Somatic Sensory System Notes

The Somatic Sensory System

Introduction to Somatic Sensation

• Somatic sensation enables the body to feel touch, pain, and temperature.
• It is sensitive to various stimuli and responsible for the sense of touch and pain.
• The somatic sensory system differs from other sensory systems in that its receptors are distributed throughout the body and respond to different kinds of stimuli.
• A single sensory receptor can encode the intensity, duration, position, and direction of stimuli.

Touch

• Types and layers of skin:
  • Hairy and glabrous (hairless) skin.
  • Epidermis (outer layer) and dermis (inner layer).
• Functions of skin:
  • Protective function.
  • Prevents evaporation of body fluids.
  • Provides direct contact with the world.
• Mechanoreceptors: Most somatosensory receptors are mechanoreceptors.

Mechanoreceptor Types

• Pacinian corpuscle:
  • Located in the deep dermis; looks like an onion.
  • Large receptive field.
  • Rapidly adapting.
  • Best frequency: approximately $250$ Hz.
• Meissner's corpuscle:
  • Embedded within the epidermal ridges (fingerprints).
  • Small receptive field.
  • Rapidly adapting.
  • Best frequency: approximately $50$ Hz.
• Hair follicle receptor:
  • Nerve endings around the root of hair in hairy skin.
  • Small receptive field.
  • Either slowly or rapidly adapting (two kinds).
  • Vibrissae (whiskers) are specialized hair follicle receptors on cats, rats, etc.
• Ruffini's ending:
  • Small lobed blobs.
  • Large receptive field.
  • Slowly adapting.
• Merkel's disks:
  • Small arrays of tiny disks at the dermal/epidermal border.
  • May have synapses onto the nerve ending.
  • Small receptive fields.
  • Slowly adapting.
• Free nerve endings:
  • Respond to very light touch.
  • Some are possibly specific for tickle or itch.
  • Type I, slow adapting
• Krause end bulbs:
  • Look like light knotted balls of string in the skin.
  • Located at the border between dry skin and mucous membrane in the mouth, genitals, and anus.

Mechanoreceptor Properties (Receptive Field)

• Ake Vallbo recorded from single sensory axons in the human arm to map single mechanoreceptor's receptive field.
• Small receptive field: Merkel's disk
• Large receptive field: Ruffini's endings

Mechanoreceptor Properties (Adaptation)

• Fast adapting receptors (FA): Phasic, provide accurate information about changes in skin contact.
• Slow adapting receptors (SA): Tonic, provide information about long-term contact with the skin.
• Type I receptors: Located superficially in the skin with small receptive fields.
• Type II receptors: Sensitive endings located deep in the skin with broad and diffuse receptive fields.
• Skin receptors rarely act alone; contact with the skin usually activates multiple classes of receptors. The total pattern of responses yields sensations.

Mechanoreceptor Sensitivity

• Pacinian corpuscle:
  • 10-15% of mechanoreceptors.
  • Highly sensitive (lower response threshold than Meissner corpuscle; action potential generation by as little displacement as $10$ nm).
  • Sensitive to high-frequency vibrations; responds best to vibration.
• Meissner's corpuscle:
  • Approximately 40% of mechanoreceptors.
  • Closest to the surface.
  • $2-6$ afferent nerve fibers are encapsulated by several lamellae of Schwann cells.
  • Highly sensitive (more than 4 times as sensitive to skin deformation as Merkel afferents).
  • Sensitive to low-frequency vibrations.
  • Particularly responsible for detecting slippage of an object in hands; responds best to light touch.
• Ruffini's ending:
  • 20% of mechanoreceptors.
  • Present not only in the skin but also in ligaments and tendons.
  • Sensitive to stretching.
  • Responsible for detecting internally generated stimuli such as bending or moving fingers.
• Merkel's disks:
  • 25% of the mechanosensory afferents in the hand.
  • Highest spatial resolution ($0.5$ mm).
  • Highly sensitive to points, edges, curvature, form, and texture.

