Nervous system Flashcards
Two coordination systems
Hormonal and nervous
Differences between them
Hormonal: slow, controlled by hormones in blood, long-lasting, permanent and irreversible, travel all body.
Nervous: fast, neurones, specific cells, localised, short lived
Neurone structure
Cell body with usual organelles, a lot of RER to produce proteins and neurotransmitter.
Dendron, carry impulse toward cell body
Axon, away from
Schwann cell. surround axon, providing insulation and carry out phagocytosis of cell debris.
Myelin sheath to insulate cell and speed up impulse.
nodes of ranvier - constructions between schwann cells.
Sensory neurone
Connect receptor to intermediate or motor neurone
motor neurone
From relay to effector (gland or muscle)
Relay neurone
Between neurones
Resting potential voltage
-65mV
How is resting potential maintained
Sodium ions actively transported out of axon by Na-K pump.
K pumped into axon by pump.
3 Na out for every 2 K in.
More sodium ions in tissue fluid than axon and more potassium ions in the cytoplasm, so electrochemical gradient.
Na diffuses back into axon and K into tissue fluid but more K moving as more K gates open.
How is action potential
1) Some voltage gated K channels are open but Na closed.
2) energy from stimulus causes some V gated Na channels to open in axon which triggers reversal in potential difference across membrane.
3) More Na in by diffusion, increasing voltage, stimulating more gates to open.
4) When +40mV, Na ion channels close and K ion channels open.
5) Stimulates K to move out, repolarisation.
6) repolarise more than usual as overshoot of outward dif of K, so K gates close and pump brings back to -65mV.
Passage of action potential along unmyelinated
Starts as -ve inside and +ve outside.
Stimulus causes Na+ influx and depolarisation.
Creates localised electrical currents that cause opening of sodium voltage-gated channels further along axon and further back, membrane is repolarising and sodium-potassium pump can return sodium back to tissue fluid to prepare for next action potential.
Passage of action potential along myelinated
Myelin sheath acts as an insulator preventing action potentials from forming. At intervals of 1mm to 3mm, there are nodes of Ranvier where action potentials occur. Localised currents arise between nodes (saltatory conduction) making it faster.
Factors affecting speed of action potential
Myelin sheath through saltatory conduction.
Diameter of neurone, higher = less leakage = faster, also less resistance to flow of ions
Temp affects rate of diffusion (also enzymes)
All or nothing principle
When there is certain level of stimulus (threshold value), there is an action potential triggered.
All action potential same size.
Purposes of refractory period
Ensures that action potential propagated in one direction only as cannot sodium gates are closed during repolarisation.
Discrete impulses.
Limits number of action potentials.
How does Pacinian corpuscle work
Stretch mediated sodium channels are too narrow in normal state for Na ions to pass along. Neurone of Pacinian corpuscle has resting potential.
When pressure applied, membrane around corpuscle deformed and membrane around neurone is stretched. Widens sodium channels so they diffuse into membrane, depolarising, creating generator potential.
