Peripheral Nervous System: Structure, Function, and Divisions

Peripheral Nervous System (PNS)

Overview of the Nervous System

The nervous system is divided into two main parts: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). The CNS consists of the brain and spinal cord, while the PNS comprises nerves and ganglia that connect the CNS to the rest of the body, including receptors, muscles, and glands.

• CNS: Brain and spinal cord; responsible for processing and integrating information.

• PNS: Nerves and ganglia outside the CNS; transmits signals between the CNS and the body.

Peripheral Nerves

The PNS includes cranial nerves (12 pairs arising from the brain) and spinal nerves (31 pairs arising from the spinal cord). These nerves are essential for communication between the CNS and peripheral tissues.

• Cranial nerves: Serve the head and neck region; some are sensory, some motor, most are mixed.

• Spinal nerves: All are mixed nerves, containing both sensory and motor fibers.

Structure of Peripheral Nerves

Sensory, Motor, and Mixed Nerves

Nerves in the PNS can be classified based on the direction of impulse transmission:

• Sensory (Afferent) fibers: Carry impulses into the CNS from receptors.

• Motor (Efferent) fibers: Carry impulses away from the CNS to effectors (muscles/glands).

• Mixed nerves: Contain both sensory and motor fibers.

Spinal Nerve Roots

Each spinal nerve is attached to the spinal cord by two roots:

• Ventral root: Contains axons of motor neurons; cell bodies are in the spinal cord's gray matter.

• Dorsal root: Contains axons of sensory neurons; cell bodies are in the dorsal root ganglion.

Divisions of the Peripheral Nervous System

Functional Organization

The PNS is functionally divided into afferent (sensory) and efferent (motor) divisions:

• Afferent (Sensory) Division: Transmits sensory information to the CNS.

  • Somatic sensory neurons: From skin, muscles, and joints.

  • Visceral sensory neurons: From internal organs.

• Efferent (Motor) Division: Transmits motor commands from the CNS to effectors.

  • Somatic division: Controls voluntary movements of skeletal muscles.

  • Autonomic division: Controls involuntary actions of smooth muscle, cardiac muscle, and glands.

Autonomic Nervous System (ANS)

The ANS regulates involuntary physiological functions to maintain homeostasis. It operates largely without conscious control and is regulated by the medulla oblongata, hypothalamus, and cerebral cortex.

• Sympathetic division: Prepares the body for 'fight-or-flight' responses; releases noradrenaline.

• Parasympathetic division: Maintains 'rest-and-digest' functions; releases acetylcholine.

Neural Pathways in the PNS

Somatic vs. Autonomic Pathways

• Somatic pathway: One motor neuron from CNS to skeletal muscle; neurotransmitter is acetylcholine.

• Autonomic pathway: Two motor neurons (preganglionic and postganglionic); neurotransmitters are acetylcholine or noradrenaline.

Comparison of Autonomic and Somatic Divisions

Key Differences

Characteristic	Autonomic Division	Somatic Division
Effectors	Heart muscle, involuntary muscles, glands	Skeletal (voluntary) muscles
General Function	Adjusts internal environment	Response to external environment
Efferent (outward) pathway	Two neurons from CNS to effector with a synapse in a ganglion	One neuron from CNS to effector
Neurotransmitter at effector	Acetylcholine or noradrenaline	Acetylcholine
Control	Usually involuntary	Usually voluntary
Nerves to target organ	Two sets: sympathetic and parasympathetic	One set
Effect on target organ	Excitation or inhibition	Always excitation

Effects of the Autonomic Nervous System

Sympathetic vs. Parasympathetic Stimulation

Structure	Effect of Sympathetic Stimulation	Effect of Parasympathetic Stimulation
Heart	Increases rate and strength of contraction	Decreases rate and strength of contraction
Lungs	Dilates bronchioles	Constricts bronchioles
Liver	Increases breakdown of glycogen and release of glucose	Increases uptake of glucose and synthesis of glycogen
Eye (pupil)	Dilates pupil	Constricts pupil
Sweat glands	Increases secretion	No effect
Blood vessels (skeletal muscle)	Dilates vessels	Little effect
Adrenal medulla	Stimulates hormone secretion	No effect

Fight-or-Flight Response

Sympathetic Activation

The fight-or-flight response is triggered by the sympathetic division during situations of fear, anger, stress, or danger. This response prepares the body for increased physical activity by:

• Increasing heart rate and blood pressure

• Dilating airways and blood vessels in muscles

• Raising blood glucose levels

• Stimulating sweat gland secretion

• Releasing adrenaline and noradrenaline from the adrenal medulla

Neural Circuitry: Reflex Arc

Identification of Neurons

In a typical reflex arc, the afferent neuron (sensory neuron) transmits impulses to the CNS, the interneuron processes the information, and the efferent neuron (motor neuron) carries the response to the effector.

Description	Marks
Afferent neuron = B	1
Interneuron = A	1
Total	2

Additional info: The PNS is essential for integrating sensory input and motor output, and its divisions allow for both voluntary and involuntary control of body functions. Understanding the structure and function of the PNS is fundamental for studies in neurobiology, physiology, and medicine.