Afferent Systems and Their Properties (Table 9.2 Summary)

• Merkel:
  • Location: Tip of epidermal sweat ridges.
  • Axon diameter: $7-11$ μm.
  • Conduction velocity: $40-65$ m/s.
  • Sensory function: Form and texture perception.
  • Effective stimuli: Edges, points, corners, curvature.
  • Receptive field area: $9$ mm\textsuperscript{2}.
  • Innervation density (finger pad): $100/$cm\textsuperscript{2}.
  • Spatial acuity: $0.5$ mm.
  • Response to sustained indentation: Sustained (slow adaptation).
  • Frequency range: $0-100$ Hz.
  • Peak sensitivity: $5$ Hz.
• Meissner:
  • Location: Dermal papillae (close to the skin surface).
  • Axon diameter: $6-12$ μm.
  • Conduction velocity: $35-70$ m/s.
  • Sensory function: Motion detection; grip control.
  • Effective stimuli: Skin motion.
  • Receptive field area: $22$ mm\textsuperscript{2}.
  • Innervation density (finger pad): $150/$cm\textsuperscript{2}.
  • Spatial acuity: $3$ mm.
  • Response to sustained indentation: None (rapid adaptation).
  • Frequency range: $1-300$ Hz.
  • Peak sensitivity: $50$ Hz.
• Pacinian:
  • Location: Dermis and deeper tissues.
  • Axon diameter: $6-12$ μm.
  • Conduction velocity: $35-70$ m/s.
  • Sensory function: Perception of distant events through transmitted vibrations; tool use.
  • Effective stimuli: Vibration.
  • Receptive field area: Entire finger or hand.
  • Innervation density (finger pad): $20/$cm\textsuperscript{2}.
  • Spatial acuity: $10+$ mm.
  • Response to sustained indentation: None (rapid adaptation).
  • Frequency range: $5-1000$ Hz.
  • Peak sensitivity: $200$ Hz.
• Ruffini:
  • Location: Dermis.
  • Axon diameter: $6-12$ μm.
  • Conduction velocity: $35-70$ m/s.
  • Sensory function: Tangential force; hand shape; motion direction.
  • Effective stimuli: Skin stretch.
  • Receptive field area: $60$ mm\textsuperscript{2}.
  • Innervation density (finger pad): $10/$cm\textsuperscript{2}.
  • Spatial acuity: $7+$ mm.
  • Response to sustained indentation: Sustained (slow adaptation).
  • Frequency range: $0-?$ Hz.
  • Peak sensitivity: $0.5$ Hz.
• Threshold for rapid indentation or vibration:
  • Merkel: Best - $8$ μm, Mean - $30$ μm.
  • Meissner: Best - $0.01$ μm, Mean - $2$ μm.
  • Pacinian: Best - $0.08$ μm, Mean - $6$ μm.
  • Ruffini: Best - $40$ μm, Mean - $300$ μm.

Mechanoreceptors (Cont'd)

• Receptive field size and adaptation rate.
  

• Hairs are part of a receptor system.
• Follicles are richly innervated by free nerve endings that sense the deformation of follicles.
• Whiskers are important navigation tools.

Mechanoreceptors - Structure

• Largely determined by the structure of endings.
• Layers of connective tissue with viscous fluid between them help in dissipating the stimulus energy, restoring the axonal membrane that was deformed initially.
• Naked (without connective tissue) nerve terminal is much less sensitive to vibration but more sensitive to steady pressure.

Mechanoreceptors - Ion Channels

• Some ion channels are sensitive to stretching of the lipid membrane.
• Some ion channels open with force applied to extracellular structures.
• Some ion channels open with deformation and stress on the cell's cytoskeleton.

Mechanoreceptors - Spatial resolution

• Two-point discrimination.
• Ability to discriminate the detailed features of a stimulus is determined by:
  • Receptor density.
  • Receptive field size.
  • Number of neurons (size of area) in the cortex that are responsible for the processing of information from a given region.

