Sensory and Motor Pathways Overview

Overview of Events in Sensory and Motor Pathways

• Sensory Pathway:
  • Depolarization of Sensory Receptor: A stimulus (e.g., touch, temperature, pressure) causes a graded change in the membrane potential of the receptor.
  • Action Potential Generation: Transduction converts stimuli into action potentials by sensory receptors.
    • General senses: Temperature, pain, touch, pressure, vibration, proprioception
    • Special senses: Olfaction, gustation, vision, equilibrium, hearing
  • Receptor specificity: Each receptor has a unique sensitivity.
  • Receptive field: Area monitored by a single receptor cell.
    • Larger receptive fields make localization of stimuli harder.
• Stimulus Detection
  • Transduction: Stimuli are converted into action potentials.
    • Fast-adapting receptors (phasic): Respond strongly initially, then decrease (e.g., temperature).
    • Slow-adapting receptors (tonic): Exhibit little adaptation (e.g., pain).
  • Central adaptation (CNS): Inhibition of nuclei along the sensory pathway.
    • Sensory information is often processed in the spinal cord, brainstem, or thalamus before reaching the cortex.
    • Sensory awareness can be consciously or subconsciously increased or decreased (e.g., tuning out background noise).
• Pain Modulation: Endorphins and enkephalins inhibit pain pathways in the CNS.
• Variations in Pain Perception: Differences may be due to facilitation of neurons in the pain pathway.

Types of Pain

• Fast Pain ("prickling pain"): Carried by myelinated Type A fibers.

Chemoreceptors

• Sensitivity: Respond to water-soluble and lipid-soluble substances dissolved in body fluids, including pH, CO2, and O2 levels.
• Adaptation: Exhibit fast peripheral adaptation, meaning they quickly reduce sensitivity to constant stimuli.

Locations and Functions of Chemoreceptors

• Chemoreceptors in and Near the Medulla Oblongata
  • Function: Monitor pH and CO2 levels in the cerebrospinal fluid.
  • Role: Trigger reflexive adjustments in the depth and rate of breathing to maintain acid-base balance and proper ventilation.
• Chemoreceptors in the Carotid Bodies
  • Location: Near the origin of the internal carotid arteries.
  • Function: Sensitive to changes in blood pH, CO2, and O2.
  • Role: Send signals via cranial nerve IX (glossopharyngeal nerve) to regulate respiratory and cardiovascular activity, such as adjusting heart rate and breathing.
• Chemoreceptors in the Aortic Bodies
  • Location: Positioned between the major branches of the aortic arch.
  • Function: Sensitive to changes in blood pH, CO2, and O2.
  • Role: Similar to carotid bodies, they send signals to the CNS via cranial nerve X (vagus nerve) for regulating respiratory and cardiovascular activity.

Somatic Sensory Pathways

• Carry sensory information from the skin and muscles of the body wall, head, neck, and limbs.
• Made up of symmetrical pairs of spinal tracts.
• Correspond with specific regions of the body.
• The size of the area corresponds to the degree of fine motor control available (e.g., hands, face, and tongue appear large).
• Proportions are similar to those of the sensory homunculus.

Somatic Motor Pathways

• Control the movement of skeletal muscles.

Corticospinal Pathway

• Pyramidal system: Provides voluntary control over skeletal muscles.
• Upper motor neurons: 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: Axons synapse with lower motor neurons in the motor nuclei of cranial nerves III, IV, V, VI, VII, IX, and XII. 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: Axons synapse on lower motor neurons in the anterior horns of the spinal cord. Visible along the anterior surface of medulla oblongata as a pair of thick bands, the pyramids. Synapse with lower motor neurons in the anterior horn of the spinal cord which then innervate skeletal muscle.

Medial pathways

• Centers on the cerebrum, diencephalon, and brainstem.
  • Involved in:
    • Posture
    • Balance
    • Gross movements of the limbs
    • Head and neck movements
    • Controls proximal limb muscles

Lateral pathways

• Primarily involved in the control of fine, skilled limb movements.

Key Points:

• Involuntary responses are faster than voluntary responses because they involve direct reflex arcs.
• Voluntary responses require processing in the brain before a conscious action can be taken.
• The size of the area representing a body part on the somatosensory homunculus reflects the sensitivity of that area.
• Sensory information is often processed in the spinal cord, brainstem, or thalamus before reaching the cortex.
• The corticospinal pathway is the main pathway for voluntary control of skeletal muscles.
• The medial pathways are involved in the control of posture, balance, and gross movements.
• The lateral pathways primarily control fine, skilled limb movements.