TOPIC 6 - Nervous coordination Flashcards
describe the general structure of a motor neurone
cell body - 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 the 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 three 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
- Notes of electrical current causes 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 electrical insulator. So impulse does not travel along the whole axon length.
what is resting potential?
potential difference across neuron to membrane when not stimulated (-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
= establishing electrochemical gradient - cell contents are more negative than extracellular environment.
Name the stages in generating an action potential.
- Depolarisation.
- Repolarisation.
- Hyperpolarisation.
- Returning 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 reverse 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?
- K+ ions diffuse out of membrane - p.d becomes more - 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 hyper polarised sections of the membrane:
- ensures uni directional impulse
- ensures discrete impulses
-limits frequency of impulse transmission
What is the all or nothing principal?
Any stimulus that causes the membrane to reach threshold potential will generate an action potential. All action potentials have the same magnitude.
Name the factors that affect the speed of conduction.
- myelin sheath
- axon diameter
- temperature
how does axon diameter affect the speed of conductance?
greater diameter = faster
- less resistance to flow of ions
- 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 may 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)
Suggest appropriate units for the maximum frequency of impulse conduction
Hz
How can an organism detect the strength of a stimulus?
Largest 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 and impulses between neurons/ from neurons to effectors
-new impulses can be initiated in several different neurons were multiple simultaneous responses.
Describe the structure of a synapse.
Pre-synaptic neuron ends and synaptic knob : contains lots of mitochondria, endoplasmic reticulum and vesicles of neurotransmitter.
Synaptic left : 20-30 nm gap between neurons
Post synaptic neuron : has complimentary receptors to neurotransmitter ( Voltage gated Na+ channels )
Outline what happens in the pre-synaptic neuron when an action potential is transmitted from one neuron to another.
- Wave of depolarisation travels down the pre-synaptic neuron.
- This causes voltage gated. Ca2+ channels to open.
- Vesicles move towards and fuse with the pre-synaptic membrane.
- Exocytosis of the neurotransmitter into the 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.
- Voltage gated. Na+ channels open
- If influx of Na+ ions raises membrane to threshold potential, action potential is generated.
