Somatic Sensory Pathways

Somatic Sensory Pathways

Neural pathways relay information from somatic sensory receptors in the skin, muscles, and joints to the primary somatosensory cortex (postcentral gyrus) in the parietal lobe

Somatic Sensory Pathways: Components

A somatic sensory pathway consists of thousands of sets of three neurons: a first-order neuron, a second-order neuron, and a third-order neuron.

Somatic Sensory Pathways: First-Order Neurons

Primary sensory neurons conduct impulses from receptors into the brainstem or spinal cord, while all other neurons are interneurons within the CNS

Somatic Sensory Pathways: First-Order Neurons: Brainstem

Sensory impulses from the face, nasal cavity, oral cavity, and eyes propagate along the cranial nerves into the brainstem

Somatic Sensory Pathways: First-Order Neurons: Spinal Cord

Sensory impulses from the neck, trunk, limbs, and the back of the head propagate along the spinal nerves into the spinal cord

Somatic Sensory Pathways: Second-Order Neurons

Secondary sensory neurons conduct impulses from the brainstem or spinal cord to the thalamus, where their axons cross over (decussate) before ascending the thalamus.

Somatic Sensory Pathways: Third-Order Neurons

Tertiary sensory neurons conduct impulses from the thalamus to the primary somatosensory cortex to perceive sensation

Somatic Sensory Pathways: Third-Order Neurons: Process

Because the axons of second-order neurons decussate as they pass through the brainstem or spinal cord, sensory information from the left side of the body is perceived by the primary somatosensory cortex in the right brain, and vice versa.

Somatic Sensory Pathways: Relay Stations

Regions within the CNS where neurons synapse with other neurons (e.g., the neurons of many sensory pathways synapse with neurons in the thalamus; therefore the thalamus functions as a major relay station)

Pathway to the Cerebral Cortex

Somatic sensory impulses ascend to the cerebral cortex via three general pathways: the posterior column–medial lemniscus, the anterolateral (spinothalamic), and the trigeminothalamic pathways.

Posterior Column–Medial Lemniscus Pathway

A major sensory tract in the central nervous system that coveys nerve impulses for touch, pressure, vibration, and proprioception to the cerebral cortex

Posterior Column–Medial Lemniscus Pathway: Step 1

This pathway begins with primary neurons that extend from peripheral sensory receptors into the spinal cord

Posterior Column–Medial Lemniscus Pathway: Step 2

After entering the spinal cord, axons of these primary neurons ascend to the medulla via tracts known as the posterior columns, where they synapse with secondary neurons.

Posterior Column–Medial Lemniscus Pathway: Step 3

Axons of the secondary neurons cross to the opposite side of the medulla and enter the medial lemniscus, a thin, ribbon-like projection tract, before reaching the thalamus.

Posterior Column–Medial Lemniscus Pathway: Step 4

Axon terminals of secondary neurons synapse with tertiary neurons, which would later synapse with the primary somatosensory cortex of the cerebrum to perceive sensation.

Anterolateral (Spinothalamic) Pathway

A major sensory tract in the central nervous system that conveys nerve impulses for pain, temperature, touch, and pressure to the cerebral cortex

Anterolateral (Spinothalamic) Pathway: Step 1

This pathway begins with primary neurons that extend from peripheral sensory receptors into the spinal cord, where they synapse with secondary neurons

Anterolateral (Spinothalamic) Pathway: Step 2

Axons of the secondary neurons cross over in the spinal cord and then ascend to the spinothalamic tract before reaching the thalamus.

Anterolateral (Spinothalamic) Pathway: Step 3

Axons of secondary neurons synapse with tertiary neurons, which project their axons to the primary somatosensory cortex of the cerebrum.

Trigeminothalamic Pathway

A major sensory tract in the central nervous system that conveys nerve impulses for pain, temperature, touch, and proprioception from the face, nasal cavity, oral cavity, and teeth, which ascend to the cerebral cortex

Trigeminothalamic Pathway: Step 1

This pathway begins with primary neurons that extend from peripheral sensory receptors in the face, nasal cavity, oral cavity, and teeth into the pons through the trigeminal (V) nerves

Trigeminothalamic Pathway: Step 2

The cell bodies of primary neurons are in the trigeminal ganglion; some synapse with secondary neurons in the pons, while others descend to the medulla to synapse with secondary neurons.

Trigeminothalamic Pathway: Step 3

Axons of secondary neurons cross over to the opposite side of the pons and medulla and ascend to the trigeminothalamic tract before reaching the thalamus.

Trigeminothalamic Pathway: Step 4

Axon terminals of secondary neurons synapse with tertiary neurons, which would later synapse with the primary somatosensory cortex of the cerebrum to perceive sensation

Somatic Sensory Map

A representation of which areas of the cortex receive sensory information from each body part, while the somatic motor map shows which areas control movement of each body part.

Primary Somatosensory Cortex

The primary somatosensory cortex in the postcentral gyri of the parietal lobes precisely identifies sensations, with each region receiving sensory input from a specific body part.

Primary Somatosensory Cortex: Contralateral Representation

The right primary somatosensory cortex receives sensory input mainly from the left side of the body, while the left primary somatosensory cortex receives input mainly from the right side.

Sensory Homunculus

The sensory homunculus is a distorted body map in the primary somatosensory cortex, where the amount of cortical area for each body part depends on the number of sensory receptors it has.

Sensory Homunculus: Lips, Face, Tongue, and Hand

The lips, face, tongue, and hand occupy relatively large regions of the primary somatosensory cortex because they contain many sensory receptors.

Sensory Homunculus: Trunk and Lower Limbs

The trunk and lower limbs occupy relatively smaller regions of the primary somatosensory cortex because they have fewer specialized sensory receptors.

Sensory Cortex: Cortical Plasticity

The size of the cortical region representing a body part can increase or decrease depending on how much sensory input that body part provides.

Pathway to the Cerebellum

Somatic sensory impulses reach the cerebellum via the spinocerebellar tracts.

Spinocerebellar Tract

The anterior and posterior spinocerebellar tracts are the main pathways that carry proprioceptive nerve impulses from the spinal cord to the cerebellum.

Spinocerebellar Tract: Function

Although they are not consciously perceived, sensory nerve impulses conveyed to the cerebellum along these two pathways are critical for posture, balance, and coordination of skilled movements