a motor neuron
Cell body: contains organelles and high proportion of RER
Dendrons: branch into dendrites which carry impulses towards cell body
Axon: long, unbranched fibre carries nerve impulse away from cell body
Describe the additional features of a myelinated motor neuron
Schwann cells: wrap around axons 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 sheaths act as electrical insulator
So axon does not travel along whole axon length
What is resting potential?
Potential difference (voltage) across neuron membrane when not stimulated (-50mV to -90mV, usually about -70mV in humans)
How is resting potential established?
- Membrane is more permeable to K+ than Na+
- Sodium-potassium pump actively transports 3Na+ out of cell and 2K+ into cell
Establishes electrochemical gradient: cell contents more negative than extracellular environment
Name stages in activating 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 electrochemical gradient
- p.d across membrane becomes more nevative
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 importance of the refractory period
No action potential can be generated in hyper polarised sections of membrane:
- ensures unidirectional impulse
- ensures discrete impulses
- limits frequency of impulse transmission
What is ‘all or nothing’ principle?
Any stimulus that causes the membrane to reach the threshold potential will generate an action potential.
All action potentials have same magnitude
Name factors that could affect speed of conductance
- myelin sheath
- axon diameter
- temperature
How does axon diameter affect 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
Students t-test (comparing means of continuous data)
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 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-gates Na+ channels)
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 and fuse with presynaptic membrane
- Exocytosis of neurotransmitter into synaptic cleft
How do neurotransmitters cross synaptic cleft?
Via simple diffusion
Outline what happens in the post synaptic 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 post synaptic membrane has complementary receptors.
So impulse always travels presynaptic -> postsynaptic
Describe summation and name the two types
Neurotransmitter from several sub-threshold impulses accumulates to generate an action potential
- temporal summation
- spatial summation
NB no summation at neuromuscular junction
What is difference between temporal and spatial summation?
Temporal: one presynaptic neuron releases neurotransmitter several times in quick succession
Spatial: multiple presynaptic neurons release neurotransmitter several
What are cholinergic synapses?
Use acetylcholine as primary neurotransmitter. Excitatory or inhibitory. Located at:
- motor end plate (muscle fibres)
- preganglionic neurons (excitation)
- parasympathetic postganglion neurons (inhibition e.g. of heart or breathing rate)
What happens to acetylcholine from synaptic cleft?
- Hydrolysis into acetyl and choline by acetlycholinesterase (AChE)
- Acetyl and choline diffuse back into presynaptic membrane
- ATP is used to reform acetylcholine for storage in vesicles
Explain 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 and 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
