Sensory Pathways and the Somatic Nervous System: Study Notes

Sensory Pathways and the Somatic Nervous System

Overview

This study guide covers the structure and function of sensory pathways and the somatic nervous system, focusing on neural transduction, transmission, sensory receptors, and motor pathways. It is based on Chapter 15 of a Human Anatomy & Physiology II course.

Components of the Nervous System

Afferent and Efferent Divisions

The nervous system is divided into afferent (sensory) and efferent (motor) divisions. The somatic nervous system is part of the efferent division and controls skeletal muscles.

• Afferent Division: Delivers sensory information to the CNS for integration and processing.

• Efferent Division: Sends motor commands from the CNS to peripheral effectors.

• Somatic Nervous System (SNS): Controls skeletal muscles via motor neurons and pathways.

• Autonomic Nervous System (ANS): Controls effector tissues other than skeletal muscles.

Neural Transduction and Transmission

Definitions and Processes

Neural transduction and transmission are fundamental to sensory communication in the nervous system.

• Transduction: The conversion of a sensory stimulus (e.g., light, sound, touch) into an electrical signal within a sensory receptor cell.

• Transmission: The propagation of the electrical signal along a neuron and its relay to other neurons at synapses.

• Action Potential: The electrical signal generated by a receptor and propagated along the neuron membrane.

Example: Touching a hot surface activates thermoreceptors, which transduce the stimulus into an action potential that is transmitted to the CNS.

Sensory Receptors and Their Classification

General and Special Senses

Sensory receptors connect the internal and external environments with the nervous system. Sensation is the arriving information; perception is the conscious awareness of a sensation.

• General Senses: Sensitivity to temperature, pain, touch, pressure, vibration, and proprioception.

• Special Senses: Olfaction (smell), gustation (taste), vision (sight), hearing (sound), equilibrium (balance).

• Receptor Types: Free nerve endings (dendrites) and specialized receptor cells.

Classification of General Sensory Receptors

General sensory receptors are classified by location and nature of the stimulus.

• By Location: Exteroceptors (external), proprioceptors (body position), interoceptors (internal).

• By Stimulus:

  • Nociceptors: Pain receptors

  • Thermoreceptors: Temperature receptors

  • Mechanoreceptors: Physical distortion (touch, pressure, vibration)

  • Chemoreceptors: Chemical detection

Tonic and Phasic Receptors

Response Types

Receptors can be classified by their response to stimuli.

• Tonic Receptors: Continue to fire action potentials as long as the stimulus is present. Provide information about stimulus duration. Example: Nociceptors (pain receptors).

• Phasic Receptors: Respond with a sharp initial reaction, then decline as the stimulus continues. Provide information about changes in stimulus. Example: Thermoreceptors (temperature receptors).

Additional info: Tonic responses are important for ongoing sensations like pain, while phasic responses are crucial for detecting changes, such as entering a cold room.

Types of General Sensory Receptors

Nociceptors (Pain Perception)

Nociceptors are common in skin, joints, bones, blood vessels, and visceral organs. They are free nerve endings with large receptive fields, making it difficult to pinpoint pain sources.

• Sensitivity: Temperature extremes, mechanical damage, chemicals.

• Axon Types:

  • Type A fibers: Myelinated, fast pain (prickling).

  • Type C fibers: Unmyelinated, slow pain (aching).

Thermoreceptors

Thermoreceptors are phasic receptors, active during temperature changes and quickly adapting to stable temperatures.

• Location: Dermis, skeletal muscles, liver, hypothalamus.

• Types: Warm and cold thermoreceptors; cold receptors are more abundant.

Mechanoreceptors

Mechanoreceptors detect mechanical pressure or distortion.

• Tactile Receptors: Sense touch, pressure, vibration.

• Baroreceptors: Monitor pressure changes in organs.

• Proprioceptors: Monitor positions of joints and skeletal muscles.

Tactile Receptors in the Skin

• Free nerve endings: Detect touch, pressure, pain, temperature.

• Root hair plexuses: Sense hair movement.

• Tactile discs: Fine touch and pressure.

• Tactile corpuscles (Meissner): Fine touch, pressure, low-frequency vibration.

• Bulbous corpuscles (Ruffini): Pressure and skin distortion.

• Lamellar corpuscles (Pacinian): Deep pressure.

Additional info: Sensitivity to tactile sensations can be altered by infection, disease, or trauma. Tickle and itch sensations are related to touch and pain, carried by unmyelinated Type C fibers.

Baroreceptors

Baroreceptors respond to stretch in distensible organs, providing feedback for autonomic regulation of vascular tone, blood pressure, and heart rate.

• Location: Elastic tissues in blood vessels, respiratory, digestive, urinary tracts.

• Function: Continuous, non-conscious feedback to the brainstem.

Chemoreceptors

Chemoreceptors detect changes in chemical concentrations (e.g., pH, CO2) in bodily fluids. They do not send information to the primary somatosensory cortex, so sensations are not consciously perceived.

