Somatosensory System

1. Coding of Sensory Information
   
   • Sensory Modulation
   • Changes in Signal Strength
   • Receptive Fields
2. Organisation of the Somatosensory System
   
   • Topographical Representation
3. Pathways of the Somatosensory System
   
   • Spinothalamic Pathway
     
     • Lissauer's Tract
   • Dorsal Column Medial Lemniscus Pathway
4. Brown-Sequard Syndrome
5. Descending Modulation of Pain
6. Quiz

• Sensation in the body can be special sensation (vision, hearing, taste and smell) or general sensation (touch, pain, temperature and the sense of body position, arising in the skin, muscles, joints and internal organs). There are seven modalities (types) of somatic (body wall) sensation: temperature, pain, pressure, vibration, fine touch, proprioception (the sense of joint position) and two point discrimination (the ability to tell two close points of touch apart). 

• Receptors detect sensation in analogue (information on a scale), and these impulses are converted to digital information (frequency of action potentials) in the first order neurone (the first neurone in the pathway, running from the receptor to the spinal cord). Rapidly adapting receptors quickly reduce the frequency of action potentials during constant stimulation, whereas slowly adapting receptors keep responding for longer during a sustained stimulus. 

• A single primary sensory neurone supplies a single area of skin known as a receptive field. A neurone with a large receptive field gives lower acuity (a poorer ability to tell fine detail apart) than a neurone with a small receptive field, which gives high acuity. There is some overlap between the receptive fields of adjacent neurones. 

• The principal conscious sensory pathways from the body can be described using three neurones. The cell bodies of the first-order neurones lie in the dorsal root ganglia (clusters of nerve cell bodies beside the spinal cord). The second-order neurones relay in the dorsal horn (the back part of the spinal cord grey matter) or the medulla (the lowest part of the brainstem), and cross the midline. The third-order neurones relay in the thalamus and project to the primary somatosensory cortex (the part of the cortex that receives sensation from the body).

• The homunculus (the map of the body across the sensory cortex) is an organisation of the body’s sensory information in the primary sensory cortex. Somatotopic organisation (neighbouring parts of the body are represented next to each other) is established through ascending pathways and thalamocortical projections (connections from the thalamus to the cortex), rather than within the second order neurones themselves. 

• The spinothalamic pathway (spinal cord to thalamus) carries pain, temperature and crude touch (poorly localised touch) or pressure. First-order pain and temperature fibres may travel a short distance in Lissauer’s tract (a small bundle of fibres at the tip of the dorsal horn) before relaying in the dorsal horn. Second-order fibres then cross within the spinal cord.

• The dorsal column medial lemniscus (DCML) pathway carries impulses for vibration, proprioception, fine touch and two point discrimination. The decussation (crossing) of second order neurones occurs in the medulla. Impulses from T7 and below are carried through the gracile fasciculus (a bundle of nerve fibres in the dorsal column) of the spinal cord to the gracile nucleus of the medulla. Impulses from T6 and above are carried through the cuneate fasciculus (the other dorsal column bundle) to the cuneate nucleus. 

• Brown-Sequard syndrome refers to the characteristic pattern of neurological deficit seen with hemisection (damage across one half) of the spinal cord. It typically involves ipsilateral weakness and ipsilateral loss of DCML modalities, with contralateral loss of spinothalamic modalities. In clinical practice, lesions are often incomplete, so the pattern varies with the extent of the lesion. Contralateral pain and temperature loss begins a little below the lesion, because first order pain and temperature fibres ascend one or two segments before the second order neurones cross. 

• Pain can be modulated by stimulation of mechanoreceptors (receptors that detect touch and pressure), for example by rubbing the affected area. This is thought to act via inhibitory interneurones (small connecting neurones that dampen the signal) in the dorsal horn that reduce transmission in second order pain neurones.

Sensation can be split into two main classifications: general sensation and special sensation. 

