Chapter 15

Somatic nervous system components

The somatic nervous system contains both sensory and motor neurons.

Function of somatic sensory neurons

Receive input from receptors of the special and somatic senses; sensations are consciously perceived.

Somatic motor neuron function

Innervate skeletal muscle to produce conscious, voluntary movements; effect is always excitation.

Autonomic nervous system components

Contains both autonomic sensory and motor neurons.

Autonomic sensory neurons

Associated with interoceptors; input is not consciously perceived.

Effect of sensory input on ANS

Somatic and special senses via the limbic system affect autonomic responses.

Autonomic motor neuron function

Regulate visceral activities by increasing (exciting) or decreasing (inhibiting) cardiac muscle, smooth muscle, and gland activity.

Conscious control of autonomic responses

Most autonomic responses cannot be consciously altered or suppressed.

Neurons in autonomic motor pathways

Two motor neurons in series (preganglionic and postganglionic).

Preganglionic neuron

First neuron; axon extends from CNS to an autonomic ganglion.

Postganglionic neuron

Second neuron; axon extends from ganglion to effector.

ANS neurotransmitters

Preganglionic fibers release acetylcholine; postganglionic fibers release acetylcholine or norepinephrine.

Divisions of ANS

Sympathetic and parasympathetic divisions.

Dual innervation

Organs that receive impulses from both sympathetic and parasympathetic fibers.

Preganglionic neuron location

Cell body in brain or spinal cord; myelinated axon extends to an autonomic ganglion.

Postganglionic neuron location

Located entirely outside the CNS; unmyelinated axon terminates in a visceral effector.

Sympathetic preganglionic neuron location

Lateral gray horns of T1–L2/L3 segments (thoracolumbar division).

Parasympathetic preganglionic neuron location

Cranial nerve nuclei III, VII, IX, X, and S2–S4 segments (craniosacral division).

Sympathetic ganglia types

Sympathetic trunk (paravertebral) ganglia and prevertebral (collateral) ganglia.

Prevertebral ganglia examples

Celiac, superior mesenteric, and inferior mesenteric ganglia.

Parasympathetic ganglia location

Terminal or intramural ganglia near or within the wall of target organs.

Examples of parasympathetic ganglia

Ciliary, pterygopalatine, submandibular, and otic ganglia.

Autonomic plexuses

Tangled networks of sympathetic and parasympathetic neurons (cardiac, pulmonary, celiac, mesenteric, hypogastric, renal).

Sympathetic trunk ganglia location

Arranged anterior and lateral to vertebral column.

Adrenal medulla

Modified sympathetic ganglion; cells resemble postganglionic neurons.

Horner’s Syndrome

Loss of sympathetic innervation on one side of the face.

Parasympathetic cranial outflow

Preganglionic axons in cranial nerves III, VII, IX, and X.

Parasympathetic sacral outflow

Preganglionic axons in anterior roots of S2–S4 nerves.

Cholinergic neurons

Release acetylcholine (ACh); include all preganglionic neurons, all parasympathetic postganglionic neurons, and sympathetic postganglionic neurons to sweat glands.

Types of cholinergic receptors

Nicotinic (always excitatory) and muscarinic (excitatory or inhibitory).

Adrenergic neurons

Release norepinephrine (NE); include most sympathetic postganglionic neurons.

Adrenergic receptor types

Alpha (α) and beta (β) receptors; activation can cause excitation or inhibition depending on subtype.

Effect duration

Adrenergic effects last longer than cholinergic effects.

Agonist

Substance that binds and activates a receptor, mimicking a neurotransmitter.

Antagonist

Substance that binds and blocks a receptor, preventing neurotransmitter action.

Dual innervation

Most organs receive input from both sympathetic and parasympathetic divisions, creating autonomic tone.

Sympathetic division nickname

Fight-or-flight division.

Four E’s of sympathetic response

Emergency, Exercise, Excitement, Embarrassment.

Sympathetic response characteristics

Longer-lasting, more widespread; norepinephrine degraded slowly; more postganglionic neurons.

Sympathetic effects

Pupils dilate, heart rate and BP increase, decreased GI and kidney blood flow, increased blood flow to muscles and liver, glucose and fat breakdown increase.

Parasympathetic division nickname

Rest-and-digest or energy conservation-restorative system.

Parasympathetic response characteristics

Shorter-lasting, less widespread; acetylcholine degraded quickly.

Parasympathetic effects (SLUDD)

Salivation, Lacrimation, Urination, Digestion, Defecation; also decreased heart rate, airway diameter, and pupil size.

Visceral autonomic reflex

Unconscious adjustment of visceral effector activity.

Components of visceral reflex arc

Sensory receptor → sensory neuron → integrating center → motor neurons → visceral effector.

Major control center of ANS

Hypothalamus; integrates sympathetic and parasympathetic activity.

Cerebral cortex control

Occurs primarily during emotional stress.

