The somatosensory system feeds into the afferent division (the input division) of the nervous system. It carries information to the central nervous system so that the CNS can regulate motor behavior. It is essential for efferent output, and, it helps to regulate our behavior in accordance with external circumstances.

It is also important for coordinating internal activities that are directed at maintaining homeostasis.

An example of this is if it is really cold outside and you are not dressed warm, the thermoreceptors on your skin inform your CNS that the temperature of the environment is very low and you are at risk of getting hypothermic. The CNS then sends signals to your muscles to induce shivering and can also cause you to go look for a warm place. All of this is to maintain temperature homeostasis.

The somatosensory system is arguably the most diverse of the sensory systems and it mediates a range of sensations that are transduced by receptors and conveyed to a variety of targets in the central nervous system.

This just shows the range of the somatosensory system

Sensory receptors:

Receptors are transducers (convert various forms of energy to action potentials) and can be classified according to:

1. Adequate stimulus (the form of energy that a receptor can respond to)
   1. Mechanoreceptors – Stimulated by changes in pressure or movement. Examples include cutaneous receptors for touch and pressure
   2. Chemoreceptors – stimulated by changes in the chemical environment
   3. Thermoreceptors – stimulated by changes in temperature
   4. Photoreceptors – stimulated by light energy
2. Location:
   1. Interoceptors/visceroceptors– respond to stimuli within the body (e.g. pressure receptors in blood vessels)
   2. Exteroceptors - respond to stimuli outside the body
3. Duration:
   1. Rapidly adapting receptors (e.g touch receptors)
   2. Slow adapting receptors

Receptors can either be specialized endings of afferent neurons or a separate receptor cell:

Specialised nerve endings = a certain part of the afferent neuron becomes a receptor. In the diagram above the round part at the end of the axon is acting as the receptor.

How it works:

• When a stimulus occurs (pressure, heat etc.) it opens stimulus-sensitive ion channels which allows the influx of sodium into receptor, causing a depolarization known as receptor potential.
• the local depolarization (due to Na⁺ influx) spreads to the adjacent voltage-gated Na⁺ channels which open if the receptor potential is strong enough (reaches threshold)- this amplifies the depolarization and starts the action potential.
• the action potential is then transmitted along the afferent nerve fiber towards the CNS

In the separate receptor cell, it's basically almost the same as before except that the stimulus changes or distorts the membrane (some stimuli distort the membrane (like sound), others change it through chemical or photic signaling).

This results in the entry of sodium or cations (mainly sodium) which cause a depolarization and a change in voltage. This voltage change, in turn, will cause the entry of Ca into the receptor cell through voltage-gated Ca channels. It’s this entry of Ca that causes exocytosis of neurotransmitter vesicles which release their neurotransmitter towards the afferent neuron.

The neurotransmitter binds to chemically gated receptor channels on the afferent neuron causing the entry of Na which depolarises the afferent neuron to threshold generating an action potential that propagates towards the CNS.