Nervous System: Structure, Function, and Organization

Nervous System Overview

Central and Peripheral Nervous System

The nervous system is divided into the Central Nervous System (CNS) and Peripheral Nervous System (PNS). The CNS consists of the brain and spinal cord, while the PNS includes cranial and spinal nerves. The PNS is further subdivided into the Somatic Nervous System (controls skeletal muscles and skin), Autonomic Nervous System (ANS) (controls smooth muscle, cardiac muscle, organs, and glands), and Enteric Nervous System (ENS) (regulates gastrointestinal function).

• CNS: Brain and spinal cord

• PNS: Cranial and spinal nerves

• Somatic NS: Voluntary control of skeletal muscles

• Autonomic NS: Involuntary control of organs and glands

• Enteric NS: Controls GI tract function

Neurons and Nervous Tissue

Structure of Neurons

Neurons are the structural and functional units of the nervous system. Each neuron consists of dendrites, a cell body (soma), and an axon. The axon transmits impulses away from the cell body toward the axon terminal. The axon hillock is the region where impulses are initiated.

• Dendrites: Receive signals

• Cell Body: Contains nucleus and metabolic machinery

• Axon: Conducts impulses away from cell body

• Axon Hillock: Initiates action potentials

Neuroglia (Glial Cells)

Neuroglia are support cells in nervous tissue. They provide nutrients, remove debris, assist in cerebrospinal fluid (CSF) production, and myelinate axons. In the PNS, Schwann cells myelinate axons, enabling saltatory conduction. In the CNS, Oligodendrocytes perform this function. The myelin sheath insulates axons and increases transmission speed. The neurilemma is the outer surface of the myelin sheath.

• Schwann cells: Myelinate PNS axons, allow regeneration

• Oligodendrocytes: Myelinate CNS axons

• Myelin sheath: Insulation, speeds up transmission

• Neurilemma: Surface of myelin sheath

• Connective tissue layers: Epineurium, perineurium, endoneurium

Nodes of Ranvier and Synapses

Nodes of Ranvier are gaps in the myelin sheath where ion exchange occurs, enabling rapid impulse transmission via saltatory conduction. Synapses are junctions between neurons or between neurons and effector cells (muscle, gland, etc.).

• Nodes of Ranvier: Facilitate fast impulse transmission

• Synaptic Cleft: Gap between neurons for signal transmission

• Saltatory Conduction: Impulse jumps from node to node

Types of Neurons

Classification by Function

Neurons are classified based on their function:

• Sensory (Afferent): Unipolar or bipolar; transmit impulses to CNS

• Motor (Efferent): Multipolar; transmit impulses away from CNS

• Interneurons (Association): Multipolar; found only in CNS, most abundant, mediate complex actions

Neurotransmitters

Chemical Messengers

Neurotransmitters are chemicals released from axon terminals that alter the permeability of the postsynaptic membrane, allowing sodium ions to enter and depolarize the cell. Examples include:

• Acetylcholine: Neuromuscular junction

• Epinephrine/Norepinephrine: Adrenal medulla

• Dopamine: Brain, regulates movement and expression

• Serotonin, Histamine, GABA, Endorphins, Prostaglandin, Glutamate: Various functions

Spinal Cord Structure and Function

Spinal Cord Anatomy

The spinal cord contains 31 pairs of spinal nerves and consists of gray and white matter. It serves as a pathway for impulses to and from the brain and is involved in reflex activity.

• Gray matter: Cell bodies, nuclei, reflex centers

• White matter: Axon tracts (ascending sensory, descending motor)

• Ascending tracts: Carry sensory impulses to cerebrum/cerebellum

• Descending tracts: Carry motor impulses from CNS

Spinal Roots and Nerve Plexuses

Each spinal nerve is attached by two roots:

• Ventral root: Motor signals

• Dorsal root: Sensory signals

• Mixed nerves: Contain both motor and sensory fibers

Major nerve plexuses:

• Cervical: Neck, diaphragm (Phrenic nerve)

• Brachial: Arms, hands (Radial nerve)

• Lumbosacral: Lower abdomen, genitalia, legs, feet (Sciatic nerve)

Brain Structure and Functional Areas

Cerebrum and Diencephalon

The cerebrum has two hemispheres and six functional lobes responsible for conscious thought, reasoning, emotion, and cognition. The diencephalon includes the thalamus, hypothalamus, and epithalamus, separated by the third ventricle. The choroid plexus produces CSF.

