The Nervous System: Overview

Introduction to the Nervous System

The nervous system is the master controlling and communicating system of the body. It receives, integrates, and responds to information through rapid and specific electrical and chemical signals, resulting in almost immediate responses.

• Central Nervous System (CNS): Composed of the brain and spinal cord; serves as the integrative and control center.

• Peripheral Nervous System (PNS): Consists mainly of nerves extending from the brain and spinal cord, including the enteric nervous system in the gastrointestinal tract.

Functions of the Nervous System

The nervous system performs three primary functions:

1. Sensory Input: Detects stimuli from inside or outside the body via sensory receptors.

2. Integration: Processes and interprets sensory input, deciding on a response at a control center.

3. Motor Output: Sends signals to effectors (muscles and glands) to cause a response.

Anatomical Subdivisions of the Nervous System

Central and Peripheral Nervous System

The nervous system is divided into two principal parts:

• CNS: Brain and spinal cord.

• PNS: Cranial nerves, spinal nerves, and ganglia.

Functional Divisions of the PNS

The PNS is functionally divided into:

• Sensory (Afferent) Division: Conducts impulses from receptors to the CNS.

• Motor (Efferent) Division: Conducts impulses from the CNS to effectors.

Motor Division Subdivisions

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

• Autonomic Nervous System (ANS): Controls involuntary responses in cardiac muscle, smooth muscle, and glands. Subdivided into:

  • Sympathetic Division: Mobilizes body systems during activity.

  • Parasympathetic Division: Conserves energy and promotes housekeeping functions during rest.

Nervous Tissue: Cellular Components

Principal Cell Types

Nervous tissue consists of two main cell types:

• Neuroglia (Glial Cells): Support and protect neurons; play a central role in nervous system health.

• Neurons (Nerve Cells): Excitable cells that transmit electrical signals.

Neuroglial Cells of the CNS

There are four types of glial cells in the CNS:

• Astrocytes: Most abundant; support neurons, regulate the blood-brain barrier, and maintain extracellular environment.

• Microglia: Small phagocytic cells; act as immune defense by removing pathogens and debris.

• Ependymal Cells: Line fluid-filled cavities; produce and circulate cerebrospinal fluid (CSF).

• Oligodendrocytes: Form myelin sheaths around CNS axons, increasing impulse speed.

Neuroglial Cells of the PNS

Two major glial cells in the PNS:

• Satellite Cells: Surround neuron cell bodies; similar to astrocytes in function.

• Schwann Cells: Form myelin sheaths around PNS axons; vital for regeneration of damaged nerve fibers.

Neurons: Structure and Function

Basic Structure of a Neuron

Neurons are large, highly specialized cells that conduct impulses. They have extreme longevity, are mostly amitotic, and possess a high metabolic rate. All neurons have a cell body (soma) and one or more slender processes.

• Cell Body (Soma): Contains nucleus, biosynthetic and metabolic centers, and cytoskeletal elements.

• Dendrites: Receptive regions; branch extensively and contain dendritic spines for increased connectivity.

• Axon: Single process; transmits impulses away from the cell body. Distal ends are called axon terminals.

Myelin Sheath

The myelin sheath is a white, fatty substance that coats many axons, providing protection and electrical insulation, and increasing the speed of nerve impulse transmission.

• Myelinated Axons: Segmented sheath surrounds most long or large-diameter axons; impulses travel faster.

• Non-myelinated Axons: Lack sheath; conduct impulses more slowly.

Myelin Sheath in PNS vs. CNS

• PNS: Formed by Schwann cells; each cell forms one segment. Gaps between cells are called nodes of Ranvier.

• CNS: Formed by oligodendrocyte processes; each cell can wrap up to 60 axons. No outer collar of perinuclear cytoplasm.

Classification of Neurons

Structural Classification

Neurons are classified based on the number of processes arising from the cell body:

• Multipolar: One axon, multiple dendrites; most common type.

• Bipolar: One axon, one dendrite; found in sense organs.

• Unipolar: Single process branches into a T; dendrites receive signals from skin and joints, axon leads to spinal cord.

Functional Classification

Neurons are grouped by the direction of impulse travel relative to the CNS:

• Sensory (Afferent) Neurons: Transmit impulses from sensory receptors toward CNS; almost all are unipolar.

• Motor (Efferent) Neurons: Carry outgoing signals from CNS to effectors; usually multipolar.

• Interneurons: Perform integrative functions; only found in CNS; most abundant and usually multipolar.

Summary Table: Structural and Functional Classes of Neurons

Key Terms and Concepts

• Neuron: Excitable cell transmitting electrical signals.

• Neuroglia: Supportive cells in nervous tissue.

• Myelin Sheath: Insulating layer around axons.

• Nodes of Ranvier: Gaps in myelin sheath facilitating rapid impulse conduction.

• Astrocyte: Most abundant CNS glial cell.

• Microglia: CNS immune defense cell.

• Ependymal Cell: Lines CNS cavities, produces CSF.

• Oligodendrocyte: Forms CNS myelin sheath.

• Schwann Cell: Forms PNS myelin sheath.

Equations and Academic Context

Nerve Impulse Transmission

The speed of nerve impulse transmission is increased by myelination due to saltatory conduction, where impulses jump from node to node.

• Resting Membrane Potential:

• Ohm's Law (for neurons):

Additional info:

The nervous system's rapid communication is essential for maintaining homeostasis, responding to environmental changes, and coordinating complex behaviors. Neuroglial cells, though less understood than neurons, are critical for neural health, repair, and function.