Chapter 15: Sensory Pathways and the Somatic Nervous System

Overview of the Somatic Nervous System

The somatic nervous system is a division of the peripheral nervous system responsible for voluntary control of body movements via skeletal muscles. It consists of afferent (sensory) and efferent (motor) pathways that transmit information to and from the central nervous system (CNS).

• Afferent division: Transmits sensory information from receptors to the CNS.

• Efferent division: Carries motor commands from the CNS to effectors (skeletal muscles).

• Somatic motor pathways: Control voluntary movements.

Example: Moving your hand away from a hot surface involves sensory input (pain receptors) and motor output (muscle contraction).

Events Along Sensory and Motor Pathways

Sensory and motor pathways involve a series of steps that allow the body to detect stimuli and respond appropriately. The process includes sensory receptor activation, action potential generation, propagation, and CNS processing.

• Depolarization of Sensory Receptor: A stimulus causes a change in membrane potential at the receptor cell.

• Action Potential Generation: If the stimulus is strong enough, it triggers an action potential.

• Propagation: Action potentials travel along sensory neurons to the CNS.

• CNS Processing: Information is processed in the spinal cord or brain, leading to perception or reflex responses.

• Immediate Involuntary Response: Reflex centers in the spinal cord or brainstem may trigger rapid, automatic responses.

• Voluntary Response: More complex responses are processed in the cerebral cortex, allowing conscious control.

Additional info: Only about 1% of sensory information reaches conscious awareness; most is processed subconsciously.

Receptors and Receptive Fields

Types of Sensory Receptors

Sensory receptors are specialized cells that detect specific types of stimuli. The area monitored by a single receptor is called its receptive field.

• Receptive field: The region of the body where a stimulus will activate a particular sensory neuron.

• Sensitivity: Smaller receptive fields allow for greater sensitivity and more precise localization of stimuli.

Example: Fingertips have small receptive fields, allowing for fine touch discrimination.

Tonic and Phasic Sensory Receptors

Sensory receptors can be classified based on their response to stimuli over time.

• Tonic receptors: Always active; generate action potentials at a frequency proportional to stimulus strength.

• Phasic receptors: Normally inactive; generate action potentials only in response to changes in stimulus conditions.

Example: Pain receptors are typically tonic, while olfactory receptors are phasic.

Classification of Sensory Receptors

By Location and Function

Sensory receptors are classified by the type of stimulus they detect and their location.

• Exteroceptors: Provide information about the external environment (e.g., touch, temperature).

• Proprioceptors: Report the position of skeletal muscles and joints.

• Interoceptors: Monitor conditions inside the body (e.g., internal organs).

Example: Thermoreceptors in the skin detect temperature changes, while baroreceptors in blood vessels monitor pressure.

Types of General Sensory Receptors

• Nociceptors: Detect pain.

• Thermoreceptors: Detect temperature changes.

• Mechanoreceptors: Detect physical distortion, contact, or pressure.

• Chemoreceptors: Detect chemical composition of body fluids.

Additional info: Mechanoreceptors include tactile receptors (touch), baroreceptors (pressure), and proprioceptors (body position).

Major Sensory Pathways

Neuronal Organization

Sensory pathways typically involve three types of neurons:

• First-order neuron: Sensory neuron delivering information to the CNS.

• Second-order neuron: Interneuron in the spinal cord or brainstem; may cross to the opposite side (decussation).

• Third-order neuron: Located in the thalamus; relays information to the cerebral cortex for conscious perception.

Additional info: Decussation explains why sensations from one side of the body are processed by the opposite hemisphere.

Somatic Sensory Pathways

There are several major somatic sensory pathways, each carrying different types of sensory information.

• Spinothalamic pathway: Carries sensations of crude touch, pressure, pain, and temperature.

• Posterior column pathway: Carries sensations of fine touch, vibration, pressure, and proprioception.

• Spinocerebellar pathway: Conveys information about position of muscles, tendons, and joints to the cerebellum (not consciously perceived).

Example: Phantom limb pain is an abnormality in the spinothalamic pathway, where pain is perceived in an amputated limb.

Comparison of Somatic Sensory Pathways

Somatic Motor Pathways

Components and Functions

Somatic motor pathways control voluntary movements by transmitting signals from the motor cortex to skeletal muscles.

• Motor cortex: Initiates voluntary movement.

• Descending tracts: Pathways in the spinal cord that carry motor commands.

• Lower motor neurons: Directly innervate skeletal muscle fibers.

Example: The corticospinal tract is a major descending pathway for voluntary movement.

Major Motor Pathways

• Corticospinal pathway: Provides voluntary control over skeletal muscles; begins in the motor cortex and descends through the spinal cord.

• Medial and lateral pathways: Involved in subconscious regulation of balance, muscle tone, and posture.

Additional info: Motor pathways are integrated to allow both conscious and subconscious control of movement.

Key Terms and Concepts

• Decussation: Crossing over of nerve fibers from one side of the CNS to the other.

• Receptive field: Area monitored by a single sensory neuron.

• Proprioception: Sense of body position and movement.

• Baroreceptor: Mechanoreceptor that detects pressure changes in blood vessels.

• Chemoreceptor: Detects chemical changes in body fluids.

Relevant Equations

• Action Potential Generation:

• Frequency Coding: