Chapter 15: Sensory Pathways and the Somatic Nervous System – Study Notes

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Chapter 15: Sensory Pathways and the Somatic Nervous System

15-1 Sensory and Motor Pathways

The nervous system processes sensory information through specialized pathways that relay signals from sensory receptors to the central nervous system (CNS), and then generate motor responses. Sensory pathways transmit information from the body and environment, while motor pathways control skeletal muscle activity.

Sensory pathways: Series of neurons relaying sensory information from receptors to the CNS.
Sensory receptors: Specialized cells or neuron processes that monitor specific conditions and generate action potentials when stimulated.
Afferent division: Includes somatic (body wall, limbs) and visceral (internal organs) sensory pathways.
Efferent division: Somatic motor pathways control skeletal muscles (effectors).
Motor commands may be modified by higher-order brain functions.

Diagram of sensory and motor pathways, showing sensory pathway steps and motor responses

15-2 Sensory Receptors

Sensory receptors detect changes in the environment and convert stimuli into electrical signals (action potentials) through a process called transduction. Sensations are the arrival of sensory information in the CNS, while perception is the conscious awareness of these sensations.

General senses: Temperature, pain, touch, pressure, vibration, proprioception.
Special senses: Olfaction, gustation, vision, equilibrium, hearing (with specialized sense organs).
Receptor specificity: Each receptor is sensitive to a particular type of stimulus.
Receptive field: Area monitored by a single receptor cell; larger fields make localization harder.

Diagram of receptive fields and free nerve endings in the skin

Transduction and Receptor Potentials

Stimulus changes the receptor membrane potential (receptor potential).
Can be depolarizing (generator potential) or hyperpolarizing.
Strength of stimulus affects the size of the receptor potential.
Special sense receptors communicate with sensory neurons at synapses.

Interpretation of Sensory Information

Labeled line: Pathway linking a specific receptor to a specific cortical neuron; determines the perceived modality.
Sensory coding: Frequency and pattern of action potentials encode stimulus strength, duration, and variation.

Tonic vs. Phasic Receptors

Tonic receptors: Always active; frequency of action potentials changes with stimulus intensity.
Phasic receptors: Normally inactive; respond to changes in stimulus intensity with bursts of action potentials.

Graphs comparing tonic and phasic receptor responses to stimuli

Adaptation

Adaptation: Reduction in receptor sensitivity during constant stimulation.
Peripheral adaptation: Occurs at the receptor level (e.g., phasic receptors adapt quickly).
Central adaptation: Occurs in the CNS, inhibiting sensory pathway nuclei.

15-3 General Sensory Receptors

Sensory receptors can be classified by the location of the stimulus or the nature of the stimulus they detect.

Exteroceptors: Monitor external environment.
Proprioceptors: Monitor position of muscles and joints.
Interoceptors: Monitor visceral organs and functions.

Types of General Sensory Receptors

Nociceptors: Detect pain; free nerve endings with large receptive fields, tonic, little peripheral adaptation.
Thermoreceptors: Detect temperature; free nerve endings, phasic, adapt quickly.
Mechanoreceptors: Detect physical distortion (touch, pressure, vibration); contain mechanically-gated ion channels.
Chemoreceptors: Detect chemical concentrations in body fluids; fast peripheral adaptation.

Tactile Receptors in the Skin

Free nerve endings: Tonic, small receptive fields, detect touch, pressure, pain, temperature.

Illustration of free nerve endings in the skin

Root hair plexus: Free nerve endings around hair follicles, detect hair movement, adapt rapidly.

Illustration of root hair plexus around a hair follicle

Tactile discs (Merkel discs): Tonic, fine touch and pressure, sensitive to shape and texture, small receptive fields.

Illustration of tactile discs (Merkel cells) in the skin

Bulbous corpuscles (Ruffini corpuscles): Tonic, sensitive to pressure and skin distortion, located in deep dermis, little adaptation.

Illustration of bulbous (Ruffini) corpuscle in the dermis

Lamellar corpuscles (Pacinian corpuscles): Phasic, deep pressure and high-frequency vibration, concentric layers of collagen.

Illustration and micrograph of lamellar (Pacinian) corpuscle

Tactile corpuscles (Meissner corpuscles): Phasic, fine touch, pressure, low-frequency vibration, abundant in sensitive areas (fingertips, lips).

Illustration and micrograph of tactile (Meissner) corpuscle

Baroreceptors

Detect pressure changes in blood vessels and hollow organs (digestive, respiratory, urinary tracts).
Free nerve endings in elastic tissues, adapt rapidly.

Diagram showing baroreceptors in various organs

Proprioceptors

Monitor position of joints and muscles; only somatic sensation.
Types: Muscle spindles (muscle length), Golgi tendon organs (tension), joint capsule receptors (pressure, tension, movement).

Chemoreceptors

Respond to water- and lipid-soluble substances in body fluids.
Monitor pH, CO2, and O2 in arterial blood (carotid and aortic bodies).

Diagram showing chemoreceptors in the body

15-4 Sensory Pathways

Sensory information is relayed to the CNS through a series of neurons, typically involving three orders of neurons for somatic sensation.