Touch - Primary Afferent Axons

• Course through the vast network of peripheral nerves on their way to the central nervous system.
• Pseudo-unipolar cells with cell bodies lying in the dorsal root ganglia.

Touch - Primary Afferent Axons

• Size correlates with the type of receptors.
• Named according to axonal size and innervating tissues.
• Conduction velocity is correlated with the diameter of the axon; pain sensing is slower than touch sensation.

The Spinal Cord

• Spinal segments (30) - spinal nerves within 4 divisions of spinal cord.
• Dermatomes: The region of the body surface innervated by each spinal cord nerve; one-to-one correspondence with segments.
• Shingles.
• Sensory Organization of the spinal cord - 4 Divisions; Cervical (C), Thoracic (T), Lumbar (L), Sacral (S).

Dermatome

• The area of skin innervated by the right and left dorsal roots of a single spinal segment.
• One-to-one correspondence between dermatome and spinal segment.
• Overlapping innervation.
• Herpes zoster virus causes shingles in a dermatome-specific manner.

Touch - Spinal Cord Organization

• Division of spinal gray matter: Dorsal horn; Intermediate zone; Ventral horn.
• Gray matter - Dorsal horn: A branch of Aβ axons makes synapses to second-order sensory neurons, which modify or initiate many unconscious reflexes.
• Another branch ascends straight to the brain which is percieved.
• Ventral horn & intermediate zone.

Touch - Dorsal Column–Medial Lemniscal Pathway

• Touch sensation takes a distinct path from the path taken by pain and temperature information.
• Touch information ascends through the dorsal column, dorsal nuclei, medial lemniscus, and ventral posterior nucleus to the primary somatosensory cortex.
• Not a mere transfer of information: every synaptic connection transforms information (e.g., lateral inhibition).

Somatosensory Central Pathways - Touch

• Dorsal column - medial lemniscus pathway:
  1. A β sensory afferents from the skin enter the dorsal root and bifurcate. A short branch goes to the intermediate zone of the spinal cord, and a long branch goes to the brain.
  2. The long branch ascends in the dorsal columns to the dorsal column nuclei at the top of the spinal cord.
  3. Second-order axons decussate and form the contralateral medial lemniscus.
  4. Medial lemniscus fibers terminate in the contralateral ventral posterior nucleus (VP) of the thalamus.
  5. VP neurons project to the primary somatosensory cortex (S1).

Trigeminal Pathway

• Supplies the somatic sensation of the face.
  1. The trigeminal nerve (n. V) innervates the skin of most of the face. Three branches innervate the face, mouth areas, outer two-thirds of the tongue, and dura mater covering the brain. Small portions of the skin on the face are innervated by the facial nerve (n. VII), glossopharyngeal nerve (n. IX), and vagus nerve (n. X).
  2. Fibers from cutaneous receptors synapse in the ipsilateral principal sensory trigeminal nucleus.
  3. Second-order fibers decussate and project to VP in the thalamus.
  4. VP neurons project to the primary somatosensory cortex (S1).

Touch - Somatosensory Cortex

• Primary somatosensory cortex (S1):
  • Located in the parietal lobe, postcentral gyrus (behind the central sulcus).
  • Area 3b:
    • Dense inputs from VP.
    • Very responsive to somatosensory stimuli.
    • Lesions impair somatic sensation.
    • Electrical stimulation evokes somatic sensory experiences.
  • Area 3a receives VP inputs as well, but those inputs carry information about body position.
  • 3b projects to area 1 (texture info) and area 2 (size and shape).
• Secondary somatosensory cortex - S2 (Brodmann areas 40 and 43).

Secondary Somatosensory Cortex - S2

• Located at the bottom of the Post-central Gyrus of the Parietal lobe.
• Receives bilateral input: both sides of the body are represented.
• Receives pathways for touch, pressure, pain, and position sense.
• Exact function is unknown yet.

Somatosensory Cortex : Columnar organization

• First described cortical column: vertical stacking of neurons that have similar inputs and responses.

Cortical Somatotopy - Mapping the Somatosensory Cortex

• Electrical stimulation during brain surgery – Wilder Penfield.
• Recording from S1 neurons - or imaging - Somatotopy, mapping of the body’s surface sensations onto a structure in the brain.
• Importance of mouth tactile sensations for speech, lips and tongue are important as last line of defense - represented as Homunculus in the homonculus man. .

Cortical Somatotopy - Unequal Representation

• Not always continuous.
• Unequal representation is correlated with the density of sensory inputs, importance of the sensory input, and frequency of usage.
• Rodent S1 - Vibrissae are important navigation tools.
• Barrel cortex comprises 5 rows of whiskers which make 5 rows of barrels.

Whisker Barrel

• The somatosensory pathways bring sensory information from the periphery into the brain.
• The pathway to the Information from the whisker pad of the rat (or mouse) passes along the trigeminal nerve, projecting to the trigeminal complex in the brainstem, which sends projections to the medial ventral posterior nucleus of the thalamus (VPm), then to the somatosensory cortex.

Cortical Map Plasticity

• Michael Merzenich performed experiments regarding the changes in somatosensory cortex after removing digits or overstimulating digits.
• Examine somatotopy before and after the changes.
• Reorganization of cortical maps can occur.
• Sensory experience-dependent dynamic changes of cortical maps can occur.
• Phantom limb can occur also.
• Expansion of bordering skin regions makes confusion as well
• Musician’s hands have shown that plasticity could occur.

S1 - Owl monkey

Area 3b, Area 1: Diagram showcasing representation of the Leg, Sole, Thigh, Trunk, Arm, Wrist, Palm, Chin, Caudal Vibrissae, Lower Lip, Upper Lip, Teeth, Fingers, M. vib and Palmar pads.

Secondary Somatosensory Cortex - S2

• Ventral surface of the postcentral gyrus overlying the auditory cortex on top of the temporal lobe.
• Posterior parietal cortex - posterior to S1 along the midline:
  • Brodmann’s areas 5 and 7.
  • Place where submodalities come together for complex somatosensory sensations.
  • Damage to the posterior parietal cortex can cause:
    • Agnosia (인지 불능증) - inability to recognize objects despite normal simple sensory skills.
    • Astereognosia - inability to recognize objects by feeling them, but can recognize by sight and sound and have normal cutaneous thresholds (inability to identify an object by touch without visual input; a form of tactile agnosia).
    • Neglect syndrome: part of body or part of the visual world is ignored.

Pain

• Nociceptors:
  • Nocere - ‘to hurt’.
  • Distinct pathway from mechanoreceptors’ tactile pathway.
  • Starts from free nerve endings.
  • Pain and nociception are vital to life.
• Pain and nociception:
  • Pain - feeling (perception) of sore, aching, throbbing, miserable sensation.
  • Nociception - sensory process, provides signals that trigger pain
  • The cognitive qualities of nociception can be controlled from within by the brain itself.

Nociception and the Transduction of Painful Stimuli

• Activation of nociceptors: Stimuli with the potential to cause tissue damage.
  • Mechanical force: stretching and bending.
  • Released chemicals from damaged cells:
    • Proteases - breakdown kininogen to bradykinin - bind to receptors that activate ionic conductances in some nociceptors.
    • ATP - ATP-gated channels.
    • K+ - Direct depolarization.
  • Other chemicals:
    • Anaerobic ATP production from exercise generates lactic acid.
    • H+-gated channels.
    • Histamine is released from mast cells (e.g., a bee sting).
  • Extreme heat (over 43°C) or cold.

