Sensory Pathways and the Somatic Nervous System

15-1 Sensory and Motor Pathways

  • Sensory pathways relay sensory information from receptors to the CNS.
    • Sensory receptors are specialized cells that monitor specific conditions and generate action potentials along sensory pathways when stimulated.
  • Afferent division includes somatic and visceral sensory pathways.
    • Somatic sensory information goes to the cerebral cortex.
    • Visceral sensory information goes to the brainstem and diencephalon.
  • Efferent division includes somatic motor pathways that control peripheral effectors.
    • Motor commands, potentially modified by higher-order functions, travel from the brain to skeletal muscles (effectors).

15-2 Sensory Receptors

  • Sensation is sensory information arriving in the CNS.
  • Perception is the conscious awareness of a sensation.
  • Transduction is the conversion of a stimulus into an action potential by a sensory receptor.
  • General senses:
    • Temperature
    • Pain
    • Touch
    • Pressure
    • Vibration
    • Proprioception (body position).
  • Special senses:
    • Olfaction (smell)
    • Gustation (taste)
    • Vision (sight)
    • Equilibrium (balance)
    • Hearing
    • Special sensory receptors are located in sense organs.

Detection of Stimuli

  • Receptor specificity: Each receptor is sensitive to specific stimuli.
  • Receptive field: Area monitored by a single receptor cell.
    • Larger fields make it harder to localize stimuli.
  • Transduction: Receptors convert stimuli to action potentials.
  • Receptor potential: Stimulus changes the receptor membrane potential.
    • Can be depolarizing (generator potential).
    • Can be hyperpolarizing.
    • Size depends on stimulus strength.
  • Special sense receptors (specialized cells) communicate with sensory neurons via synapses.
    • Receptor potential occurs in the receptor cell.
    • Generator potential occurs in the sensory neuron.

Interpretation of Sensory Information

  • Sensory information arrives at specific sites in the cerebral cortex based on location and nature of stimulus.
  • Labeled line: Sensory neurons that link specific peripheral receptors to specific cortical neurons.
    • Each carries information about one modality (e.g., touch or light).
  • Sensory coding: Strength, duration, and variation of the stimulus determine the frequency and pattern of action potentials.
    • Perception depends on the labeled line used.
  • Tonic receptors: Always active.
    • Changes in stimulation cause changes in action potential frequency.
  • Phasic receptors: Normally inactive, activated by a stimulus.
    • Provide information about intensity and rate of change.
    • Stimulation causes a burst of action potentials that end when the stimulus stops or is constant.

Adaptation

  • Adaptation: Reduction of receptor sensitivity in the presence of a constant stimulus.
  • The nervous system adapts quickly to painless, constant stimuli.
  • Peripheral adaptation (in the PNS): Receptor activity changes.
    • Fast-adapting (phasic) receptors: Respond strongly at first, then activity decreases (e.g., temperature).
    • Slow-adapting (tonic) receptors: Show little peripheral adaptation (e.g., pain).
  • Central adaptation (in the CNS): Inhibition of nuclei along a sensory pathway.
    • Most sensory information is processed in the spinal cord, brain stem, or thalamus before reaching the cerebral cortex.
    • Includes conscious and subconscious increase or decrease of sensory awareness.
    • Example: Tuning out background noise.

15-3 General Sensory Receptors

  • Classification by stimulus location:
    • Exteroceptors: External environment.
    • Proprioceptors: Position of skeletal muscles and joints.
    • Interoceptors: Visceral organs and functions.
  • Classification by stimulus nature:
    • Nociceptors: Detect pain.
    • Thermoreceptors: Detect temperature.
    • Mechanoreceptors: Detect physical distortion.
    • Chemoreceptors: Detect chemical concentration.

