The peripheral nervous system (PNS) connects your brain and spinal cord to the rest of your body. It's made up of nerves, ganglia, and receptors that work together to send and receive signals, allowing you to move, feel, and react to your environment.
This network is divided into somatic and autonomic divisions. The somatic system controls voluntary movements, while the autonomic system manages involuntary functions like heart rate and digestion. Understanding these components helps explain how your body responds to various stimuli.
Peripheral Nervous System
Structures of the Peripheral Nervous System
The PNS is built from four main types of structures: nerves, ganglia, receptors, and effectors. Each plays a distinct role in carrying information to or from the central nervous system (CNS).
Nerves are bundles of axons wrapped in layers of connective tissue. From outermost to innermost, these layers are:
- Epineurium: tough outer covering that surrounds the entire nerve
- Perineurium: wraps around each fascicle (bundle of axons) within the nerve
- Endoneurium: delicate tissue surrounding each individual axon
Nerves are classified by where they originate:
- Cranial nerves (12 pairs) originate from the brain
- Spinal nerves (31 pairs) originate from the spinal cord
- Peripheral nerves is a general term for any nerve extending outside the brain and spinal cord, including branches of cranial and spinal nerves
Ganglia are clusters of neuron cell bodies located outside the CNS. (Cell body clusters inside the CNS are called nuclei, not ganglia.)
- Sensory ganglia house cell bodies of sensory neurons. Examples include the dorsal root ganglia along the spinal cord and the trigeminal ganglia associated with cranial nerve V.
- Autonomic ganglia house cell bodies of autonomic motor neurons. Examples include the sympathetic chain ganglia (running alongside the vertebral column) and the ciliary ganglia (near the eye).
Receptors are specialized structures that detect stimuli and generate nerve impulses:
- Sensory receptors in the skin detect touch, pressure, temperature, and pain. Meissner's corpuscles respond to light touch, while Pacinian corpuscles respond to deep pressure and vibration.
- Proprioceptors in muscles and tendons detect body position and movement. Muscle spindles monitor muscle stretch, and Golgi tendon organs monitor tension in tendons.
Effectors are the structures that carry out responses to nerve impulses:
- Muscles contract when stimulated by motor neurons (skeletal, smooth, and cardiac muscle)
- Glands secrete substances when stimulated by autonomic neurons (e.g., salivary glands, sweat glands)
Somatic vs. Autonomic Nervous Systems
The PNS has two major functional divisions, plus a third semi-independent system in the gut.
Somatic Nervous System provides voluntary control of skeletal muscles.
- Afferent (sensory) components carry information from receptors to the CNS
- Efferent (motor) components carry commands from the CNS to skeletal muscles
- This system enables conscious movement (walking, grasping) and conscious sensation (touch, pain)
- A single motor neuron extends from the CNS directly to the skeletal muscle with no ganglion in between
Autonomic Nervous System (ANS) provides involuntary control of smooth muscle, cardiac muscle, and glands. It has two main divisions:
- Sympathetic division: activates "fight or flight" responses. It increases heart rate, dilates pupils, and redirects blood flow to skeletal muscles.
- Parasympathetic division: promotes "rest and digest" functions. It slows heart rate, constricts pupils, and stimulates digestion.
- Most organs receive dual innervation from both divisions, which typically have opposing effects. This allows fine-tuned control of organ function.
- Unlike the somatic system, the ANS uses a two-neuron chain: a preganglionic neuron synapses in an autonomic ganglion, and then a postganglionic neuron continues to the target organ.
Enteric Nervous System is sometimes called the "brain of the gut." It's a subdivision of the ANS embedded in the walls of the gastrointestinal tract.
- Regulates motility (peristalsis), secretion (digestive enzymes), and local blood flow
- Contains two major nerve plexuses: the myenteric plexus (controls motility) and the submucosal plexus (controls secretion and blood flow)
- Can function independently of the CNS, though the sympathetic and parasympathetic divisions can modulate its activity
Cranial Nerves and Functions
There are 12 pairs of cranial nerves. Each is identified by a Roman numeral and a name. A classic mnemonic for the names is: Oh, Oh, Oh, To Touch And Feel Very Green Vegetables, AH (first letter of each nerve in order).
You should also know whether each nerve is sensory (S), motor (M), or both (B). A mnemonic for that: Some Say Marry Money, But My Brother Says Big Brains Matter More.
