Neural Integration: Sensory Pathways, Receptors, and Somatic Sensory Tracts

Neural Integration

Introduction

This topic explores how the peripheral nervous system (PNS) and central nervous system (CNS) work together to process sensory and motor information. Sensory pathways begin at receptors and end at the postcentral gyrus, while motor pathways begin in the precentral gyrus and end at skeletal muscles.

Receptors

Definition and Function

• Stimulus: A change in the environment that can produce a response. Examples include mechanical changes (touch), chemical changes (pain), and temperature changes.

• Receptor: A structure that detects a stimulus and acts as a transducer, converting the stimulus into an action potential.

• Action Potential: Whether or not an action potential is fired depends on the strength of the stimulus.

Adaptation

• Adaptation: Decreased sensitivity to a stimulus over time. Some receptors adapt quickly (e.g., touch), while others do not (e.g., pain).

• Example: Touch receptors decrease their response if the stimulus is constant, such as pressure on the skin.

Classification of Receptors

By Location

• Exteroceptors: Detect changes outside the body (e.g., touch, pressure).

• Interoceptors: Detect changes within the body (e.g., pain, pressure).

• Proprioceptors: Detect changes in muscles, tendons, and joints, providing information about body position and movement.

By Type of Stimulus

• Mechanoreceptors: Respond to mechanical deformation (e.g., touch, vibration, pressure).

• Nociceptors: Respond to temperature and mechanical damage; found in skin and internal organs.

• Chemoreceptors: Respond to concentration of chemicals dissolved in surrounding fluids (e.g., taste buds, olfactory receptors).

• Photoreceptors: Respond to photons of light (e.g., rods and cones in the retina).

Touch Receptors / Types of Mechanoreceptors

Major Types

• Tactile (Merkel) Cell/Disc: Located in the dermis; slow-adapting (tonic), detects sustained pressure.

• Tactile (Meissner) Corpuscle: Located in the dermal papilla; adapts quickly, detects fine touch.

• Hair Root Plexus: Free dendritic endings; adapt rapidly, detect hair movement.

• Pacinian Corpuscle: Deep pressure receptor.

Proprioceptors

Types and Functions

• Muscle Spindle: Specialized muscle cells that detect changes in muscle length; important for maintaining balance.

• Golgi Tendon Organ: Located at the muscle/tendon junction; detects skeletal muscle contraction and can trigger relaxation.

• Joint Receptors: Located in synovial joints; detect joint position.

• Internal Ear Receptors: Sometimes included with proprioceptors; provide information about head position.

Vocabulary Review

	BUNDLE OF AXONS	BUNDLES OF NEURON CELL BODIES
CNS	TRACTS	NUCLEUS
PNS	NERVE	GANGLION

Sensory (ascending) tracts carry information to the brain. Motor (descending) tracts carry commands from the brain to muscles.

Motor and Sensory Tracts

TRACT NAME	TYPE	FUNCTION
Spinothalamic tract	Sensory	Carries pain, temperature, crude touch to the thalamus.
Corticospinal tract	Motor	Controls voluntary movement of skeletal muscles.
Rubrospinal tract	Motor	Assists in motor coordination and muscle tone.
Spinocerebellar tract	Sensory	Transmits proprioceptive information to the cerebellum.

Somatic Sensory Pathways

General Pathway

The general sensory pathway transmits information from the receptor to the postcentral gyrus in the parietal lobe. It involves three neurons: first-order, second-order, and third-order.

• First-order neuron: Unipolar sensory neuron with its cell body in the dorsal root ganglion.

• Second-order neuron: Cell body in the posterior horn of the spinal cord; axon crosses to the opposite side.

• Third-order neuron: Cell body in the thalamus; axon extends to the postcentral gyrus.

Spinothalamic Pathways

• Anterior spinothalamic pathway: Carries crude touch and pressure sensations.

• Lateral spinothalamic pathway: Carries pain and temperature sensations.

• Both tracts are bilateral, meaning they exist on both sides of the spinal cord, providing redundancy and preserving function if one side is damaged.

Neuron Locations

• First-order neuron: Dorsal root (spinal) ganglion; unipolar shape.

• Second-order neuron: Posterior horn of the gray matter in the spinal cord.

• Third-order neuron: Thalamic nucleus; axon ends in the postcentral gyrus of the parietal lobe (primary somatosensory cortex).

Key Concepts

• Regardless of the pathway, the information transmitted is an action potential.

• The location of neuron cell bodies is consistent across sensory pathways.

• By analyzing which sensations are lost and on which side, clinicians can pinpoint the location of spinal cord damage.

Example

If a patient loses pain and temperature sensation on one side of the body, the lateral spinothalamic tract on that side may be damaged.

Additional info: The notes cover material relevant to Ch. 15 Sensory Pathways and the Somatic Nervous System, Ch. 12 Nervous Tissue, and Ch. 13 The Spinal Cord, Spinal Nerves, and Spinal Reflexes. All equations and tables have been rendered in HTML and LaTeX as required.