Peripheral Nervous System Overview

PNS:

Peripheral Nervous System Overview

• The Peripheral Nervous System (PNS) connects the Central Nervous System (CNS) to the limbs and organs.

• It consists of two primary divisions: 12 pairs of cranial nerves and 31 pairs of spinal nerves.

Nervous System Divisions and Functions

• Central and Peripheral Nervous Systems: The structure and function are observed through several divisions.

  • Key Divisions:

  • Central-Peripheral: CNS (brain and spinal cord) vs PNS (nerves outside CNS).

  • Afferent-Efferent: Afferent (sensory) neurons bring impulses to CNS, while efferent (motor) neurons carry impulses away from CNS.

  • Autonomic-Somatic: Autonomic (involuntary control) vs somatic (voluntary control) functions.

  • Sympathetic-Parasympathetic: Sympathetic prepares body for fight or flight; parasympathetic maintains normal body functions.

Neuron Types and Pathways

• Sensory Neurons (Afferent): Transmit sensory information to the CNS.

• Interneurons: Process information within the CNS.

• Motor Neurons (Efferent): Carry commands from CNS to muscles and glands.

Function of PNS: takes messages from receptors to the central nervous system (CNS) and from the CNS to muscles and glands

Types of nerves:

-          Cranial nerves: 12 pairs that connect directly to the brain i.e. optic, auditory, may be motor, sensory or mixed.

-          Spinal nerves: 31 pairs of spinal nerves, below the 2^nd lumbar vertebra the vertebral foreman is occupied by a mass of spinal nerves, the cauda equina, which serve the lower body. All mixed nerves (motor and sensory) and are joined to spinal cord by roots. Spinal nerves are close to spinal cord, the mixed spinal nerve splits into a dorsal root and a ventral root. The dorsal branch carries afferent (sensory) fibres. A swelling on the dorsal branch, the dorsal root ganglion, contains the cell bodies of the sensory neurons. The ventral branch carries efferent (motor) fibres, their cell body is in the grey matter of the spinal cord.

Structure of the PNS

• Components of PNS: Includes all nerves and ganglia outside the brain and spinal cord.

  • Nerve fibres that carry info to and from CNS and ganglia (groups of nerve cell bodies) that lie outside the brain and spinal cord.

  • Contains sensory and motor pathways (affector and effector).

Afferent and Efferent Divisions

• Afferent Division:

  • Carries impulses from receptors (skin, muscles, internal organs) to CNS. Via sensory neurons.

  • Subdivided into:

  • Somatic Sensory Neurons: Responsible for sensing external stimuli, from skin and muscles.

  • Visceral Sensory Neurons: Responsible for sensing internal stimuli (e.g., organ function).

• Efferent Division:

  • Fibres that carry impulses away from CNS to effectors (muscles and glands).

  • Subdivided into:

  • Somatic Division: takes impulses away from CNS to skeletal muscles. Controls voluntary movements of skeletal muscles.

  • Autonomic Division: takes impulses away from CNS, Controls involuntary movements (heart, smooth muscles).

    • Further divided into:

    • Sympathetic Division: Activates 'fight or flight' responses.

    • Parasympathetic Division: Activates 'rest and digest' responses.

Autonomic Nervous System (ANS)

• Function: Controls the body's internal environment and maintains homeostasis.

• Regulation: Operates without conscious control through groups of nerve cells in the medulla oblongata, hypothalamus, and cerebral cortex. (includes; blood pressure, body temp, digestion, energy release, pupil, diameter, airflow lungs, defecation + urination.

• Nerve fibre of ANS: make up part of the spina nerves and part of some of the cranial nerves, takes impulses to heart muscle, internal organs and glands. ANS Structure

• Impulses travel through two neurons:

  • First Neuron: Myelinated, connects to the CNS.

  • Second Neuron: Unmyelinated, connects to ganglion. Ganglia are clusters of nerve cell bodies outside CNS.

Differences between ANS and Somatic Nervous System (SNS)

• Pathway:

  • ANS involves two motor neurons; SNS involves one.

• Neurotransmitters:

  • ANS can use acetylcholine or noradrenaline; SNS primarily uses acetylcholine.

• Control:

  • ANS is usually involuntary; SNS is usually voluntary.

• Effectors:

  • ANS targets involuntary muscles and glands; SNS targets skeletal muscles.

Sympathetic vs Parasympathetic Effects

• Sympathetic Division:

  • Facilitates responses that prepare the body for strenuous activity; often referred to as 'fight or flight'.

  • Uses noradrenalin as a neurotransmitter. Uses lumbar nerves

• Parasympathetic Division:

  • Maintains body systems during calm conditions; often referred to as 'rest and digest'.

  • Uses acetylcholine as a neurotransmitter. Uses cranio-sacral nerves

Summary

• The PNS plays a crucial role in transmitting signals between the body and CNS, ensuring that both voluntary and involuntary actions can occur effectively. Understanding the different divisions and neuron types is essential for grasping how the body responds to both internal and external stimuli.