Nociceptors

  • Nociceptors: Free nerve endings with large receptive fields that detect pain.
    • Common in superficial skin, joint capsules, periosteum, and blood vessel walls.
    • Sensitive to temperature extremes, mechanical damage, or dissolved chemicals released by injured cells.
    • Tonic receptors, with little peripheral adaptation.
    • Central adaptation can reduce pain perception.
      • Facilitation in pain pathways may explain differing pain perceptions.
      • Endorphins and enkephalins inhibit pain pathways in the CNS.
  • Types of pain:
    • Fast pain: Prickling pain (e.g., injection).
      • Carried by myelinated Type A fibers.
      • Reaches CNS quickly, triggers somatic reflexes.
      • Relayed to somatosensory cortex for conscious attention.
    • Slow pain: Burning and aching pain.
      • Carried by unmyelinated Type C fibers.
      • Causes generalized activation of reticular formation and thalamus.
      • Awareness of pain, but with general localization.

Thermoreceptors

  • Thermoreceptors: Free nerve endings that detect temperature.
    • Located in dermis, skeletal muscles, liver, and hypothalamus.
    • Sensations conducted along pain pathways.
      • Sent to reticular formation, thalamus, and somatosensory cortex.
    • Phasic receptors, respond most to temperature changes, then adapt.

Mechanoreceptors

  • Mechanoreceptors: Sensitive to physical stimuli that distort plasma membranes.
    • Membranes contain mechanically-gated ion channels that open or close in response to stretching, compression, twisting, and other distortions.
  • Classes:
    • Tactile receptors
    • Baroreceptors
    • Proprioceptors

Tactile Receptors

  • Tactile receptors: Detect touch (shape or texture), pressure (degree of mechanical distortion), and vibration (pulsing pressure).
    • Fine touch and pressure receptors: Provide detailed information (sensitive, narrow receptive fields).
    • Crude touch and pressure receptors: Poor localization, little information (large receptive fields).
  • Types of tactile receptors in the skin:
    • Free nerve endings: Tonic receptors with small receptive fields for touch and pressure; located between epidermal cells.
    • Root hair plexus: Nerve endings that monitor distortions and movements across body surface where hairs are located; adapt rapidly.
    • Tactile discs: Very sensitive tonic receptors for fine-touch and pressure; sensitive to shape and texture; small receptive fields.
    • Bulbous corpuscles (Ruffini corpuscles): Tonic receptors sensitive to pressure and distortion of skin; located in reticular dermis; show little adaptation.
    • Lamellar corpuscles (Pacinian corpuscles): Fast-adapting receptors sensitive to deep pressure; most sensitive to pulsing or high-frequency vibrations; consists of a single dendrite within concentric layers of collagen.
    • Tactile corpuscles (Meissner corpuscles): Large, fast adapting receptors sensitive to fine touch, pressure, and low-frequency vibration; abundant in eyelids, lips, fingertips, nipples, and external genitalia.

Baroreceptors

  • Baroreceptors: Detect pressure changes in blood vessels and digestive, respiratory, and urinary tracts.
    • Free nerve endings branching within elastic tissues in the wall of distensible organs.
    • Respond immediately to pressure changes, but adapt rapidly.

Proprioceptors

  • Proprioceptors: Monitor the position of joints and skeletal muscles.
    • Proprioception is only a somatic sensation (no proprioceptors in visceral organs).
  • Types:
    • Muscle spindles: Monitor skeletal muscle length and trigger stretch reflexes.
    • Golgi tendon organs: At the junction between skeletal muscle and its tendon; monitor tension during muscle contraction.
    • Receptors in joint capsules: Free nerve endings that detect pressure, tension, and movement at the joint.

Chemoreceptors

  • Chemoreceptors: Respond to water-soluble and lipid-soluble substances dissolved in body fluids.
    • Exhibit fast peripheral adaptation.
  • Monitor pHpH, carbon dioxide, and oxygen levels in arterial blood.
    • Carotid bodies: Near the origin of the internal carotid arteries.
    • Aortic bodies: Between the major branches of the aortic arch.

15-4 Sensory Pathways

  • Sensory neurons:
    • First-order neuron: Delivers sensations from periphery to CNS.
      • Cell body in spinal or cranial nerve ganglion.
      • Synapses with a second-order neuron.
    • Second-order neuron: Interneuron in spinal cord or brainstem.
      • Crosses to the opposite side of the CNS (decussation).
      • Synapses with a third-order neuron.
    • Third-order neuron: Neuron in the thalamus.
      • Synapses with neurons of the primary somatosensory cortex.
  • Somatic sensory pathways carry sensory information from the skin and muscles of the body wall, head, neck, and limbs to the CNS.
    • Spinothalamic pathway
    • Posterior column pathway
    • Spinocerebellar pathway
    • Made up of symmetrical pairs of spinal tracts on opposite sides of the spinal cord.

