Touch Pain Motor Control

Somatosensory system

The sensory system responsible for touch, pressure, temperature, pain, and itch.

Pain

An unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage.

Why is pain more than a purely sensory experience?

Pain includes both sensory information and emotional interpretation.

Major peripheral sensory receptors discussed in the presentation

Merkel disks, Meissner’s corpuscles, Ruffini endings, Pacinian corpuscles, free nerve endings, and hair receptors.

Sensory or touch receptors

Specialized skin structures connected to axons that transmit somatosensory information toward the spinal cord and enable perception.

Mechanoreceptors

Sensory receptors that respond to mechanical stimulation such as pressure, stretch, vibration, or movement across the skin.

Merkel disks

Slowly adapting mechanoreceptors that respond to sustained pressure and help detect fine details, edges, and texture.

Meissner’s corpuscles

Rapidly adapting mechanoreceptors that respond to light touch and low-frequency vibration or movement across the skin.

Ruffini endings

Slowly adapting mechanoreceptors that respond to sustained skin stretch and pressure.

Pacinian corpuscles

Rapidly adapting mechanoreceptors that respond especially to deep pressure and high-frequency vibration.

Free nerve endings

Unencapsulated sensory endings that function as nociceptors and thermoreceptors and are important for detecting pain and temperature.

Hair receptors

Receptors associated with hair follicles that respond to movement of hairs and light contact with the skin.

Slowly adapting mechanoreceptors

Merkel disks and Ruffini endings; they remain active during sustained pressure or stretch.

Rapidly adapting mechanoreceptors

Meissner’s and Pacinian corpuscles; they respond strongly to changes such as the beginning, end, or vibration of a stimulus.

Difference between slowly and rapidly adapting receptors

Slowly adapting receptors continue signaling during a sustained stimulus, whereas rapidly adapting receptors emphasize dynamic changes.

Piezo channels

Mechanosensitive cation channels that open in response to mechanical forces and help convert touch into electrical signals.

Ions conducted through Piezo channels

Cations including Na+, K+, Ca2+, and Mg2+.

How do Piezo channels contribute to touch perception?

Mechanical force opens the channels, allowing ion movement that changes membrane potential and can generate action potentials.

Nociceptors

Sensory receptors, often free nerve endings, that detect potentially damaging or painful stimuli.

Thermoreceptors

Sensory receptors that detect changes in temperature.

Transient receptor potential (TRP) channels

Ion channels in sensory endings that open in response to particular temperatures and other chemical or physical stimuli.

Primary fibers associated with pain

Aδ fibers and C fibers.

Aβ fibers

Large, fast, myelinated fibers that primarily transmit nonpainful touch and pressure information.

Aδ fibers

Thin, myelinated fibers that conduct relatively quickly and carry sharp, fast, or pricking pain.

C fibers

Small, unmyelinated fibers that conduct slowly and carry dull, aching, burning, or lingering pain.

Fast pain

Immediate, sharp, well-localized pain carried primarily by Aδ fibers.

Slow pain

Delayed, dull, aching, or burning pain carried primarily by C fibers.

First-order somatosensory axons

Axons that carry peripheral sensory information toward the central nervous system and enter the spinal cord through similar general routes.

Why do somatosensory axons transmit at different speeds?

Fiber diameter and myelination differ; larger and more heavily myelinated axons conduct signals faster.

Gate-control theory of pain

The idea that processing within the spinal cord regulates how much nociceptive information is transmitted to the brain.

Where does pain gating occur?

Within spinal-cord circuitry before the signal ascends to the brain.

Inputs that influence the spinal pain gate

Nociceptive C-fiber input, nonnociceptive A-fiber touch input, and inhibitory interneurons.

How can nonpainful touch reduce pain?

Activity in fast touch fibers can recruit inhibitory spinal interneurons and reduce transmission of nociceptive signals.

Why might rubbing an injured area help?

Rubbing activates nonnociceptive touch fibers that can partially close the spinal pain gate.

Ascending somatosensory pathways

Neural pathways that carry sensory information from the spinal cord toward the brain and thalamus.

Two major ascending somatosensory systems

The dorsal column–medial lemniscal system and the anterolateral system.

Dorsal column–medial lemniscal system

An ascending pathway that primarily carries discriminative touch and pressure information.

Anterolateral system

An ascending system that primarily carries pain and temperature information through several supraspinal pathways.

Descending inhibitory pathways

Brain-to-spinal-cord pathways that suppress pain transmission and allow ongoing behavior despite injury or nociceptive input.

Adaptive function of descending pain inhibition

It protects ongoing performance by temporarily reducing distraction from pain when action is necessary.

Periaqueductal gray (PAG)

A midbrain region that helps initiate descending pain-inhibitory control.

Three major descending inhibitory systems

The endogenous opioid, serotonergic, and noradrenergic systems.

Endogenous opioid system

An internal pain-modulation system that uses opioid peptides to reduce nociceptive transmission.

Serotonergic pain-inhibitory system

A descending system that uses serotonin to help regulate and inhibit pain transmission.

Noradrenergic pain-inhibitory system

A descending system that uses norepinephrine to help regulate and inhibit pain transmission.

Thalamic relays for somatosensory information

The ventral posterior lateral (VPL) and ventral posterior medial (VPM) nuclei.

VPL nucleus

The thalamic nucleus that primarily relays somatosensory information from the body and trunk.

VPM nucleus

The thalamic nucleus that primarily relays somatosensory information from the face and head.

