Fundamentals of the Nervous System and Nervous Tissue

Functions and Organization of the Nervous System

Overview of Nervous System Functions

The nervous system is the master controlling and communicating system of the body. It uses electrical and chemical signals to coordinate rapid and specific responses. The nervous system performs three main functions:

• Sensory Input: Gathering information from sensory receptors about internal and external changes.

• Integration: Processing and interpreting sensory input to determine an appropriate response.

• Motor Output: Activating effector organs (muscles and glands) to produce a response.

Layout and Organization of the Nervous System

The nervous system is divided into two principal parts:

• Central Nervous System (CNS): Consists of the brain and spinal cord, serving as the integration and control center.

• Peripheral Nervous System (PNS): Composed of nerves extending from the brain and spinal cord (cranial and spinal nerves), connecting the CNS to the rest of the body.

Functional Divisions of the PNS

The PNS is further subdivided into sensory (afferent) and motor (efferent) divisions:

• Sensory (Afferent) Division: Transmits impulses from sensory receptors to the CNS. Includes somatic sensory fibers (from skin, skeletal muscles, joints) and visceral sensory fibers (from visceral organs).

• Motor (Efferent) Division: Transmits impulses from the CNS to effector organs. It is divided into:

  • Somatic Nervous System (SNS): Voluntary control of skeletal muscles.

  • Autonomic Nervous System (ANS): Involuntary control of smooth muscle, cardiac muscle, and glands. The ANS is further divided into sympathetic, parasympathetic, and enteric nervous systems.

Neurons: Structure and Classification

Structure of Neurons

Neurons are the structural and functional units of the nervous system. They are highly specialized cells capable of conducting electrical impulses. Key features include:

• Extreme longevity (can last a lifetime)

• Amitotic (do not divide after maturation, with few exceptions)

• High metabolic rate (require continuous oxygen and glucose)

• Composed of a cell body (soma) and one or more processes (dendrites and axons)

Neuron cell bodies are found in clusters called nuclei (CNS) or ganglia (PNS). Bundles of neuron processes are called tracts (CNS) or nerves (PNS).

Classification of Neurons

Neurons can be classified structurally and functionally:

• Structural Classification:

  • Multipolar: Three or more processes (one axon, multiple dendrites); most common in CNS.

  • Bipolar: Two processes (one axon, one dendrite); rare, found in retina and olfactory mucosa.

  • Unipolar (Pseudounipolar): One T-like process; mainly sensory neurons in PNS.

• Functional Classification:

  • Sensory (Afferent) Neurons: Transmit impulses toward CNS; mostly unipolar; cell bodies in PNS ganglia.

  • Motor (Efferent) Neurons: Carry impulses from CNS to effectors; multipolar; cell bodies in CNS (except some ANS neurons).

  • Interneurons (Association Neurons): Lie between sensory and motor neurons; integrate signals; found entirely in CNS.

Neuroglia (Glial Cells)

Neuroglia in the CNS

Neuroglia are supporting cells in the nervous system. They are not electrically excitable, can divide, and make up about half the volume of the nervous system. There are four main types in the CNS:

• Astrocytes: Most abundant; regulate exchanges between neurons and capillaries.

• Microglial Cells: Act as phagocytes, removing debris and pathogens.

• Ependymal Cells: Line brain and spinal cord cavities; produce and circulate cerebrospinal fluid (CSF).

• Oligodendrocytes: Form myelin sheaths around CNS nerve fibers.

Neuroglia in the PNS

There are two main types of neuroglia in the PNS:

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

• Schwann Cells (Neurolemmocytes): Surround axons and form myelin sheaths; vital for regeneration of damaged peripheral nerve fibers.

Myelination and Nervous Tissue Organization

Myelination of Neurons

The myelin sheath is a fatty insulating layer that surrounds the axons of many neurons, increasing the speed of impulse conduction. In the PNS, myelin is produced by Schwann cells; in the CNS, by oligodendrocytes. The Node of Ranvier is a gap in the myelin sheath where the axon is exposed.

Gray Matter vs. White Matter

Gray matter consists mainly of neuron cell bodies and unmyelinated fibers, found in the cortex and basal nuclei of the brain and the central region of the spinal cord. White matter consists of myelinated nerve fibers, located in deeper brain regions and the outer regions of the spinal cord.

Resting Membrane Potential

Membrane Potentials and Ion Channels

Neurons have a resting membrane potential due to differences in ion concentrations and membrane permeability. The main factors are:

• Differences in K+ and Na+ concentrations inside and outside the cell

• Selective permeability of the plasma membrane to these ions

• Presence of specific ion channels (leakage, ligand-gated, mechanically-gated, voltage-gated)

Establishment and Maintenance of Resting Membrane Potential

The resting membrane potential (typically -70 mV) is established by:

• Unequal distribution of ions across the plasma membrane

• Selective permeability of the membrane (more permeable to K+ than Na+)

• Na+/K+ ATPase pump, which actively transports 3 Na+ out and 2 K+ in

• Most anions cannot leave the cell

Key Equation:

The Nernst equation can be used to calculate the equilibrium potential for a particular ion:

$E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{outside}}{[ion]_{inside}} \right)$

Additional info: R = gas constant, T = temperature in Kelvin, z = charge of the ion, F = Faraday's constant.

Summary Table: Types of Neuroglia

Neuroglia Type	Location	Main Function
Astrocytes	CNS	Support neurons, regulate exchanges with capillaries
Microglial cells	CNS	Phagocytosis of debris and pathogens
Ependymal cells	CNS	Produce and circulate CSF
Oligodendrocytes	CNS	Form myelin sheaths
Satellite cells	PNS	Support neuron cell bodies
Schwann cells	PNS	Form myelin sheaths, aid regeneration