Peripheral Nervous System, Autonomic Nervous System, and Endocrine System: Study Guide

Peripheral Nervous System (PNS)

Divisions of the PNS

The Peripheral Nervous System is divided into afferent (sensory) and efferent (motor) pathways, which transmit information to and from the Central Nervous System (CNS).

• 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.

• Fascicles: Bundles of nerve fibers (axons).

• 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: Sensory (e.g., Olfactory), Motor (e.g., Oculomotor), Mixed (e.g., Facial).

• Example: CN II (Optic): Sensory nerve for vision.

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 Ganglia: Houses cell bodies of sensory neurons.

• Plexus: Network 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 nerve (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. Each reflex arc has five components:

• 1. Receptor

• 2. Sensory neuron

• 3. Integration center

• 4. Motor neuron

• 5. Effector

Types discussed:

• 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: Decreased sensitivity to a constant stimulus.

• Types of Sensory Receptors:

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

  • Thermoreceptors: Respond to temperature.

  • Photoreceptors: Respond to light (in eye).

  • Chemoceptors: Respond to chemicals.

  • 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, detail).

  • 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 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 and Divisions

The ANS is a part of the PNS that regulates involuntary functions. It has two main divisions:

• Sympathetic Division: "Fight or flight" responses; increases heart rate, dilates pupils.

• Parasympathetic Division: "Rest and digest" responses; decreases heart rate, promotes digestion.

Shared Characteristics

• Two Motor Neuron Relay: Preganglionic and postganglionic neurons.

• Effectors: Smooth muscle, cardiac muscle, glands.

Differences Between Divisions

• 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 and Regulation

• Dual Innervation: Most organs receive input from both divisions.

• Sympathetic Tone: Maintains blood vessel constriction.

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

Visceral Reflexes

• Long Reflexes: Involve CNS and two motor neurons.

• Short Reflexes: Bypass CNS; involve local ganglia.

• Higher Brain Center Control: Hypothalamus and other centers modulate ANS activity.

Endocrine System

Basic Functions and Comparison to Nervous System

The endocrine system regulates long-term processes via hormones, while the nervous system provides rapid, short-term control via electrical impulses.

• Hormones: Chemical messengers released into the bloodstream.

• Neurotransmitters: Chemical messengers released at synapses.

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: Act across synapses.

Hormone Structure and Action

• Chemical Structure: Determines solubility, transport, and mechanism of action.

• Two Major Mechanisms:

  • Second Messenger Systems: Used by amino acid-based hormones; e.g., cAMP.

  • Direct Gene Activation: Used by steroid hormones; hormone enters cell and activates genes.

Hormone-Receptor Binding Effects

• Alters membrane permeability

• Stimulates protein synthesis

• Activates or deactivates enzymes

• Induces secretory activity

• Stimulates mitosis

Second Messengers

• Examples: cAMP, Ca2+, IP3

• Function: Amplify hormone signal inside cell.

• Note: Lower amounts of amino acid-derived hormones can be effective due to amplification; steroid hormones require higher concentrations as they act directly.

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: Increase in receptor number in response to low hormone levels.

• Down-Regulation: Decrease in 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.

Gland	Hormone	Major Function
Pituitary (anterior)	Growth hormone (GH), ACTH, TSH, FSH, LH	Growth, metabolism, reproductive regulation
Thyroid	Thyroxine (T4), Triiodothyronine (T3)	Metabolic rate regulation
Adrenal (cortex)	Cortisol, Aldosterone	Stress response, electrolyte balance
Adrenal (medulla)	Epinephrine, Norepinephrine	Fight or flight response
Pancreas	Insulin, Glucagon	Blood glucose regulation
Ovaries/Testes	Estrogen, Testosterone	Sexual development and function

Example: Insulin increases glucose uptake in liver, skeletal muscle, and adipose tissue, lowering blood glucose levels.

Hypophyseal Portal System

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

Tropic Hormones

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

Conditions Due to Improper Regulation

• Diabetes Mellitus: Insulin deficiency or resistance.

• Hyperthyroidism: Excess thyroid hormone.

• Hypothyroidism: Deficient thyroid hormone.

• Cushing's Syndrome: Excess cortisol.

• Addison's Disease: Deficient adrenal cortex hormones.

Additional info:

• Hormone action often involves feedback mechanisms to maintain homeostasis.

• Some hormones act rapidly (e.g., epinephrine), others slowly (e.g., steroid hormones).