Nervous System Overview

Introduction to the Nervous System

The nervous system is a complex network responsible for controlling and coordinating bodily functions, maintaining homeostasis, and enabling communication throughout the body. It processes sensory information, integrates data, and initiates motor responses.

• Control System: Directs and regulates body activities.

• Homeostasis: Maintains internal stability by responding to changes.

• Communication System: Transmits electrical impulses for rapid signaling.

• Sensory Input: Receives information from internal and external environments.

• Motor Output: Initiates actions in response to processed information.

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 serves as the main integrative and control center, processing sensory input and dictating motor output.

• Brain: Responsible for higher functions such as thought, memory, and emotion.

• Spinal Cord: Conducts signals between the brain and the rest of the body.

Peripheral Nervous System (PNS)

The Peripheral Nervous System includes all nervous tissue outside the CNS, mainly nerves that extend from the brain and spinal cord. It connects the CNS to limbs and organs.

• Cranial Nerves: Connect the brain to the head and neck.

• Spinal Nerves: Connect the spinal cord to the rest of the body.

Functional Divisions of the PNS

• Sensory (Afferent) Division: Transmits sensory information from receptors to the CNS.

• Motor (Efferent) Division: Transmits motor commands from the CNS to effectors (muscles and glands).

Motor Division Subdivisions

• Somatic Nervous System: Controls voluntary movements of skeletal muscles.

• Autonomic Nervous System: Regulates involuntary functions (smooth muscle, cardiac muscle, 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").

Flow of Information in the Nervous System

Sensory input is received from sense organs, integrated in the CNS, and motor output is sent to effectors.

• Sensory Input: Detection of stimuli (e.g., seeing a glass of water).

• Integration: Processing and interpretation in the brain.

• Motor Output: Initiation of movement (e.g., reaching for the glass).

Nervous System Organization Table

Nervous Tissue

Types of Nervous Tissue

Nervous tissue is composed of two main cell types: neuroglia (supporting cells) and neurons (functional cells).

• Neuroglia (Glial Cells): Support, protect, and nourish neurons.

• Neurons: Specialized for transmitting electrical impulses.

Neuroglia Types and Functions

• Astrocytes: Anchor neurons to capillaries, regulate capillary permeability, maintain chemical environment, and facilitate exchange between capillaries and neurons.

• Microglial Cells: Act as phagocytes, removing debris and defending CNS cells.

• Ependymal Cells: Line fluid-filled cavities in the brain and spinal cord, circulate cerebrospinal fluid.

• Oligodendrocytes: Form myelin sheaths around CNS nerve fibers, increasing transmission speed.

• Schwann Cells: Form myelin sheaths around PNS nerve fibers.

• Satellite Cells: Surround neuron cell bodies in PNS, provide support and nutrition.

Neurons: Structure and Function

Neurons are the basic functional units of the nervous system, specialized for rapid communication.

• Cell Body (Soma): Contains nucleus and organelles.

• Dendrites: Receive incoming signals and transmit them toward the cell body.

• Axon: Transmits impulses away from the cell body to other neurons or effectors.

• Axon Terminals: Release neurotransmitters to communicate with other cells.

• Myelin Sheath: Insulates axons, increases speed of impulse transmission.

Types of Neurons

• Multipolar Neurons: Many dendrites, one axon; most common type, found in CNS.

• Bipolar Neurons: One dendrite, one axon; found in sensory organs (e.g., retina).

• Unipolar Neurons: Single process that splits into two branches; found in sensory ganglia of PNS.

Myelin and Nerve Impulse Transmission

Myelin Sheath

The myelin sheath is a whitish, protein-lipid layer that insulates axons and increases the speed of nerve impulse transmission.

• Myelinated Fibers: Have segmented myelin sheaths; conduct impulses rapidly.

• Unmyelinated Fibers: Lack myelin; conduct impulses more slowly.

Multiple Sclerosis (MS)

Multiple Sclerosis is an autoimmune disease characterized by the gradual destruction of myelin sheaths in the CNS, leading to hardened (sclerosed) sheaths and impaired nerve signal transmission.

• Symptoms: Vision and speech difficulties, loss of muscle control.

• Treatment: Interferon injections, drugs to slow autoimmune response.

Summary Table: Neuroglia Types and Functions

Additional info:

• Neurons communicate via action potentials, rapid changes in membrane potential that propagate along axons.

• Formula for membrane potential (Nernst equation):

• Reflex arcs are predictable, involuntary responses involving sensory input, integration, and motor output.