🩺Chapter 10- Nervous System

Intro: The Nervous System’s Role

• The nervous system is one of the two main communication systems in the body; the other is the endocrine system.

• Nervous system:

  • Transmits info very rapidly via nerve impulses conducted from one body area to another.

• Endocrine system:

  • Transmits info more slowly via chemicals (hormones) secreted into the blood and carried to target parts of the body.

Both systems control, regulate, and integrate body functions to maintain homeostasis.

• Homeostasis = balanced and controlled internal environment, vital for life.

• Nervous system structures like the brain, spinal cord, and nerves control rapid communication to maintain homeostasis.

Organization of the Nervous System

Two major divisions:

1. Central Nervous System (CNS) – brain and spinal cord (midline/central).

2. Peripheral Nervous System (PNS) – all nerves of the body outside the CNS.

PNS subdivisions:

• Cranial nerves – connect to the brain.

• Spinal nerves – connect to the spinal cord.

• Autonomic Nervous System (ANS) – regulates involuntary functions (heart rate, stomach contractions, gland secretions).

• Somatic Nervous System (SNS) – voluntary control of skeletal muscles.

• Sensory nerves – carry info from the body to CNS.

Cells of the Nervous System

Nervous tissue has two major cell types:

1. Neurons (nerve cells) – conduct impulses.

2. Glia (neuroglia) – support neurons.

Neurons

Structure:

• Cell body – main part of the neuron.

• Dendrites – branching projections that carry impulses to the cell body.

• Axon – single elongated projection that carries impulses away from the cell body.

Types of neurons:

1. Sensory neurons (afferent neurons) – carry impulses to the brain and spinal cord from all body parts.

2. Motor neurons (efferent neurons) – carry impulses away from brain/spinal cord to muscles and glands.

3. Interneurons (central/connecting neurons) – conduct impulses between sensory and motor neurons within the CNS.

Glia (Neuroglia)

• Do not transmit impulses; act as support cells.

• Hold neurons together, protect them, and help coordinate nervous tissue function.

• A common brain tumor, glioma, develops from glia.

Central Glia Types

1. Astrocytes – star-shaped; anchor small blood vessels to neurons, help form the blood-brain barrier (BBB) (protects brain tissue from harmful substances in blood).

2. Microglia – smaller; act as scavengers, move to sites of inflammation or damage, and destroy microbes via phagocytosis.

3. Oligodendrocytes – help hold nerve fibers together in CNS and form myelin sheaths (fatty insulation) around axons in CNS.

Peripheral Glia

• Schwann cells – form myelin sheaths in the PNS.

  • Myelinated fibers = axons wrapped in myelin.

  • Nodes of Ranvier = gaps between myelin segments.

  • Neurilemma = outer layer of Schwann cell; important in regeneration of injured PNS axons (absent in CNS axons).

Myelin Disorders

• Multiple Sclerosis (MS) – autoimmune disease where myelin in CNS is destroyed by inflammation and immune attack; slows or blocks nerve conduction.

• Neurofibromatosis – inherited disorder causing benign tumors of Schwann cells in the skin, visible as lumps.

Quick Check (p. 257)

1. What is a myelin disorder? How does a myelin disorder disrupt nervous system function?

• A myelin disorder is a disease involving damage to the myelin sheath, the protective fatty layer around axons.

• It disrupts nerve function by slowing or blocking impulse conduction, which affects coordination, movement, and sensory functions.

2. What is a common disease that is characterized by myelin loss and destruction of varying degrees of the oligodendrocytes?

• Multiple sclerosis (MS).

3. What is a neuroma?

• A general term for tumors arising in nervous system structures, often from glia, membrane tissues, or blood vessels (not directly from neurons).

Conditions of Nervous Tissue

Multiple Sclerosis (MS)

• Definition: Autoimmune condition in which myelin in the CNS is destroyed by the immune system.

