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, usually 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 that extend 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

CNS and PNS

The nervous system is divided into two principal parts:

• CNS: Brain and spinal cord.

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

Divisions and Subdivisions of the PNS

The PNS is functionally divided into two main divisions:

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

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

The motor division is further subdivided into:

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

• Autonomic Nervous System (ANS): Controls involuntary functions, such as cardiac muscle, smooth muscle, and glands. The ANS is further divided 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 principal cell types:

• Neuroglia (Glial Cells): Small cells that surround and wrap delicate neurons, supporting their function.

• 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 glial cells, constituting over 90% of brain tissue in some areas. They support neurons, regulate the blood-brain barrier, and maintain the extracellular environment.

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• Microglia: Small phagocytic cells that destroy pathogens, debris, or foreign matter. They become concentrated in areas damaged by infection, trauma, or stroke.

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• Ependymal Cells: Line internal fluid-filled cavities of the brain and spinal cord, produce cerebrospinal fluid (CSF), and have cilia to help circulate CSF.

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• Oligodendrocytes: Resemble an octopus with multiple arm-like processes. They form the myelin sheath around axons in the CNS, insulating and increasing the speed of nerve impulse transmission.

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Neuroglial Cells of the PNS

Two major glial cells in the PNS:

• Satellite Cells: Similar to astrocytes in the CNS, they support and regulate the environment around neuron cell bodies in ganglia.

• Schwann Cells: Similar to oligodendrocytes, they form myelin sheaths around axons in the PNS and are vital to regeneration of damaged peripheral 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 have a high metabolic rate. All neurons have a cell body (soma) and one or more slender processes.

• Cell Body (Soma): Contains the nucleus and is the biosynthetic and metabolic center of the neuron.

• Dendrites: Thick arms arising from the soma, branching further. They are the receptive regions of the neuron, with dendritic spines increasing connectivity and plasticity.

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• Axon: Each neuron has only one axon, starting at the axon hillock. Axons can be long (nerve fibers), branch, and end in axon terminals. Bundles of axons in the CNS are called tracts; in the PNS, they are called nerves.

Myelin Sheath

Structure and Function

The myelin sheath is a white, fatty substance that coats many axons.

• Function: Protects and electrically insulates axons, increasing the speed of nerve impulse transmission.

• Myelinated Axons: Segmented sheath surrounds most long or large-diameter axons.

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

Myelin Sheath in the PNS

• Formed by Schwann cells wrapping around the axon in a jelly roll fashion.

• One Schwann cell forms one segment of myelin sheath.

• Myelin sheath gaps (nodes of Ranvier) are sites where axon collaterals can emerge.

Myelin Sheath in the CNS

• Formed by processes of oligodendrocytes, not whole cells.

• Each oligodendrocyte can wrap up to 60 axons at once.

• Myelin sheath gap is present; no outer collar of perinuclear cytoplasm.

• Thinnest fibers are unmyelinated but covered by long extensions of adjacent neuroglia.

Structural and Functional Classes of Neurons

Structural Classification

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

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

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

• Unipolar: One process leading away from the soma, branches into a T; dendrites receive signals from sources such as skin and joints, axon leads to the spinal cord.

Functional Classification

Neurons are grouped by the direction nerve impulse travels relative to the CNS:

• Sensory (Afferent) Neurons: Transmit impulses from sensory receptors toward the CNS; almost all are unipolar, with cell bodies located in ganglia in the PNS.

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

• Interneurons (Association Neurons): Perform integrative functions; only found in the CNS; most abundant and usually multipolar.

Summary Table: Structural and Functional Classes of Neurons

Key Terms and Concepts

• Neuron: Excitable cell that transmits electrical signals.

• Neuroglia: Supportive cells in nervous tissue.

• Myelin Sheath: Insulating layer around axons.

• Axon: Long process that conducts impulses away from the neuron cell body.

• Dendrite: Branching process that receives signals.

• Nodes of Ranvier: Gaps in the myelin sheath.

• Ganglia: Collections of neuron cell bodies in the PNS.

Important Equations

Resting Membrane Potential

The resting membrane potential of a neuron is determined by the difference in ion concentrations across the membrane: Where:

• = membrane potential

• = gas constant

• = temperature

• = Faraday's constant

• = extracellular potassium concentration

• = intracellular potassium concentration

Example: Sympathetic vs. Parasympathetic Effects

• Sympathetic: Dilates pupils, accelerates heartbeat, inhibits digestion.

• Parasympathetic: Constricts pupils, slows heartbeat, stimulates digestion.

Additional info:

• Neuroglia play a central role in maintaining the health and function of neurons, including immune defense and metabolic support.

• Plasticity of dendritic spines is crucial for learning and memory.