Peripheral Nervous System and Special Senses: Structure and Function

Peripheral Nervous System and Special Senses

Overview

The peripheral nervous system (PNS) and special senses are essential components of the human nervous system, responsible for transmitting information between the body and the central nervous system (CNS), and for processing sensory input from the environment. This guide reviews the divisions of the nervous system and introduces the organization and function of the PNS and special senses.

Divisions of the Nervous System

Central Nervous System (CNS)

• Definition: The CNS consists of the brain and spinal cord.

• Function: Integrates sensory information and coordinates bodily responses.

Peripheral Nervous System (PNS)

• Definition: The PNS connects the CNS to the rest of the body.

• Components: Includes nerves and ganglia outside the CNS.

• Function: Transmits sensory and motor signals between the CNS and peripheral tissues.

PNS Functional Divisions

Afferent (Sensory) System

The afferent system carries sensory information from receptors to the CNS.

• Somatic Sensory Division: Transmits information from skeletal muscles, bones, joints, and skin (including special sensory organs) to the CNS.

• Visceral Sensory Division: Transmits information from organs (viscera) such as the heart and stomach, as well as from smooth muscles and glands to the CNS.

Efferent (Motor) System

The efferent system carries motor commands from the CNS to effectors (muscles and glands).

• Somatic Motor Division: Controls voluntary movements by transmitting signals from the CNS to skeletal muscles.

• Autonomic Motor Division: Regulates involuntary functions by transmitting signals from the CNS to cardiac muscle, smooth muscle, and glands. Maintains homeostasis.

  • Sympathetic Nervous System: Prepares the body for stressful situations and energy expenditure ("fight or flight").

  • Parasympathetic Nervous System: Maintains normal functions and conserves energy ("rest and digest").

Organization of the Peripheral Nervous System

Functional Pathways

The PNS is organized into sensory and motor divisions, each with somatic and visceral components. The following table summarizes the main divisions and their functions:

Division	Type	Function	Example
Somatic Sensory	Afferent	Transmits sensory info from skin, muscles, joints	Touch, pain, proprioception
Visceral Sensory	Afferent	Transmits sensory info from organs	Fullness of stomach, blood pressure
Somatic Motor	Efferent	Controls voluntary muscle movement	Walking, writing
Autonomic Motor	Efferent	Controls involuntary functions	Heart rate, digestion

Key Terms

• Afferent Neurons: Carry sensory signals to the CNS.

• Efferent Neurons: Carry motor signals from the CNS to effectors.

• Effectors: Muscles or glands that respond to nervous stimulation.

Special Senses (Preview)

Introduction

The special senses include vision, hearing, taste, smell, and equilibrium. These senses rely on specialized receptors and are primarily located in the head. They provide detailed information about the external environment and are processed by specific regions of the brain.

• Vision: Detection of light by photoreceptors in the eye.

• Hearing: Detection of sound waves by mechanoreceptors in the ear.

• Taste: Detection of chemicals by chemoreceptors on the tongue.

• Smell: Detection of airborne chemicals by chemoreceptors in the nasal cavity.

• Equilibrium: Detection of head position and movement by mechanoreceptors in the inner ear.

Example: Somatic vs. Autonomic Motor Pathways

• Somatic Motor Pathway: CNS → Lower motor neuron → Skeletal muscle (voluntary movement)

• Autonomic Motor Pathway: CNS → Preganglionic neuron → Autonomic ganglion → Postganglionic neuron → Cardiac/smooth muscle or gland (involuntary movement)

Additional info: The autonomic nervous system is further divided into sympathetic and parasympathetic branches, which often have opposing effects on target organs. For example, sympathetic stimulation increases heart rate, while parasympathetic stimulation decreases it.