Types of Nociceptors

• Polymodal nociceptors vs. Selective nociceptors:

  • Mechanical: Activated by strong stimuli such as pinch and sharp objects that penetrate, squeeze, and pinch the skin. Sharp or pricking pain via A-delta fibers.
  • Thermal: Activated by noxious heat (temperature above 45°C), noxious cold (temperature below 5°C), and strong mechanical stimuli via A-delta fibers.
  • Chemical.

• Nociceptors are present in most body tissues (absent in the brain except for the meninges).

• Hyperalgesia (통각과민증): Hypersensitivity of damaged tissue. Reduced threshold for pain, increased intensity of painful stimuli, or spontaneous pain. Primary (within the area of damage) and secondary (surroundings) hyperalgesia.

Hyperalgesia - Peripheral Sensitization

• Various chemicals released by damaged tissue modulate the excitability of nociceptors.
  • Bradykinin: Long-lasting intracellular changes make heat-activated ion channels more sensitive.
  • Prostaglandins:
    • Produced by lipid membrane breakdown.
    • Increase the sensitivity of nociceptors.
    • Aspirin: Cyclooxygenase (COX) inhibitor prevents arachidonic acids' conversion into prostaglandins.
  • Substance P:
    • Produced by nociceptors themselves.
    • Secreted from other branches than activated branches.
    • Vasodilation and histamine release from mast cells.
  • Neurotransmitters [ATP, CGRP (calcitonin gene-related peptide)], histamine, and NGF all contribute to sensitization.

Hyperalgesia - Central Sensitization

• Contribution of the CNS to secondary hyperalgesia.
• Activity-dependent increase in the excitability of dorsal horn neurons.
• Allodynia: Induction of pain by normally innocuous stimuli.
• Mechanoreceptive Aβ activation can cause pain: cross-talk between the touch and pain pathways.
• Transcription-independent process: ‘wind-up’ - progressive increase in the firing rate of dorsal horn neurons; lasts only during the period of stimulation; arises from the summation of slow synaptic potentials.
• LTP-like potentiation of postsynaptic potential:
  • Transcription-dependent process.
  • Longer-lasting.
  • Induced by neuronal activity changes or by humoral signals.

Pain - Primary Afferents and Spinal Mechanisms

• First pain and second pain - Due to differences in the action potential conduction velocity
  1. Because Aδ are myelinated, they mediate a fast, sharp, first pain.
  2. Because C fibers are unmyelinated and slower, they mediate a duller, longer-lasting second pain.
  3. Both types are believed to use glutamate as their neurotransmitter; both have their cell bodies in the dorsal root ganglion.

Pain - Primary Afferents - Spinal Connections

• Visceral nociceptor axons take the same route as the cutaneous ones.
• Referred pain:
  • Angina: When the heart receives insufficient oxygen, patients often complain of pain in the upper chest wall and left arm.
  • Pain from other visceral organs is often referred to other parts of the body surface.
  • Apparently caused when visceral nerves and somatic nerves synapse in the spinal column with the same nerve going to the cortex.

Ascending Pain Pathways

1. Incoming axons branch immediately in the dorsal horn of the spinal cord and travel up and down the spinal cord for short distances in the zone of Lissauer.
2. Synapse in the adjacent substantia gelatinosa.
3. Nociceptors in the viscera join those from the skin - cross-talk causes referred pain.
4. Second-order neurons in the substantia gelatinosa decussate immediately and ascend in the contralateral spinothalamic tract, continuing through the medulla to the thalamus alongside the medial lemniscus, but separate and distinct from it.
5. Synapse in VP and intralaminar nuclei of the thalamus.
6. Thalamic neurons project to S1.
7. Thus, pain axons, Aδ and C, travel up the contralateral side of the spinal cord.
8. Injuries to one side of the spinal cord affect touch on that side and pain on the other side.

Ascending Pain Pathways - Spinal vs Touch

• Differences between touch and pain pathways:
  • Nerve endings in the skin: Structured vs. Free endings.
  • Diameter of axons: Aβ vs. Aδ / C.
  • Connections in spinal cord: Deep dorsal horn vs. substantia gelatinosa.
• Spinothalamic pain pathway:
  • Immediate decussation of second-order neurons’ axons.
  • Ascending through the spinothalamic tract along the ventral surface of the spinal cord.

