Content Reviewers:Rishi Desai, MD, MPH
Broadly speaking, the nervous system can be split into an afferent or sensory division and an efferent or motor division.
The afferent division brings sensory information from the outside world into the brain.
Sensory information involves special senses - like vision, hearing, taste, and smell - as well as general somatic senses which make up the somatosensory system, which is involved in the sense of touch, proprioception, pain, and temperature. These sensations are felt by sensory neurons all over the body.
These sensory neurons have receptors that are classified according to the stimulus they respond to - there are mechanoreceptors for touch and proprioception, nociceptors for pain, and thermoreceptors for temperature.
Now, neurons are the main cells of the nervous system. They’re composed of a cell body, which contains all the cell’s organelles, and nerve fibers, which are projections that extend out from the neuron cell body. These are either dendrites that receive signals from other neurons, or axons that send signals along to other neurons.
Where two neurons come together is called a synapse, and that’s where one end of an axon sends neurotransmitters to the dendrites or directly to the cell body of the next neuron in the series.
The somatosensory pathways are made up of a relay of four neurons.
The first neuron is called the first order neuron or sensory neuron, which has the sensory receptors and converts stimuli from the outside world into an impulse that can be passed through a synapse to the next neuron in series.
Next is the second order neuron, and it may have its cell body in the spinal cord or up in the brainstem.
The second order neuron then takes the impulse to the third order neuron, which has its cell body in the thalamus.
Finally, the third order neuron takes the impulse to the fourth order neuron, which has its cell body further up in the sensory cortex of the brain.
Now let’s zoom into first order or sensory neurons. First order neurons are called pseudounipolar neurons, which means that they don’t have separate dendrites and axons; instead, there’s only one axon that extends out from the cell body, and it has two branches: a peripheral branch and a central branch.
The peripheral branch goes from the cell body - located at the dorsal root ganglia right next to the spinal cord - to the peripheral tissues.
Every first order neuron has a receptive field, which is the area that it receives sensory input from.
The end of the peripheral branch is full of sensory receptors and ion channels.
When a stimulus from the outside world hits the sensory receptors - for instance if somebody pokes you - ion channels open and close and that allows ions to flow in and out of the neuron.
Overall, if enough positive charge flows into the cell, that’s called depolarization.
And if the first order neuron reaches a certain threshold of depolarization, another set of ion channels that are voltage-gated sense it and flip open, letting even more positive charge enter the cell. This triggers an action potential that’s sent through the peripheral branch and back to the central branch, and from there it goes to the spinal cord.
Now, receptive fields vary in size, and the smaller the receptive field, the higher the resolution, which means that stimuli can be localized or identified more precisely.
As an example, we have smaller receptive fields in the fingertips than at the back. This is very important to read Braille, which is a method of reading through touch - specifically feeling raised dots with the tip of the index finger.
Also, to understand that two stimuli are distinct, the receptive fields of two sensory neurons need to be separated by at least one neuron’s receptive field.
This third neuron will send a negative signal between the two positive ones, allowing two-point discrimination.
Now, in the case of a strong stimulus, the nearby sensory neurons might get mildly activated by deformation of the surrounding skin.
To help minimize these collateral stimuli, the neuron with the strongest activation will activate inhibitory interneurons, which are neurons that project onto surrounding first order neurons to suppress their activity. This is called lateral inhibition, because it helps the precise localization of a stimulus by defining its boundaries, and that helps us recognize objects.
Two more features - the strength and duration of a stimulus - are encoded by the frequency of nerve firing.
A strong stimulus - like boiling water - will make a first order neuron fire at a really high frequency.
But sensory neurons also have a tendency to adapt - which means that if a stimulus doesn’t change for a while, sensory neurons get used to it and stop firing.
Receptors are either fast adapting or phasic, or they’re slow adapting or tonic.
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