The Nervous System: Structure, Function, and Organization

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The Nervous System

Overview

The nervous system is the master control and communication system of the body. It is responsible for maintaining homeostasis, processing sensory information, and coordinating voluntary and involuntary responses.

Master control system: Regulates and coordinates body activities.
Maintains homeostasis: Ensures stable internal conditions.
Communication system: Transmits electrical and chemical signals throughout the body.
Processes sensory input: Receives information from sensory receptors.
Dictates motor output: Initiates responses by activating muscles and glands.

Divisions of the Nervous System

Central Nervous System (CNS)

The central nervous system consists of the brain and spinal cord, located in the dorsal body cavity. It acts as the main integrative and control center of the body.

Brain: Processes information and is responsible for higher functions such as thought, memory, and emotion.
Spinal cord: Conducts signals to and from the brain and controls reflex activities.

Peripheral Nervous System (PNS)

The peripheral nervous system includes all nervous tissue outside the CNS. It consists mainly of nerves that extend from the brain and spinal cord.

Cranial nerves: Carry signals to and from the brain.
Spinal nerves: Carry signals to and from the spinal cord.
Function: Serves as communication lines between the CNS and the rest of the body.

Functional Organization of the PNS

Sensory (Afferent) Division: Transmits sensory information from receptors to the CNS.
Motor (Efferent) Division: Transmits commands from the CNS to effector organs (muscles and glands).

Motor Division Subdivisions

Somatic Nervous System (SNS): Controls voluntary movements by innervating skeletal muscles.
Autonomic Nervous System (ANS): Regulates involuntary functions by innervating smooth muscle, cardiac muscle, and glands.

Autonomic Nervous System Subdivisions

Sympathetic Division: Mobilizes body systems during activity ("fight or flight").
Parasympathetic Division: Conserves energy and promotes housekeeping functions during rest ("rest and digest").

Summary Table: Organization of the Nervous System

Division	Components	Main Function
Central Nervous System (CNS)	Brain, Spinal Cord	Integration, control center
Peripheral Nervous System (PNS)	Cranial nerves, Spinal nerves	Communication between CNS and body
Sensory (Afferent) Division	Somatic and visceral sensory nerve fibers	Conducts impulses from receptors to CNS
Motor (Efferent) Division	Motor nerve fibers	Conducts impulses from CNS to effectors
Somatic Nervous System	Somatic motor (voluntary)	Conducts impulses to skeletal muscles
Autonomic Nervous System	Visceral motor (involuntary)	Conducts impulses to cardiac muscle, smooth muscle, and glands
Sympathetic Division	Part of ANS	Mobilizes body systems during activity
Parasympathetic Division	Part of ANS	Conserves energy, promotes rest functions

Nervous Tissue

Types of Nervous Tissue

Neuroglia (Glial Cells): Support, protect, and nourish neurons.
Neurons: Specialized cells that transmit electrical impulses.

Neuroglia (Glial Cells) in the CNS

Astrocytes: Most abundant; anchor neurons to capillaries, regulate capillary permeability, and maintain the chemical environment.
Microglial Cells: Act as phagocytes, removing debris and defending CNS cells.
Ependymal Cells: Line fluid-filled cavities of the brain and spinal cord; help circulate cerebrospinal fluid.
Oligodendrocytes: Produce myelin sheaths around CNS nerve fibers, increasing the speed of impulse transmission.

Neuroglia in the PNS

Schwann Cells: Form myelin sheaths around peripheral nerve fibers.
Satellite Cells: Surround neuron cell bodies in ganglia, providing support and regulating the environment.

Neurons: Structure and Function

Cell Body (Soma): Contains the nucleus and organelles.
Dendrites: Receive incoming signals and transmit them toward the cell body.
Axon: Conducts impulses away from the cell body to other neurons or effectors.
Axon Terminals: Release neurotransmitters to communicate with target cells.
Myelin Sheath: Whitish, protein-lipid layer that insulates axons and increases the speed of impulse transmission.

Types of Neurons (by Structure)

Multipolar Neurons: Many dendrites, one axon; most common type; all motor and interneurons.
Bipolar Neurons: One dendrite, one axon; found in special sensory organs (e.g., retina, olfactory mucosa).
Unipolar Neurons: Single process that splits into two branches; mainly sensory neurons in the PNS.

Myelination and Nerve Impulse Transmission

Myelin Sheath

Function: Electrically insulates axons, increasing the speed of nerve impulse conduction.
Myelinated fibers: Have segmented myelin sheaths; conduct impulses rapidly.
Unmyelinated fibers: Lack myelin; conduct impulses more slowly.

Multiple Sclerosis (MS)

Definition: An autoimmune disease characterized by the gradual destruction of myelin sheaths in the CNS.
Pathology: Myelin sheaths become hardened (scleroses), impairing nerve signal transmission.
Symptoms: Vision and speech difficulties, loss of muscle control.
Treatment: Interferon injections and drugs to slow the autoimmune response.

Functional Classification of Neurons

Sensory (Afferent) Neurons: Transmit impulses from sensory receptors to the CNS.
Motor (Efferent) Neurons: Carry impulses from the CNS to effectors (muscles and glands).
Interneurons (Association Neurons): Connect sensory and motor neurons within the CNS; involved in integration.

Neural Communication and Reflexes

Basic Steps in Neural Communication

Sensory Input: Sensory receptors detect changes and send information to the CNS.
Integration: CNS processes and interprets sensory input, deciding on a response.
Motor Output: CNS sends commands to effectors to produce a response.

Reflex Arcs

Definition: Rapid, predictable, involuntary responses to stimuli.
Components: Receptor, sensory neuron, integration center, motor neuron, effector.
Example: Knee-jerk (patellar) reflex.

Summary Table: Types of Reflexes

Type	Effector	Example
Somatic Reflex	Skeletal muscle	Withdrawal reflex
Autonomic Reflex	Smooth muscle, cardiac muscle, glands	Pupil constriction, heart rate regulation

Key Terms and Definitions

Neuron: A nerve cell specialized for the transmission of electrical impulses.
Neuroglia: Supporting cells in the nervous system.
Myelin: A fatty substance that insulates axons.
Reflex: An automatic, rapid response to a stimulus.
Synapse: The junction between two neurons or a neuron and an effector cell.

Additional info:

Some diagrams and tables were inferred and expanded for clarity and completeness.
Definitions and examples were added to ensure the notes are self-contained and suitable for exam preparation.