Comprehensive Study Notes: The Nervous System (Chapter 7)

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The Nervous System: Overview

Introduction to Nervous System Function

The nervous system is the master control and communication system of the body. It regulates every thought, action, and emotion by communicating with body cells using electrical impulses. Its functions are rapid and specific, causing almost immediate responses. The nervous system works alongside the endocrine system to maintain homeostasis, with the nervous system acting quickly and the endocrine system acting more slowly via hormones.

Key Functions: Sensory input, integration, and motor output.
Example: When driving and seeing a red light, sensory input is processed (integration), and motor output is sent to the leg muscles to press the brake.

Diagram of nervous system pathways throughout the body

Organization of the Nervous System

Structural and Functional Classification

The nervous system is divided structurally into the central nervous system (CNS) and peripheral nervous system (PNS). Functionally, it is divided into sensory (afferent) and motor (efferent) divisions, with the motor division further subdivided into somatic (voluntary) and autonomic (involuntary) systems.

CNS: Brain and spinal cord; integration and command centers.
PNS: Nerves outside the CNS; communication lines linking the body to the CNS.
Somatic Nervous System: Controls voluntary movements of skeletal muscles.
Autonomic Nervous System (ANS): Controls involuntary functions (smooth muscle, cardiac muscle, glands); subdivided into sympathetic and parasympathetic divisions.

Eye, leg muscle, and brain illustrating sensory input, motor output, and integration

Nervous Tissue: Structure and Function

Supporting Cells (Neuroglia)

Nervous tissue consists of neurons and supporting cells called neuroglia. Neuroglia support, insulate, and protect neurons. Types include:

Astrocytes: Star-shaped cells; form blood-brain barrier, regulate chemical environment.
Microglia: Phagocytes; dispose of debris and defend CNS cells.
Ependymal Cells: Line brain and spinal cord cavities; circulate cerebrospinal fluid.
Oligodendrocytes: Form myelin sheaths in CNS.
Schwann Cells: Form myelin sheaths in PNS.
Satellite Cells: Cushion and protect neuron cell bodies in PNS.

Astrocyte supporting cell in nervous tissue Structure of a typical motor neuron

Neurons: Anatomy and Terminology

Neurons are specialized cells for transmitting nerve impulses. They consist of a cell body (metabolic center) and processes (dendrites and axons).

Dendrites: Carry impulses toward the cell body.
Axons: Carry impulses away from the cell body; end in axon terminals containing neurotransmitters.
Myelin Sheath: Insulates axons, increases impulse speed; formed by Schwann cells in PNS and oligodendrocytes in CNS.
Nodes of Ranvier: Gaps in myelin sheath; facilitate rapid impulse transmission.

Schwann cell forming myelin sheath around axon

Classification of Neurons

Functional and Structural Classification

Neurons are classified by function (direction of impulse) and structure (number of processes).

Sensory (Afferent) Neurons: Carry impulses to CNS; cell bodies in ganglia outside CNS.
Motor (Efferent) Neurons: Carry impulses from CNS to effectors; cell bodies in CNS.
Interneurons (Association Neurons): Connect sensory and motor neurons; found in CNS.
Multipolar: Several processes; most common, includes motor and interneurons.
Bipolar: One axon, one dendrite; rare, found in special sense organs.
Unipolar: Single process; sensory neurons in PNS ganglia.

Functional classification of neurons: sensory, motor, interneuron

Physiology: Nerve Impulses

Generation and Conduction of Nerve Impulses

Neurons exhibit irritability (response to stimulus) and conductivity (transmission of impulse). The plasma membrane of a resting neuron is polarized, with more positive ions outside than inside.

Depolarization: Sodium ions enter cell, making inside more positive.
Action Potential: If depolarization is strong enough, an action potential is generated and propagated along the axon.
Repolarization: Potassium ions exit cell, restoring negative charge inside.
Sodium-Potassium Pump: Restores original ion concentrations using ATP.
Saltatory Conduction: In myelinated fibers, impulse jumps from node to node, increasing speed.

Diagram of nerve impulse generation and propagation

Transmission at Synapses

Impulses are transmitted across synapses by neurotransmitter chemicals. When an action potential reaches the axon terminal, neurotransmitters are released into the synaptic cleft and bind to receptors on the next neuron, initiating a new impulse.

Neurotransmitter Removal: Quickly removed by diffusion, reuptake, or enzymatic breakdown.
Electrochemical Event: Transmission is electrical along the neuron, chemical at the synapse.