- Olfactory (I) [S] — carries smell information from the nasal cavity to the brain
- Optic (II) [S] — carries visual information from the retina to the brain
- Oculomotor (III) [M] — controls most eye movements, pupillary constriction, and lens accommodation
- Trochlear (IV) [M] — innervates the superior oblique muscle (moves the eye downward and laterally)
- Trigeminal (V) [B] — provides sensation from the face; motor control of chewing muscles
- Abducens (VI) [M] — innervates the lateral rectus muscle (abducts the eye)
- Facial (VII) [B] — controls facial expression, taste from the anterior two-thirds of the tongue, salivation, and tear production (lacrimation)
- Vestibulocochlear (VIII) [S] — carries hearing (cochlear branch) and balance (vestibular branch) information
- Glossopharyngeal (IX) [B] — mediates taste from the posterior one-third of the tongue, swallowing, and salivation from the parotid gland
- Vagus (X) [B] — the major parasympathetic nerve; controls thoracic and abdominal organ function, swallowing, and speech. It's the longest cranial nerve and the most widely distributed.
- Accessory (XI) [M] — controls the sternocleidomastoid and trapezius muscles (turning the head, shrugging shoulders)
- Hypoglossal (XII) [M] — controls tongue muscles for speech and swallowing
Composition of Spinal Nerves
31 pairs of spinal nerves exit the spinal cord. All spinal nerves are mixed nerves, meaning they carry both sensory and motor fibers.
They're organized by vertebral region:
- 8 cervical (C1–C8)
- 12 thoracic (T1–T12)
- 5 lumbar (L1–L5)
- 5 sacral (S1–S5)
- 1 coccygeal (Co1)
Note that there are 8 cervical nerves but only 7 cervical vertebrae. C1–C7 exit above their corresponding vertebra, while C8 exits below the 7th cervical vertebra. All nerves below C8 exit below their corresponding vertebra.
Formation of a spinal nerve:
Each spinal nerve forms from two roots that merge near the intervertebral foramen:
- Dorsal root carries afferent (sensory) fibers into the spinal cord. The dorsal root ganglion sits on this root and contains the cell bodies of sensory neurons.
- Ventral root carries efferent (motor) fibers out of the spinal cord.
Nerve plexuses are networks where ventral rami (branches) of spinal nerves intermingle and redistribute fibers. This means that a single peripheral nerve leaving a plexus typically contains fibers from multiple spinal nerve levels.
- Cervical plexus (C1–C4): innervates the neck and upper shoulder; includes the phrenic nerve (C3–C5), which controls the diaphragm
- Brachial plexus (C5–T1): innervates the upper limb; gives rise to major nerves like the median, ulnar, radial, and musculocutaneous nerves
- Lumbar plexus (L1–L4): innervates the lower abdomen, anterior thigh, and medial leg; includes the femoral nerve
- Sacral plexus (L4–S4): innervates the posterior thigh, most of the lower leg, and the foot; includes the sciatic nerve, the largest nerve in the body
Thoracic spinal nerves (T2–T12) generally do not form plexuses. They run as intercostal nerves between the ribs.
Nerve Conduction and Synaptic Transmission
Signals travel along peripheral nerves thanks to the structure of myelinated axons and the chemistry of synapses.
Schwann cells are the glial cells of the PNS that wrap around axons to form the myelin sheath. This fatty insulation dramatically increases the speed of nerve impulse conduction. (In the CNS, oligodendrocytes perform this role instead.)
Nodes of Ranvier are small gaps between adjacent Schwann cells where the axon membrane is exposed. Action potentials "jump" from node to node in a process called saltatory conduction, which is much faster than continuous conduction along an unmyelinated fiber.
At the end of an axon, signals must cross a synapse to reach the next neuron or target cell:
- An action potential arrives at the axon terminal (synaptic knob)
- Voltage-gated calcium channels open, and calcium ions enter the terminal
- Calcium triggers synaptic vesicles to fuse with the membrane and release neurotransmitters into the synaptic cleft
- Neurotransmitters bind to receptors on the postsynaptic cell, triggering a response
The neuromuscular junction (NMJ) is a specialized synapse between a motor neuron and a skeletal muscle fiber. The neurotransmitter at the NMJ is acetylcholine (ACh).
Reflex arcs are neural pathways that produce rapid, automatic responses to stimuli without requiring conscious thought. A basic reflex arc includes five components: receptor, sensory neuron, integration center (often in the spinal cord), motor neuron, and effector.
Disorders of the Peripheral Nervous System
Peripheral neuropathy refers to damage or dysfunction of peripheral nerves. It can result from diabetes (the most common cause), trauma, infections, toxins, or autoimmune conditions. Symptoms depend on which nerve fibers are affected:
- Sensory fiber damage: pain, numbness, tingling (often starting in the hands and feet)
- Motor fiber damage: muscle weakness, cramping, or loss of coordination
- Autonomic fiber damage: abnormal blood pressure, digestive problems, or excessive/reduced sweating