Fundamentals of the Nervous System and Nervous Tissue

Functions of the Nervous System

Overview of Nervous System Functions

The nervous system is the master controlling and communicating system of the body. It operates through rapid and specific electrical and chemical signals, producing almost immediate 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.

Organization of the Nervous System

Central and Peripheral Nervous Systems

The nervous system is divided into two principal parts:

• Central Nervous System (CNS): Composed of the brain and spinal cord, located in the dorsal body cavity. It serves as the integration and control center, interpreting sensory input and dictating motor output.

• Peripheral Nervous System (PNS): Consists mainly of nerves extending from the brain and spinal cord. It connects the CNS to the rest of the body via spinal and cranial nerves.

Functional Divisions of the PNS

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

• Motor (Efferent) Division: Transmits impulses from the CNS to effector organs. Includes the somatic nervous system (voluntary control of skeletal muscles) and the autonomic nervous system (involuntary control of smooth muscle, cardiac muscle, and glands).

Autonomic Nervous System Subdivisions

• Sympathetic Division: Mobilizes body systems during activity.

• Parasympathetic Division: Promotes housekeeping functions during rest.

Nervous Tissue Cells

Types of Cells in Nervous Tissue

Nervous tissue consists of two principal cell types:

• Neuroglia (Glial Cells): Small cells that surround and support neurons.

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

Neuroglia of the CNS

• Astrocytes: Most abundant, versatile, and highly branched glial cells. They support and brace neurons, regulate exchanges between capillaries and neurons, control the chemical environment, respond to nerve impulses, and participate in information processing in the brain.

• Microglial Cells: Small, ovoid cells with thorny processes. They monitor neurons, migrate toward injured neurons, and can transform to phagocytize microorganisms and neuronal debris.

• Ependymal Cells: Line the central cavities of the brain and spinal cord. Their cilia help circulate cerebrospinal fluid (CSF), and they form a permeable barrier between CSF and tissue fluid.

• Oligodendrocytes: Branched cells whose processes wrap CNS nerve fibers, forming insulating myelin sheaths in thicker nerve fibers.

Neuroglia of the PNS

• Satellite Cells: Surround neuron cell bodies in the PNS, functioning similarly to astrocytes.

• Schwann Cells (Neurolemmocytes): Surround all peripheral nerve fibers and form myelin sheaths in thicker nerve fibers. They are vital for the regeneration of damaged peripheral nerve fibers.

Neurons: Structure and Function

Neuron Cell Body

The neuron cell body is the biosynthetic center, synthesizing proteins, membranes, and chemicals. It contains a spherical nucleus with a nucleolus and may contain pigments. The plasma membrane is part of the receptive region, receiving input from other neurons. Most neuron cell bodies are located in the CNS (called nuclei), while clusters in the PNS are called ganglia.

Neuron Processes

Neurons have armlike processes extending from the cell body. The CNS contains both neuron cell bodies and their processes, while the PNS contains chiefly neuron processes. Bundles of neuron processes are called tracts in the CNS and nerves in the PNS. The two types of processes are dendrites and axons.

Dendrites and Axons

• Dendrites: Receptive (input/afferent) regions of the neuron, conveying incoming messages as graded potentials.

• Axon: Each neuron has one axon, which starts at the axon hillock. Axons are the conducting region, generating and transmitting nerve impulses to axon terminals, where neurotransmitters are released.

Myelin Sheath

The myelin sheath is composed of myelin, a whitish, protein-lipid substance. Its functions are to protect and electrically insulate the axon and increase the speed of nerve impulse transmission. Myelinated fibers have segmented sheaths, while nonmyelinated fibers are thin and not wrapped in myelin.

• Myelination in the PNS: Schwann cells form myelin sheaths, with gaps called nodes of Ranvier.

• Myelination in the CNS: Oligodendrocytes form myelin sheaths, with each cell able to wrap multiple axons. White matter consists of myelinated fibers, while gray matter is mostly neuron cell bodies and nonmyelinated fibers.

Key CNS versus PNS Terminology

The following table summarizes important terminology differences between the CNS and PNS:

Term	Definition
Nucleus	A collection of neuron cell bodies in the CNS
Ganglion	A collection of neuron cell bodies in the PNS
Tract	A bundle of axons in the CNS
Nerve	A bundle of axons in the PNS

Classification of Neurons

Structural Classification

Neurons are classified by the number of processes extending from the cell body:

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

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

• Unipolar: One T-like process (two axons); also called pseudounipolar, associated with sensory receptors.

Functional Classification

Neurons are also classified by the direction in which nerve impulses travel relative to the CNS:

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

• Motor (Efferent): Carry impulses from the CNS to effectors (muscles/glands); multipolar, with most cell bodies in the CNS.

• Interneurons (Association Neurons): Lie between motor and sensory neurons, shuttling signals through CNS pathways; most are entirely within the CNS.

Membrane Potentials and Signal Transmission

Resting Membrane Potential

Neurons have a resting membrane potential (RMP), which can rapidly change. Changes in membrane potential occur when ion concentrations across the membrane change or when membrane permeability to ions changes. These changes produce two types of signals:

• Graded Potentials: Incoming signals operating over short distances.

• Action Potentials: Long-distance signals of axons.

Terms describing membrane potential changes relative to RMP:

• Depolarization: Decrease in membrane potential (moves toward zero and above, e.g., from -70mV to -55mV); increases probability of producing an action potential.

• Hyperpolarization: Increase in membrane potential (moves away from zero, e.g., from -70mV to -90mV); decreases probability of producing an action potential.

Graded Potentials

Graded potentials are short-lived, localized changes in membrane potential. The stronger the stimulus, the more voltage changes and the farther current flows. They are triggered by stimuli that open gated ion channels, resulting in depolarization or hyperpolarization. Graded potentials dissipate quickly and decay, serving as signals only over short distances.

Example: Receptor potentials occur in sensory neurons, while postsynaptic potentials occur in neurons receiving input.

Additional info: The notes above expand on brief lecture points with academic context, definitions, and examples to ensure completeness and clarity for exam preparation.