Chapter 7: Nervous System – Spinal Cord and Peripheral Nervous System

Spinal Cord

Structure and Location

The spinal cord is a major component of the central nervous system (CNS), serving as a conduit for information between the brain and the rest of the body. It extends from the foramen magnum of the skull to the first or second lumbar vertebra.

• Cauda equina: A collection of spinal nerves at the inferior end of the spinal cord.

• The spinal cord provides a two-way conduction pathway to and from the brain.

Example: Damage to the spinal cord can result in loss of sensation or motor function below the level of injury.

Gray Matter of the Spinal Cord and Spinal Roots

The spinal cord contains regions of gray matter and white matter. The gray matter is centrally located and shaped like a butterfly or the letter 'H' in cross-section.

• Dorsal (posterior) horns: Receive information from sensory neurons.

• Anterior (ventral) horns: House motor neurons of the somatic (voluntary) nervous system and send information out via the ventral root.

Additional info: The dorsal root contains sensory fibers, while the ventral root contains motor fibers.

White Matter of the Spinal Cord

White matter surrounds the gray matter and consists of myelinated fiber tracts that transmit signals up and down the spinal cord.

• Three regions: dorsal, lateral, and ventral columns.

• Sensory (afferent) tracts: Conduct impulses toward the brain.

• Motor (efferent) tracts: Carry impulses from the brain to skeletal muscles.

Example: The dorsal columns carry touch and proprioceptive information to the brain.

Cross-Sectional Anatomy of the Spinal Cord

The spinal cord is protected by three layers of meninges: dura mater (outermost), arachnoid mater (middle), and pia mater (innermost). The central canal runs through the center, containing cerebrospinal fluid.

• Dorsal root ganglion: Contains cell bodies of sensory neurons.

• Central canal: Contains cerebrospinal fluid.

• Pia mater: Delicate inner layer adhering to the spinal cord.

• Arachnoid mater: Web-like middle layer.

• Dura mater: Tough outer layer.

Additional info: The arrangement of gray and white matter allows for efficient processing and transmission of neural signals.

Peripheral Nervous System (PNS)

Overview

The Peripheral Nervous System (PNS) consists of nerves and ganglia outside the CNS. It connects the CNS to limbs and organs, facilitating communication throughout the body.

• Nerves: Bundles of axons found outside the CNS.

• Ganglia: Clusters of neuron cell bodies located outside the CNS.

Example: The sciatic nerve is the largest nerve in the body and is part of the PNS.

Structure of a Nerve

Nerves are classified based on the direction of impulse transmission:

• Mixed nerves: Contain both sensory and motor fibers.

• Sensory (afferent) nerves: Carry impulses toward the CNS.

• Motor (efferent) nerves: Carry impulses away from the CNS.

Additional info: Most nerves in the body are mixed nerves, containing both types of fibers.

Cranial Nerves

There are 12 pairs of cranial nerves that primarily serve the head and neck. Most are mixed nerves, but three are purely sensory.

Nerve	Type	Main Function
Olfactory (I)	Sensory	Smell
Optic (II)	Sensory	Vision
Oculomotor (III)	Motor	Most eye movement
Trochlear (IV)	Motor	Moves eye
Trigeminal (V)	Both	Face sensation, mastication
Abducens (VI)	Motor	Abducts the eye
Facial (VII)	Both	Facial expression, taste
Vestibulocochlear (VIII)	Sensory	Hearing, balance
Glossopharyngeal (IX)	Both	Taste, gag reflex
Vagus (X)	Both	Gag reflex, parasympathetic innervation
Accessory (XI)	Motor	Shoulder shrug
Hypoglossal (XII)	Motor	Swallowing, speech

Additional info: The vagus nerve is unique in that it extends beyond the head and neck to the thoracic and abdominal cavities.

Spinal Nerves and Plexuses

There are 31 pairs of spinal nerves, named for the region of the spinal cord from which they arise. Spinal nerves are formed by the combination of the ventral and dorsal roots.

• Plexuses: Networks of nerves serving motor and sensory needs of the limbs.

• Main plexuses: Cervical, Brachial, Lumbar, Sacral.

Plexus	Important Nerves	Body Areas Served	Clinical Significance
Cervical	Phrenic	Diaphragm, skin and muscles of shoulder and neck	Paralysis of diaphragm impairs breathing
Brachial	Axillary, Radial, Median, Musculocutaneous, Ulnar	Shoulder, arm, forearm, hand	Injury can cause loss of movement or sensation in upper limb
Lumbar	Femoral, Obturator	Lower abdomen, anterior and medial thigh	Impaired hip flexion or thigh adduction
Sacral	Sciatic, Superior and Inferior Gluteal	Lower trunk, posterior thigh, leg, foot	Sciatic nerve injury causes foot drop

Additional info: Damage to plexuses can result in characteristic motor and sensory deficits.

Dermatomes

Dermatomes are areas of skin innervated by sensory fibers of a single spinal nerve. They are clinically important for diagnosing nerve injuries.

• Each spinal nerve (except C1) innervates a specific region of skin.

• Dermatome maps are used to localize neurological lesions.

Example: Loss of sensation in a dermatome can indicate damage to the corresponding spinal nerve.

Autonomic Nervous System (ANS)

Overview

The Autonomic Nervous System (ANS) is a subdivision of the PNS that controls involuntary body functions. It consists only of motor nerves and regulates cardiac muscle, smooth muscle, and glands.

• Controls automatic functions such as heart rate, digestion, and respiratory rate.

• Divided into sympathetic and parasympathetic divisions.

Additional info: The ANS maintains homeostasis by balancing the activity of its two divisions.

Somatic vs. Autonomic Nervous Systems

The somatic nervous system controls voluntary movements, while the autonomic nervous system regulates involuntary functions.

• Somatic: Motor neuron cell bodies originate inside the CNS; axons extend to skeletal muscles.

• Autonomic: Chain of two motor neurons; includes sympathetic and parasympathetic divisions.

Additional info: The autonomic system uses a two-neuron chain (preganglionic and postganglionic neurons).

Anatomy of the Parasympathetic Division

The parasympathetic division is also known as the craniosacral division. It conserves energy and maintains daily body functions.

• Originates from brainstem and sacral spinal cord.

• Promotes "rest-and-digest" activities.

Example: Stimulates digestion and slows heart rate.

Anatomy of the Sympathetic Division

The sympathetic division is also known as the thoracolumbar division. It prepares the body for "fight or flight" responses.

• Originates from thoracic and lumbar spinal cord.

• Increases heart rate, dilates bronchioles, inhibits digestion.

Example: Activates during stress or emergency situations.

Autonomic Functioning

The two divisions of the ANS have opposing effects on target organs.

• Sympathetic: "E" division – Exercise, Excitement, Emergency, Embarrassment.

• Parasympathetic: "D" division – Digestion, Defecation, Diuresis.

Additional info: Most organs receive dual innervation from both divisions.

Effects of the Sympathetic and Parasympathetic Divisions

Organ/System	Sympathetic Effect	Parasympathetic Effect
Digestive System	Decreases activity, constricts sphincters	Increases motility, relaxes sphincters
Heart	Increases rate and force	Decreases rate, slows and steadies
Bronchioles	Dilates	Constrics
Urinary Bladder	Relaxes sphincters, inhibits voiding	Contracts sphincters, promotes voiding
Pupils	Dilates	Constricts
Metabolism	Increases metabolic rate	No effect

Example: During exercise, the sympathetic division increases heart rate and redirects blood flow to muscles.