• Damage: Myelin loss, scarring, and varying degrees of oligodendrocyte injury/death.

• Effect: Nerve conduction is slowed or blocked.

• Onset: Most common in women aged 20–40 years.

• Course: Often relapsing-remitting; may become chronic and progressive.

• Symptoms: Weakness, poor coordination, vision problems, speech issues.

• Cause: Likely autoimmune, possible viral trigger.

• Prognosis: No cure, but early diagnosis and treatment can slow progression.

Tumors

• Neuroma: General term for nervous system tumors.

• Glioma: Common type of brain tumor arising from glia.

  • Usually benign but can be life-threatening depending on location.

• Multiple Neurofibromatosis: Inherited disease causing numerous fibrous benign tumors in Schwann cells of peripheral nerves; can cause disfigurement and organ involvement.

Nerves and Tracts

• Nerve = group of peripheral axons bundled like a cable.

• Bundled structures:

  • Endoneurium: Wraps each axon.

  • Perineurium: Wraps fascicles (groups of axons).

  • Epineurium: Wraps the entire nerve.

• Myelinated fibers look white — make up white matter in CNS.

• Gray matter = cell bodies and unmyelinated fibers.

Reflex Arcs

• Definition: Pathway of nerve impulses for reflexes (involuntary responses).

• Simplest form: Two-neuron arc (sensory + motor).

• Three-neuron arc: sensory neuron → interneuron → motor neuron.

• Receptors: Detect stimulus, often in tendons, skin, or mucous membranes.

• Ganglion: Group of neuron cell bodies in PNS.

• Synapse: Gap between neurons where neurotransmitters cross.

• Effector: Muscle or gland that responds to motor neuron signals.

Quick Check (p. 260)

1. How is white matter different from gray matter?

• White matter contains myelinated axons; gray matter contains cell bodies and unmyelinated fibers.

2. Can you explain the function of a reflex arc?

• It conducts impulses in one direction from a receptor to an effector, producing an involuntary, rapid response.

3. What is a sensory receptor? How does it relate to the reflex arc?

• A sensory receptor detects external stimuli and initiates nerve impulses; it is the starting point of a reflex arc.

4. What is a ganglion?

• A cluster of neuron cell bodies located in the PNS.

5. What is an effector? How does it relate to the reflex arc?

• An organ (muscle or gland) that responds to motor neuron impulses in a reflex arc, carrying out the action.

Reflex Responses

• A reflex = an involuntary response to impulse conduction over a reflex arc.

• When a reflex arc conducts an impulse, it triggers the reflex automatically.

• Example: In the knee-jerk reflex, impulses travel to the quadriceps muscle (the effector), causing contraction and producing the "kick."

• Some reflexes use only two neurons (sensory neuron + motor neuron).

• Others involve three neurons:

  • Sensory neuron

  • Interneuron

  • Motor neuron

• In three-neuron reflexes, the sensory neuron’s axon first synapses with an interneuron before connecting to the motor neuron.

• Example of a three-neuron reflex: the withdrawal reflex, where touching something painful triggers pulling your hand/limb away.

Withdrawal reflex (three‑neuron reflex arc)

Trigger: An irritating stimulus to the skin of the thigh starts the arc.
Goal: Fast, automatic flexor response that pulls the leg away from the irritant.

Pathway (in order):

1. Receptor in the skin detects the irritating stimulus and starts the impulse.

2. Sensory (afferent) neuron

   • Dendrite carries the impulse toward its cell body in the dorsal root ganglion (a cluster of neuron cell bodies in the PNS located near the spinal cord).

   • The sensory axon enters the spinal cord and ends on the dendrites of another neuron in the gray matter.

3. First synapse — the tiny gap where the sensory neuron communicates with an interneuron.

4. Interneuron (in the gray matter of the spinal cord) conducts the impulse to a motor neuron.

5. Second synapse — interneuron to motor (efferent) neuron.

6. Motor neuron

   • Its axon exits via the ventral root of the spinal nerve and ends in the effector.

