Classification of somatic sensitivity
The body's sensitivity is divided into global categories, among which special modalities (vision, hearing, taste, smell) and somatic sensitivity are distinguished. The latter encompasses sensations from receptors of the skin, muscles, tendons, joint capsules, and internal organs.
According to the localization of the receptor apparatus, the following are distinguished:
- Superficial sensitivity (exteroception) — tactile, pain, and temperature.
- Deep sensitivity (proprioception) — vibration and joint-muscle sense.
- Interoception — perception of stimuli by chemoreceptors and baroreceptors of internal organs.
In addition, neurology distinguishes complex modalities of sensitivity, which include stereognosis and two-dimensional point localization.
Receptor apparatus and mechanisms
Signal perception is ensured by specialized receptor formations:
- Tactile sensitivity is mediated through encapsulated nerve endings (Meissner, Pacinian, and Ruffini corpuscles, and Merkel discs) located in the skin and subcutaneous tissue.
- Deep sensitivity is provided by muscle spindles of intrafusal fibers in skeletal muscles, tendon receptors, and joint receptors.
Depending on the strength of external stimulation, these receptors generate nerve impulses at varying frequencies.
Pathways of deep and tactile sensitivity
The pathway for impulse transmission from the periphery to the cerebral cortex is organized as follows:
- First neuron: Cell bodies lie in the dorsal root ganglia. Dendrites extend to the receptors, and axons enter the spinal cord.
- Spinal cord: Fibers ascend on the same side (ipsilaterally) as part of the posterior funiculi. The lower extremities form the medial gracile fasciculus (fasciculus gracilis, Goll's tract), and the upper extremities form the lateral cuneate fasciculus (fasciculus cuneatus, Burdach's tract).
- Second neuron: In the medulla oblongata, axons synapse in the gracile and cuneate nuclei. Their fibers cross to the opposite side and form the medial lemniscus (lemniscus medialis).
- Third neuron: Located in the ventrolateral nucleus of the thalamus.
- Cortical representation: Axons pass through the posterior limb of the internal capsule to the postcentral gyrus of the parietal lobe.
Somatotopic organization of the cortex
The projection of body parts in the postcentral gyrus (sensory homunculus) is inverted:
- Upper part and medial region: genitalia and lower extremity (toes, foot, leg, thigh).
- Middle part: trunk, neck, head, and upper extremity. Notably, the projection of the hand and fingers occupies the largest area.
- Lower part: face, oral cavity, swallowing elements, and internal organs in the lowest region.
Briefly: answers to topical questions
Which modalities of sensitivity are classified as complex?
Complex modalities of sensitivity include stereognosis (stereognostic sensitivity) and two-dimensional tactile localization. Somatotropic sensitivity encompasses sensations arising from the stimulation of receptors in the skin, subcutaneous tissues, muscles, tendons, joints, and internal organs.
Where are the cell bodies of the second neurons of the superficial (pain and temperature) sensitivity pathway located?
The cell bodies of the second neurons of the superficial pain and temperature sensitivity pathway are located in the dorsal horns of the spinal cord. Axons of these neurons cross to the opposite side via the anterior white commissure, enter the lateral funiculus, and form the lateral spinothalamic tract, which transmits pain and temperature sensitivity.
As part of which tracts do superficial sensitivity impulses ascend to the brain?
Superficial sensitivity impulses ascend to the brain as part of the spinothalamic tracts. The conductors of superficial sensitivity include:
- Lateral spinothalamic tract (tractus spinothalamicus lateralis) — located in the lateral funiculus of the spinal cord. Its function is to transmit pain and temperature sensitivity.
- Anterior spinothalamic tract — located in the anterior funiculus. It transmits tactile sensitivity (pressure and touch).
Fibers of the lateral tract decussate at the level of the spinal cord, crossing to the opposite side.
Goll is lower, Burdach is higher: the gracile fasciculus (gracilis) ascends from the legs and lies medially, while the cuneate fasciculus (cuneatus) ascends from the arms and lies laterally.
Class and exam questions
Where are the cell bodies of the first-order neurons of the deep sensory pathways located?
The cell bodies of the first-order neurons are located in the dorsal root ganglia (spinal ganglia).
What area do the nuclei of Goll and Burdach occupy and where are they located?
These nuclei are located in the medulla oblongata. The fibers of the gracile and cuneate fasciculi terminate in them, synapsing onto the second neuron.
Where are somatic sensitivity impulses projected in the cerebral cortex?
The final destination of the pathways is the postcentral gyrus of the parietal lobe cortex.
What else to study on this topic
- Anatomy and functions of Pacinian and Meissner corpuscles and Merkel discs
- The role of the segmental apparatus of the spinal cord and reflex arcs
- Characteristics of complex types of sensitivity (stereognosis)
- Topography of pathways in the posterior limb of the internal capsule
- Features of the somatotopic representation of the hand in the cortex
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