Special sensation refers to vision, hearing, taste and smell. General sensation arises from receptors throughout the body: in the skin, muscles and joints, and in the internal organs. It includes touch, pain, temperature and the sense of body position. General sensation can be divided into somatic sensation (arising from the body wall and usually consciously perceived) and visceral sensation (arising from the internal organs and often not consciously perceived). 

Coding of Sensory Information 

There are a number of different modalities (types) of sensation, and each is detected through a particular type of receptor. 

Sensory Modulation 

Sensation is detected in different ways based on which receptor is activated. 

The body can detect somatic sensation in seven main categories: 

1. Temperature 
2. Pain 
3. Pressure or crude touch (touch that is poorly localised) 
4. Vibration 
5. Fine touch 
6. Proprioception or joint position sense 
7. Two point discrimination (telling two close points of touch apart) 

In the central nervous system, the main pathways for conscious sensation carry these seven modalities in two separate systems. Unconscious proprioception (position sense that we are not aware of, used by the cerebellum to coordinate movement) travels separately, in the spinocerebellar tracts. 

Temperature, pain, and pressure are carried through the spinothalamic tract. 

Vibration, fine touch, proprioception and two point discrimination are carried through the dorsal column-medial lemniscus tract. 

Changes in Signal Strength 

The sensory system of the body detects stimuli that have levels of gradation. This means that the body can detect temperature as cold-cool-comfortable-warm-hot, rather than just cold-hot. This variety in strength of signal detection means that the receptors are producing analogue signals. 

The conversion from analogue to digital signals occurs in the primary sensory neurone, also called the first order sensory neurone, as graded receptor potentials (small changes in voltage that vary with the strength of the stimulus) are translated into action potentials. This is the neurone that connects the receptor to the central nervous system, as shown in the diagram below. It travels down the spinal nerve, through the dorsal root, and into the spinal cord. The cell body of the first order sensory neurone is in the dorsal root ganglion, as shown below.

Diagram - The Path of a First Order Sensory Neurone from the Receptor to the Central Nervous System

SimpleMed original by Dr. Maddie Swannack

The receptor detects a sensation, and the strength of the receptor activation sets the frequency of action potentials. Strong receptor activation causes high frequency of action potentials in the primary sensory neurone, and weak receptor activation causes lower frequency of action potentials. For example, bumping your head lightly on a doorframe and being hit by a cricket ball both stimulate the crude touch receptors (along with others, for example pain receptors). The light bump causes weak receptor activation, leading to fewer action potentials. The cricket ball causes strong receptor activation, leading to a high frequency of action potentials. 

Some rapidly adapting receptors, including certain mechanoreceptors (receptors that detect touch and pressure), respond best to changes in signal strength. Their frequency of firing decreases during a long-lasting stimulus, which helps to stop ‘normal’ sensations (like clothes on the skin) from becoming overwhelming. 

Slowly adapting receptors such as nociceptors (pain receptors) change the frequency of firing very little with continuous stimulation, which helps to explain why pain can persist. Persistent pain also involves sensitisation (nociceptors becoming more responsive) and processing in the central nervous system.

Diagram - Comparison of Action Potential Frequencies from Strong and Weak Stimulations

SimpleMed original by Dr. Maddie Swannack

Receptive Fields 

A single primary sensory neurone supplies a single area of skin known as its receptive field. Most primary sensory neurones branch near their endings, so more than one receptor inputs to them, but all of these receptors are of the same type. This makes sense because the neurone can only output digital signals through action potentials, and a train of action potentials cannot show which type of receptor produced it. 

If an area of skin is supplied by a primary sensory neurone with a large receptive field, the area has a low sensory acuity (a poorer ability to tell fine detail apart), meaning its two-point discrimination is poor. In the two-point test, two needle tips are touched to the skin together. With poor discrimination, the tips would have to be quite far apart to be detected as two separate points. The skin of the back is an example of a region with poor two point discrimination. 

