Chapters 4 - Sensory systems 02. The somatosensory system

02. The somatosensory system

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Somesthesia [ 2 , 5 , 72 ] (also called body sensitivity, overall sensitivity or somatic sensitivity) differs from other sensory systems. Indeed, its receptor organs are distributed throughout the body [ 39 ] and it is completely different sensory modalities [ 5 ].

1. Somatosensory modalities:

Somesthesia is in fact a multi-sensory system that provides information on:

  • fine tact (epicritic) [ 36 ] detector delicate shapes and fine textures of objects.
  • Coarse tact (protopathic) gives us a general idea of the geometry of objects.
  • Pressure.
  • Vibration (pallesthesia) [ 107 ].
  • Temperature [ 4 ].
  • Pain (nociception) [ 5 ].
  • And the sense of position of the limbs in space (statokinésie).

In general, there are three broad categories of general sensitivity [ 5 , 96 ]:

  • The exteroception: Sensitivity linked to the outside world.
  • Proprioception [ 4 ]: Perception of the relative position of parts of the body (deep sensitivity).
  • The interoception [ 74 ]: Sensitivity of vegetative organs and systems.

2. Receptors [ 41 , 57 ]: somesthesie1

There are several types of receptors involved in the conversion of the various signals (mechanical, thermal and chemical) in action potentials: language understood by neurons. Thus we distinguish:

  • Mechanoreceptors (which react to pressure),
  • Chemoreceptors stimulated by chemicals,
  • The thermoreceptors (heat sensitive)
  • And nociceptors (which collect data on pain) [ 57 ].

In the category of mechanoreceptors [ 5 ], there are several varieties depending on the discriminating power (ability to distinguish between two points close stimulation), and time to adapt to the stimulus (time after which the receiver stops sending potential action).

  We distinguish: The hair receptors [ 44 ], Merkel discs [ 38 , 41 ], Meissner's corpuscles (very important for fine tact) [ 38 ] and Ruffini [ 143 ].

Receptors temperature are two in number [ 36 ]: Receivers warm and cold receptors.


For proprioception, there are three varieties of receptors: The Golgi tendon organs [ 54 , 109 ], muscle spindles [ 3 , 38 , 109 ] and joint receptors [ 54 , 109 ].

There are also polymodal receptors [ 57 , 144 ] and free nerve endings [ 41 ] which provide information primarily on pain.

3. Transmission:

3.1. Peripheral pathways:

The receptors are linked to the nerve fibers that carry sensory information from the receivers to the CNS. There are four types of fibers according to their diameter and myelination potential [ 57 ]: somesthesie3

  • A alpha fibers are myelinated fibers of large diameter (proprioception).
  • A beta fibers, mean diameter myelinated (Mechanoreception).
  • A delta fibers, small diameter myelinated fibers.
  • Amyélinisées and C fibers of small diameter (for nociception and thermoception) [ 3 ].

The cell bodies (pseudo-unipolar) of these fibers are located in the dorsal root ganglia for the spinal nerves, and in the trigeminal ganglia (ganglion trijéminal) [ 45 ] to the trigeminal nerve that is responsible for the sensitivity of the face.

3.2. Receptive fields: somesthesie4

Is called (receptive field) [ 5 ] innervated by the dendritic cell to a single nerve extensions anatomical region. These regions are particularly small and numerous at the ends (fingertips, lips, tongue), which explains the smoothness of the sensitivity in these regions.

3.3. Dermatomes:

Each sensory root contains sensory fibers connected to a part of the skin is called: A dermatome [ 41 ]. There are 31 pairs of spinal nerves, however, was that 30 pairs of dermatomes. This is because the first spinal nerve root C1 often do not contain sensory fibers [ 13 , 18 ].

3.4. Central pathways:

There has conventionally a chain of three neurons which ensures the conduction of sensory signal to the cerebral cortex.

In the CNS, the fibers of the sensitivity are organized into two main clusters: the lemniscal system (lane dorsal columns) and the extra-lemniscal system (spinothalamic or anterolateral) (= spine spine).  somesthesie5

  • The fibers of the lemniscal system [ 41 ] route information fine tact, vibration and proprioception. They are the posterior columns of the spinal cord and ascend to the slender and cuneiform nuclei [ 36 ] in the medulla. At this level, they are on the following neuron synapses, they intersect the center line (decussation) and ascend along the ribbon Reil median (median lemniscus) to the thalamus, where they will make a second synapse.
  • The extra-lemniscal system [ 145 ] carries afferents from the pain, and coarse thermoception tact. The first system neurons synapse directly upon return to the medulla at the level of the gelatinous substance, the second neuron will cross the centerline and which form the anterolateral beam will reach the thalamus where these fibers will make a second synapse. Fibers of this system are relays at different nuclei of the brain including the reticular formation and the periaqueductal gray trunk.
  • There is also a system that connects the third fiber cerebellum reducing information unconscious proprioception (spinocerebellar beam) [ 38 , 75 ].

Both systems (lemniscal and extra-lemniscal) join the thalamus at the ventral basal complex [ 50 ]. At this level, there is a somatotopic map different parts of the body, head in the ventral posteromedial nucleus (VPM) [ 71 ] and the rest of the body at the ventral posterolateral nucleus (VPL) [ 2 ] .

Thalamus will project the third neurons to the primary somatosensory cortex.

4. Perception:

The primary somatosensory cortex S1 is at the level of a bottom parietal gyrus (postcentral gyrus) and meets the areas 3, 2 and 1 of Brodmann classification [ 38 ]. At this level, there is also a somatotopic representation [ 57 ] parts of the whole body. This representation is based on the disproportionate sensitive fineness and distribution of receptive fields at each body part. This somatotopy is illustrated by the famous Penfield homunculus [ 119 ] which has a giant mouth and hands and a tiny trunk.

Fibers from the primary somatosensory cortex S1 will be projected on the secondary somatosensory cortex S2 [ 38 ] involved in memory processes.

Others will join the associative somatosensory cortex back at the posterior parietal cortex (area 5 and 7) [ 57 ], this is what will be the integration of sensory information with visual information to construct a reality consistent.

In total, each somatosensory cortex receives and analyzes sensory information from the contralateral side of the body.


Chapters 4 - Sensory systems 02. The somatosensory system