Spinothalamic Pathway

  • Spinothalamic pathway: Carries sensations of crude touch, pressure, pain, and temperature.
    • First-order neurons enter spinal cord and synapse with second-order neurons within the posterior horns.
    • Second-order neurons cross to the opposite side of spinal cord and ascend to synapse with third-order neurons in the ventral nuclei of the thalamus.
      • Sort and process sensations, then carry information to neurons in the primary somatosensory cortex.
  • Tracts:
    • Anterior spinothalamic tract—crude touch and pressure.
    • Lateral spinothalamic tract—pain and temperature.
    • Which sensation is perceived depends on which second- and third-order neurons are stimulated.

Referred Pain

  • Painful sensations not produced where they are perceived.
    • Example: phantom limb syndrome - continued feeling of pain in amputated limb because the labeled line still exists and is activated
  • Referred pain: Visceral pain manifested as body surface pain.
    • Internal organ and body surface innervated by the same spinal segment.
    • Example: Heart attack felt as pain in the left arm.

Posterior Column Pathway

  • Posterior column pathway: Carries sensations of fine touch, vibration, pressure, and proprioception.
    • First-order neurons reach CNS and ascend grouped by region they innervate, synapsing with second-order neurons in the medulla oblongata.
    • Second-order neurons decussate in the brain stem and ascend to the thalamus where they synapse with third-order neurons in the ventral nuclei of the thalamus.
      • Sort and process sensations, then carry information to neurons in the primary somatosensory cortex.
  • Tracts:
    • Left and right gracile fasciculus—axons carry sensation from the inferior half of the body and synapse in the gracile nucleus of the medulla oblongata.
    • Left and right cuneate fasciculus—axons carry sensation from the superior half of the body and synapse in the cuneate nucleus of the medulla oblongata.
    • Medial lemniscus—axons of second-order neurons after they decussate.

Sensory Homunculus

  • Sensory homunculus: Functional map of primary somatosensory cortex.
    • Corresponds with specific regions of the body.
    • The area devoted to a region is proportional to the density of sensory neurons, not actual size.

Spinocerebellar Pathway

  • Spinocerebellar pathway: Carries information about the positions of muscles, tendons, and joints.
    • First-order neurons reach the CNS and synapse with second-order neurons in the posterior horn of the spinal cord.
    • Second-order neurons ascend to the cerebellum and often decussate twice (in spinal cord and cerebellum).
    • Information arrives in the cerebellum (Purkinje cells of the cerebellar cortex) and does not reach our awareness.
  • Tracts:
    • Posterior spinocerebellar tracts—travel through inferior cerebellar peduncle.
    • Anterior spinocerebellar tracts—travel via superior cerebellar peduncle.

Visceral Sensory Pathways

  • Visceral sensory information is collected by interoceptors monitoring visceral tissues and organs within the thoracic and abdominopelvic cavities.
  • Interoceptors include nociceptors, baroreceptors, thermoreceptors, tactile receptors, and chemoreceptors.
  • Axons are first-order neurons form the sensory portion of cranial nerves and posterior roots of spinal nerves.
  • Axons of second-order interneurons ascend within the spinothalamic pathway and deliver the information to the solitary nuclei of the medulla oblongata.
  • The solitary nuclei have extensive connections with cardiovascular and respiratory centers and the reticular formation.

15-5 Somatic Motor Pathways

  • Somatic nervous system (SNS): Controls skeletal muscles.
  • Somatic motor pathways: Involve at least two motor neurons.
    • Upper motor neuron: Cell body lies in a CNS processing center (primary motor cortex or premotor cortex and axon synapses on lower motor neuron).
    • Lower motor neuron: Cell body lies in a nucleus of the brainstem or spinal cord and axon extends outside of the CNS to innervate a single motor unit in a skeletal muscle.
  • Somatic motor pathways:
    • Corticospinal pathway
    • Medial pathway
    • Lateral pathway
    • Carry conscious and subconscious motor commands.

