Control & communication · Topic 8 of 18

Brain, Spinal Cord & Peripheral Nervous System

Brain & Spinal Cord

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Anatomy · brain & spinal cord

Brain & Spinal Cord

The cerebrum thinks and commands, the cerebellum coordinates, the brainstem runs vital functions, and the spinal cord carries signals and handles reflexes.

01 · CerebrumCerebrum = thinking + voluntary movement

02 · CerebellumCerebellum = balance + coordination

03 · MedullaMedulla = heart rate, BP, breathing

What it is

Spinal reflex: fast and automatic, handled in the cord, not the brain

Parts & jobs

4 brain regions: cerebrum, diencephalon, brainstem, cerebellum must know

Cerebrum = largest; cortex does thinking, sensing, voluntary movement must know

Cerebellum = balance and coordination (smooth, accurate movement) must know

Medulla oblongata = vital centers: heart rate, blood pressure, breathing must know

Spinal cord = two-way path between brain and body + reflex center must know

Frontal lobe: voluntary movement, planning/judgment, speech (Broca's area)

Parietal lobe: body sensation (touch, temperature, pain, position)

Temporal lobe: hearing and memory (also smell)

Occipital lobe: vision

Thalamus = sensory relay station to the cortex (all senses but smell)

Hypothalamus = homeostasis: temperature, hunger, thirst; runs ANS + pituitary

Brainstem = midbrain + pons + medulla; links brain to spinal cord

Meninges, outside in: dura → arachnoid → pia mater

CSF cushions the brain and cord (in ventricles + subarachnoid space)

Step by step

Reflex arc: receptor → sensory neuron → integration center → motor neuron → effector

Worked examples

Knee-jerk = stretch reflex (tap stretches the muscle → it contracts)

Don't confuse

Gray = cell bodies, white = myelinated axons; brain gray outside, cord gray inside

Cerebrum or cerebellum?

Commonly confused

	Cerebrum	Cerebellum
Where	Top of the brain; two hemispheres	Back of the brain, behind the brainstem
Size	Largest part of the brain	About 10% of brain mass
Main job	Thinking, sensation, voluntary movement commands	Balance, coordination, smooth movement
Name means	"Brain"	"Little brain"

More detail

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Cord horns: posterior = sensory, anterior = motor

Ascending tracts = sensory up; descending tracts = motor down

Cord ends near the upper lumbar spine; roots below = cauda equina

Hippocampus + amygdala (limbic system): memory and emotion

Basal nuclei: start wanted movement, block unwanted movement

"Left-brained vs right-brained" people is a myth; the hemispheres work together

Pearl

Cerebrum decides, cerebellum smooths it out, brainstem keeps you alive.

Sources · OpenStax Anatomy & Physiology 2e §13.2 The Central Nervous System · OpenStax Anatomy & Physiology 2e §13.3 Circulation and the Central Nervous System · OpenStax Anatomy & Physiology 2e §14.3 Motor Responses (reflexes) · OpenStax Anatomy & Physiology 2e §14.1 Sensory Perception (olfaction) · OpenStax Anatomy & Physiology 2e §12.1 Basic Structure and Function of the Nervous System · Union Test Prep — HESI A2 Anatomy & Physiology study guide, p. 3: brain and spinal cord · Union Test Prep — HESI Anatomy and Physiology Review: A Comprehensive Guide (parts of the brain, reflex arcs) · NurseHub — HESI A2 Anatomy & Physiology review course (lessons: central nervous system, reflexes) · Built from the verified NurseSavvy HESI A2 fact map (facts reviewed 2026-10-16). HESI is a trademark of Elsevier; NurseSavvy is not affiliated with Elsevier.

Peripheral & Autonomic Nervous System

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Anatomy · peripheral & autonomic

Peripheral & Autonomic Nervous System

The PNS is every nerve outside the brain and spinal cord; its autonomic part runs involuntary organs through sympathetic (fight-or-flight) and parasympathetic (rest-and-digest) divisions.

01 · Two systemsCNS = brain + cord; PNS = the nerves

02 · Who's in controlSomatic = voluntary; autonomic = involuntary

03 · Two divisionsSympathetic = fight or flight

What it is

CNS = brain + spinal cord; PNS = all the nerves and ganglia outside them must know

Sympathetic: faster heart, wide pupils, open airways, sweat, slow digestion

Parasympathetic: slower heart, small pupils, narrower airways, more digestion

Dual innervation: most organs get both divisions, with opposite effects

PNS: nerve = axon bundle, ganglion = cell-body cluster (CNS: tract, nucleus)

Parts & jobs

Vagus nerve (CN X) = main parasympathetic nerve to heart, lungs, gut

Spinal nerves are mixed: sensory in the dorsal root, motor out the ventral root

Know by heart

12 pairs of cranial nerves (I–XII) from the brain; mostly head and neck

31 pairs of spinal nerves, named for their spinal cord level

Don't confuse

Somatic = voluntary (skeletal muscle); autonomic = involuntary (smooth, cardiac, glands) must know

Sympathetic = fight or flight; parasympathetic = rest and digest must know

Afferent (sensory) = into the CNS; efferent (motor) = out to muscles and glands

Sympathetic or parasympathetic?

Commonly confused

	Sympathetic	Parasympathetic
Nickname	Fight or flight	Rest and digest
Heart rate	Speeds up	Slows down
Pupils	Dilate	Constrict
Airways (bronchioles)	Dilate	Constrict
Digestion	Slows; blood sent to skeletal muscle	Increases
Leaves the CNS from	Thoracic + upper lumbar cord	Brainstem (e.g. vagus nerve) + sacral cord

Mix-ups to avoid

Both divisions are autonomic; skeletal muscle is somatic, not autonomic

More detail

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Spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal

Sympathetic = thoracolumbar; parasympathetic = craniosacral

Autonomic = 2-neuron chain: preganglionic (CNS → ganglion) → postganglionic (→ organ)

Parasympathetic → acetylcholine; most sympathetic → norepinephrine

Sense nerves: I olfactory (smell), II optic (sight), VIII hearing + balance

Enteric nervous system = nerve network in the gut wall

Pearl

Running from danger speeds the heart and stops digestion; resting after a meal slows the heart and starts digestion.

Sources · OpenStax Anatomy & Physiology 2e §12.1 Basic Structure and Function of the Nervous System · OpenStax Anatomy & Physiology 2e §15.1 Divisions of the Autonomic Nervous System · OpenStax Anatomy & Physiology 2e §15.2 Autonomic Reflexes and Homeostasis · OpenStax Anatomy & Physiology 2e §13.4 The Peripheral Nervous System · OpenStax Biology 2e §35.4 The Peripheral Nervous System (sympathetic effects: bronchi dilate) · Pearson Channels A&P study guide — Autonomic nervous system structure and function (effects table) · Union Test Prep — HESI A2 Anatomy & Physiology study guide, p. 3: CNS, PNS and autonomic divisions · Union Test Prep — HESI Anatomy and Physiology Review: A Comprehensive Guide (study tips: cranial nerves) · NurseHub — HESI A2 Anatomy & Physiology review course (lesson: the peripheral nervous system) · Built from the verified NurseSavvy HESI A2 fact map (facts reviewed 2026-10-16). HESI is a trademark of Elsevier; NurseSavvy is not affiliated with Elsevier.

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