7. Effector (skeletal flexor muscles) contracts, producing the withdrawal (pulls the limb away).

Key facts to remember (from the text):

• It’s a three‑neuron arc → two synapses (sensory→interneuron, interneuron→motor).

• Reflex arcs are one‑way streets (receptor → CNS → effector).

• The action is involuntary and rapid; brain input isn’t required for the immediate movement.

• The interneuron and its synapses are in the gray matter; bundles of myelinated axons form white matter.

Nerve Impulses

Definition

• A nerve impulse is a self-propagating wave of electrical disturbance traveling along the surface of a neuron’s plasma membrane.

• Think of it like a tiny spark racing along a wire — except it’s the neuron’s membrane carrying the signal.

Mechanism of a Nerve Impulse

• Polarization (resting state):

  • Outside = slightly positive (more Na⁺)

  • Inside = negative (more K⁺ and proteins)

• Depolarization:

  • Stimulus opens sodium channels, Na⁺ rushes in → inside becomes positive.

• Repolarization:

  • Sodium channels close, potassium channels open → K⁺ leaves → inside negative again.

• This change in charge = action potential.

Conduction Types

1. Continuous conduction – In unmyelinated fibers, the action potential travels step-by-step along the axon.

2. Saltatory conduction – In myelinated fibers, the action potential “jumps” between nodes of Ranvier, making transmission much faster.

Synapses

• Definition: Junction between two neurons where impulses are transmitted from the presynaptic neuron to the postsynaptic neuron.

• Parts:

  • Synaptic knob – bulge at axon terminal containing neurotransmitter vesicles.

  • Synaptic cleft – space between neurons.

  • Plasma membrane of postsynaptic neuron – has receptor proteins for neurotransmitters.

Steps in synaptic transmission:

1. Action potential reaches synaptic knob.

2. Neurotransmitters released from vesicles into synaptic cleft.

3. Neurotransmitters bind to receptors on postsynaptic neuron.

4. Ion channels open, generating a new impulse in postsynaptic neuron.

Neurotransmitter Termination

• After release, neurotransmitters must be quickly removed:

  • Reuptake – transported back into presynaptic knob.

  • Enzyme breakdown – destroyed by enzymes in the cleft.

• This stops the signal and resets the synapse.

Neurotransmitters & Receptors

• Chemicals neurons use to communicate.

• Examples:

  • Acetylcholine (ACh) – at neuromuscular junctions.

  • Norepinephrine (NE), dopamine, serotonin – influence mood, motor control, pleasure.

• Many belong to chemical group amines.

Blood-Brain Barrier (BBB)

• Formed by astrocyte “feet” around blood vessels in CNS.

• Allows passage of water, oxygen, CO₂, alcohol, some drugs.

• Blocks toxins, pathogens, many drugs (e.g., antibiotics, dopamine).

• L-dopa can cross and be converted to dopamine (important for Parkinson’s treatment).

Antidepressants

• SSRIs (serotonin-specific reuptake inhibitors) – block serotonin reuptake, increasing levels in synapse (e.g., Paxil, Prozac, Zoloft).

• Other antidepressants affect NE or dopamine pathways.

• Endorphins & enkephalins – natural painkillers; inhibit pain conduction.

• Nitric oxide (NO) – diffuses directly through membranes; involved in male sexual response (e.g., Viagra enhances NO effect).

Parkinson Disease (PD)

• Chronic, progressive disorder caused by dopamine deficiency in certain brain areas.

• Signs: rigidity, tremor of head/extremities, forward trunk tilt, shuffling gait, reduced arm swing.

• Cause: dopamine can’t cross BBB, so treatments use L-dopa.

• Some advanced cases use dopamine agonists, brain implants, or stimulation to increase dopamine.

Quick Check (p. 264)

1. Why are nerve impulses often referred to as action potentials?

• Because they are brief changes in the electrical potential across a neuron’s membrane that propagate along its length.