• Location: Interstitial fluid, blood plasma, cerebrospinal fluid.

• Function: Autonomic control of respiratory and cardiovascular functions.

Organization of Sensory Pathways

Neuronal Structure

Sensory pathways relay information from receptors to the CNS through a series of neurons:

• First-order neuron: Sensory neuron, cell body in spinal or cranial ganglion (PNS).

• Second-order neuron: Interneuron in spinal cord or brainstem (CNS).

• Third-order neuron: Located in the thalamus (CNS).

• Decussation: Second-order neuron axon crosses to the opposite side of the CNS.

Additional info: Sensory pathways include first-, second-, and third-order neurons; somatic and visceral sensory signals are transmitted by different pathways.

Somatic and Visceral Sensory Pathways

Somatic Sensory Pathways

Somatic sensory pathways carry information from skin and muscles to the CNS. Three major pathways:

• Spinothalamic pathway

• Posterior column pathway

• Spinocerebellar pathway

These pathways are made up of pairs of spinal tracts, symmetrically arranged on opposite sides of the spinal cord.

Visceral Sensory Pathways

Visceral sensory information is collected by interoceptors in internal organs. The pathway does not include a third-order neuron, so sensory information does not reach conscious awareness.

• Interoceptors: Nociceptors, thermoreceptors, tactile receptors, baroreceptors, chemoreceptors.

• Location: Mouth, palate, pharynx, larynx, trachea, esophagus, vessels, glands, internal organs.

Somatic Motor Pathways

Structure and Function

The somatic nervous system (SNS) controls skeletal muscles through somatic motor pathways, which consist of motor nuclei, tracts, and nerves.

• Nuclei: Clusters of neurons in the CNS.

• Tracts: Bundles of axons within the CNS.

• Nerves: Bundles of axons in the PNS.

• Key Point: A single peripheral nerve contains axons from multiple neural tracts.

Motor Neurons

• Upper motor neuron: Cell body in CNS processing center.

• Lower motor neuron: Cell body in brainstem or spinal cord; axon extends outside CNS to innervate skeletal muscle.

• Activation: Upper motor neuron may facilitate or inhibit lower motor neuron; activation triggers muscle contraction.

• Damage: Destruction of lower motor neuron eliminates voluntary and reflex control over the muscle.

Motor Control Pathways

Three integrated motor pathways control skeletal muscles:

• Corticospinal pathway (pyramidal system): Voluntary control over skeletal muscles; originates in the primary motor cortex.

• Medial pathway: Controls muscle tone and gross movements of neck, trunk, and proximal limbs.

• Lateral pathway: Controls distal limb muscles for precise movements.

Clinical Example: Amyotrophic Lateral Sclerosis (ALS)

ALS is a progressive, degenerative disorder affecting motor neurons in the spinal cord, brainstem, and cerebral hemispheres. Both upper and lower motor neurons are affected, leading to muscle atrophy.

• Defect: Axonal transport is thought to underlie the disease.

• Result: Destruction of motor neurons causes associated skeletal muscles to atrophy.

Summary Table: Types of General Sensory Receptors

Type	Stimulus	Location	Response
Nociceptors	Pain	Skin, joints, bones, blood vessels, organs	Tonic
Thermoreceptors	Temperature	Dermis, muscles, liver, hypothalamus	Phasic
Mechanoreceptors	Touch, pressure, vibration	Skin, muscles, organs	Tonic/Phasic
Chemoreceptors	Chemicals (pH, CO2)	Blood, CSF, interstitial fluid	Phasic

Key Equations

Propagation Speed of Action Potential

The speed of action potential propagation depends on axon diameter and myelination:

• Type A fibers: $\text{Diameter} = 4-20\ \mu m$, $\text{Speed} = 120\ \text{m/sec}$

• Type B fibers: $\text{Diameter} = 2-4\ \mu m$, $\text{Speed} = 18\ \text{m/sec}$

• Type C fibers: $\text{Diameter} < 2\ \mu m$, $\text{Speed} = 1\ \text{m/sec}$

Formula: $\text{Propagation Speed} \propto \text{Axon Diameter} \times \text{Myelination}$

Summary and Review Questions

• What are the body’s specialized cells that monitor specific internal or external conditions?

• Can you lay out the basic events that occur during neural transduction and transmission?

• Is it possible for somatic motor commands to occur at the subconscious level?

• What is a tonic receptor, and can you give examples?

• What is a phasic receptor, and can you give examples?

• Can you list the types of general sensory receptors?

• Can you list the three classes of mechanoreceptors?

• What are the two pathways that make up the afferent division of the PNS?

• What are the three major somatic sensory pathways?

• Why do we not notice the visceral sensory information?

• What is the anatomical basis for the fact that the left side of the brain controls motor function on the right side of the body?

• What are the three somatic motor pathways in the somatic nervous system?