First-order neuron: Delivers sensation from receptor to CNS (cell body in spinal or cranial nerve ganglion).
Second-order neuron: Interneuron in spinal cord or brainstem; axon crosses to opposite side (decussation).
Third-order neuron: Located in thalamus; relays information to primary somatosensory cortex.

Somatic Sensory Pathways

Spinothalamic pathway: Crude touch, pressure, pain, temperature.
Posterior column pathway: Fine touch, vibration, pressure, proprioception.
Spinocerebellar pathway: Proprioceptive information to cerebellum (not conscious).

Diagram of ascending sensory tracts in the spinal cord

Spinothalamic Pathway

First-order neurons synapse in posterior horns; second-order neurons cross and ascend to thalamus; third-order neurons project to cortex.
Tracts: Anterior (crude touch, pressure), Lateral (pain, temperature).
Referred pain: Visceral pain perceived as originating from body surface (e.g., heart attack felt in left arm).

Diagram of spinothalamic pathway Diagram of lateral spinothalamic tract for pain and temperature Diagram of referred pain patterns

Posterior Column Pathway

First-order neurons ascend to medulla; second-order neurons decussate and ascend to thalamus; third-order neurons project to cortex.
Tracts: Gracile fasciculus (lower body), Cuneate fasciculus (upper body), Medial lemniscus (after decussation).
Sensory homunculus: Map of cortex regions corresponding to body areas; size reflects sensory neuron density.

Diagram of sensory homunculus Diagram of spinocerebellar pathway

Spinocerebellar Pathway

First-order neurons synapse in posterior horn; second-order neurons ascend to cerebellum (may decussate twice).
Tracts: Posterior and anterior spinocerebellar tracts.
Information does not reach conscious awareness.

Diagram of spinocerebellar pathway

Visceral Sensory Pathways

Interoceptors monitor visceral tissues and organs; information relayed via cranial and spinal nerves to medulla oblongata.
Solitary nuclei integrate visceral sensory information for autonomic regulation.

Diagram of ascending visceral sensory pathways

Somatic Motor Pathways

The somatic nervous system (SNS) controls voluntary and reflexive movements of skeletal muscles through descending motor pathways.

Always involve at least two motor neurons: upper motor neuron (CNS) and lower motor neuron (brainstem/spinal cord to muscle).
Major pathways: Corticospinal (pyramidal), medial, and lateral pathways.
Basal nuclei and cerebellum provide coordination and feedback control.

Diagram of descending motor tracts in the spinal cord

Corticospinal Pathway (Pyramidal System)

Voluntary control over skeletal muscles; upper motor neurons in primary motor cortex.
Tracts: Corticobulbar (cranial nerves), lateral corticospinal (decussate at pyramids), anterior corticospinal (decussate at spinal segment).
Motor homunculus: Map of primary motor cortex; area size reflects degree of fine motor control.

Diagram of motor homunculus and corticospinal pathway

Medial and Lateral Pathways

Medial pathway: Controls muscle tone and gross movements of trunk/proximal limbs (vestibulospinal, tectospinal, reticulospinal tracts).
Lateral pathway: Controls muscle tone and precise movements of distal limbs (rubrospinal tract).
Upper motor neurons in these pathways can stimulate, facilitate, or inhibit lower motor neurons also innervated by corticospinal tracts.

Diagram of medial and lateral motor pathways in the spinal cord

Basal Nuclei and Cerebellum

Basal nuclei: Provide background patterns for voluntary movement, modulate upper motor neuron activity via thalamus and reticular formation.
Cerebellum: Monitors proprioceptive, visual, and vestibular input; adjusts movement and inhibits unnecessary motor commands; patterns learned by repetition.

Summary Table: Major Sensory Pathways

Pathway/Tract	Sensation(s)	First-Order Neuron	Second-Order Neuron	Third-Order Neuron	Destination	Site of Decussation
Lateral spinothalamic	Pain, temperature	Spinal ganglion, enters CNS in posterior roots	Posterior horn, crosses to opposite side	Ventral nuclei of thalamus	Primary sensory cortex	Axons of second-order neurons at level of entry
Anterior spinothalamic	Crude touch, pressure	Spinal ganglion, enters CNS in posterior roots	Posterior horn, crosses to opposite side	Ventral nuclei of thalamus	Primary sensory cortex	Axons of second-order neurons at level of entry
Gracile fasciculus	Fine touch, vibration, pressure, proprioception (lower body)	Spinal ganglion, enters CNS in posterior roots	Gracile nucleus of medulla oblongata, crosses to opposite side	Ventral nuclei of thalamus	Primary sensory cortex	Axons of second-order neurons in medulla oblongata
Cuneate fasciculus	Fine touch, vibration, pressure, proprioception (upper body)	Spinal ganglion, enters CNS in posterior roots	Cuneate nucleus of medulla oblongata, crosses to opposite side	Ventral nuclei of thalamus	Primary sensory cortex	Axons of second-order neurons in medulla oblongata
Posterior spinocerebellar	Proprioception	Spinal ganglion, enters CNS in posterior roots	Posterior horn, axons ascend uncrossed	Not present	Cerebellar cortex on same side	None
Anterior spinocerebellar	Proprioception	Spinal ganglion, enters CNS in posterior roots	Posterior horn, axons cross to opposite side	Not present	Cerebellar cortex on opposite side	Axons of second-order neurons at level of entry