Step by step process of nervous communication from a sensory neuron
to the effector (muscle fibres)
Step 1: Production of an action potential
Resting potential is disrupted by an external stimuli to a receptor cell. The causes the opening
of voltage-gated channels in the cell surface membrane which allows sodium ions to pass to the
inside of the cell. This causes the internal environment to become less negative which is referred to
as depolarization.
Depolarization triggers more channels to open and more sodium ions enter leading to more
depolarization. If the potential difference reaches -50mV then more channels open = positive
feedback
An action potential is only triggered if the potential difference reaches between -60mV and -50mV
(Threshold potential)
Step 2: Transmission of an action potential
An action potential at any point in an axon’s cell surface membrane triggers the production of an
action potential next to it. The temporary depolarization of the membrane at the site of the action
potential causes a ‘local circuit’ to set up between the depolarised region and the resting regions to
the side of it.
These electrical impulses travel along the neurones but cannot jump between neurones.
Step 3: Chemical transmission at synapses from motor neurone to effector (muscle fibres).
When the electrical impulse reaches the end of a neurone (in this case the motor neurone at a
neuromuscular junction), the depolarization also triggers the opening of calcium ion voltage
channels in the presynaptic neurone (motor neurone).
The influx of calcium ions stimulates the release of ACh from vesicles. The ACh diffuses across the
synaptic cleft (the neuromuscular junction) and binds to receptor proteins (chemically gated ion
channels) on the cell surface membrane of the postsynaptic membrane (in this case the
sarcolemma).
The binding of ACh changes the shape of the protein allowing sodium ions to pass through allowing
depolarization to occur once again and initiate and continue the action potential (electrical
impulses).
Step 4: How the action potential in the Sarcolemma causes muscle contraction.
The depolarization spreads down the T-tubules of the muscle fibre. Channel proteins for calcium
ions open and calcium ions diffuse out of the sarcoplasmic reticulum.
Calcium ions bind to troponin which changes the shape of the troponin
molecules. Tropomysin moves to expose myosin-binding sites on the thin actin filaments due to the
troponin shape change.
Myosin heads form cross-bridges with the thin actin filaments and moves the thin filaments closer
to each other by power strokes. This causes the sarcomere to shorten and contraction happens.