Peripheral Nervous System and Special Senses: Structure and Function

Peripheral Nervous System and Special Senses

Divisions of the Nervous System

The nervous system is divided into two main parts: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). Each division has specialized roles in processing and transmitting information throughout the body.

• Central Nervous System (CNS): Consists of the brain and spinal cord. It is responsible for integrating, processing, and coordinating sensory data and motor commands.

• Peripheral Nervous System (PNS): Connects the CNS to limbs and organs. It serves as a communication relay between the brain/spinal cord and the rest of the body.

Afferent (Sensory) Division

The afferent division of the PNS transmits sensory information from receptors to the CNS.

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

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

Efferent (Motor) Division

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

• Somatic Motor Division: Sends signals from the CNS to skeletal muscles, enabling voluntary movements.

• Autonomic Motor Division: Regulates involuntary functions by transmitting signals from the CNS to cardiac muscle, smooth muscle, and glands. It is further divided into:

  • Sympathetic Nervous System (SNS): Prepares the body for stressful or emergency situations ("fight or flight" response).

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

Summary Table: Organization of the Peripheral Nervous System

Division	Function	Target Structures
Somatic Sensory	Transmits sensory information	Skeletal muscles, skin, joints, special senses
Visceral Sensory	Transmits sensory information	Internal organs (viscera), smooth muscle, glands
Somatic Motor	Controls voluntary movement	Skeletal muscles
Autonomic Motor	Controls involuntary responses	Cardiac muscle, smooth muscle, glands
Sympathetic	Prepares for stress, increases alertness	Various organs (heart, lungs, etc.)
Parasympathetic	Conserves energy, promotes maintenance	Various organs (digestive tract, etc.)

Key Terms and Definitions

• Neuron: A nerve cell specialized for the transmission of electrical impulses.

• Effector: A muscle or gland that responds to neural stimulation.

• Receptor: A specialized cell or structure that detects changes in the environment and initiates a neural response.

• Homeostasis: The maintenance of a stable internal environment within the body.

Example: Reflex Arc

A classic example of PNS function is the reflex arc, where a sensory receptor detects a stimulus (e.g., touching a hot object), sends a signal via afferent neurons to the CNS, which then processes the information and sends a response via efferent neurons to effectors (muscles) to withdraw the hand.

Additional info: The provided diagram visually summarizes the flow of information in the nervous system, showing how sensory input is received and processed, and how motor output is directed to effectors. The autonomic division's sympathetic and parasympathetic branches often have opposing effects on target organs, maintaining physiological balance.