If an area of skin is supplied by a primary sensory neurone with a small receptive field, the area has a high sensory acuity, meaning its two point discrimination is good. This means that the two needle tips can be detected as separate even when they are placed quite close. The skin of the fingertip has very good sensory acuity. 

It is also important to note that there is some overlap between the receptive fields of adjacent first order sensory neurones. This means that adjacent spinal nerves supply slightly overlapping areas of skin, so adjacent dermatomes (each dermatome is the strip of skin supplied by one spinal nerve) overlap a little at their edges. This is why, when examining dermatomes, it is best to test a spot in the middle of each dermatome, as this is unlikely to overlap with a neighbouring dermatome.

Diagram - The Receptive Fields of Two Adjacent Nerves

SimpleMed original by Dr. Maddie Swannack

Organisation of the Somatosensory System 

The principal conscious sensory pathways from the body, including the spinothalamic and dorsal column–medial lemniscus pathways, can be described as a three-neurone chain. The following account describes these pathways rather than every somatosensory connection.

The first neurone in the chain of the somatosensory system is called the first order sensory neurone or primary sensory neurone, noted with the symbol 1°. 

These first order sensory neurones: 

• Have their cell bodies in the dorsal root ganglion 
• Communicate with a receptor 
• Project to the ipsilateral side of the CNS (on one side of the midline) 

The second order neurones: 

• Have their cell bodies in the spinal cord dorsal horn or medulla 
• Decussate (cross the midline) 
• Project onto third order neurones 

The third order neurones: 

• Have their cell bodies in the thalamus 
• Project to the primary sensory cortex in the post-central gyrus (the ridge of cortex behind the central sulcus).

Diagram - A simplified three-neurone schematic of conscious body sensation. The intermediate relay sites are not labelled: second-order neurones relay in the dorsal horn or medulla, and third-order neurones relay in the thalamus before projecting to primary somatosensory cortex.

SimpleMed original by Dr. Maddie Swannack

Topographical Representation

Somatotopic organisation means that neighbouring neurones represent neighbouring body regions. This spatial organisation is maintained through successive sensory relays, ascending tracts and thalamocortical projections (connections from the thalamus to the cortex) to the cortical sensory homunculus. There is no single stage, such as the second-order neurones, that converts the dermatomal map (the map of the skin by spinal nerve) into the homuncular map (the map of the body across the cortex).

Along this entire path, impulses from neighbouring receptive fields are carried on neighbouring axons (for example, impulses from the hand are carried on axons next to those carrying impulses from the wrist). One proposed reason is that this reduces the amount of ‘wiring’ (axons and dendrites) needed. The idea comes mainly from studies of maps in the cortex, and the functional significance of such maps is still uncertain.

Diagram - The Primary Sensory Cortex

Licensed under Creative Commons, Source.

Pathways of the Somatosensory System 

There are a variety of modalities of somatic sensation, with a modality being defined as a ‘unit’ of sensation that is usually detected mainly by one class of receptor (although some, such as touch, involve several receptor types). These various modalities can be split into systems depending on which pathway their impulses follow. 

Spinothalamic Pathway 

The first pathway is the spinothalamic pathway. This is responsible for the following modalities: 

• Temperature - thermoreceptors (temperature receptors) 
• Pain - nociceptors 
• Pressure or crude touch - mechanoreceptors 

First order neurones of the spinothalamic pathway: 

• Relay to second-order neurones in the ipsilateral dorsal horn. First-order pain and temperature fibres may ascend or descend a short distance in Lissauer’s tract before this relay, rather than necessarily synapsing at the level of entry.
• Cell  bodies in the dorsal root ganglion (same as all first order neurones). 

Second order neurones of the spinothalamic pathway: 

• Cell bodies in the dorsal horn of the spinal cord. 
• The axons decussate in the ventral white commissure of the spinal cord (a band of fibres in front of the central canal), then ascend as part of the spinothalamic tract to the spinal lemniscus (the path through the brainstem). 
• They project to the thalamus. 

