Peripheral Nervous System (PNS)

Divisions of the PNS

The Peripheral Nervous System consists of all neural structures outside the Central Nervous System. It is divided into afferent (sensory) and efferent (motor) pathways.

• Afferent Pathways: Carry sensory information from receptors to the CNS.

• Efferent Pathways: Transmit motor commands from the CNS to effectors (muscles and glands).

Nerve Structure & Organization

Nerves are organized into bundles called fascicles, surrounded by connective tissue wrappings.

• Endoneurium: Surrounds individual axons.

• Perineurium: Surrounds each fascicle.

• Epineurium: Surrounds the entire nerve.

Cranial Nerves

There are 12 pairs of cranial nerves, each with specific functions and classifications as sensory, motor, or mixed.

• Names and Numbers: I (Olfactory) to XII (Hypoglossal).

• Functions: E.g., CN II (Optic) is sensory for vision; CN III (Oculomotor) is motor for eye movement.

• Classification: Some are purely sensory (e.g., Olfactory), some motor (e.g., Hypoglossal), most are mixed.

Structure of Spinal Nerves

Spinal nerves connect the CNS to the body via roots and ganglia.

• Ventral Root: Contains motor fibers.

• Dorsal Root: Contains sensory fibers.

• Dorsal Root Ganglion: Houses cell bodies of sensory neurons.

• Plexus: Networks of nerves formed by ventral rami.

Major Nerve Plexuses

• Cervical Plexus: Major nerve: Phrenic nerve (innervates diaphragm).

• Brachial Plexus: Major nerves: Radial, Ulnar, Median (innervate upper limb).

• Lumbar Plexus: Major nerves: Femoral, Obturator (innervate thigh).

• Sacral Plexus: Major nerve: Sciatic (innervates lower limb).

Dermatome

A dermatome is an area of skin innervated by a single spinal nerve.

• Clinical Application: Used to diagnose nerve or spinal cord injuries.

Spinal Reflexes

Reflexes are rapid, automatic responses to stimuli. They involve five components:

• Receptor

• Sensory neuron

• Integration center

• Motor neuron

• Effector

Types of Spinal Reflexes

• Crossed-Extensor Reflex: Maintains balance during withdrawal.

• Withdrawal (Flexor) Reflex: Pulls body part away from painful stimulus.

• Stretch Reflex: Maintains muscle tone and posture (e.g., knee-jerk).

Reciprocal Inhibition

When one muscle contracts, its antagonist is inhibited to allow smooth movement.

Sensory Receptors and Senses

• General Senses: Touch, pain, temperature, proprioception.

• Special Senses: Vision, hearing, equilibrium, taste, smell.

Adaptation

Adaptation is the decrease in sensitivity to a constant stimulus.

• Example: Mechanoreceptors adapt quickly; nociceptors (pain receptors) do not adapt.

Types of Sensory Receptors

• Mechanoreceptors: Respond to mechanical force (touch, pressure).

• Thermoreceptors: Respond to temperature changes.

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

• Chemoceptors: Respond to chemicals (taste, smell).

• Nociceptors: Respond to pain; do not adapt.

Special Senses: Eye and Ear

Eye

• Layers: Fibrous (sclera, cornea), vascular (choroid, ciliary body, iris), neural (retina).

• Photoreceptors: Rods (dim light), cones (color vision).

• Fovea Centralis: Area of sharpest vision.

• Optic Disk: Blind spot; no photoreceptors.

• Pupillary Constriction/Dilation: Regulates light entry.

• Accommodation: Lens changes shape for near/far vision.

• Refraction: Bending of light by cornea and lens.

• Myopia: Nearsightedness; image focuses in front of retina.

• Hyperopia: Farsightedness; image focuses behind retina.

• CN II (Optic Nerve): Transmits visual information to the brain.

Ear

• Regions: Outer (auricle, external auditory canal), middle (ossicles), inner (cochlea, semicircular canals, saccule, utricle).

• Functions: Hearing (cochlea), equilibrium (semicircular canals, saccule, utricle).

• CN VIII (Vestibulocochlear Nerve): Transmits auditory and balance information.

Autonomic Nervous System (ANS)

Overview

The ANS is a division of the PNS that regulates involuntary functions (e.g., heart rate, digestion).

Sympathetic vs Parasympathetic Divisions

• Sympathetic: "Fight or flight"; increases heart rate, dilates pupils, inhibits digestion.

