Primary Somatosensory (Somatic Sensory) Cortex
The primary somatosensory cortex is located in the post-central gyrus of the parietal lobe and is concerned with discriminative aspects of reception and appreciation of somatic sensory impulses. It consists of at least four functionally distinct areas, each containing a complete somatotopic map. fibers terminate in the post central gyrus in an organized fashion, with the lower extremity represented on the medial surface of the hemisphere, and the arm and hand represented on the lateral surface. The face, mouth, and tongue are represented in the supra sylvian region. Sensory signals reach the cortex from the peripheral receptors through peripheral nerves, the spinal cord and brainstem via a number of parallel pathways mediating different functions. Brodmann's areas 1, 2, 3a and 3b. 2 and 3a process information from mechanoreceptors of the skin; there is a separate representation of the body in theses areas. In areas 1 and 3b the representations are complete and highly detailed whereas in 2 and 3a the representations are coarser. Areas 2 and 3a play important roles in limb position sense and shape discrimination of grasped objects and 1 and 3b play a major role in superficial touch and texture discrimination.
The Primary Somatic Sensory Cortex has a somatotopic organization and receives somatotopically organized inputs from the ventral posterior lateral and medial nuclei. This thalamocortical projection imposes a body map on the post central gyrus, the sensory homunculus, originally described in the human by the neurosurgeon Wilder Penfield.
Somatosensory receptors include cutaneous receptors, joint receptors and muscle receptors. Cutaneous receptors consist of low-threshold mechanoreceptors which are innervated by large myelinated fibers and transmit touch sensation and high-threshold mechanoreceptors, chemoreceptors, and thermoreceptors, which are innervated by small myelinated or unmyelinated fibers and mediate pain and temperature sensation. Joint and muscle receptors are mainly innervated by large rapidly conducting myelinated fibers. Muscle receptors include muscle spindles, which signal muscle length and rate of change in length. Golgi tendon organs, which respond to changes in muscle tension, and free nerve endings, which respond to muscle pressure and pain.
Information from somatic receptors is transmitted to the spinal cord by the first-order neurons. The cell bodies of these neurons are located in the dorsal root ganglia. Each of these neurons has a single nerve process that divides into two branches. The distal, or peripheral, branch corresponds to the sensory afferent that innervates the receptor. The proximal, or central branch enters the spinal cord via the dorsal root. The areas of the skin innervated by individual dorsal roots are called dermatomes. Their nerve roots include axons from peripheral nerves and spinal nerves: the latter are arranged in a highly ordered way on the body surface that differs from the peripheral nerves.
The two main types of neurons in a dorsal root ganglion are large neurons, with large myelinated axons that innervate low-threshold mechanoreceptors (touch) and proprioceptors, and small neurons, with small myelinated or unmyelinated axons that innervate nociceptors, thermoreceptors, and visceral receptors. Touch fibers are intermediate in size; these fibers in the dorsal root are separated medial to lateral as they enter the dorsal horn of the spinal cord. This subdivision is relevant clinically because diseases that selectively effect large sensory fibers or large dorsal root ganglion neurons produce severe loss of all tactile modalities and proprioception but leave pain and temperature sensation intact. Diseases of small sensory fibers or small dorsal root ganglion neurons affect pain and temperature but spare touch and proprioceptive sensation.
The pathways for the different sensory modalities diverge as they ascend in the spinal cord to higher centers. The medially located large myelinated fibers bifurcate into branches that may 1 ascend directly in the ipsilateral dorsal columns, without synapsing in the spinal cord, as the direct dorsal column pathway; 2 synapse on dorsal horn neurons that in turn contribute axons to the dorsal column (the post synaptic dorsal column pathway), dorsolateral funiculus, and spinothalamic tract; 3 synapse in the intermediate gray matter on neurons that give rise to the spinocerebellar tract; 4 synapse on interneurons and motor neurons in the ventral horn for segmental, or myostatic, reflexes; and 5 synapse in the dorsal horn on interneurons that provide segmental modulation of pain transmission. The laterally located small myelinated and unmyelinated fibers bifurcate into ascending and descending branches that run longitudinally in Lissauer's tract, part of the dorsolateral funiculus. Within several segments, these axons leave Lissauer's tract to enter the dorsal horn and the intermediate gray matter of the spinal cord. In the gray matter, they may 1 synapse on different groups of dorsal horn and intermediate gray matter neurons that form the spinothalamic and other tracts ascending in the contralateral ventrolateral quadrant, 2 synapse on dorsal horn interneurons involved in segmental modulation of pain and in intrinsic (propriospinal) intersegmental pathways, and 3 synapse on interneurons and activate somatic and preganglionic autonomic motor neurons to initiate segmental visceral and somatic reflexes. The second-order spinal somatosensory neurons occupy the dorsal horn and the intermediate gray matter of the spinal cord. These neurons contribute to all somatosensory pathways except the direct dorsal column pathway.
Somatosensory pathways can be subdivided into three groups. First, the direct, contralateral, somatotopically organized pathways for tactile discrimination, (spinal segment T-6 and higher) conscious proprioception and discriminative aspects of pain and temperature that synapse in the ventral posterior complex of the thalamus. Second, the indirect pathways with poor somatotopy that ascend bilaterally, have multiple interconnections with the reticular formation and other subcortical regions, relay in midline thalamic nuclei, and affect limbic and paralimbic cortical areas. These indirect pathways are not helpful for localization, but they are important for transmission of affective arousal components of pain and visceral sensation and for initiation of reflex somatic, autonomic, and hormonal responses to external stimuli. third, the spinocerebrellar tracts are two-neuron pathways that transmit unconscious proprioceptive information to the ipsilateral cerebellum.