• Frontal lobe: Reasoning, emotion, motor control

• Parietal lobes: Sensory processing

• Temporal lobes: Hearing, smell

• Occipital lobe: Vision

• Insula: Links conscious thought and emotion

• Thalamus: Relay center

• Hypothalamus: Homeostasis, endocrine control

• Epithalamus: CSF production, waste elimination

Functions of the Hypothalamus

• Regulates body temperature

• Controls emotions (fear, pleasure)

• Controls ANS

• Regulates pituitary gland/endocrine system

• Maintains water balance and thirst

• Controls appetite and hunger

• Regulates sleep

• Center for mind-body phenomena

Cerebral Cortex Functional Areas

• Sensory perception

• Voluntary movement

• Language

• Personality traits

• Higher mental functions: thinking, memory, decision making, creativity, self-consciousness

• Gyri and sulci increase surface area

Lobes of the Brain

• Frontal lobe: Speech, motor control, judgment

• Parietal lobe: Sensory area for pain, touch, temperature; Broca's area (speech)

• Temporal lobe: Hearing, smell

• Occipital lobe: Vision

• Insula: Emotional response

Brainstem and Cerebellum

Brainstem Structure

The brainstem connects the cerebrum, cerebellum, and spinal cord. It acts as a relay center for sensory information and regulates vital functions.

• Midbrain: Reflexes for sight and hearing

• Pons: Connects cerebrum and spinal cord

• Medulla oblongata: Regulates respiration, heart rate, blood pressure, and reflexes

Medulla Oblongata Functions

• Respiratory center: Rate, depth, rhythm

• Cardiac center: Heart rate and force

• Vasomotor center: Blood vessel diameter

• Other centers: Cough, vomiting, swallowing, hiccups

• Decussation: Right brain controls left body and vice versa

• Reticular Activating System (RAS): Maintains wakefulness, muscle tone

Cerebellum

• Maintains balance (input from inner ear)

• Coordinates skeletal muscle function

• Integrates impulses from cerebrum

Disorders and Protection of CNS

Cerebrovascular Accident (CVA)

• Stroke: Death of brain tissue due to blood clot or vessel rupture

• Infarct: Area of dead tissue

• Ischemia: Deficiency of blood supply

• Transient Ischemic Attack (TIA): Temporary loss of blood supply

Protection of CNS

• Skull and vertebrae

• Meninges: Dura mater (toughest), arachnoid membrane (web-like, CSF flow), pia mater (delicate, follows contours)

Cerebrospinal Fluid (CSF)

• Cushions and nourishes brain and spinal cord

• Produced by choroid plexuses

• Pathway: Lateral ventricle → Foramen of Munro → Third ventricle → Aqueduct of Sylvius → Fourth ventricle → Subarachnoid space → Circulatory system

Cranial Nerves

Overview and Functions

There are twelve pairs of cranial nerves, each with specific sensory, motor, or mixed functions. The mnemonic "On Old Olympus, Towering Tops, A Fin, Viking, And German Viewed Some Hops" helps recall their order.

Number	Name	Function	Type
I	Olfactory	Smell	Sensory
II	Optic	Sight	Sensory
III	Oculomotor	Eye muscles	Motor (Mixed)
IV	Trochlear	One eye muscle	Motor
V	Trigeminal	Pain, touch, temperature of face; chewing	Both
VI	Abducens	One eye muscle	Motor
VII	Facial	Facial expression, taste, glands	Mixed
VIII	Vestibulocochlear	Hearing, balance	Sensory
IX	Glossopharyngeal	Swallowing, tongue, parotid gland	Mixed
X	Vagus	Thoracic/abdominal organs, glands	Mixed
XI	Spinal Accessory	Neck and back muscles	Motor
XII	Hypoglossal	Tongue movement	Motor

Autonomic Nervous System (ANS)

Structure and Function

The ANS is part of the PNS and controls involuntary functions of smooth muscle, cardiac muscle, and glands. It maintains homeostasis and is divided into the Sympathetic and Parasympathetic systems.

• Sympathetic NS: Thoracolumbar origin (T1-L2), energy expenditure, "fight or flight"

• Parasympathetic NS: Craniosacral origin (cranial nerves 3, 7, 9, 10; sacral region), restorative processes

Sympathetic vs. Parasympathetic Effects

Organ	Sympathetic Response	Parasympathetic Response
Heart	Increase rate	Decrease rate (to normal)
Bronchioles	Dilate	Constrict (to normal)
Pupils (Iris)	Dilate	Constrict (to normal)
Salivary glands	Decrease secretion	Increase secretion (to normal)
Stomach & Intestines (Smooth muscle)	Decrease peristalsis	Increase peristalsis for normal digestion
Stomach & Intestines (Glands)	Decrease secretion	Increase secretion for normal digestion
Internal Anal Sphincter	Contracts to prevent defecation	Relaxes to permit defecation
Urinary Bladder	Relaxes to prevent urination	Contracts for normal urination

Fight or Flight Syndrome

• Under stress, sympathetic system increases heart rate, dilates pupils, increases respiratory rate, inhibits digestive and urinary tract

• Most organs receive both sympathetic and parasympathetic stimulation (like gas and brake pedals)

Example:

During a stressful event, the sympathetic nervous system prepares the body for rapid action by increasing heart rate and redirecting blood flow to muscles, while the parasympathetic system restores normal function after the stress subsides.