Somatosensory system

Topics

• The general organization of sensory systems in the brain.
• The somatosensory system as an example of a sensory system.
• Plasticity in cortical maps.

Preparation for Class

• PN6 Chapter 9

Topic Slide

Vernon B. Mountcastle (1918–2015) was a major figure in the study of the somatosensory system and in neuroscience in general. He introduced the use of the microelectrode to record action potentials from single units in the brain in experimental animals. He described the columnar organization of somatosensory cortex, which was later extended to visual cortex by David Hubel and Torsten Wiesel. Hubel and Wiesel won the Nobel Prize for their work on the organization of visual cortex, and many thought Mountcastle should have been similarly honored.

General concepts

There are general concepts that apply to the major sensory systems including the somatosensory, auditory, visual systems. I will except the chemical sensory systems (which include taste and smell) from this general overview, as there are some organizational differences for these evolutionarily more ancient systems.

Receptors

Receptors transduce stimulus modality-specific information into neural codes. Sensory receptors are often neuron-like, but have special ion gates that are sensitive to the physical properties of the modality being transduced.

• Photoreceptors in the retina have channels that are responsive to photons of light.
• Mechanoreceptors for tactile sensation have channels that open when physically stretched.
• Hair cells in the cochlea move when struck by waves of fluid that are set into motion by sound waves.

Sensory Submodalities

There are often submodalities of sensation conveyed from the receptor surface. For example, in the visual system there are submodalities derived from photoreceptors that lead to pathways for color, and pathways for motion. There are several forms of somatosensation such as light touch, vibration, muscle sense that I will discuss further below.

Thalamus

Pathways (axonal tracts) convey sensory information from receptors to the rest of brain. These pathways often decussate (cross over) in part or in whole so that the primary sensory brain region is contralateral to (opposite side of) the body receiving sensory input. Thus, our left cerebrum receives its somatosensory input from the right body.

There is a main thalamic nucleus for each sensory modality and sometimes a secondary thalamic nucleus for alternate pathways:

• Visual input -> lateral geniculate nucleus (LGN) of the thalamus. The pulvinar is an example of a thalamic nucleus that is provides an alternative pathway for some visual stimuli.
• Somatosensory input -> ventral posterior thalamic nucleus (medial, VPM, and lateral, VPL)
• Auditory input -> medial geniculate nucleus (MGN) of the thalamus

Primary sensory cortex:

The primary sensory cortices receive the greatest sensory projection from the sensory specific thalami. The sensory projections from thalamus to primary sensory cortex terminate primarily in layer 4 of the six-layered cortex.

• For the visual system, the LGN (lateral geniculate nucleus) of the thalamus projects to Brodmann area 17, also known as V1. V1 is located in the occipital lobe.
• For the somatosensory system, VPL (ventral posterior lateral) thalamus projects to Brodmann areas 3a (primarily muscle spindle information) and 3b (primarily tactile information). We also refer to these areas as S1 (primary somatosensory cortex). S1 is located in the post-central gyrus, the most anterior part of the parietal lobe.
• For the auditory system, the MGN (medial geniculate body) of the thalamus projects to Brodmann areas 41 and 42 (and part of 22), also known as A1. A1 is located within the temporal lobe.

The primary sensory cortices are arranged as maps for some stimulus quality. I will sometimes refer to this map as a feature space.

• Somatosensory maps are somatotopic; i.e., the body surface is represented from medial (lower body) to lateral (face). This is the sensory homunculus.
  • These maps were first identified by cortical stimulation studies first performed by Fritsch and Hitzig and David Ferrier, but continuing through Wilder Penfield, a neurosurgeon working at the Montreal Neurological Institute in the mid–20th century.

Alternative and intermediate pathways

While the main pathway, and the most fibers, go to the thalamus and then to primary sensory cortex, there are smaller alternative pathways for some sensory information. For example:

• The superior colliculus (tectum, midbrain) is a target for visual, auditory, and somatosensory information and plays an important role in orienting head and eye movements toward relevant stimuli in the environment.
• The inferior colliculus (tectum, midbrain) is a target for auditory input.
• The cerebellum is a major target for somatosensory information – particularly that related to the state of muscles. We will discuss the modulatory role of the cerebellum in detail later this semester.

I will return to a discussion of intermediate and alternative projections of sensory systems as we discuss each system in the upcoming lectures.

