03.03 Peripheral Nervous System

Module 03 — Coordination and Control · quiz worth 18 pts · 2026-10-20

The big idea Peripheral pathways carry sensory input and motor output; somatic reflexes and autonomic circuits organize fast responses without requiring conscious control.

Put every PNS question on an arrow

Direction and target distinguish sensory, somatic motor, and autonomic pathways.

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The PNS links receptors, CNS, and effectors

The peripheral nervous system is made up of the nerves that extend beyond the brain and spinal cord. It is divided into the somatic and autonomic nervous systems. Receptors detect a change; effectors respond to it. Afferent (sensory) neurons carry signals toward the spinal cord. Efferent (motor) neurons carry signals out to muscles.

Control. Somatic: Voluntary reactions; Autonomic: Involuntary reactions

Effector organs. Somatic: Skeletal muscles; Autonomic: Cardiac muscle, smooth muscle, blood vessels, glands

Neurons from CNS to effector. Somatic: One; Autonomic: Two

Effect on muscle. Somatic: Excites; Autonomic: Excites or inhibits

Simple receptors and complex receptors

The somatic system is made up of some cranial nerves and all spinal nerves, and its nerves contain both sensory and motor neurons. Receptors in the skin, eyes, mouth, nose, ears, and muscles detect the change first.

Simple receptors are fairly small and spread throughout the body; they handle information such as a temperature change on your skin. Complex receptors serve the special senses: taste, smell, hearing, balance, and vision. Each sensory organ is built to receive one kind of stimulus well.

The special senses route a stimulus to the brain

Smell: vapors enter the nasal cavity, which is lined with millions of olfactory sensors. These nerve cells turn the chemical message into a nerve impulse that travels to the olfactory bulbs, which send it to the brain.Taste: papillae on the tongue hold bundles of taste receptor cells called taste buds. The lesson video lists four basic flavors (sweet, salty, sour, bitter); current descriptions add umami (savory) as a fifth. The nose adds the finishing touches to flavor.Vision: the iris contracts and relaxes to control how much light enters. The lens focuses an upside-down image on the retina, the retina converts light to nerve impulses, the optic nerve carries them, and the brain turns the image right-side up.Hearing: sound passes the pinna and ear canal to the eardrum, which vibrates. The vibration crosses the three ossicles (the tiniest bones in the body) to the cochlea, whose cilia vibrate and stimulate the cochlear nerve.Skin: tactile nerve receptors sense pressure, vibration, temperature, and pain. The CNS analyzes that information and orders a reaction.
Flow chart of five senses. Smell: vapors, olfactory sensors in the nose, olfactory bulbs, brain. Taste: flavor, taste buds on papillae, nerve impulse, brain, with smell adding to flavor. Vision: light, iris adjusts the pupil, lens focuses the image, retina changes light to impulses, optic nerve, brain flips the image upright. Hearing: sound waves, pinna and ear canal, eardrum vibrates, ossicles, cochlea cilia vibrate, cochlear nerve to brain. Skin: tactile receptors, impulse to the CNS, CNS analyzes and orders a reaction.
Each row follows the lesson's Senses video from stimulus to the brain or CNS.
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A reflex arc favors speed

A reflex is an involuntary response to an internal or external stimulus. It is the one involuntary movement the somatic system controls. In the withdrawal reflex, the spinal cord, not the brain, processes the “my hand is on fire” signal and sends the “pull it away” response, which saves time. The signal also travels on to the brain, so you feel the pain, but the withdrawal does not wait for the brain to decide.

The withdrawal reflex uses five parts, in order: the receptor picks up the stimulus; the sensory neuron carries the impulse through the dorsal root into the spinal cord; an interneuron connects the afferent and efferent neurons; the motor neuron leaves through the ventral root; the effector (a muscle, gland, or organ) responds.

The knee-jerk is different. Tapping the patellar tendon stretches the quadriceps, and stretch receptors (muscle spindles) in the quadriceps send a sensory signal to the spinal cord. There the sensory neuron connects directly to the motor neuron for the quadriceps, with no interneuron in that link (a monosynaptic reflex). The quadriceps contracts and the lower leg kicks forward. The course video traces the signal from the patellar tendon and uses the five-part arc for every reflex; the five-part chain fits the withdrawal reflex.

Most reflexes are internal and unseen, such as digestion signals after you eat or a rise in blood pressure during exercise.

Withdrawal reflex arc for a hand on a hot surface: a receptor in the skin, a sensory neuron entering the spinal cord through the dorsal root, an interneuron in the spinal cord, a motor neuron leaving through the ventral root, and skeletal muscle as the effector that pulls the hand away.
Withdrawal reflex: receptor → sensory neuron → interneuron → motor neuron → effector. The spinal cord starts the withdrawal; the brain also receives the signal and becomes aware of the pain.