Ascending Pain Pathways

*A table comparison

Ascending Pain Pathways - Trigeminal Pain Pathway

• Face or head --> trigeminal nerve --> spinal trigeminal nucleus (brain stem) --> decussate and ascend (trigeminal lemniscus) to thalamus.
• The Thalamus and the Cortex:
  • Wider innervation in the thalamus.
  • Touch and pain systems remain segregated.
  • Pain and temperature information is sent to various cortical areas.

Ascending Pain Pathways - Parallel Pathways

• Innervate a variety of structures in the brainstem.
  Parallel pathways leading to structures in the brainstem. The sensory-discriminative pathway goes to the Somatosensory cortex (S1, S2) then to the Ventral posterior nucleus. The Affective-motivational pathway goes to the Insular cortex, Anterior cingulate cortex, Amygdala, Hypothalamus then to the Midline thalamic nuclei, Periaqueductal grey, Superior colliculus, and Reticular formation. These parallel pathways make up the ANTEROLATERAL SYSTEM.

The Regulation of Pain

• The same level of nociceptor activity generates different levels of pain - pain modulation.
• Depending on the concurrent level of nonpainful sensory input and the behavioral context.
• Afferent Regulation: Pain is reduced when low-threshold mechanoreceptors are activated simultaneously (rubbing).
• Gate theory of pain.

Melzack and Wall's Gate Theory of Pain

• A spinothalamic projection neuron in the dorsal horn receives:
  • Excitatory input from the nociceptor - C fiber.
  • Excitatory input from the normal mechanoreceptor (Aα or Aβ).
  • Inhibitory input from the interneuron, which is excited by the mechanoreceptor but inhibited by the nociceptor.
• When only nociceptors are active, the gate is open, and pain is perceived.
• Non-nociceptive inputs from mechanoreceptors can close the gate and reduce the pain that is perceived.
• This is why rubbing a painful spot or joint can help reduce the pain.

The Regulation of Pain - Descending Regulation

• Suppression of pain by strong emotion, stress.
• The periaqueductal gray matter (PAG) has inputs from brain structures that convey information about emotional status.
• Serotonergic Raphe nuclei inputs can effectively depress the activity of nociceptive neurons.
• The endogenous opiates:
  • Opioids (opium, morphine, heroine, codeine) have a strong analgesic effect in addition to mood changes, drowsiness, and mental clouding.
  • Endogenous opioids, collectively called endorphins, bind to opioid receptors in the brain (blocked by naloxone).
  • Distributed widely in the brain but particularly concentrated in areas that modulate nociceptive information.

Temperature

Thermoreceptors

• Non-uniform temperature sensitivity of skin “Hot” and “cold” receptors
• Capsaicin (Trpv1) and menthol (Trpm8)
• Varying sensitivities
• Different thermoreceptive neurons appear to express only a single type of channel
• Some cold receptors coexpress Trpv1

Thermoreceptors - Capsaicin and Menthol

• “Hot” and “cold” receptors.
• Capsaicin (Trpv1) and menthol (Trpm8).
• Vanilloid receptor (VR-1, =TRPV1) has endogenous ligands (endovanilloids) that are produced by peripheral injuries. This may explain why plants developed such receptors.

Thermoreceptors - Adaptations and Pathways

• Adaptations: It is the sudden change in the quality of a stimulus that generates the most intense neural activity and perceptual response.
• Temperature pathway:
  • Virtually identical to the pain pathway.
  • Cold receptors to Aδ and C fibers and warm receptors only to C fibers.

Concluding Remarks

• Sensory systems exhibit similar organization and function.
• Sensory types are segregated within the spinal cord and cerebral cortex.
• Repeated theme: Parallel processing of information.
• Perception of an object involves the seamless coordination of somatic sensory information.