Neurotransmitter release at synapse

Reflexes and Reflex Arcs

Types and Elements of Reflexes

Reflexes are rapid, predictable, and involuntary responses to stimuli, occurring over neural pathways called reflex arcs. Reflexes are classified as somatic (skeletal muscle) or autonomic (smooth muscle, glands).

Elements of Reflex Arc: Receptor, sensory neuron, integration center, motor neuron, effector.
Two-Neuron Reflex Arc: Example: Patellar (knee-jerk) reflex.
Three-Neuron Reflex Arc: Example: Withdrawal reflex.

Five basic elements of reflex arc

Central Nervous System: Brain and Spinal Cord

Major Regions of the Brain

The brain consists of four major regions: cerebral hemispheres, diencephalon, brain stem, and cerebellum.

Cerebral Hemispheres: Largest part; divided into lobes; responsible for higher functions (speech, memory, voluntary movement).
Diencephalon: Includes thalamus (relay station), hypothalamus (autonomic control, emotions), epithalamus (pineal gland, choroid plexus).
Brain Stem: Midbrain, pons, medulla oblongata; controls vital functions (breathing, heart rate).
Cerebellum: Coordinates muscle activity and balance.

Development and regions of the human brain Left lateral view of the brain: structural areas Sensory and motor areas of the cerebral cortex (homunculi)

Protection of the Central Nervous System

Meninges, Cerebrospinal Fluid, and Blood-Brain Barrier

The CNS is protected by bone, three meningeal layers (dura mater, arachnoid mater, pia mater), cerebrospinal fluid (CSF), and the blood-brain barrier.

Meninges: Connective tissue membranes covering CNS; dura mater (outer), arachnoid mater (middle), pia mater (inner).
Cerebrospinal Fluid: Watery cushion produced by choroid plexuses; circulates in ventricles and subarachnoid space.
Blood-Brain Barrier: Tight junctions in capillaries prevent harmful substances from entering brain tissue.

Meninges of the brain: dura mater, arachnoid mater, pia mater Ventricles and location of cerebrospinal fluid

Spinal Cord and Peripheral Nervous System

Spinal Cord Structure and Function

The spinal cord is a two-way conduction pathway and major reflex center. It is protected by vertebral column and meninges, and consists of gray matter (butterfly shape) and white matter (fiber tracts).

Gray Matter: Contains interneurons, motor neuron cell bodies.
White Matter: Myelinated fiber tracts; divided into dorsal, lateral, and ventral columns.
Spinal Nerves: 31 pairs; formed by union of dorsal and ventral roots; serve body regions.

Anatomy of the spinal cord, posterior view

Structure of a Nerve

A nerve is a bundle of neuron fibers outside the CNS, wrapped in connective tissue coverings: endoneurium (individual fiber), perineurium (fiber bundle), epineurium (entire nerve).

Classification: Sensory (afferent), motor (efferent), or mixed nerves.

Autonomic Nervous System

Sympathetic and Parasympathetic Divisions

The ANS controls involuntary functions and is divided into sympathetic (fight-or-flight) and parasympathetic (rest-and-digest) divisions. Both divisions serve the same organs but have opposite effects.

Sympathetic: Mobilizes body during stress; increases heart rate, blood pressure, dilates bronchioles.
Parasympathetic: Promotes digestion, conserves energy; decreases heart rate, increases digestive activity.
Neurotransmitters: Sympathetic uses norepinephrine; parasympathetic uses acetylcholine.

Developmental Aspects and Disorders

Brain Development and Aging

The nervous system forms early in embryonic development and is sensitive to maternal and environmental factors. Disorders include cerebral palsy, anencephaly, hydrocephalus, and spina bifida. Aging leads to decreased brain mass and efficiency, with senility caused by circulatory problems or other factors.

Summary Table: Major Brain Regions and Functions

Region	Function
Cerebral Hemispheres	Interpret sensory input, control voluntary movement, intellectual/emotional processing
Diencephalon	Relay sensory impulses, regulate autonomic functions, hormone production
Brain Stem	Control vital functions (heart rate, breathing), relay information
Cerebellum	Coordinate muscle activity, maintain balance and posture

Key Terms and Definitions

Neuron: Nerve cell specialized for impulse transmission.
Neuroglia: Supporting cells in nervous tissue.
Myelin Sheath: Insulating layer around axons.
Synapse: Junction between two neurons.
Reflex Arc: Neural pathway for reflexes.
Action Potential: Electrical impulse transmitted along neuron.
Autonomic Nervous System: Controls involuntary functions.

Additional info: Academic context was added to clarify structural and functional classifications, neuron anatomy, and the physiological basis of nerve impulses and reflexes. Images were included only when directly relevant to the explanation of the paragraph.