The autonomic system controls involuntary effectors

The autonomic system controls cardiac muscle, smooth muscle, and glands. The sympathetic division prepares the body for an emergency (“fight or flight”). The parasympathetic division brings the body back to homeostasis.

Eye. Sympathetic: Dilates pupil; Parasympathetic: Constricts pupil

Sweat gland. Sympathetic: Stimulates secretion; Parasympathetic: No effect

Heart. Sympathetic: Increases heart rate; Parasympathetic: Decreases heart rate

Blood vessels. Sympathetic: Constrict to raise blood pressure; Parasympathetic: Dilate to lower blood pressure (course chart; most vessels are mainly under sympathetic control)

Lungs. Sympathetic: Increases respiration; relaxes airways; Parasympathetic: Decreases respiration; constricts airways

Gastrointestinal tract. Sympathetic: Inhibits movement (slows digestion); Parasympathetic: Stimulates movement

Fat cells. Sympathetic: Stimulates breakdown; Parasympathetic: No effect

Mouth (salivary glands). Sympathetic: Inhibits salivation; Parasympathetic: Stimulates saliva

Liver. Sympathetic: Stimulates release of glucose; Parasympathetic: Inhibits release of glucose

Bladder. Sympathetic: Relaxes; Parasympathetic: Contracts

This is the course's simplified comparison. The first seven rows come from the lesson's comparison chart; mouth, liver, and bladder come from its nervous-system diagram, which the guided notes ask you to add.

Autonomic output uses a two-neuron chain

Ganglia are clusters of cell bodies in the PNS. A preganglionic motor neuron sits in the CNS and sends its fiber into a ganglion. A postganglionic motor neuron runs from the ganglion to the body organ. In both divisions, the preganglionic neuron releases acetylcholine at the ganglion.

Sympathetic: starts in the thoracic and lumbar spinal cord. Many of its ganglia form a chain just outside the spinal cord, which lets a strong emergency signal reach many organs together (the course calls this a whole-body response). Most of its postganglionic neurons release norepinephrine; those to sweat glands release acetylcholine instead.Parasympathetic: starts in nuclei of the brain stem, whose fibers leave in cranial nerves such as the vagus nerve (to the heart, lungs, digestive tract, and liver), and in the sacral spinal cord. Its ganglia are terminal ganglia near or within the target organ, so its effects are localized to specific organs. Its postganglionic neurons release acetylcholine.

A neurotransmitter's effect depends on the receptor and target tissue, not on the chemical alone. Norepinephrine can excite or inhibit; acetylcholine stimulates digestion but slows the heart.

Wording note: the course lesson describes parasympathetic ganglia as grouped along cranial nerves and calls norepinephrine the sympathetic transmitter without exceptions. If a course question uses that wording, it is pointing to the parasympathetic or sympathetic division; the placement and exception above are the anatomically accurate version.

Three lanes. Somatic: one motor neuron from the spinal cord to skeletal muscle. Sympathetic: a preganglionic neuron releasing acetylcholine runs from the thoracic and lumbar spinal cord to chain ganglia beside the cord; a postganglionic neuron then carries the signal to many organs and releases mostly norepinephrine (acetylcholine at sweat glands). Parasympathetic: a long preganglionic neuron from the brain stem or sacral spinal cord releases acetylcholine at a terminal ganglion near or in the target organ; a short postganglionic neuron releases acetylcholine at that specific organ.
Somatic output uses one motor neuron; autonomic output uses two, with a ganglion between them.
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Disorders of the peripheral nervous system

Autoimmune disorders, injury, or environmental factors can change the pathway the nerve signal travels.

Guillain-Barré syndrome is an autoimmune condition in which the body attacks nerves of the PNS, usually set off by an illness or stress to the body. Mild cases can bring severe pain or limited use of a body part; some cases make breathing difficult enough that the patient needs help.Diabetes is not itself a PNS disorder, but too much glucose in the blood can damage peripheral nerves, causing numbness or paralysis. Damage to peripheral blood vessels can mean blood no longer reaches the extremities, which can require amputation of toes or feet.

The lesson names these supports when the damage is known: treatments that help the nerve regrow or be stimulated, physical therapy, medications such as steroids that reduce inflammation, and devices that stimulate the nerve or block neurotransmitters to lower pain.

Practice the peripheral routes

Track direction, neuron count, and target. Some questions use numbered diagrams: name the numbered part.

Final checklist

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