Nervous Coordination Flashcards
describe 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
schwann cells - wrap around axon many times
myelin sheath - made from myeli-rich membranes of schwann cells
nodes of ranvier - very short gaps between neighbouring schwann cells where there is no myelin sheath
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
- 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 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 and 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
- potential difference 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 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 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 and 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 and 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 a factor has a 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 neurones
- neurotransmitters send impulses between neurones/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 and vesicles of neurotransmitter
synaptic cleft: 20 - 30 nm 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
- 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 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