• Parasympathetic: "Rest and digest"; decreases heart rate, constricts pupils, stimulates digestion.

Shared Characteristics

• Both use a two-neuron relay (preganglionic and postganglionic neurons).

• Regulate effectors: smooth muscle, cardiac muscle, glands.

Differences

• Preganglionic Fibers: Sympathetic (short), Parasympathetic (long).

• Postganglionic Fibers: Sympathetic (long), Parasympathetic (short).

• Ganglia Location: Sympathetic (near spinal cord), Parasympathetic (near or within target organs).

• Origin: Sympathetic (thoracolumbar), Parasympathetic (craniosacral).

• Neurotransmitters: Sympathetic (preganglionic: acetylcholine, postganglionic: norepinephrine), Parasympathetic (both: acetylcholine).

Receptors

• Adrenergic Receptors: Bind norepinephrine; found in sympathetic targets.

• Cholinergic Receptors: Bind acetylcholine; found in parasympathetic targets.

Effects of Each Division

• Effects depend on receptor type and tissue.

• Same neurotransmitter can have different effects in different tissues.

Dual Innervation

Most organs receive input from both divisions, allowing fine control.

Sympathetic and Parasympathetic Tone

• Sympathetic Tone: Maintains blood pressure.

• Parasympathetic Tone: Maintains normal activity of digestive and urinary tracts.

Visceral Reflexes

• Similar elements as somatic reflexes.

• Long Reflexes: Involve two motor neurons.

• Short Reflexes: Bypass CNS; occur within autonomic ganglia.

Higher Brain Center Control

Hypothalamus and other brain centers modulate ANS activity.

Endocrine System

Basic Functions

The endocrine system regulates long-term processes via hormones, complementing the nervous system's rapid control.

• Hormones: Chemical messengers released into the bloodstream.

• Comparison: Nervous system acts quickly and locally; endocrine system acts slowly and systemically.

Types of Messengers

• Autocrines: Act on the same cell that secretes them.

• Paracrines: Act on nearby cells.

• Hormones: Act on distant targets via blood.

• Neurotransmitters: Released by neurons; act across synapses.

Hormone Structure and Action

• Structure determines solubility, transport, and mechanism of action.

• Two Major Mechanisms:

  • Second Messenger Systems: Used by water-soluble hormones (e.g., cAMP).

  • Direct Gene Activation: Used by lipid-soluble hormones (e.g., steroids).

Possible Effects of Hormone-Receptor Binding

• Alter membrane permeability

• Stimulate protein synthesis

• Activate or deactivate enzymes

• Induce secretory activity

• Stimulate mitosis

Second Messengers

• Examples: cAMP, Ca2+, IP3

• Water-soluble hormones can be effective at low concentrations due to amplification.

• Steroid hormones require higher concentrations as they act directly on genes.

Regulation of Hormone Secretion

• Humoral: Changes in blood levels of ions/nutrients (e.g., Ca2+ regulates parathyroid hormone).

• Neural: Nerve fibers stimulate hormone release (e.g., sympathetic stimulation of adrenal medulla).

• Hormonal: Hormones stimulate other glands (e.g., pituitary hormones).

Up-Regulation and Down-Regulation

• Up-Regulation: Target cells increase receptor number in response to low hormone levels.

• Down-Regulation: Target cells decrease receptor number in response to high hormone levels.

Endocrine Glands and Hormones

Major glands include pituitary, thyroid, adrenal, pancreas, etc. Each secretes specific hormones with distinct functions.

Hypophyseal Portal System

A network of blood vessels connecting the hypothalamus to the anterior pituitary, allowing hormone transport.

Tropic Hormones

Hormones that regulate other endocrine glands (e.g., TSH stimulates thyroid).

Conditions Due to Improper Regulation

• Diabetes Mellitus: Insulin deficiency or resistance.

• Hypothyroidism: Low thyroid hormone.

• Hyperthyroidism: Excess thyroid hormone.

• Additional info: See lecture slides for more examples.

Pancreas and Blood Glucose Regulation

• Insulin: Secreted by pancreas; increases glucose uptake in liver, skeletal muscle, adipose tissue, lowering blood glucose.

• Glucagon: Raises blood glucose by stimulating glycogen breakdown.

Example: After a meal, insulin is released to promote glucose uptake and storage.