Peripheral Nervous System (PNS)

Functional Divisions of the PNS

The Peripheral Nervous System (PNS) connects the Central Nervous System (CNS) to the rest of the body and is functionally divided into sensory and motor divisions. These divisions allow the CNS to receive information and initiate action throughout the body.

• Sensory Division: Transmits sensory inputs from receptors to the CNS.

• Motor Division: Carries motor outputs from the CNS to effectors (muscles and glands).

• Subdivisions:

  • Somatic: Relates to body wall and limbs.

  • Visceral: Relates to internal organs.

  • General: Includes senses like touch, pain, temperature.

  • Special: Includes senses like vision, hearing, taste, smell.

Autonomic Nervous System (ANS)

The general visceral motor part of the PNS is known as the Autonomic Nervous System (ANS), which regulates involuntary functions.

• Sympathetic Division: Prepares the body for 'fight or flight' responses.

• Parasympathetic Division: Promotes 'rest and digest' activities.

Functional Organization of the PNS and CNS

The PNS is organized to relay sensory information to the CNS and transmit motor commands from the CNS to the body. The following table summarizes the main divisions and their functions:

Organization of the Peripheral Nervous System

Key Components

The PNS consists of sensory receptors, nerves, ganglia, and motor endings, each playing a distinct role in neural communication.

• Sensory Receptors: Detect stimuli from inside or outside the body.

• Nerves: Bundles of peripheral axons that transmit signals.

• Ganglia: Clusters of peripheral neuronal cell bodies.

• Motor Endings: Axon terminals of motor neurons that innervate effectors (muscles and glands).

Peripheral Sensory Receptors

Types and Functions

Peripheral sensory receptors are specialized structures that detect sensory stimuli and initiate signals in sensory axons. They are classified into two main categories:

• Free Nerve Endings of Sensory Neurons: Monitor general sensory information such as pain and temperature.

• Complete Receptor Cells: Specialized epithelial cells or small neurons that monitor most types of special sensory information (e.g., taste, vision).

Classification of Sensory Receptors

Sensory receptors can be classified by location, type of stimulus detected, and structure.

• By Location:

  • Exteroceptors: Sensitive to stimuli arising from outside the body (e.g., touch, pressure, pain, temperature).

  • Interoceptors: Receive stimuli from internal organs (e.g., chemical concentration, stretching, temperature).

  • Proprioceptors: Located in muscles, tendons, joints, and ligaments; monitor degree of stretch and body movement.

• By Stimulus Type:

  • Mechanoreceptors: Respond to mechanical forces (touch, pressure, stretch, vibration).

  • Thermoreceptors: Respond to temperature changes.

  • Chemoreceptors: Respond to chemicals in solution.

  • Photoreceptors: Respond to light (located in the eye).

  • Nociceptors: Respond to harmful stimuli that result in pain.

• By Structure:

  • General Sensory Receptors: Widely distributed; monitor touch, pressure, vibration, stretch, pain, temperature, proprioception.

  • Free Nerve Endings: Abundant in epithelia and connective tissue; respond to pain and temperature.

  • Encapsulated Nerve Endings: Mechanoreceptors enclosed in connective tissue; include tactile (Meissner's) corpuscles, lamellar (Pacinian) corpuscles, bulbous corpuscles (Ruffini endings), and proprioceptors.

Encapsulated Nerve Endings

Encapsulated nerve endings are specialized mechanoreceptors that detect various forms of mechanical stimuli.

• Tactile (Meissner's) Corpuscles: Spiraling nerve endings surrounded by Schwann cells; occur in dermal papillae; rapidly-adapting receptors for discriminative touch, especially in sensitive, hairless skin areas.

• Lamellar (Pacinian) Corpuscles: Single nerve ending surrounded by layers of flattened Schwann cells; located in subcutaneous tissue; sensitive to deep pressure and vibration; rapidly-adapting.

• Bulbous Corpuscles (Ruffini Endings): Located in the dermis; monitor continuous pressure and stretch; slowly-adapting receptors.

• Proprioceptors: Monitor stretch in locomotory organs; different types monitor muscle length and degree/rate of stretch.

Summary Table: Sensory Receptor Types

Key Terms and Definitions

• Peripheral Nervous System (PNS): The part of the nervous system outside the brain and spinal cord.

• Central Nervous System (CNS): Composed of the brain and spinal cord.

• Autonomic Nervous System (ANS): Regulates involuntary bodily functions.

• Sensory Receptor: Specialized cell or structure that detects changes in the environment.

• Mechanoreceptor: Sensory receptor that responds to mechanical pressure or distortion.

• Proprioception: Sense of body position and movement.