The Somatosensory System: Touch and Proprioception

NSC4354 Lecture 2 - Touch and Proprioception

Somatosensation

• Diverse range of sensations → touch, pressure, vibration, limb position, heat, cold, itch, pain

• Transduced by receptors in skin or muscles to CNS

Cutaneous mechanoreceptors

Part of the somatosensory system

• Fine touch

• Vibration

• Pressure

Specialized receptors

Part of the somatosensory system

Associated with muscles, tendons and joints

Proprioception → Our ability to sense the position of our own limbs and other body parts in space

Free nerve endings

Part of the somatosensory system

• Pain

• Temperature

• Coarse touch/ non-discriminative

Somatosensory afferents

Convey information from the skin surface to central circuits

Includes Pseudounipolar neuron (in DRG) and Bipolar neuron

Dermatomes

Innervation from a single dorsal root ganglion and its spinal nerve

• Defined in patients suffering from shingles or after surgical interruption

Ia, Ib, and II afferents

Largest and fastest, supply sensory receptors to muscles for proprioception – heavily myelinated

Aβ afferents

Smaller afferents, convey touch –myelinated

Aδ and C afferents

Small and slow afferents, pain and temperature - little to no myelination

Slowly adapting afferents

• Generate sustained discharge during ongoing stimulus

• Provide spatial info: size and shape of stimulus

Rapidly adapting afferents

• Fire rapidly when a stimulus is first presented (and at end)

• Fall silent with continual stimulation

• Convey changes (stimulus movement)

Free Nerve Endings

Located in the Epidermis

Responsible for pain

Meissner Corpuscle

Located in the Epidermis

Responsible for motion detection and grip

Textured objects that move across the skin

Rapid adaptation

Merkel cell-neurite complex

Located in the Epidermis

Responsible for shape and texture

Highest spatial resolution of all sensory afferents + especially in the fingertips

Slow adaptation

Ruffini corpuscle

Located in the Dermis

Responsible for force, shape, and internally generated motion

Internally generated stimuli → Movement of fingers

Slow adaptation

Pacinian corpuscle

Located in the subcutaneous layer + dermis and deeper tissues

Detect vibrations transmitted through held objects

Skilled use of tools (using a wrench, writing, cutting bread with a knife)

Rapid adaptation

Proprioception

Ability to sense position of our own limbs and body parts in space

Proprioceptors

Information about the position of limbs & other body parts in space

Low threshold mechanoreceptors provide this information → Muscle spindles, Golgi tendon organs, and Joint receptors

Muscle spindles

• Signal changes in muscle length

• When muscle is stretched, the tension activates the nerve endings, triggering an action potential

Primary endings (group Ia afferents)

Largest myelinated sensory axons have rapidly adapting responses to changes in muscle length.

Muscle spindles that transmit information about limb dynamics → velocity and direction of movement.

Secondary endings (group II afferents)

Muscle spindles that produce sustained responses to constant muscle lengths. Static position of limbs.

Golgi tendon organs

Signal changes in muscle tension

These Group Ib afferents are distributed among the collagen fibers that form the tendons

Joint receptors

• Relay finger position for range of motion protection

• Mechanoreceptors in and around the joints

Dorsal column-medial lemniscal pathway

Central pathways → Tactile from body

Dorsal column-medial lemniscal pathway First order neurons

• Info ascends in columns ipsilaterally in spinal cord to the medulla (dorsal columns)

• Topographic organization (medial–lateral) bundles

→ Fasiculus gracilisto the gracile nucleus (subdivision dorsal column nuclei): lower limbs

→ Medial bundle

→ Fasciculus cuneatus to cuneate nucleus (subdivision dorsal column nuclei): upper limbs, trunk and neck

→ Lateral bundle

Dorsal column-medial lemniscal pathway

• Second order neurons

→ Internal arcuate fibers cross (decussate) the midline and form medial lemniscus → remains separated

→ Synapse at ventral posterior lateral nucleus (VPL) of the thalamus

Dorsal column-medial lemniscal pathway Third order neurons

• VPL neurons send axons to synapse in somatosensory cortex (SI & SII)

Trigeminothalamic pathway

Central pathways → Tactile from face

____ nerve (cranial nerve V)

Trigeminothalamic pathway First order neurons

• Cell bodies in trigeminal ganglia (CN V)

• Nerve has 3 subdivisions: opthalamic, maxillary, mandibular

• Enter brainstem at pons to synapse on trigeminal brainstem complex; different nuclei process different stimuli submodalities (principal and spinal nuclei)

Spinocerebellar tract

Central pathways → proprioception

• Travel with the axons in dorsal column with some differences

Spinocerebellar tract First order neurons

• Bifurcate into ascending and descending branches (dorsal and ventral horns)

• Lower limbs innervate Clarke’s nucleus in the medial dorsal horn (red)

Spinocerebellar tract Second order neurons

• Travel to medulla and into the ipsilateral cerebellum via the dorsal spinocerebellar tract

• Send collaterals to synapse in proprioceptive neurons of the dorsal column nuclei

Spinocerebellar tract Third order neurons

• Decussate and travel in medial lemniscus (with fibers from cutaneous mechanoreceptors) to VPL thalamus

Somatotopic maps

The foot, leg, trunk, forelimbs and face are represented in a medial to lateral arrangement

Homunculus

Illustrates the proportion of representation in cortical processing

Facial expression, speaking and hand use require lots of cortical circuitry

Areas of the primary somatosensory cortex (S1)

• 3b and 1 → respond to cutaneous stimuli

• 3a → proprioceptors

• 2 → tactile and proprioception

Area 3b

Obligatory 1st step in cortical processing

→ Lesions produce profound deficits in all forms of tactile sensations

Area 1

Lesions produce inability to discriminate texture of objects

Area 2

Lesions produce inability to discriminate size and shape of objects

Intracortical microstimulation

• Ultimate goal: to create a sensory neuroprosthesis to restore tactile sensation

• Goal of this paper: to determine how S1 stimulation is perceived

Thalamus

Includes:

Ventral posterior medial nuclues (VPM) → Face

Ventral posterior lateral nucleus (VPL) → Body

Postcentral Gyrus

The postcentral gyrus contains the primary somatosensory cortex, a significant brain region responsible for proprioception.

This region perceives various somatic sensations from the body, including touch, pressure, temperature, and pain