Autonomic dysreflexia

Exaggerated sympathetic response in spinal cord injury at or above T6.

Raynaud’s phenomenon

Excessive sympathetic stimulation causes reduced blood flow to fingers and toes.

Dysautonomia

Group of disorders resulting from damage to the autonomic nervous system.

Brain development

The brain and spinal cord arise from embryological ectoderm beginning as a hollow neural tube.

Major parts of the brain

Brain stem, diencephalon, cerebrum, and cerebellum.

Brain protection

The brain is protected by cranial bones and cranial meninges.

Cranial meninges

The three layers: dura mater, arachnoid mater, and pia mater.

Extensions of dura mater

Falx cerebri, falx cerebelli, and tentorium cerebelli separate brain parts.

Brain blood supply

Blood flows to the brain via internal carotid and vertebral arteries; venous blood drains into internal jugular veins.

Brain oxygen use

Although only 2% of body weight, the brain uses about 20% of total body oxygen.

Effect of oxygen interruption

Interruptions cause weakness, brain damage, or death; lack during childbirth can cause paralysis or death.

Glucose supply to brain

Must be continuous; deficiency causes confusion, dizziness, convulsions, or unconsciousness.

Blood-brain barrier (BBB)

Selective barrier protecting brain cells from harmful substances and pathogens.

Breaching the BBB

Injury, trauma, or toxins can break down the BBB, allowing harmful substances into brain tissue.

BBB and drug therapy

The BBB can prevent drug entry, complicating treatment of brain cancers or CNS disorders.

Cerebrospinal fluid (CSF)

A clear, colorless liquid protecting the brain and spinal cord and carrying nutrients and chemicals.

CSF cavities

Four CSF-filled ventricles in the brain.

Functions of CSF

Provides mechanical protection, chemical protection, and circulation.

CSF formation

Formed by filtration from choroid plexuses in ventricles.

Blood–CSF barrier

Formed by ependymal cells that regulate material exchange between blood and CSF.

CSF circulation

Flows through ventricles, subarachnoid space, and central canal.

CSF absorption

Absorbed by arachnoid villi into the superior sagittal sinus at the same rate it’s produced.

Hydrocephalus

If CSF drainage is blocked, fluid buildup increases brain pressure; treated by shunt surgery.

Medulla oblongata

Continuous with spinal cord; contains sensory and motor tracts and cranial nerves VIII–XII.

Medulla pyramids

Contain motor tracts; decussation causes left cortex to control right body side and vice versa.

Inferior olivary nucleus

Relays proprioceptive impulses to the cerebellum.

Medulla reflex centers

Regulate heart rate, respiration, and vasoconstriction; control swallowing, coughing, vomiting, etc.

Medulla injury

Can be fatal or cause severe dysfunction.

Pons

Located above the medulla; connects spinal cord with brain and relays signals between cerebellum and cortex.

Pons function

Relays motor impulses for voluntary movement and helps control breathing.

Cranial nerves in pons

CN V–VII and vestibular branch of VIII.

Midbrain (mesencephalon)

Connects pons to diencephalon; conveys motor impulses and regulates visual/auditory reflexes.

Midbrain structures

Cerebral peduncles, substantia nigra, and red nuclei.

Midbrain cranial nerves

CN III (oculomotor) and CN IV (trochlear).

Reticular formation

Network of gray and white matter throughout brainstem; regulates muscle tone and consciousness.

Reticular activating system (RAS)

Alerts the cortex to sensory input and maintains wakefulness.

Cerebellum

Occupies inferior/posterior cranial cavity; has two hemispheres and a vermis.

Cerebellar peduncles

Three pairs connecting cerebellum to brainstem.

Cerebellar function

Coordinates skeletal muscle contractions, balance, posture, and muscle tone.

Cerebellar injury

Results in ataxia (loss of coordination).

Thalamus

Located superior to midbrain; relay station for all sensory input except smell.

Thalamic functions

Registers conscious awareness of pain, temperature, touch, and pressure; involved in cognition.

Hypothalamus

Below thalamus; major regulator of homeostasis with four regions: mammillary, tuberal, supraoptic, preoptic.

Hypothalamus functions

Regulates emotion, behavior, eating, drinking, body temperature, circadian rhythms, and consciousness.

Feeding and satiety centers

Control hunger and fullness sensations.

Thirst center

Monitors water balance and stimulates fluid intake.

Epithalamus

Contains pineal gland and habenular nuclei; superior/posterior to thalamus.

Pineal gland

Secretes melatonin, regulating sleep–wake (diurnal) cycles.

Habenular nuclei

Involved in emotional responses to odors.

Circumventricular organs (CVOs)

Regions lacking BBB that monitor blood chemistry.

Examples of CVOs

Hypothalamus, pineal gland, pituitary gland, and nearby structures.

CVO functions

Coordinate endocrine and autonomic activities; possible entry site for HIV into brain.

Cerebrum

The largest brain part; responsible for intelligence, memory, and voluntary actions.