Sensory Receptor/Sensory input

Somatosensory system

• Provide information to CNS about state of body and contact with the environment

  • Exteroceptive (external sense)

    • Sensing stimulis outside the body by sight, sound, smell, taste and touch. How we perceive our environment

  • Proprioceptive (body position sense)

    • Refers to awareness of body position, posture and movement in space

    • Enable coordination and balance

  • Interoceptive (internal sense)

    • Sensing the internal physiological condition of the body

    • Hunger, thirst, heart rate, respiration and feelings of pain

• Receptors transduce mechanical (pressure/stretch/vibration) or thermal stimuli

• Receptor potentials generated in distal tips of first order neuron (cell bodies in dorsal root)

• First order neurons are pseudo unipolar

• Aps relay information about magnitude and duration

• Located immediately posterior to central sulcus in postcentral guyus

• Amount of cortical tissue dedicated to body region is proportional to how richly innervated the area is

Structure of somatosensory system

• First order synapse to second order in spinal cord/brain stem

• Second order synapse to third in thalamus

• Third order projects to somatosensory cortex (S1)

• Ascending pathways include

  • Dorsal column medial lemniscus (fine discriminatory touch, proprioception)

  • Spinothalamic (pain and temp)

Tactile

• Touch information sensed by specialized receptors in skin

• Dermatomes are specific cutaneous regions supplied by a single dorsal root (sensory axon enter spinal cord)

  • Each root carries tactile input from its dermatome to CNS

  • Rostral = enter spinal tract

  • Caudal = afferent limb of reflex arc

• Axons of PNS enter and leave CNS are spinal roots

  • Dorsal roots = sensory

  • Ventral - motor neurons

Tactile Information is Carried by type 3 (A-delta) and 4 fibres C

• 3 is faster than C

• Both are skin mechanoreceptors, thermal receptors, noiciceptors

• Type A-delta,

  • Diameter 1-5 micrometer

  • 5-30m/s

  • For pain/temp

• Type C

  • Diameter 0.2-1.5 micrometer

  • 0.5-2m/s

  • Temperature, pain, itch

Tactile Receptors

• Meissner's corpuscles (glabrous skin, superficial; FA1 - fast adapting, small field; detects low-frequency vibration, flutter).

• Pacinian corpuscles (glabrous skin, deeper; FA2 - fast adapting, large field; high-frequency vibration).

• Merkel's disks (glabrous skin, superficial; SA1 - slow adapting, small field; sustained pressure, texture).

• Ruffini endings (glabrous skin, deeper; SA2 - slow adapting, large field; skin stretch).

• Hairy skin: hair follicles + free nerve endings.

• Fast-acting receptors act to filter steady or slowly changing stimuli

  • FA is sensitive to changing stimuli

• Receptors transduce mechanical stimuli into receptor potentials in pseudo unipolar first-order neuron tips (A-beta fibers for touch). APs signal magnitude/duration. 

Tactile information uses dorsal column lemniscus pathway

Feature Extraction

S1 cortex performs feature extraction for tactile (and other somatosensory) input. 

• Initial processing: S1 cortical neurons handle basic input analysis.

• Higher-order processing: Begins feature extraction (beyond simple sensation).

• Direction selectivity: Specific S1 neurons respond to stimuli moving in preferred directions:

  • UR (ulna to radius).

• WF (wrist to fingers).

This enables perception of stimulus orientation, direction, and texture via DCML pathway input to S1

Pain/temperature Information is carried by Type 3 (a-delta) and 4, C

• It takes a different pathway to the cortex

• The sharp/intense pain stimuli is carried by the fast A-delta axon

• Longer and less painful sensation is mediated by slow C fibers

• Visceral nociceptors and skin/muscle nociceptors converged on the same second-order neuron in SC dorsal horn

  • Brain can't distinguish the source

  • Pain is "referred" to skin like having heart attack pain I left arm

• Uses the spinothalamic tract

Temperature sensation

• Transient receptor potential ion channels

  • Heat sensitive: TRPV1

  • Cold sensitive: TRPM8

• Firing rate increase with temperature in activation range

• Multiple TRP potential sum = AP in afferent neuron

• Capsaicin

  • Activate the same nerve receptor that responds to painful heat that is triggered above 43 degrees

  • Opens ion channel called TRPv1 in certain sensory neurons

  • Channel opens = Ca2+ and Na+, makes neuron fire and sending a hot/pain signal

Pain/Temperature Receptor

• Pain (nociception): Free nerve endings (A-delta for fast/sharp "first pain"; C fibers for slow/dull "second pain"). Nociceptors express TRP channels (chemo-, mechano-, thermo-sensitive). Referred pain from visceral/somatic convergence in dorsal horn. 

