The nervous system is the body’s primary control and communication network. It is organized structurally into the central nervous system (CNS) and peripheral nervous system (PNS), and functionally into somatic and autonomic divisions.
Structural Organization
Central Nervous System (CNS)
The CNS consists of the brain and spinal cord, which serve as the integrating and command centers.
Brain:
- Location: Cranial cavity
- Weight: ~1300-1400 g (adult)
- Components: Cerebrum, cerebellum, brainstem
- Protected by: Skull, meninges, blood-brain barrier
Spinal Cord:
- Location: Vertebral canal (C1-L1/L2)
- Length: ~45 cm (adult)
- Segments: 31 (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal)
- Protected by: Vertebral column, meninges
Peripheral Nervous System (PNS)
The PNS consists of nerves and ganglia outside the CNS that connect it to the rest of the body.
Cranial nerves: 12 pairs arising from the brain Spinal nerves: 31 pairs arising from the spinal cord
| Component | Structure | Function |
|---|---|---|
| Nerves | Bundles of axons with connective tissue sheaths | Conduct impulses between CNS and periphery |
| Ganglia | Clusters of neuron cell bodies outside CNS | Relay and processing stations |
| Plexuses | Networks of intersecting nerves | Redistribution of nerve fibers |
Functional Organization
Afferent Division (Sensory)
Carries information from the periphery to the CNS.
Sensory modalities:
| Modality | Receptor Type | Examples |
|---|---|---|
| Somatic sensory | Skin, muscles, joints | Touch, pain, temperature, proprioception |
| Visceral sensory | Internal organs | Stretch, pain, chemical changes |
| Special senses | Specialized organs | Vision, hearing, taste, smell, balance |
Efferent Division (Motor)
Carries commands from the CNS to effectors (muscles and glands).
| Component | Effector | Control |
|---|---|---|
| Somatic motor | Skeletal muscle | Voluntary |
| Autonomic (visceral) motor | Smooth muscle, cardiac muscle, glands | Involuntary |
Autonomic Nervous System
The autonomic nervous system (ANS) regulates involuntary physiological functions.
| Feature | Sympathetic | Parasympathetic | Enteric |
|---|---|---|---|
| Origin | Thoracolumbar (T1-L2) | Craniosacral (CN III, VII, IX, X; S2-S4) | GI tract wall |
| Preganglionic length | Short | Long | Intramural |
| Postganglionic length | Long | Short | Intramural |
| Ganglia location | Paravertebral, prevertebral | Near or within target organs | Within GI wall |
| Neurotransmitter (postganglionic) | Norepinephrine (mostly) | Acetylcholine | Various |
| Response | Fight or flight | Rest and digest | Local reflexes |
Sympathetic Division
Preganglionic neurons: Intermediolateral cell column (T1-L2)
Ganglia:
- Sympathetic chain (paravertebral): 22-23 pairs
- Prevertebral (collateral): Celiac, superior mesenteric, inferior mesenteric, aorticorenal
Distribution:
- Every spinal nerve receives sympathetic fibers via gray rami communicantes
- Splanchnic nerves: Preganglionic fibers to prevertebral ganglia
Parasympathetic Division
Cranial outflow:
- CN III (oculomotor): Ciliary ganglion → pupil constriction, lens accommodation
- CN VII (facial): Pterygopalatine, submandibular ganglia → lacrimal, salivary glands
- CN IX (glossopharyngeal): Otic ganglion → parotid gland
- CN X (vagus): Extensive distribution to thoracic and abdominal viscera
Sacral outflow:
- S2-S4: Pelvic splanchnic nerves → pelvic ganglia → pelvic viscera
Enteric Nervous System
The enteric nervous system is a semi-autonomous network within the GI tract wall.
Components:
- Myenteric plexus (Auerbach): Between circular and longitudinal muscle layers
- Submucosal plexus (Meissner): Within the submucosa
Functions:
- Motility (peristalsis, segmentation)
- Secretion
- Blood flow regulation
- Local reflexes independent of CNS input
Cells of the Nervous System
Neurons
Neurons are excitable cells specialized for electrical and chemical signaling.
| Component | Function |
|---|---|
| Cell body (soma) | Metabolic center, contains nucleus |
| Dendrites | Receive signals from other neurons |
| Axon | Conducts action potentials away from cell body |
| Axon hillock | Trigger zone for action potentials |
| Synaptic terminals | Release neurotransmitters |
Classification by structure:
| Type | Description | Example |
|---|---|---|
| Unipolar | Single process (peripheral + central) | Sensory neurons (dorsal root ganglia) |
| Bipolar | Two processes (dendrite + axon) | Retinal neurons, olfactory epithelium |
| Multipolar | Multiple dendrites, one axon | Most CNS neurons, motor neurons |
Glial Cells
Glial cells outnumber neurons 10:1 and provide support, insulation, and protection.
CNS glia:
| Cell | Function |
|---|---|
| Astrocytes | Blood-brain barrier, metabolism, ion buffering, scar formation |
| Oligodendrocytes | Myelination of CNS axons |
| Microglia | Immune surveillance, phagocytosis |
| Ependymal cells | CSF production, ventricular lining |
PNS glia:
| Cell | Function |
|---|---|
| Schwann cells | Myelination of PNS axons |
| Satellite cells | Support of sensory and autonomic ganglia |
Meninges
The meninges are three connective tissue layers that cover the brain and spinal cord.
| Layer | Description | Space |
|---|---|---|
| Dura mater | Thick, fibrous outer layer | Epidural space (between dura and bone) |
| Arachnoid mater | Delicate, web-like middle layer | Subdural space (between dura and arachnoid) |
| Pia mater | Thin, vascular inner layer | Subarachnoid space (between arachnoid and pia; contains CSF) |
Ventricular System and CSF
The ventricular system is a network of cavities within the brain that produces and circulates cerebrospinal fluid.
| Ventricle | Location | Connections |
|---|---|---|
| Lateral ventricles (2) | Cerebral hemispheres | Interventricular foramen (Monro) → third ventricle |
| Third ventricle | Diencephalon | Cerebral aqueduct (Sylvius) → fourth ventricle |
| Fourth ventricle | Brainstem (pons/medulla) | Median/lateral apertures → subarachnoid space |
CSF production: ~500 mL/day (by choroid plexus) CSF volume: ~150 mL total at any time
Blood-Brain Barrier
The blood-brain barrier (BBB) regulates the passage of substances from the blood into the brain.
Components:
- Capillary endothelial cells (tight junctions)
- Basement membrane
- Pericytes
- Astrocyte foot processes
Permeability:
- Lipid-soluble substances: Cross freely
- Glucose: Via GLUT1 transporter
- Amino acids: Via specific transporters
- Proteins, large molecules: Very limited
- Most drugs: Limited unless lipid-soluble