2. How does myelin increase the speed of nerve impulse conduction?

• By allowing saltatory conduction — impulses jump between nodes of Ranvier instead of traveling continuously.

3. How do neurotransmitters transmit signals across the synapse?

• Released from presynaptic vesicles → cross synaptic cleft → bind to postsynaptic receptors → open ion channels → start new impulse.

4. How are the terms reuptake and recycle used when discussing the synapse?

• Reuptake: neurotransmitters taken back into presynaptic neuron.

• Recycle: neurotransmitters repackaged into vesicles for reuse.

5. What are the characteristics of parkinsonism?

• Rigidity, tremors of head/extremities, forward trunk tilt, shuffling gait, reduced arm swing.

Central Nervous System (CNS)

Overview

• The CNS includes the brain and spinal cord — centrally located along the body’s axis.

• Brain is protected by the skull; spinal cord is protected by vertebrae and meninges.

• CNS integrates sensory information, processes it, and coordinates body responses.

Divisions of the Brain

The brain is divided into major sections:

1. Brainstem – medulla oblongata, pons, midbrain

2. Cerebellum

3. Diencephalon – hypothalamus, thalamus, pineal gland

4. Cerebrum

Brainstem

• Medulla oblongata – lowest part of brainstem; connects to spinal cord.

  • Contains vital centers for heartbeat, breathing, blood vessel diameter.

• Pons – above medulla; acts as a bridge to the midbrain.

• Midbrain – above pons; relays sensory/motor information.

• Brainstem also contains reticular formation (network mixing gray and white matter) that regulates consciousness and alertness.

Key function: Two-way conduction path between spinal cord and higher brain regions; houses reflex centers.

Cerebellum

Structure:

• Second largest part of the brain.

• Lies under occipital lobe of cerebrum.

• Thin outer layer of folded gray matter → large surface area for processing.

• Interior mostly white matter arranged in a branching pattern = arbor vitae (“living tree”).

Function:

• Produces smooth, coordinated muscle movements.

• Maintains equilibrium and posture.

• Newer research: may also coordinate with cerebrum to influence whole-brain function.

Diencephalon

Hypothalamus

• Located below thalamus.

• Connected to pituitary gland via stalk; controls many endocrine functions.

• Vital roles:

  • Controls heartbeat, blood vessel diameter, stomach/intestine contractions.

  • Maintains water balance via antidiuretic hormone (ADH).

  • Regulates temperature, appetite, emotions, sleep cycles, sexual arousal.

  • Links nervous system to endocrine system.

Thalamus

• Above hypothalamus, forms wall of third ventricle.

• Functions:

  1. Relays sensory info to cerebral cortex.

  2. Associates sensations with emotions.

  3. Regulates consciousness/alertness.

  4. Plays role in motor reflexes.

Pineal Gland

• Small gland behind thalamus.

• Receives light info from eyes → regulates melatonin production for sleep-wake cycles.

Cerebrum

Structure:

• Largest part of the brain.

• Outer layer (cerebral cortex) = gray matter (cell bodies, dendrites).

• Inner portion = white matter (myelinated axons).

• Surface features:

  • Gyri – ridges.

  • Sulci – grooves; deeper ones = fissures (e.g., longitudinal fissure dividing hemispheres).

• Hemispheres connected by corpus callosum.

Major Functions:

• Consciousness, thinking, memory, sensations, emotions, willed movements.

• Specialized areas control speech, vision, hearing, motor skills.

Lobes of the Cerebrum (Fig. 10-14)

• Frontal lobe – voluntary movement, reasoning, speech production (Broca’s area), personality.

• Parietal lobe – sensory perception (touch, pressure, temperature, pain), spatial awareness.

• Temporal lobe – hearing, smell, memory, language comprehension (Wernicke’s area).

• Occipital lobe – vision.