• Temperature: Free nerve endings with TRP channels (TRPV1 for heat >43°C, capsaicin; TRPM8 for cold, menthol). Receptor potentials sum to trigger APs. 

Dorsal Column Lemniscus Pathway

Carries fine touch and proprioception

Projection pathway

• First order enters SC and project to DCML

• Second order decussates at medulla and project to thalamus

• Third order neurons are located in the thalamus and project to S1

• Other tactile and proprioceptive pathway project to thalamus

  • Trigeminal = facial sensation

  • Spinocerebellar = muscle tone and coordination

 

• 1st-order neurons: Enter spinal cord via dorsal roots, ascend ipsilateral in dorsal columns to medulla nuclei (gracile/cuneate). 

• 2nd-order: Decussate in medulla, ascend as medial lemniscus to thalamus (VPL).

• 3rd-order: Thalamus to somatosensory cortex (S1, postcentral gyrus).
  Crosses midline at medulla. S1 does feature extraction (e.g., direction-specific neurons).

Spinothalamic Pathway

• Transmitted from PNS to CNS

• Morphologically distinct from afferents, slow and fast pain

1. First order afferent project to second order afferent in spinal cord

2. Decussates in the level of spinal cord it enters

3. Second order project to third in VPI thalamus

4. Neuron in thalamus projects to S1 and other areas responsible for affective response (emotional)

• Free nerve ending of nociceptors express chemo- , mechano- , or thermo- sensitive ion channels

 

Carries pain, temperature, crude touch.

• 1st-order: Enter dorsal horn, synapse on 2nd-order.

• 2nd-order: Decussate immediately in spinal cord, ascend contralateral anterolateral tract to thalamus (VPL).

• 3rd-order: Thalamus to S1 + affective areas (emotion).
  Crosses midline at spinal level

Proprioceptive information

• Carried by 1a, 1b and 2 fibers

• 1a

  • 13-20 micrometers

  • 80-120 m/s

  • Primary muscle spindles

• 1b and 2

  • 6012 micrometers

  • 35-75m/s

  • Golgi tendon organ, secondary muscle spindles, skin mechanoreceptors

Muscle spindles

• Muscle spindles is near tendon insertions. Fusiform-shaped muscle spindles. Lie parallel with regular muscle fibers

• 1alpha afferent nerves innervate nuclear bag1 and chain fibers

  • Detects dynamic length changes

  • AP is always firing, frequency increases when stretch increases, no AP when there is released

  • Is also sensitive to static length

• Type 2 afferent nerves innervate nuclear bag2 and chain fibers

  • Detects static length

  • AP frequency is related to amount of stretch, frequency is high when muscle is very stretched, frequency is low when it is not. AP is always firing, even if there is a release just means frequency slows down

• Afferent nerves enter through dorsal horn, interneurons connect afferent to efferent. Alpha and gamma motor neuron exits through ventral

• Gamma motoneurons innervate intrafusal muscle fibers

• Extrafusal fibers are innervated by 1 alpha moto neuron

  • Intrafusal has 2 sensory and 1 motor innervation

Golgi Tendon Organs

• Monitor muscle force

• Similar to spindles but only has 1b afferent nerves

• Axon wraps around dense connective tissue in tendons

• It is in series with muscles (GTO's is in tendon)

• Force is proportional to stretch of GTO

• Has force decreases, firing rate of GTO decreases

Describe how spindle activity can be influenced by gamma motor neurons

γ-motor neurons (dynamic/static) innervate spindle intrafusal fibers, contracting them to keep spindles taut during muscle shortening (maintains sensitivity to length changes). Prevents spindles from going slack. Essential for continuous proprioceptive feedback.