The Peripheral Nervous System & Reflex Activity
Sensory receptors, nerves, the 12 cranial nerves, spinal nerves and plexuses, reflexes
By the end of this unit you can…
- ✓Classify sensory receptors by stimulus type, location and structure
- ✓Describe the structure of a nerve and how peripheral nerves regenerate
- ✓Name the 12 cranial nerves, their numbers, type and main functions
- ✓Describe spinal nerves, rami and the four major plexuses and their key nerves
- ✓Explain dermatomes, the components of a reflex arc and the stretch, flexor and plantar reflexes
Key terms
Practice →1Sensory receptors
The PNS includes all neural structures outside the brain and spinal cord: sensory receptors, peripheral nerves and their ganglia, and motor endings. Sensation is awareness of a change; perception is conscious interpretation of its meaning.
| Classified by stimulus | Detects |
|---|---|
| Mechanoreceptors | Touch, pressure, vibration, stretch |
| Thermoreceptors | Temperature changes |
| Photoreceptors | Light (retina) |
| Chemoreceptors | Chemicals in solution (smell, taste, blood chemistry) |
| Nociceptors | Potentially damaging stimuli → pain |
| Classified by location | Where / what |
|---|---|
| Exteroceptors | Near the body surface — stimuli from outside (touch, pain, special senses) |
| Interoceptors (visceroceptors) | Internal viscera and blood vessels — stretch, chemicals, discomfort |
| Proprioceptors | Skeletal muscles, tendons, joints, ligaments — body position and movement (muscle spindles, tendon organs) |
Structurally, general sensory receptors are unencapsulated (free) nerve endings — pain, temperature, light touch (tactile discs, hair follicle receptors) — or encapsulated endings: tactile (Meissner) corpuscles, lamellated (Pacinian) corpuscles, bulbous (Ruffini) endings, muscle spindles (detect muscle stretch) and tendon organs (detect tendon tension).
Adaptation: receptors decrease their response to a constant stimulus — phasic receptors adapt quickly (you stop feeling your clothes); tonic receptors adapt slowly or not at all (nociceptors, proprioceptors).
2Nerve structure & regeneration
| Wrapping | Surrounds |
|---|---|
| Endoneurium | Each individual axon |
| Perineurium | A fascicle (bundle of axons) |
| Epineurium | The whole nerve |
Nerves are classified by direction: mixed nerves (both sensory and motor — all spinal nerves and most cranial nerves), sensory (afferent) nerves and motor (efferent) nerves.
Peripheral axons can regenerate if the cell body is intact: the distal part degenerates (Wallerian degeneration), macrophages clean up, and Schwann cells form a regeneration tube that guides the regrowing axon (~1–5 mm/day). CNS axons rarely regenerate — oligodendrocytes don't form guiding tubes and they secrete growth-inhibiting proteins, and astrocytes form scar tissue.
3The cranial nerves
12 pairs of cranial nerves arise from the brain and are numbered (Roman numerals) from anterior to posterior. Most serve the head and neck; only the vagus (X) extends into the thorax and abdomen.
| # | Nerve | Type | Main function |
|---|---|---|---|
| I | Olfactory | Sensory | Smell |
| II | Optic | Sensory | Vision |
| III | Oculomotor | Motor | Most eye movements; pupil constriction; lens shape; raises eyelid |
| IV | Trochlear | Motor | Superior oblique eye muscle |
| V | Trigeminal | Both | Sensation of the face; muscles of chewing (mastication) |
| VI | Abducens | Motor | Lateral rectus — abducts the eye |
| VII | Facial | Both | Facial expression; taste (anterior 2/3 of tongue); lacrimal and salivary glands |
| VIII | Vestibulocochlear | Sensory | Hearing and balance |
| IX | Glossopharyngeal | Both | Taste (posterior 1/3), swallowing, gag reflex; parotid gland; chemo/baroreceptors |
| X | Vagus | Both | Parasympathetic to thoracic & abdominal organs; swallowing, speech; carries ~90% of all parasympathetic preganglionic fibers in the body |
| XI | Accessory | Motor | Trapezius and sternocleidomastoid |
| XII | Hypoglossal | Motor | Tongue movements |
4Spinal nerves & plexuses
31 pairs of spinal nerves — all mixed — named for their point of exit: 8 cervical (C1–C8), 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal. (There are 8 cervical nerves but only 7 cervical vertebrae: C1 exits above the atlas.)
Each spinal nerve forms from a dorsal (sensory) root and a ventral (motor) root, then quickly splits into a dorsal ramus (serves the back) and a larger ventral ramus (serves the limbs and the anterolateral trunk). Except for T2–T12 (which form the intercostal nerves), ventral rami interlace to form plexuses.
| Plexus | Origin | Key nerves & what they serve |
|---|---|---|
| Cervical | C1–C4 | Phrenic nerve (C3–C5) → diaphragm — the most important nerve of the plexus |
| Brachial | C5–T1 | Axillary (deltoid, teres minor), musculocutaneous (arm flexors), median (most forearm flexors, thumb), ulnar (wrist flexion, intrinsic hand muscles — “funny bone”), radial (all extensors of the upper limb) |
| Lumbar | L1–L4 | Femoral (quadriceps — knee extension; anterior thigh skin), obturator (adductors) |
| Sacral | L4–S4 | Sciatic nerve — the largest, thickest nerve in the body; splits into the tibial (posterior leg, foot) and common fibular (anterior & lateral leg) nerves; superior & inferior gluteal nerves |
A dermatome is the area of skin innervated by the cutaneous branches of a single spinal nerve. Mapping lost sensation to dermatomes helps locate spinal cord or nerve damage. (Shingles rashes follow dermatomes.)
5Reflex activity
A reflex is a rapid, predictable, involuntary motor response to a stimulus. Somatic reflexes activate skeletal muscle; autonomic (visceral) reflexes activate smooth muscle, cardiac muscle and glands. Reflexes may be inborn or learned (acquired).
- 1Receptor — site of the stimulus.
- 2Sensory neuron — carries afferent impulses to the CNS.
- 3Integration center — one synapse (monosynaptic) or several with interneurons (polysynaptic) in the CNS.
- 4Motor neuron — carries efferent impulses to the effector.
- 5Effector — muscle or gland that responds.
| Reflex | How it works | Example |
|---|---|---|
| Stretch reflex | Monosynaptic: a stretched muscle spindle triggers contraction of the same muscle (and reciprocal inhibition of its antagonist). Maintains muscle tone and posture | Patellar (knee-jerk) reflex — tests L2–L4 |
| Tendon reflex | Polysynaptic: tendon organs sense excessive tension → muscle relaxes (protects tendons) | — |
| Flexor (withdrawal) reflex | Polysynaptic, ipsilateral: painful stimulus → withdraw limb | Stepping on a tack |
| Crossed-extensor reflex | Flexor reflex on one side + extension of the opposite limb for balance | Shifting weight to the other leg |
| Plantar reflex | Stroking the sole curls the toes down (tests corticospinal tracts, L4–S2) | — |