• Insula (hidden lobe) – taste, visceral sensations, emotion integration.

Brain Conditions

Destruction of Brain Tissue

Injury or disease can destroy neurons in the brain.

Concussion (Physical Injury)

• Definition: Type of traumatic brain injury (TBI) from a jolt or blow to the head that bends the brain stem, causing temporary chemical changes in the brain.

• Symptoms:

  • Thinking/concentration problems

  • Headache

  • Nausea

  • Light sensitivity

  • Mood changes

  • Sleep disturbances

• Symptoms may appear immediately or later (hours to months).

• Postconcussion syndrome: Persistent symptoms over months.

• Severe concussions or repeated injuries can cause bleeding/swelling, possibly life-threatening.

• Most concussions: Mild, heal with rest; prevention includes helmets & avoiding risky movements.

Stroke (Cerebrovascular Accident, CVA)

• Definition: Destruction of neurons from interrupted blood flow in the brain due to:

  • Hemorrhage (bleeding) or

  • Clot (blocked blood flow)

• Effect: Lack of oxygen to brain tissue → neurons cease functioning.

• Motor damage: Damage in one side of the brain affects the opposite side of the body (cross-over in brainstem).

• Symptoms vary based on location of brain injury.

Cerebral Palsy (CP)

• Definition: Chronic condition from brain tissue damage (often during childhood).

• Cause: Damage to motor control areas → variable muscle tension (spasticity).

• Spastic paralysis: Involuntary muscle contractions.

• Can affect:

  • One side of body (hemiplegia)

  • Legs (paraplegia)

  • Legs + one arm (triplegia)

  • All four limbs (quadriplegia)

• Condition is permanent but non-progressive.

Degenerative Diseases

• Dementia: General decline in memory, attention, personality, motor control.

  • Alzheimer Disease (AD):

    • Lesions develop in cortex (mid–late adult years).

    • Cause unknown; possible genetic + environmental.

    • Misfolded proteins accumulate → neuron death.

    • Symptoms: Memory loss, cognitive decline, personality change.

    • Treatment: No cure; drugs like donepezil (Aricept) or memantine (Namenda) slow progression.

Chronic Traumatic Encephalopathy (CTE)

• Repeated brain trauma → protein buildup in brain cells.

• Causes mood, behavior, memory changes.

Huntington Disease (HD)

• Genetic disorder causing neuron death.

• Chorea: Involuntary purposeless movements.

• Symptoms appear 30–40 years old, progress to dementia, death ~15 years later.

HIV-Associated Dementia

• AIDS can cause progressive dementia from HIV infection damaging brain neurons.

Seizure Disorders

• Definition: Nervous system conditions with sudden bursts of abnormal neuron activity → temporary brain function changes.

• Causes: Tumors, trauma, chemical imbalance, idiopathic.

• Types:

  • Idiopathic: Unknown cause

  • Epilepsy: Recurrent seizures

• Mechanism: Neurons fire abnormally → uncontrolled electrical bursts.

• Drugs:

  • Phenytoin (Dilantin)

  • Valproic acid (Depakene)

  • Gabapentin (Neurontin)

  • Lamotrigine (Lamictal)

• EEG: Measures brain’s electrical activity — chaotic spikes during seizure.

Spinal Cord

Structure

• Average length: 42–45 cm (17–18 inches) in adults

• Location: Lies inside the spinal cavity; extends from occipital bone to bottom of first lumbar vertebra (L1)

• Shape: Slight bulge in cervical and lumbar regions, tapers to an end

• Conus medullaris → tapered end of the spinal cord

• Cauda equina → bundle of spinal nerve roots below conus medullaris

• Filum terminale → fibrous extension from conus medullaris to coccyx

• Spinal nerves:

  • 31 pairs named for the region they emerge from

    • Cervical (C1–C8) — includes cervical plexus, brachial plexus

    • Thoracic (T1–T12) — thoracic nerves

    • Lumbar (L1–L5) — lumbar plexus

    • Sacral (S1–S5) — sacral plexus

    • Coccygeal nerve — 1 pair

Spinal Cord Cross Section

Gray Matter

• H-shaped core of spinal cord — made of dendrites & neuron cell bodies

• Contains many synapses & interneurons

• Involved in reflex arcs

White Matter

• Outer portion; made of spinal tracts (bundles of myelinated axons)

• Ascending tracts (blue) → conduct impulses up to brain

• Descending tracts (red) → conduct impulses down from brain

Major ascending tracts:

• Posterior & anterior spinocerebellar

• Lateral & ventral spinothalamic

• Gracilis & cuneatus tracts

• Spinotectal

Major descending tracts:

• Lateral & anterior corticospinal

• Rubrospinal, reticulospinal, vestibulospinal

• Tectospinal

Functions

• Two-way conduction: Sensory input to brain (ascending), motor output from brain (descending)

• Reflex center for spinal reflexes

  • Withdrawal reflex — pull away from painful stimulus

  • Jerk reflex — knee jerk

• Spinal cord reflexes: automatic, quick, protective

• If spinal cord is severed:

  • Anesthesia — loss of sensation

  • Paralysis — loss of voluntary movement

Coverings & Fluid Spaces

Meninges

Three layers:

1. Dura mater — tough outer layer

2. Arachnoid mater — middle, cobweb-like; space underneath contains CSF

3. Pia mater — delicate inner layer, directly on spinal cord

Meningitis

• Inflammation of meninges

• Causes: bacterial (Neisseria meningitidis, Streptococcus pneumoniae, Haemophilus influenzae), viral, fungal, tumors

• Can be spinal meningitis if only spinal meninges affected

Cerebrospinal Fluid (CSF) Spaces

• Location: Fills subarachnoid space between pia & arachnoid mater

• Also in brain’s ventricles:

  • Lateral ventricles → interventricular foramen → third ventricle → cerebral aqueduct → fourth ventricle → subarachnoid space

• Formation: Produced by choroid plexus (capillary network in ventricles)

• Flow: Lateral ventricles → 3rd ventricle → cerebral aqueduct → 4th ventricle → subarachnoid space → central canal → back to blood via dural venous sinuses

• Functions:

  • Cushion CNS

  • Maintain stable environment

  • Remove waste

• Hydrocephalus: Excess CSF causes pressure on brain

Peripheral Nervous System (PNS)

• Definition: Nerves connecting the brain and spinal cord to other parts of the body.

• Components:

  • Cranial nerves (connect brain to peripheral structures)

  • Spinal nerves (connect spinal cord to peripheral structures)

• Function: Carry sensory and motor impulses to/from skin surface, skeletal muscles, cardiac muscles, and smooth muscles of thorax/abdomen.

• Autonomic Nervous System (ANS): Part of PNS; connects brain/spinal cord to glands and internal organs.

Cranial Nerves

• 12 pairs emerge from underside of brain.

• Attachment sites: Brainstem & diencephalon.

• Impulse pathways: Brain ↔ head, neck, thoracic & abdominal cavities.

• Examples:

  • CN II – Optic nerve: Eye to brain; vision.

  • CN III – Oculomotor nerve: Brain to eye muscles; moves eyes.

  • CN X – Vagus nerve: Brain ↔ thoracic/abdominal structures.