The Somatosensory System

Somatothesis as three senses

C.S. Sherrington described somatothesis as composed of three senses:

1. Exteroception – sense of the external world
2. Proprioception – sense of oneself – posture and movement
3. Interoception – sense of internal organs
   • Normally not conscious, but can be made to be so. Ondine’s curse refers to a condition whereby the afflicted breathes only under conscious control. Individuals with this disorder require machine ventilation as they stop breathing once they fall asleep.

These ‘senses’ are imperfectly mapped onto different types of receptors, different ascending pathways, and different cortical targets. Perhaps the commonality across these ‘senses’ is that the cell bodies of the receptors are in the dorsal root ganglia. (However, this is not true of the cranial nerve inputs).

Receptors

There are different receptors types in the somatosensory system that convey different submodalities of somatothesis:

Tactile mechanoreceptors

• Tactile mechanoreceptors
  • Meissner – superficial rapidly adapting (RA1)
  • Pacinian – deep rapidly adapting (RA2)
  • Merkel – superficial slowly adapting (SA1)
  • Ruffini – deep slowly adapting (SA2)

Mechanoreceptors have membrane channels that open with physical stretch or deformation.

Other receptors

• Muscle spindle receptors and Golgi Tendon organs convey state of the muscles – i.e., whether contracted or relaxed, or muscle tension at the joint.
• Free nerve endings convey light touch, pain, temperature
• Visceral input can come from stretch receptors in organs like the lungs, or from chemoreceptors in the aortic arch.

Spinal cord

The spinal cord gray matter is butterfly-shaped in cross section. A pair of nerve bundles called roots enter/exit the spinal cord on each side. Below is a drawing of the spinal cord with its anterior (ventral) and posterior (dorsal) roots.

• The Anterior (ventral, or belly side) of the spinal cord is primarily motor in function. The ventral root consists of axons from motor neurons whose cell bodies are located in the gray matter.
• The Posterior (dorsal, or back side) of the spinal cord is primarily sensory in function. The dorsal roots contain axons of sensory nerves that enter the spinal cord. The cell bodies of the sensory neurons are located in the dorsal root ganglia which appears as a bulge on the dorsal roots. This can be seen in the image below.

The dorsal and ventral nerve roots fuse and, combined, form the spinal nerve. Remembering the functional division between the anterior (ventral) and posterior (dorsal) spinal cord can be helped by the mnemonic AMPS Law (Anterior Motor, Posterior Sensory).

Sensory input to the spinal cord

We briefly outlined the white matter pathways in the supplemental anatomical lecture on white matter. Here we review these pathways in more detail.

Dorsal Column system:

The ascending (sensory) and descending (motor) tracts of the spinal cord are illustrated in the figure below.

Tactile (touch) and proprioceptive (muscle sense) nerves enter the spinal cord through the dorsal roots and ascend in the dorsal columns on the same side. They later decussate (cross over) in the medulla (just above the top of spinal cord).

The information from lower limbs run medially in the dorsal columns, and the information from upper limbs join laterally in the dorsal columns. This arrangement of nerves (medial-lateral) is maintained in the VPL thalamus, and in the projection to S1. This is the basis for the sensory homunculus (sensory map).

Anterolateral system:

Nociceptors (pain) and thermoreceptors (temperature) enter through the dorsal roots and immediately cross over at that level of the spinal cord and ascend to the brain in the spinothalamic tracts, also known as the anterolateral system.

The difference in where the tactile information and pain/temperature cross over can lead to the Brown-Sequard syndrome following damage to one side (left or right) of the spinal cord. In this syndrome, tactile and proprioception information is lost ipsilateral to the lesion, and pain and temperature sensitivity is lost contralateral to the lesion.

Spinothalamic tracts:

Proprioceptive input can also go via the ‘unconscious’ spinocerebellar pathway that ascends to the cerebellum rather than to primary somatosensory cortex (see alternative pathways below).

Reflexes

Some ‘intelligence’ is off-loaded to the spinal cord.

• Spinal reflexes: I provided an examples in lecture whereby sensory input (from the muscle spindles) enters the spinal cord through the dorsal roots and synapses in the gray matter of the spinal cord with alpha motor neurons that innervate complementary muscles. Thus, if a load is placed on a muscle and the muscle spindle is stretched, the muscle spindle nerves will activate motor neurons in the spinal cord that oppose the load.
• Central pattern generator – example of spinal cord transection in a cat that can still walk on a treadmill with its hind legs (below the level of the cord transection).

