Somatosensory pathways

Definitions & Key takeaways

The somatosensory pathways are a complex network of nerves that transmit sensory information. Sensory information may involve special senses such as vision, hearing, taste, and smell, as well as general somatic senses like the sense of touch, proprioception, pain, and temperature. These sensations are transduced by sensory receptors, which are present in the cell membrane of highly specialized cells found all over the body.

After transduction, next is the transmission through a series of neurons and synapses to the central nervous system. The pathways consist of three main pathways: the dorsal column-medial lemniscus pathway, which conveys information about fine touch, vibration, and proprioception; the spinothalamic pathway which conveys information about pain and temperature, and the trigeminal pathway which conveys information about sensations from the face, including touch, pressure, and pain.

Chapters:

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Introduction0:00–0:50

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 may involve special senses - so vision, hearing, taste, and smell - as well as general somatic senses, so the somatosensory system, which is involved in the sense of touch, proprioception, pain, and temperature.
These sensations are transduced by sensory receptors, which are present in the cell membrane of highly specialized cells found all over the body.
According to the stimulus they respond to, sensory receptors are classified as mechanoreceptors for touch and proprioception, nociceptors for pain, and thermoreceptors for temperature.

Neurons and nerve fibers0:50–2:42

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.
To trigger the release of neurotransmitters, neurons use an electrical signal that races down the axon, known as the action potential.
To help speed up that electrical signal, some axons are intermittently wrapped by a fatty protective sheath called myelin, which comes from glial cells like oligodendrocytes in the central nervous system, and Schwann cells in the peripheral nervous system.
Since the axons in the somatosensory system can be very long, the fact that myelin helps speed up action potentials is super important!
But myelin requires energy to generate and takes up space, so not all fibers are myelinated. Somatosensory fibers are classified as non myelinated fibers, small myelinated fibers, and large myelinated fibers.
Non myelinated or type C fibers are the slowest, and they sense burning pain and hot temperatures. Small myelinated or type A delta fibers are faster than C fibers, because they’re myelinated, and they sense sharp pain, gross touch, and cold temperatures.
Finally, large myelinated or A alpha and A beta fibers are the fastest fibers, and they sense proprioception, vibration, and fine touch.

Somatosensory pathways2:42–4:53

Through these fibers, somatosensory information travels up the spinal cord to the brain, forming the somatosensory pathways.
If you take a cross-section of the spinal cord, there’s grey matter on the inside and that contains the nerve cell bodies arranged in three grey columns or horns that look a bit like a butterfly, when you put both sides together.
The three pairs of grey horns are divided into anterior or ventral horns, posterior or dorsal horns, and lateral horns. The anterior or ventral horns receive information from the motor cortex of the brain, and then send it to the skeletal muscles to trigger voluntary movement.
The neurons in the posterior or dorsal horns take sensory information from the outside world and process it. Finally the lateral horns are mainly involved with the sympathetic division of the autonomic motor system.
Surrounding the grey matter is white matter, which consists of the myelinated axons that form paired funiculi, which are bundles of nerve fibers that make up the neural tracts or pathways that carry information to and from the brain.
The somatosensory system consists of the two main paired pathways that take somatosensory information up to the brain: the medial lemniscal or posterior pathway, and the spinothalamic or anterolateral pathway.
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.

Medial lemniscal (posterior) pathway4:53–6:59

The medial lemniscal pathway carries information about fine touch and proprioception through the large myelinated A alpha and A beta fibers of the first order neuron to the spinal cord.
Most first order neurons do go directly up to the brain through an area of the spinal cord white matter called the posterior or dorsal funiculus.
The dorsal funiculus is divided into a more lateral column called the cuneate fascicle - which carries somatosensory information from the arms and chest - and a more medial column called the gracilis fascicle, which carries information from the trunk and legs.
The dorsal funiculus goes up to the medulla, where the first synapse of this pathway occurs between first order neuron fibers of the cuneate and gracilis fascicles and the second order neurons in the cuneate and gracilis nuclei, respectively.
After this synapse, the fibers decussate at the level of the medial lemniscus, which means that fibers cross to the opposite side, so fibers on the right side bring somatosensory information from the left side of the body and vice versa.
After decussating, the fibers go up through the pons and midbrain, to the thalamus, where a synapse occurs between the second order neurons and the third order neurons.
Fibers of the third order neurons then keep going up from the thalamus, and ultimately reach the sensory cortex of the brain, which is located in the parietal lobe; here, a final synapse takes place between the third order neuron and the fourth order cortical neuron.
Now, it turns out that some first order neurons do not go up to the brain, and instead they synapse with interneurons located at the posterior horn.
The axons of these interneurons then run towards the anterior horn of the spinal cord where they synapse with motor neurons.
This is important for reflexes, which are fast involuntary responses to a stimulus - like getting banged on the knee with a hammer - and shorter distances mean faster signals!

Spinothalamic tract (anterolateral pathway)6:59–8:50

Now the spinothalamic tract carries information on sharp pain and cold temperatures through first order neurons which are small myelinated A delta fibers, as well as information on hot temperatures, burning pain, and crude touch - which can’t be localized - through first order neurons with non myelinated C fibers.
Both types of fibers take the information to the posterior horn of the spinal cord, which is where there’s a synapse between first order neurons and second order neurons.
The small myelinated A delta fibres enter via the dorsal root, bend upwards, travel for two vertebral segments, and then enter the posterior horn, where they synapse directly with the second order neuron.
The small non myelinated C fibres follow the same pathway, but there’s an interneuron in between them and the second order neuron rather than a direct synapse.
After this synapse, the fibers of the second order neurons decussate and cross to the anterior part of the spinal cord through the central canal of the spinal cord.
At this point, the spinothalamic tract is divided into two parts: the lateral tract carries sensory information for pain, pressure, and temperature through the lateral funiculus, while the anterior tract carries information for crude touch through the anterior funiculus.
The lateral and anterior tracts go up the spinal cord to reach the thalamus, where the second synapse occurs between the second order neurons and the third order neurons.
Fibers of the third order neurons then keep going up from the thalamus, and ultimately reach the sensory cortex of the brain; here, the third and final synapse takes place between the third order neuron and the fourth order cortical neuron.
Alright, as a quick recap, the neurons of the somatosensory system form two neural tracts in the spinal cord that carry information to the sensory cortex of the brain.

Review8:50–9:18

The medial lemniscal tract is carries information about pressure, vibration, fine touch, and proprioception. And the spinothalamic tract is divided into the lateral tract - which carries sensory information for pain, pressure, and temperature - and the anterior tract, which carries information for crude touch.