• Table 10-2:

  Nerve	Impulses	Function
  I Olfactory	Nose → Brain	Smell
  II Optic	Eye → Brain	Vision
  III Oculomotor	Brain → Eye muscles	Eye movement
  IV Trochlear	Brain → Eye muscles	Eye movement
  V Trigeminal	Face/mouth ↔ Brain	Sensations of face, chewing
  VI Abducens	Brain → Eye muscles	Eye movement
  VII Facial	Tongue/face ↔ Brain	Taste, facial expression
  VIII Vestibulocochlear	Ear → Brain	Hearing, balance
  IX Glossopharyngeal	Tongue/throat ↔ Brain	Taste, swallowing, saliva secretion
  X Vagus	Brain ↔ Thoracic/abdominal organs	Swallowing, heartbeat, digestion
  XI Accessory	Brain → Shoulder/neck	Shoulder movement
  XII Hypoglossal	Brain → Tongue	Tongue movement

Spinal Nerves

• 31 pairs emerge from spinal cord:

  • 8 cervical

  • 12 thoracic

  • 5 lumbar

  • 5 sacral

  • 1 coccygeal

• Naming: Letter + number (e.g., C1, T8).

• Function: Carry sensory & motor impulses between spinal cord & rest of body.

• Plexuses: Interconnected nerve networks.

• Dermatomes: Skin surface areas supplied by specific spinal nerves (mapped in Fig. 10-25).

• Injury: Can cause loss of sensation/movement in specific body part.

Peripheral Nerve Conditions

• Neuritis: Inflammation of a nerve/branch.

• Sciatica: Neuritis of sciatic nerve (largest in body) → pain in thigh/leg, possible atrophy.

Clinical Application – Lumbar Puncture

• Purpose: Withdraw cerebrospinal fluid (CSF) for analysis or to relieve pressure.

• Procedure:

  • Needle inserted between lumbar vertebrae (below L4) into subarachnoid space.

  • Patient lies on side, knees to chest.

• Findings:

  • Normal CSF = clear/yellowish

  • Abnormal CSF (e.g., red) → hemorrhage.

Peripheral Nerve Disorders

• Trigeminal Neuralgia (tic douloureux)

  • Affects fifth cranial nerve (trigeminal nerve).

  • Episodes of stabbing pain along nerve branches:

    • Over forehead & eyes

    • Cheek, nose, upper lip

    • Tongue & lower lip

  • Pain triggered by irritation/degeneration.

• Bell Palsy

  • Involves seventh cranial nerve (facial nerve).

  • Causes paralysis of some or all facial features on one side (eyelids, mouth).

  • Can be temporary or permanent.

  • Plastic surgery possible for irreversible damage.

• Herpes Zoster (Shingles)

  • Viral infection affecting skin along a single dermatome.

  • Caused by varicella zoster virus (VZV) — same as chickenpox.

  • Nearly 15% of people get it by age 80.

  • Results from reactivation of dormant virus in dorsal root ganglion.

  • Risk factors: weakened immunity, stress, radiation therapy, immunosuppressive drugs.

  • Symptoms: painful eruption of red vesicles, burning, itching.

  • Complications: bacterial infection, scarring, permanent nerve pain.

  • Treatment: antivirals, pain control.

  • Vaccine: Recombinant zoster vaccine (RZV), 90% effective, for ages 50+.

Autonomic Nervous System (ANS) Overview

• Controls involuntary functions of:

  1. Cardiac muscle

  2. Smooth muscle

  3. Glandular epithelial tissue

• Regulates automatic functions like heart rate, digestion, gland secretion.

• Motor neurons controlling skeletal muscle = Somatic Nervous System (SNS).

• Two main divisions:

  • Sympathetic division (emergency/stress responses)

  • Parasympathetic division (rest & digest functions)

Functional Anatomy of ANS

• Autonomic neurons: Motor neurons in the ANS.

• Preganglionic neurons:

  • Cell bodies in spinal cord or brainstem.

  • Axons extend to autonomic ganglia.

• Postganglionic neurons:

  • Cell bodies in autonomic ganglia.

  • Axons extend to visceral effectors (smooth muscle, cardiac muscle, glands).

• Autonomic effectors:

  • Cardiac muscle

  • Smooth muscle (vessel walls, hollow organs)

  • Glandular epithelial tissue (glands)

Sympathetic Division

• Structure

  • Preganglionic neuron cell bodies in thoracic & lumbar spinal cord segments (thoracolumbar system).