The second order neurones are organised so that the most superior spinal levels are carried medially in the spinal cord. Progressively inferior spinal levels are added onto the lateral side of the pathway. 

Third order neurones of the spinothalamic pathway: 

• Thalamic neurones receiving information from more inferior parts of the body project to the medial portion of the primary sensory cortex. 
• Thalamic neurones receiving information from more superior parts of the body project to the lateral portion of the primary sensory cortex. 

This means that a cross occurs in the primary sensory cortex. Impulses from the lower portions of the body are carried laterally in the spinal cord but project to the medial primary sensory cortex (according to the homunculus). Impulses from the higher portions of the body are carried medially in the spinal cord but project to the lateral primary sensory cortex. It is important to remember that this cross is NOT a decussation, as the fibres do NOT cross the midline.

Diagram - A schematic spinothalamic pathway from two spinal levels. The relay and crossing are drawn within one cross-section for simplicity; first-order pain and temperature fibres may ascend or descend a short distance before relaying in the dorsal horn, after which second-order fibres cross in the spinal cord.

SimpleMed original by Dr. Maddie Swannack

Lissauer’s Tract

Information on Lissauer’s tract (a small bundle of fibres at the tip of the dorsal horn) is advanced detail and is not necessary for a basic understanding of the neuroanatomy of the sensory system tracts. It is included here as a very simple explanation.

Lissauer’s tract allows first-order pain and temperature fibres to ascend or descend a short distance before relaying in the dorsal horn. This helps explain why a spinal cord lesion can produce contralateral pain and temperature loss beginning below the lesion; the precise pattern depends on its extent.

Dorsal Column Medial Lemniscus (DCML) Pathway 

The second pathway is the dorsal column medial lemniscus pathway. 

It is named like this as the dorsal columns are the regions in the spinal cord that carry these neurones, and the medial lemniscus is the corresponding region in the brainstem. 

This is responsible for the following modalities: 

• Vibration - mechanoreceptors 
• Proprioception (joint position sense) - variety of receptors such as muscle spindle fibres (stretch receptors in muscle) 
• Fine touch - mechanoreceptors 
• Two-point discrimination - mechanoreceptors 

First order neurones of the DCML pathway: 

• Ascend ipsilaterally through the dorsal columns of the spinal cord. 
• Those from the lower body (spinal level T7 and below) ascend through the gracile fasciculus (or the fasciculus gracilis in Latin) to the gracile nucleus in the medulla. 
• Those from the upper body (spinal level T6 and above) ascend through the cuneate fasciculus (or the fasciculus cuneatus in Latin) to the cuneate nucleus in the medulla. 
• Axons from the lowest spinal level run most medially in the dorsal columns, with axons from more superior body parts added on laterally. 

Second order neurones of the DCML pathway: 

• Neurones in the gracile nucleus project to the contralateral thalamus through the medial lemniscus pathway. 
• Neurones in the cuneate nucleus project to the contralateral thalamus through the medial lemniscus pathway. 

As shown in the image below, the second order neurones from both nuclei decussate (cross the midline) in the medulla. 

Third order neurones of the DCML pathway: 

• Thalamic neurones receiving information from the lower half of the body via the gracile nucleus project medially into the primary sensory cortex. 
• Thalamic neurones receiving information from the upper half of the body via the cuneate nucleus project laterally into the primary sensory cortex.

Diagram - The Dorsal Column Medial Lemniscus Pathway from Two Different Spinal Levels

SimpleMed original by Dr. Maddie Swannack

It is important to note that the DCML pathway and spinothalamic pathway organise their order of fibres differently: 

The spinothalamic pathway has superior spinal levels carried medially through the spinal cord and inferior levels added on laterally.

The DCML pathway has superior spinal levels carried laterally through the spinal cord and inferior levels are added medially. 

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