Nervous System: Structure, Function, and Pathways

Overview of the Nervous System

Introduction

The nervous system is a complex network responsible for coordinating sensory input, motor output, and higher functions such as thinking, learning, and memory. It is divided into the central nervous system (CNS) and peripheral nervous system (PNS).

• CNS: Brain and spinal cord

• PNS: Cranial and spinal nerves connecting CNS to the body

The Brain

Structure and Protection

• Central control center of the nervous system

• Located in the cranial cavity

• Protected by skull, meninges, and cerebrospinal fluid (CSF)

• Connected to the spinal cord via the brainstem

Functions

• Coordinates sensory input, motor output, and higher functions

• Responsible for thinking, learning, memory, emotions

Brain Structure Overview

Main Regions

• Cerebrum: Paired hemispheres, conscious thought, memory, sensation

• Diencephalon: Thalamus (relay center), hypothalamus (homeostasis, hormone control), epithalamus (sleep-wake cycles)

• Brainstem: Midbrain, pons, medulla oblongata; relay station, autonomic functions

• Cerebellum: Motor coordination, balance

Cerebrum

Features and Functions

• Largest part of the brain

• Divided into four lobes: frontal, parietal, temporal, occipital

• Cerebral cortex: Outer layer of gray matter

• Functions: conscious thought, reasoning, sensory perception, memory storage, voluntary movement planning

Cerebral Cortex Lobes

Lobe	Main Functions
Frontal	Voluntary motor control, planning, personality, speech
Parietal	Somatic sensory processing (touch, pressure, pain, temperature)
Temporal	Hearing, memory, language comprehension
Occipital	Vision

Diencephalon

Divisions & Functions

• Thalamus: Relays sensory information to cortex, motor control

• Hypothalamus: Homeostasis, autonomic control, hormone production

• Epithalamus: Pineal gland, melatonin, sleep-wake cycles

Brainstem

Overview

• Connects brain to spinal cord

• Composed of midbrain, pons, medulla oblongata

• Functions: conduit for sensory/motor pathways, vital automatic functions, alertness

Midbrain

• Regulates alertness, wakefulness

• Visual/auditory reflexes

• Motor control, dopamine signaling

Pons

• Relay of sensory/motor info

• Regulation of breathing

• Facial movements, swallowing

Medulla Oblongata

• Vital centers for heart rate, blood pressure, breathing

• Regulates reflexes: swallowing, coughing, sneezing, vomiting

Cerebellum

Functions

• Automatic processing center

• Adjusts postural muscles for balance

• Coordinates voluntary movements

• Fine-tunes motor activity and timing

Ventricles of the Brain

Structure & Function

• Brain cavities filled with CSF

• Two lateral ventricles (one in each hemisphere)

• Third ventricle (in diencephalon)

• Fourth ventricle (between cerebellum and brainstem)

• Ventricles connected, CSF circulates continuously

Cerebrospinal Fluid (CSF)

Functions

• Surrounds and cushions CNS, protects against trauma

• Supports brain, allows it to "float"

• Transports nutrients, chemicals, wastes

• Produced by choroid plexus (~500 mL/day)

• Total CSF volume ~150 mL, replaced every ~8 hours

Peripheral Nervous System (PNS)

Overview

• Links CNS to body via peripheral nerves

• Cranial nerves: Originate from brain

• Spinal nerves: Connect to spinal cord

• Wrapped in connective tissue layers

Cranial Nerves

Classification & Mnemonic

• Twelve pairs, named I-XII

• Classified as sensory, motor, or mixed

• Mnemonic: "Oh, Once One Takes The Anatomy Final, Very Good Vacations Are Heavenly"

Nerve	Main Function
I - Olfactory	Smell
II - Optic	Vision
III - Oculomotor	Eye movement, pupil response
IV - Trochlear	Eye movement
V - Trigeminal	Facial sensation, chewing
VI - Abducens	Eye movement
VII - Facial	Facial expressions, taste, salivary/tear glands
VIII - Vestibulocochlear	Hearing, balance
IX - Glossopharyngeal	Taste, swallowing, gag reflex
X - Vagus	Parasympathetic control, heart rate, digestion
XI - Accessory	Head, shoulder movement
XII - Hypoglossal	Tongue movement, speech, swallowing

Spinal Cord Overview

Structure & Function

• Cylindrical structure within vertebral column

• Continuous with brainstem

• Major neural pathway between brain and PNS

• Conveys sensory info to brain (ascending), motor commands from brain (descending)

• Protected by meninges and CSF

• Central canal filled with CSF

Gray and White Matter of the Spinal Cord

Type	Features
Gray matter	Central "H" shape, neuron cell bodies, glial cells, sensory/motor nuclei
White matter	Surrounds gray matter, myelinated/unmyelinated axons, organized into columns, contains ascending/descending tracts

Spinal Nerves

Features

• 31 pairs of mixed nerves

• Each pair supplies a specific region: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal

• Carry sensory info to CNS and motor info away from CNS

• Nerves L2 and below form cauda equina

Roots and Ganglia

• Dorsal (posterior) root: Sensory neuron axons

• Dorsal root ganglion: Sensory neuron cell bodies

• Ventral (anterior) root: Motor neuron axons

• Roots join to form mixed spinal nerve

Nerve Plexuses

Overview

• Network where spinal nerves blend and reorganize

• Occur in: cervical, brachial, lumbar, sacral regions

• After mixing, nerves travel to limbs/organs

• Injury at one level may affect multiple areas

Dermatome

Definition & Clinical Importance

• Specific region of body surface monitored by a pair of spinal nerves

• Important for determining location of nerve damage/infection

Spinal Cord Tracts & Pathways

Sensory Pathways (Ascending)

• Carry sensory info from body to brain

• Types: pain, touch, pressure, temperature, proprioception, vibration

• Crossover in brainstem to opposite side

• Example: Posterior column pathway (fine touch, vibration, proprioception)

• Sensory homunculus: Cortical map of body, size reflects receptor density

Motor Pathways (Descending)

• Carry commands from brain to muscles/glands

• Types: voluntary (skeletal muscle), involuntary (reflexes, autonomic)

• Example: Corticospinal (pyramidal) pathway (precise voluntary movements, crosses in medulla)

• Motor homunculus: Cortical map, size reflects fine motor control

Reflexes and Reflex Arc

Definition

• Rapid, automatic response to specific stimulus

• Helps maintain homeostasis

• Simple reflex: One synapse (e.g., patellar reflex)

• Complex reflex: Multiple synapses, withdrawal reflexes

Steps of a Reflex Arc

1. Stimulus activates sensory receptor

2. Sensory neuron sends signal to spinal cord

3. Interneuron processes information

4. Motor neuron carries signal to effector

5. Effector responds, usually opposes stimulus (negative feedback)

Example Equation

Reflex arc pathway: $\text{Stimulus} \rightarrow \text{Receptor} \rightarrow \text{Afferent Neuron} \rightarrow \text{Integrating Center} \rightarrow \text{Efferent Neuron} \rightarrow \text{Effector}$

Additional info:

• Homunculus diagrams illustrate the proportional representation of body parts in the sensory and motor cortex.

• CSF circulation is essential for CNS health and protection.