Physiology - Nerve and Muscle

A synapse is an anatomically specialised junction between two neurons at which the electrical activity in one neuron (presynaptic) influences the electrical activity of the other neuron (postsynaptic). At most synapses the signal is transmitted by neurotransmitters (chemicals) that bind to specific receptors. Most synapses occur between the axon terminal and the dendrite or cell body.

1. Differences in specific ion concentrations in the intercellular and extracellular fluids.
2. Differences in membrane permeabilities to different ions- (open channels in plasma membrane).

- small changes in membrane potential confined to small region of the plasma membrane
-die out within 1-2mm from origin - decremental
-produced by a specific change in cell's environment acting on specialised region of membrane - receptor/ channel
- "graded potentials" because magnitude of potential change can vary (is graded).

1.Reuptake of NT into the presynaptic terminal
2. Breakdown of NT by enzymes
3. Diffusion of NT out of synaptic cleft

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Fatigue in skeletal muscle is caused by build-up of various byproducts from cross-bridge cycling:
1. Accumulation of potassium during repeated repolarizations of muscle membrane along t-tubules can prevent further AP conduction down t-tubule
2. Accumulation of lactic acid reduces the functioning of proteins necessary for cross- bridge cycling
3. Accumulation of ADP and Pi may cause slower rates of cross-bridge cycling

Receptor potenital - peripheral endings of afferent neurons
synaptic potential - in post-synaptic neuron in response to neurotransmitter released from pre-synaptic terminal.

- Action potential travels in one direction in axons i.e unidirectional - regions which were recently active are refractory
-propagation depends on ion movement to activate adjacent areas of axon membrane -->AP

-Velocity of propagation
-temperature, metabolism
-whether or not the axon is myelinated.

Where action potentials only occur at Nodes of ranviera where myelin is interrupted and sodium is high because concentration of voltage-gated sodium channels in myelinated regions axons is lower (less leakage of charge across the myelin)

Convergence- thousands of synapses from many different cells can affect a single post synaptic cell. Allows information from many sources to influence a cell's activity.
divergence - a single presynaptic cell can send branches to affect many other postsynaptic cells. Allow information from one source to affect multiple pathways.

- neurotransmitter stored in synaptic vescicles
- some vesicles are docked are docked at specialised regions of the synaptic membrane.
- Arriving action potential depolarise axon terminal and opens voltage-gated calcium channels.
- calcium induces vesicle fusion and release of vesicle contents into synaptic cleft.

Tetanus toxin
- toxin destroys SNARE protein in presynaptic terminal
-prevents vesicle fusion for NT release
-affects inhibitory neurons causing increased contraction = tetanus
Botulism
-toxin interferes with SNARE protein in presynaptic terminal
- prevents vesicle fusion for NT release
- Affect excitatory neurons causing decreased muscle activity
Parkinson's disease
- Dopamine deficiency with CNS - movement problems.

Nicotinic
- respond to nicotine
- ionotrpic (receptor is also ion channel)
- NMJ (PNS) -role in muscle contraction
- Brain (CNS)
Muscarinic
- respond also to muscarine
- metabotropic (coupled to Gproteins that affect ion channels)
- prevalent in brain and innervating junctions for glands and organs (especially heart)

- HYDROLYSIS of ATP by myosin energises the cross bridges.
- BINDING of ATP to myosin dissociates the cross bridges
- ATP provides the energy for active transport of calcium ions back into SR, lowering conc of Ca2+ to pres-release levels, ending contraction.