Corticospinal Pathway

  • Corticospinal pathway (pyramidal system): Provides voluntary control over skeletal muscles.
    • Upper motor neurons are pyramidal cells of the primary motor cortex.
      • Their axons descend into the brainstem and spinal cord and synapse on lower motor neurons that control skeletal muscles.
  • Tracts:
    • Corticobulbar tracts
    • Lateral corticospinal tracts
    • Anterior corticospinal tracts

Corticobulbar Tracts

  • Corticobulbar tracts
    • Axons of this tract synapse with lower motor neurons in the motor nuclei of cranial nerves.
    • Provide conscious control of the movement of the eyes, jaw, face, and some muscles of the neck and pharynx.
    • Innervate the motor centers of the medial and lateral pathways.

Corticospinal Tracts

  • Corticospinal tracts
    • Axons synapse on lower motor neurons in the anterior horns of the spinal cord.
    • Visible along the anterior surface of the medulla oblongata as a pair of thick bands, the pyramids.
      • Lateral corticospinal tracts—contain axons that decussate at the pyramids.
      • Anterior corticospinal tracts—contain axons that cross over at the anterior white commissure of spinal segments.
      • Synapse with lower motor neurons in the anterior horn of the spinal cord which then innervate skeletal muscle.

Motor Homunculus

  • Motor homunculus: Functional map of the primary motor cortex.
    • Corresponds with specific regions of the body.
    • Size of the area corresponds to the degree of fine motor control available.
    • The proportions are similar to those of the sensory homunculus.

Medial and Lateral Pathways

  • Centers in cerebrum, diencephalon, and brainstem issue somatic motor commands in response to subconscious processing.
  • Medial pathway: Controls muscle tone and gross movements of the trunk and proximal limb muscles.
  • Lateral pathway: Controls muscle tone and movements of the distal limb muscles that perform precise movements.
    • Axons of the upper motor neurons in the medial and lateral pathways synapse on the same lower motor neurons innervated by the corticospinal pathways and can stimulate, facilitate or inhibit them.

Medial Pathway

  • Medial pathway
    • Upper motor neurons are located in the vestibular nuclei, superior and inferior colliculi, and the reticular formation.
      • Vestibular nuclei:
        • Located between the pons and the medulla.
        • Receive information for the position and movement of the head from the vestibulocochlear nerve.
        • Primary goal is to maintain posture and balance.
        • Descending fibers form the vestibulospinal tracts.
    • Superior and inferior colliculi:
      • Located in the tectum (roof of the midbrain).
      • The superior colliculi receive visual sensations.
      • The inferior colliculi receive auditory sensations.
        • Axons of the upper motor neurons descend in tectospinal tracts and decussate immediately, before descending to synapse on lower motor neurons.
  • Reticular formation:
    • Loosely organized network of neurons that extends throughout brainstem.
      • Interconnected with the cerebellum and cerebrum.
      • Axons of upper motor neurons descend into medial and lateral reticulospinal tracts without crossing to opposite side.

Lateral pathway

  • Upper motor neurons are in the red nuclei of the midbrain
    • Axons decussate in the brain and descend into the spinal cord in rubrospinal tracts
    • The rubrospinal tracts only extend to the cervical spinal cord

Basal Nuclei and the Cerebellum

  • Basal nuclei and the cerebellum: Responsible for conscious or subconscious coordination and feedback control over muscle contractions.
    • Basal nuclei:
      • Provide the background patterns of movement involved in voluntary motor activities.
      • Stimulate or inhibit upper motor neurons.
      • Synapse on thalamic neurons, whose axons extend to the premotor cortex, which directs the activities of the primary motor cortex.
      • Synapse in the reticular formation to alter the excitatory or inhibitory output of the reticulospinal tracts.
    • Cerebellum: 0
      • Monitors proprioceptive (position) sensations, visual information from eyes and vestibular (balance) sensations from internal ear to adjust movement.
      • The cerebellum adjust the activity of upper motor neurons and inhibits unnecessary motor commands.
      • Patterns of cerebellar activity are learned by trial and error over many repetitions. 0