Primary somatosensory cortex (S1)

The first cortical area receiving detailed somatosensory information; it supports localization and discrimination of bodily sensations.

Secondary somatosensory cortex (S2)

A cortical area that receives processed somatosensory input from S1 and contributes to higher-order integration.

Cortical processing sequence for somatosensory information

S1 → S2 → posterior parietal cortex → temporal association cortex → parahippocampal cortex → cingulate cortex.

Posterior parietal cortex in somatosensation

Helps integrate sensory information with spatial representation and action.

Temporal association cortex in somatosensation

Contributes to higher-order interpretation and recognition of sensory information.

Parahippocampal cortex in somatosensation

Helps link somatosensory experience with contextual and memory-related information.

Cingulate cortex in somatosensation and pain

Contributes to the affective, motivational, and emotional dimensions of bodily sensations and pain.

Limbic system’s role in somatosensation

Processes emotional responses to somatosensory information.

Itch

Also called pruritus; an unpleasant protective sensation that promotes scratching, although its neural mechanisms are not fully understood.

Shared peripheral pathways of pain and itch

Both begin partly through small-diameter C fibers and Aδ fibers in the periphery.

Chronic pain and psychiatric disorders

They show overlap in neural circuitry, helping explain their frequent interaction and comorbidity.

Muscle fiber or myofiber

A single muscle cell.

Myofibrils

Elongated rods packed within a muscle fiber that contain the contractile machinery.

Sarcomere

The smallest functional unit of muscle contraction; a repeating protein arrangement bounded by Z-discs.

Z-discs

Structures marking the boundaries of a sarcomere that are pulled closer together during contraction.

Actin

A thin filament that interacts with myosin during muscle contraction.

Myosin

A motor protein whose heads attach to and pull along actin filaments during contraction.

Cross-bridge cycle

A repeating contraction cycle in which myosin heads bind to actin and pull the filaments so the Z-discs move closer together.

What happens to a sarcomere during contraction?

Actin and myosin slide relative to each other, pulling the Z-discs closer and shortening the sarcomere.

How are skeletal-muscle action potentials initiated?

Lower motor neurons synapse with muscle fibers and release acetylcholine at the neuromuscular junction.

Acetylcholine in motor control

The neurotransmitter released by lower motor neurons to activate skeletal muscle fibers.

Neuromuscular junction

The synapse between a lower motor neuron and a skeletal muscle fiber.

Motor unit

One lower motor neuron and all of the muscle fibers it controls.

How is a whole muscle organized functionally?

A muscle contains many motor units, and each motor unit may include many muscle fibers.

Antagonistic muscle pairs

Pairs of muscles that produce opposing movements; coordinated activation allows controlled movement.

Lower motor neurons (LMNs)

Caudal motor neurons that directly implement muscle contractions.

Upper motor neurons (UMNs)

More rostral neurons that plan, select, and coordinate decisions to move.

Primary motor cortex (M1)

The cortical area that sends a large proportion of motor commands into descending motor tracts.

Lower versus upper motor systems

Lower motor systems execute contractions, while upper motor systems coordinate and issue movement commands.

Size principle

Motor units are generally recruited from smaller, lower-force units to progressively larger, higher-force units as force demands increase.

Proprioception

The sensory system that provides information about limb position, muscle length, load, and movement.

Why is proprioception essential for motor control?

Movement requires continuous sensory feedback about whether the body is doing what was intended and whether adjustments are needed.

Movement as a two-way process

The brain sends motor commands to the body while proprioceptive and other sensory information returns to update and correct the movement.

Muscle spindles

Stretch receptors located within muscles that detect changes in muscle length.

Intrafusal muscle fibers

Specialized fibers within muscle spindles that function as sensory receptors for muscle length and stretch.

Extrafusal muscle fibers

Ordinary skeletal muscle fibers that generate force and produce movement or contraction.

Golgi tendon organ

A proprioceptive receptor associated with tendons that detects skeletal-muscle tension.

How does a Golgi tendon organ sense tension?

Muscle force stretches collagen fibers in the tendon organ, activating sensory endings.

Three broad cortical levels involved in voluntary movement

Prefrontal cortex; premotor cortices and subcortical structures; and primary motor cortex (M1).

Prefrontal cortex in motor control

Contributes to goals, intentions, decision-making, and planning what action should occur.

Premotor cortices in motor control

Help prepare, organize, and sequence movements before execution.

Basal ganglia in motor control

Contribute especially to selecting and initiating appropriate movements.

Cerebellum in motor control

Contributes especially to timing, coordination, prediction, and correction of movement.

Primary motor cortex in motor control

Issues major descending commands for voluntary movement.

Three major descending motor tracts in the presentation

Corticobulbar, lateral corticospinal, and anterior corticospinal tracts.

Corticobulbar tract

A descending pathway that controls muscles of the jaw, face, and other cranial musculature.

Lateral corticospinal tract

A descending pathway particularly important for voluntary control of the limbs and distal body muscles.

Anterior corticospinal tract

A descending pathway particularly involved in control of trunk and axial muscles.

Clinical health psychology applications of pain science

Use a biopsychosocial perspective, assess interactions between depression and chronic pain, and apply strategies such as mindfulness and progressive muscle relaxation.

Biopsychosocial perspective on pain

Pain reflects interacting biological, psychological, and social influences rather than tissue damage alone.

Relationship between depression and chronic pain

Depression and chronic pain frequently reinforce one another and share some neural and behavioral mechanisms.

Mindfulness for pain

A strategy that can change attention and emotional responses to pain rather than requiring the sensation to disappear.