Central and Peripheral Nervous System, Special Senses: Study Notes

Central Nervous System: The Spinal Cord and Brain

Overview of the Spinal Cord

The spinal cord is a major part of the central nervous system (CNS), extending from the foramen magnum to the L1-L2 vertebrae. It serves as a conduit for information between the brain and the rest of the body and is involved in reflex actions.

• Functions: Conduction of nerve impulses, integration of reflexes, and communication between the brain and peripheral nerves.

• Protection: Surrounded by vertebrae, meninges (dura mater, arachnoid mater, pia mater), and cerebrospinal fluid (CSF).

• Enlargements: Cervical and lumbar enlargements correspond to the origins of nerves for the limbs.

• Conus medullaris: Tapered, lower end of the spinal cord.

• Cauda equina: Bundle of spinal nerves below the conus medullaris.

• Filum terminale: Fibrous extension of the pia mater anchoring the spinal cord to the coccyx.

Spinal Cord Cross-Sectional Anatomy

• Gray matter: Butterfly-shaped region containing neuron cell bodies, dendrites, and unmyelinated axons. Organized into horns (dorsal, ventral, lateral).

• White matter: Surrounds gray matter; consists of myelinated axons organized into columns (funiculi).

• Central canal: Small channel in the center of the spinal cord containing CSF.

Spinal Nerves

Spinal nerves are mixed nerves formed by the union of dorsal (sensory) and ventral (motor) roots. There are 31 pairs, named according to their vertebral level.

• Cervical: 8 pairs

• Thoracic: 12 pairs

• Lumbar: 5 pairs

• Sacral: 5 pairs

• Coccygeal: 1 pair

Reflexes

Reflexes are rapid, involuntary responses to stimuli. The basic unit is the reflex arc, which includes a receptor, sensory neuron, integration center, motor neuron, and effector.

• Monosynaptic reflex: Involves a single synapse (e.g., patellar reflex).

• Polysynaptic reflex: Involves multiple synapses (e.g., withdrawal reflex).

Peripheral Nervous System (PNS)

Overview

The PNS consists of all neural structures outside the CNS, including cranial nerves, spinal nerves, ganglia, and sensory receptors. It connects the CNS to limbs and organs.

Cranial Nerves

There are 12 pairs of cranial nerves, each with specific sensory, motor, or mixed functions.

• I. Olfactory (Sensory): Smell

• II. Optic (Sensory): Vision

• III. Oculomotor (Motor): Eye movement, pupil constriction

• IV. Trochlear (Motor): Eye movement

• V. Trigeminal (Both): Facial sensation, chewing

• VI. Abducens (Motor): Eye movement

• VII. Facial (Both): Facial expression, taste

• VIII. Vestibulocochlear (Sensory): Hearing, balance

• IX. Glossopharyngeal (Both): Taste, swallowing

• X. Vagus (Both): Autonomic control of viscera

• XI. Accessory (Motor): Head and shoulder movement

• XII. Hypoglossal (Motor): Tongue movement

Spinal Nerve Plexuses

Spinal nerves form networks called plexuses (cervical, brachial, lumbar, sacral) that supply the limbs and body wall.

Somatic vs. Autonomic Nervous System

Feature	Somatic Nervous System	Autonomic Nervous System
Effector	Skeletal muscle	Cardiac muscle, smooth muscle, glands
Control	Voluntary	Involuntary
Neurons	One motor neuron	Two-neuron chain (preganglionic and postganglionic)
Neurotransmitter	Acetylcholine (ACh)	ACh, norepinephrine (NE)

Autonomic Nervous System (ANS)

Divisions of the ANS

• Sympathetic Division: Prepares the body for 'fight or flight' responses. Increases heart rate, dilates pupils, inhibits digestion.

• Parasympathetic Division: Promotes 'rest and digest' activities. Decreases heart rate, stimulates digestion, constricts pupils.

Neurotransmitters and Receptors

• Cholinergic fibers: Release acetylcholine (ACh).

• Adrenergic fibers: Release norepinephrine (NE).

• Receptors: Nicotinic and muscarinic (for ACh); alpha and beta adrenergic (for NE).

Special Senses

General Overview

Special senses include vision, hearing, equilibrium, taste, and smell. These senses rely on specialized sensory organs and receptors.

Vision

• Eye Anatomy: Three layers: fibrous (sclera, cornea), vascular (choroid, ciliary body, iris), and sensory (retina).

• Photoreceptors: Rods (dim light, black and white vision), cones (color vision, sharpness).

• Lens: Focuses light on the retina; changes shape for accommodation.

• Pathway: Light → cornea → lens → retina → optic nerve → brain.

Hearing and Equilibrium

• Ear Anatomy: Outer ear (auricle, auditory canal), middle ear (ossicles), inner ear (cochlea, vestibule, semicircular canals).

• Cochlea: Contains hair cells that transduce sound vibrations into nerve impulses.

• Vestibular apparatus: Detects head position and movement for balance.

Taste (Gustation) and Smell (Olfaction)

• Taste buds: Located on the tongue; detect sweet, sour, salty, bitter, and umami flavors.

• Olfactory receptors: Located in the nasal cavity; detect airborne chemicals.

Key Tables

Comparison of Somatic and Autonomic Nervous Systems

Feature	Somatic	Autonomic
Effector Organs	Skeletal muscle	Cardiac muscle, smooth muscle, glands
Control	Voluntary	Involuntary
Number of Neurons	One	Two (pre- and postganglionic)
Neurotransmitter	Acetylcholine	Acetylcholine, norepinephrine

Major Cranial Nerves and Functions

Nerve	Type	Function
Olfactory (I)	Sensory	Smell
Optic (II)	Sensory	Vision
Oculomotor (III)	Motor	Eye movement
Trigeminal (V)	Both	Facial sensation, chewing
Facial (VII)	Both	Facial expression, taste
Vestibulocochlear (VIII)	Sensory	Hearing, balance
Vagus (X)	Both	Autonomic control of viscera

Additional info:

• These notes cover content from chapters on the central and peripheral nervous systems and special senses, corresponding to Ch. 12, 13, 14, and 15 in standard Anatomy & Physiology textbooks.

• Some details, such as the full list of cranial nerves and all reflex types, have been summarized for clarity and brevity.