Cerebrum

Thalamic nuclei

The Ventral Posterior part of the thalamus provides input to the somatosensory cortex. It is composed of two nuclei:

• Ventral posterior medial nucleus (VPM)
  • primarily from the face and carried by the trigeminal nerve (the 5th Cranial Nerve).
• Ventral posterior lateral nucleus (VPL)
  • primarily input from the spinothalamic and medial lemniscal tracts of the spinal cord.

Primary somatosensory and motor cortices

There are several somatotopic maps in somatosensory cortex (Brodmann areas 3a, 3b, 1, and 2). Different submodalities are represented in these areas:

• Area 3a: muscle spindles – proprioception
• Area 3b: primary somatosensory input – exteroception
• Area 1: receives input from area 3b – texture
• Area 2: receives input from area 3b – size and shape

It is interesting to note that there is a similar mapping arrangement in primary motor cortex (M1) directly anterior to the central sulcus, and thus adjacent to the maps of somatosensory cortex (which are directly posterior to the central sulcus). This means that the sensory and motor representations for a particular body part (e.g., the hands) are adjacent.

Second somatosensory cortex

S1 consists of Brodmann Areas 3a, 3b, 1, and 2. There are other regions of cortex that receive elaborated sensory input. The second somatosensory cortex (SII) consisting of Brodmann Area 40 is located in the lower part of the parietal lobe.

The input to the secondary somatosensory cortex comes from SI, the ventroposterior inferior thalamic nucleus (VPI) and the ventroposterior medial nucleus (VPM). SII also receives input from somatosensory cortex of the opposite hemisphere through the corpus callosum. While the input to SI is primarily contralateral, SII appears to have a bilateral representation.

Are sensory/motor maps permanent?

The detailed representation of the body in the cortical maps of both primary sensory and motor cortices (i.e., the sensory and motor homunculi) are striking. Taken on their own, they strongly suggest that these regions of the brain are ‘hard-wired’.

However, several lines of evidence suggest a degree of plasticity in these maps.

• Transplants of visual cortex to somatosensory cortex in developing brain show that the transplanted cortex develops typical somatosensory features – i.e., barrels.
  • This suggests that the somatosensory function is not intrinsic to the cortex, but is driven by the pattern of input.
• Studies by Kaas and colleagues show reorganization of sensory cortex following amputation of digits in monkeys. The area previously innervated by the amputated digit is ‘invaded’ and taken over by input from the adjacent digits.
• Phantom limb studies by Ramachandran (covered in an assigned reading) changes the somatosensory map – with representations for amputated limbs appearing on the face of those individuals.

Alternative pathways

The pathways from receptor to thalamus to somatosensory cortex carries signals that form the substrate of our conscious sensation of touch, pressure, and pain. However, there is an alternative pathway that carries proprioceptive information from the muscles to the cerebellum. The spinocerebellar tracts conveys information from Golgi tendon organs and muscle spindles to the cerebellum. There are four tracts that compose the spinocerebellar tracts: the dorsal spinocerebellar tract, the ventral spinocerebellar tract, the rostral cerebellar tract, and the cuneocerebellar tract. You are not responsible for this level of detail, but you might see these different tracts on a diagram. These different tracts bring information from the left and right body trunks and the left and right arms.

The axons comprising the spinocerebellar tracts are ipsilateral. That is, the innervate the same side of the cerebellum as the receptors that give rise to them. This is different from the tracts that ascend to the thalamus. There are some axons in the ventral spinocerebellar tract that cross twice to maintain the ipsilateral projection.

Decussation

An interesting organizational principle to ponder is why some pathways decussate (or cross-over) and others do not. A theory of decussation called the ‘axial twist’ proposed by de Lussanet and Osse. In this theory, decussation was seen as a consequence of the repositioning of the eyes in fish about 450 mya. The eyes of flat bodied fish that lay on the surface were located to the left and right. However, during evolution, two axial shifts of the embryo (one in the head region was 90 degrees counterclockwise, while another in the body region was 90 degrees clockwise) resulted in a fish with its eyes on its sides and a body axis adapted for swimming.

Videos

Prerecorded lectures for Fall 2020

The lecture below was recorded live in fall 2019.