Peripheral Nervous System Physiology: Structure, Function, and Reflexes

Peripheral Nervous System (PNS) Overview

Divisions of the PNS

The Peripheral Nervous System (PNS) connects the Central Nervous System (CNS) to limbs and organs, serving as a communication relay. It is divided into sensory (afferent) and motor (efferent) divisions, each with distinct roles in transmitting information.

• Sensory (Afferent) Division: Carries sensory information from receptors to the CNS.

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

• Somatic Nervous System: Controls voluntary movements via skeletal muscles.

• Autonomic Nervous System (ANS): Regulates involuntary functions, targeting smooth muscle, cardiac muscle, and glands.

Major Neurotransmitters in the PNS

Types and Functions

Neurotransmitters are chemical messengers that transmit signals across synapses. They can be classified by their chemical structure and function, influencing excitatory or inhibitory responses in target cells.

• Acetylcholine (ACh): Main neurotransmitter at neuromuscular junctions and autonomic ganglia; usually excitatory.

• Biogenic Amines: Includes norepinephrine, epinephrine, dopamine, serotonin, and histamine; involved in mood, arousal, and autonomic regulation.

• Amino Acids: Glutamate (excitatory), GABA and glycine (inhibitory); major neurotransmitters in the CNS.

• Neuropeptides: Substance P, opioids, neuropeptide Y; modulate pain, stress, and other functions.

Neurotransmitter	Precursor	Main Effect	Location	Receptor Type
Acetylcholine	Acetyl-CoA + choline	Excitatory	CNS, PNS	Ionotropic/Metabotropic
Norepinephrine	Tyrosine	Excitatory/Inhibitory	CNS, PNS	Metabotropic
Serotonin	Tryptophan	Excitatory/Inhibitory	CNS	Metabotropic
GABA	Glutamate	Inhibitory	CNS	Ionotropic/Metabotropic
Substance P	Amino acids	Excitatory	CNS, PNS	Metabotropic
Neuropeptide Y	-	Excitatory/Inhibitory	CNS, PNS	Metabotropic

Types of Neurotransmitter Receptors

Ionotropic vs. Metabotropic Receptors

Neurotransmitter receptors determine the cellular response to neurotransmitters. They are classified as:

• Ionotropic Receptors: Ligand-gated ion channels that mediate fast synaptic transmission by directly altering membrane potential.

• Metabotropic Receptors: G-protein-coupled receptors that initiate slower, longer-lasting effects via second messenger systems.

Example: Nicotinic acetylcholine receptors are ionotropic, while muscarinic acetylcholine receptors are metabotropic.

Somatic vs. Autonomic Motor Divisions

Structural and Functional Differences

The motor division of the PNS is divided into somatic and autonomic branches, each with unique pathways and targets.

• Somatic Motor Division:

  • One neuron from CNS to skeletal muscle

  • No ganglia

  • Releases acetylcholine (ACh) at the target

  • Primarily voluntary control

• Autonomic Motor Division:

  • Two-neuron pathway (preganglionic and postganglionic)

  • Ganglia present

  • Releases ACh or norepinephrine (NE) at the target

  • Targets smooth muscle, cardiac muscle, and glands

  • Involuntary control

Preganglionic vs. Postganglionic Autonomic Neurons

Pathways in the Autonomic Nervous System

Autonomic pathways consist of two neurons:

• Preganglionic Neuron: Cell body in the CNS; axon projects to an autonomic ganglion.

• Postganglionic Neuron: Cell body in the ganglion; axon projects to the target organ.

Sympathetic and Parasympathetic Motor Neurons

Organization and Function

The autonomic nervous system is divided into sympathetic and parasympathetic branches, each with distinct anatomical origins and effects.

• Sympathetic Division: Originates from thoracic and lumbar spinal cord; prepares body for 'fight or flight' responses.

• Parasympathetic Division: Originates from brainstem and sacral spinal cord; promotes 'rest and digest' activities.

Adrenergic and Cholinergic Receptor Subtypes

Sympathetic and Parasympathetic Effects

Autonomic neurotransmitters act on specific receptor subtypes to produce varied physiological effects.

