Explain the roles of synapses in the nervous system. [6] #
- ensure one-way transmission ;
- receptor (proteins) only in postsynaptic, membrane / neurone ;
- vesicles only in presynaptic neurone ;
- ref. adaptation ;
- increased range of actions ;
- due to interconnection of many nerve pathways ;
- ref. inhibitory synapses ;
- involved in memory / learning ;
- due to new synapses being formed ;
- e.g. summation / discrimination
Describe the structure of a myelinated sensory neurone.[8] #
- nucleus;
- (long) dendron ;
- (shorter) axon ;
- many mitochondria (in cell body) ;
- many RER/nissl’s granules, (in cell body) ;
- synaptic knobs ;
- detail of synaptic knob ;
- (terminal) dendrites ;
- Schwann cells ;
- detail of myelin sheath ;
- nodes of Ranvier ;
Describe the structure of a myelin sheath and explain its role in the speed of transmission of a nerve impulse. [8] #
- Schwann cells ;
- wrap around axon ;
- sheath mainly lipid ;
- (sheath) insulates axon (membrane) ;
- Na+ / K+, cannot pass through sheath / can only pass through membrane at nodes ;
- depolarisation (of axon membrane) cannot occur where there is sheath / only at nodes of Ranvier ;
- local circuits between nodes ;
- action potentials ‘jump’ between nodes ;
- saltatory conduction ;
- increases speed / reduces time, of impulse transmission ;
- up to 100 ms-1 ;
- speed in non-myelinated neurones about 0.5 ms-1 ;
How an action potential is transmitted along myelinated neurone[6] #
[ZIMSEC June j2004/2/13(a)]
- arrival of an impulse
- increase in membrane permeability to Na+
- Na+ diffuse into axoplasm
- change in membrane potential diffence from -70mv to +40mv/depolirization
- k+ later diffuse out
- Wave of depolirization transmitted to next node of ranvier/saltatory conduction
- increase in speed transmission
Describe the role of sodium ion channels in the transmission of a nerve impulse.[6] #
- ref. to voltage-gated sodium ion channels / ref. ligand gated channels ;
- channels change shape (when, pd / voltage, changes) ;
- open when, membrane depolarises / action potential arrives / neurotransmitter
- binds to receptors ;
- sodium ions flood in ;
- diffuses / down concentration gradient ;
- channels close when membrane, repolarises / potential reaches +30mV ;
- ref. to sodium-potassium pump ;
Explain the mechanism of transmission of a nerve impulse across a cholinegic synapse[6] #
[ZIMSEC June 2004/2/13(b)]
Logical sequence of events is very important when explaining a mechanism
- arrival of an impulse
- Depolirisation of presynaptic membrane
- opening if calcium channels
- increase in mebrane permeability to Ca2+
- rushing of Ca2+ into presynaptic knob
- synaptic vesicle fuse with presynaptic cleft(by exocytosis)
- acetylecholine released into synaptic cleft
- attaches to a receptor on the post synaptic membrane
- entry of Na+
- depolarisation of membrane
Describe how a nerve impulse crosses a cholinergic synapse. [8] #
- action potential / depolarisation, reaches presynaptic membrane ;
- calcium (ion) channels open / presynaptic membrane becomes more permeable to Ca2+ ;
- Ca2+ flood into presynaptic neurone ;
- this causes vesicles of (neuro)transmitter to move towards presynaptic membrane ;
- ref. acetylcholine / ACh ;
- vesicle fuses with presynaptic membrane / exocytosis ;
- ACh released into synaptic cleft ;
- ACh diffuses across (cleft) ;
- ACh binds to receptor (proteins) ;
- on postsynaptic membrane ;
- proteins change shape / channels open ;
- sodium ions rush into postsynaptic neurone ;
- postsynaptic membrane depolarised ;
- action potential / nerve impulse ;
- e.g. action of acetylcholinesterase