  • Preganglionic axons leave spinal cord, pass through spinal nerve → sympathetic ganglion → synapse with postganglionic neurons.

  • Postganglionic neurons extend to visceral effectors.

  • Sympathetic chain ganglia: paired structures alongside spinal cord.

• Function

  • Emergency system — prepares body for stress (“fight or flight”).

  • Activates during exercise, fear, anger, anxiety.

  • Increases heart rate, dilates bronchioles, decreases digestion, releases epinephrine.

  • Widespread effects due to branching of axons to multiple effectors.

Autonomic Functions (Table 10-3)

• Heart muscle

  • Sympathetic: accelerates heartbeat

  • Parasympathetic: slows heartbeat

• Smooth muscle

  • Most blood vessels: constricts (sympathetic) / none (parasympathetic)

  • Skeletal muscle vessels: dilates (sympathetic) / none (parasympathetic)

  • Digestive tract: decreases peristalsis, inhibits defecation (sympathetic) / increases peristalsis (parasympathetic)

  • Anal sphincter: stimulates closure (sympathetic) / inhibits closure (parasympathetic)

  • Urinary bladder: relaxes (sympathetic) / contracts (parasympathetic)

  • Urinary sphincters: stimulates closure (sympathetic) / inhibits closure (parasympathetic)

  • Eye:

    • Iris: dilates pupil (sympathetic) / constricts pupil (parasympathetic)

    • Ciliary: far vision (sympathetic) / near vision (parasympathetic)

  • Hair muscles: goose pimples (sympathetic) / none (parasympathetic)

• Glands

  • Adrenal medulla: ↑ epinephrine secretion (sympathetic) / none (parasympathetic)

  • Sweat glands: ↑ sweat (sympathetic) / none (parasympathetic)

  • Digestive glands: ↓ secretion (sympathetic) / ↑ secretion (parasympathetic)

Parasympathetic Division

• Structure

  • Preganglionic neuron cell bodies in brainstem + sacral spinal cord (craniosacral outflow)

  • Axons travel to parasympathetic ganglia near/within target organs

  • Postganglionic neurons located in ganglia close to target organs

• Function

  • Dominates control of many viscera under normal conditions

  • Slows heartbeat, increases peristalsis, increases digestive juice and insulin secretion

  • Acts as counterbalance to sympathetic function

Autonomic Neurotransmitters

• Figure 10-29

  • Sympathetic preganglionic → releases ACh
    Sympathetic postganglionic → releases NE (adrenergic fibers)

  • Parasympathetic preganglionic → releases ACh
    Parasympathetic postganglionic → releases ACh

• Cholinergic fibers: release acetylcholine (all preganglionic + parasympathetic postganglionic)

• Adrenergic fibers: release norepinephrine (sympathetic postganglionic)

• Effect on organs

  • ACh slows heart (parasympathetic)

  • NE increases heart rate (sympathetic)

Autonomic Nervous System as a Whole

• Function: regulates involuntary functions to maintain/restore homeostasis

• Many internal organs are dually innervated (sympathetic & parasympathetic with opposing effects)

• Influenced by higher brain centers: hypothalamus, limbic system, cerebral cortex

• Emotional states (anger, fear) → ↑ sympathetic activity (fight-or-flight)

• Meditation & relaxation → ↑ parasympathetic activity

Conditions of the ANS

Stress-Induced Disease

• Heart disease: chronic stress → hypertension, heart failure risk

• Digestive problems: colitis, ulcers from stress-induced motility changes & decreased immunity

• Reduced resistance to disease: stress hormones (glucocorticoids) suppress immune function

• Spread of cancer: ↑ norepinephrine promotes metastasis

Neuroblastoma

• Malignant tumor of sympathetic division

• Often in children, spreads quickly

• Symptoms: exaggerated/inappropriate sympathetic activity (↑ HR, sweating, BP)

• Some cases have spontaneous remission