• Adrenergic Receptors (Sympathetic):

  • ⍺1: Vasoconstriction, increases blood pressure

  • ⍺2: Inhibits neurotransmitter release (feedback control)

  • β1: Increases heart rate and contractility

  • β2: Bronchodilation and vasodilation (skeletal muscle)

  • β3: Lipolysis (fat breakdown)

• Cholinergic Receptors (Parasympathetic):

  • Nicotinic (N): Fast synaptic transmission; found in autonomic ganglia and neuromuscular junctions

  • Muscarinic (M): Slower, longer-lasting effects; found on parasympathetic target organs

Reflex Components and Homeostasis

Structure of a Reflex Arc

Reflexes are rapid, automatic responses to stimuli that help maintain homeostasis. A typical reflex arc includes:

• Receptor: Detects a stimulus

• Sensory (Afferent) Neuron: Carries information to the CNS

• Integration Center: Processes information (spinal cord or brain)

• Motor (Efferent) Neuron: Carries response signal

• Effector: Muscle or gland that produces the response

Monosynaptic and Polysynaptic Reflexes

Types of Reflex Pathways

• Monosynaptic Reflex:

  • One synapse between sensory and motor neuron

  • Fastest reflex pathway

  • Example: Stretch (knee-jerk) reflex

  • Function: Maintains muscle length and posture

• Polysynaptic Reflex:

  • Two or more synapses with interneurons

  • Slower but more complex responses

  • Example: Withdrawal reflex

  • Function: Coordinates multiple muscles for protection

Patellar Tendon (Knee-Jerk) Reflex Pathway

Example of a Monosynaptic Reflex

The patellar tendon reflex is a classic example of a monosynaptic reflex, used clinically to assess nervous system function.

• Tap on patellar tendon stretches quadriceps muscle

• Muscle spindle (receptor) activates sensory neuron

• Sensory neuron synapses directly with motor neuron in spinal cord

• Motor neuron stimulates quadriceps to contract, causing leg extension

Major Sensory Receptors and Their Stimuli

Types of Sensory Receptors

Sensory receptors detect specific types of stimuli and convert them into neural signals.

• Mechanoreceptors: Touch, pressure, vibration, stretch

• Thermoreceptors: Temperature changes

• Chemoreceptors: Chemicals (taste, smell, blood chemistry)

• Baroreceptors: Pressure changes (e.g., blood pressure)

• Osmoreceptors: Osmotic pressure (fluid balance)

• Proprioceptors: Body position and movement

• Nociceptors: Pain or tissue damage

Receptive Field & Two-Point Discrimination

Spatial Resolution of Sensory Neurons

The receptive field is the area where a sensory neuron can detect a stimulus. The size of receptive fields affects tactile resolution and the ability to distinguish two points.

• Small Receptive Fields: High tactile precision (e.g., fingertips)

• Large Receptive Fields: Lower tactile precision (e.g., back, thighs)

First-Order & Second-Order Neurons and Two-Point Discrimination

Neural Pathways for Sensory Processing

First-order neurons detect stimuli at the skin and send signals to the spinal cord or brainstem. Second-order neurons carry the signal to the thalamus. The precision of two-point discrimination depends on the size and mapping of receptive fields.

• Fingertips: Many first-order neurons with small receptive fields → high tactile precision

• Back: Fewer first-order neurons with large receptive fields → lower tactile precision

Sympathetic Nervous System & Adrenal Medulla

Role in Stress Response

The adrenal medulla acts as a modified sympathetic ganglion. Sympathetic neurons stimulate the adrenal medulla to release epinephrine and norepinephrine into the bloodstream, producing a widespread, longer-lasting sympathetic response.

Stress Responses: SAM and HPA Pathways

Acute and Chronic Stress Mechanisms

• Sympathetic-Adrenal-Medullary (SAM) Response:

  • Fast response (seconds)

  • Sympathetic activation → adrenal medulla releases epinephrine

  • Effects: Increased heart rate, blood pressure, bronchodilation, energy mobilization

  • Prepares body for immediate action

• Hypothalamic-Pituitary-Adrenal (HPA) Response:

  • Slower response (minutes to hours)

  • Hypothalamus releases CRH → Pituitary releases ACTH → Adrenal cortex releases cortisol

  • Effects: Maintains blood glucose, suppresses immune response, supports long-term stress adaptation

Eustress vs. Distress

• Eustress: Positive, beneficial stress that improves focus and performance

• Distress: Negative, harmful stress that impairs function and health