The representations of the various body parts on the sensory map to not have the same proportions as the body itself. Rather, the portions of the body used in discriminative touch tasks, such as the fingers, have a disproportionately greater representation on the map than areas that are not as important for touch, such as the leg. It was once thought that these differences were fixed and simply reflected the density of peripheral sensory receptor neurons. We now now that the body map of the brain is not static but is dynamically controlled by the pattern of use of different body parts in touch exploration.
The Primary Somatic Sensory Cortex has a somatotopic organization and receives somatotopically organized inputs from the ventral posterior lateral and medial nuclei. This thalamocortical projection imposes a body map on the post central gyrus, the sensory homunculus, originally described in the human by the neurosurgeon Wilder Penfield.
Somatosensory receptors include cutaneous receptors, joint receptors and muscle receptors. Cutaneous receptors consist of low-threshold mechanoreceptors which are innervated by large myelinated fibers and transmit touch sensation and high-threshold mechanoreceptors, chemoreceptors, and thermoreceptors, which are innervated by small myelinated or unmyelinated fibers and mediate pain and temperature sensation. Joint and muscle receptors are mainly innervated by large rapidly conducting myelinated fibers. Muscle receptors include muscle spindles, which signal muscle length and rate of change in length. Golgi tendon organs, which respond to changes in muscle tension, and free nerve endings, which respond to muscle pressure and pain.
Information from somatic receptors is transmitted to the spinal cord by the first-order neurons. The cell bodies of these neurons are located in the dorsal root ganglia. Each of these neurons has a single nerve process that divides into two branches. The distal, or peripheral, branch corresponds to the sensory afferent that innervates the receptor. The proximal, or central branch enters the spinal cord via the dorsal root. The areas of the skin innervated by individual dorsal roots are called dermatomes. Their nerve roots include axons from peripheral nerves and spinal nerves: the latter are arranged in a highly ordered way on the body surface that differs from the peripheral nerves.
The two main types of neurons in a dorsal root ganglion are large neurons, with large myelinated axons that innervate low-threshold mechanoreceptors (touch) and proprioceptors, and small neurons, with small myelinated or unmyelinated axons that innervate nociceptors, thermoreceptors, and visceral receptors. Touch fibers are intermediate in size; these fibers in the dorsal root are separated medial to lateral as they enter the dorsal horn of the spinal cord. This subdivision is relevant clinically because diseases that selectively effect large sensory fibers or large dorsal root ganglion neurons produce severe loss of all tactile modalities and proprioception but leave pain and temperature sensation intact. Diseases of small sensory fibers or small dorsal root ganglion neurons affect pain and temperature but spare touch and proprioceptive sensation.
The pathways for the different sensory modalities diverge as they ascend in the spinal cord to higher centers. The medially located large myelinated fibers bifurcate into branches that may 1 ascend directly in the ipsilateral dorsal columns, without synapsing in the spinal cord, as the direct dorsal column pathway; 2 synapse on dorsal horn neurons that in turn contribute axons to the dorsal column (the post synaptic dorsal column pathway), dorsolateral funiculus, and spinothalamic tract; 3 synapse in the intermediate gray matter on neurons that give rise to the spinocerebellar tract; 4 synapse on interneurons and motor neurons in the ventral horn for segmental, or myostatic, reflexes; and 5 synapse in the dorsal horn on interneurons that provide segmental modulation of pain transmission. The laterally located small myelinated and unmyelinated fibers bifurcate into ascending and descending branches that run longitudinally in Lissauer's tract, part of the dorsolateral funiculus. Within several segments, these axons leave Lissauer's tract to enter the dorsal horn and the intermediate gray matter of the spinal cord. In the gray matter, they may 1 synapse on different groups of dorsal horn and intermediate gray matter neurons that form the spinothalamic and other tracts ascending in the contralateral ventrolateral quadrant, 2 synapse on dorsal horn interneurons involved in segmental modulation of pain and in intrinsic (propriospinal) intersegmental pathways, and 3 synapse on interneurons and activate somatic and preganglionic autonomic motor neurons to initiate segmental visceral and somatic reflexes. The second-order spinal somatosensory neurons occupy the dorsal horn and the intermediate gray matter of the spinal cord. These neurons contribute to all somatosensory pathways except the direct dorsal column pathway.
Somatosensory pathways can be subdivided into three groups. First, the direct, contralateral, somatotopically organized pathways for tactile discrimination, (spinal segment T-6 and higher) conscious proprioception and discriminative aspects of pain and temperature that synapse in the ventral posterior complex of the thalamus. Second, the indirect pathways with poor somatotopy that ascend bilaterally, have multiple interconnections with the reticular formation and other subcortical regions, relay in midline thalamic nuclei, and affect limbic and paralimbic cortical areas. These indirect pathways are not helpful for localization, but they are important for transmission of affective arousal components of pain and visceral sensation and for initiation of reflex somatic, autonomic, and hormonal responses to external stimuli. third, the spinocerebrellar tracts are two-neuron pathways that transmit unconscious proprioceptive information to the ipsilateral cerebellum.
The representations of the various body parts on the sensory map to not have the same proportions as the body itself. Rather, the portions of the body used in discriminative touch tasks, such as the fingers, have a disproportionately greater representation on the map than areas that are not as important for touch, such as the leg. It was once thought that these differences were fixed and simply reflected the density of peripheral sensory receptor neurons. We now now that the body map of the brain is not static but is dynamically controlled by the pattern of use of different body parts in touch exploration.