Chapter 11: Fundamentals of the Nervous System and Nervous Tissue – Study Notes

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 has three overlapping functions:

• Sensory input: Information is gathered by sensory receptors about internal and external changes.

• Integration: Processing and interpretation of sensory input occurs in the control center.

• Motor output: Activation of effector organs (muscles and glands) produces 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): Consists 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): The portion of the nervous system outside the CNS, mainly composed of nerves that extend from the brain and spinal cord. It includes spinal nerves (to and from the spinal cord) and cranial nerves (to and from the brain).

Functional Divisions of the PNS

• Sensory (afferent) division: Somatic sensory fibers convey impulses from skin, skeletal muscles, and joints to the CNS. Visceral sensory fibers convey impulses from visceral organs to the CNS.

• Motor (efferent) division: Transmits impulses from the CNS to effector organs (muscles and glands). It is further divided into:

  • Somatic nervous system: Conducts impulses from the CNS to skeletal muscle; voluntary control.

  • Autonomic nervous system (ANS): Regulates smooth muscle, cardiac muscle, and glands; involuntary control. The ANS is subdivided into:

    • 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 wrap delicate neurons, providing support and protection.

• 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 that monitor neurons. They migrate toward injured neurons and can transform to phagocytize microorganisms and neuronal debris.

• Ependymal cells: May be ciliated; line the central cavities of the brain and spinal column. Their cilia help circulate cerebrospinal fluid (CSF), and they form a permeable barrier between CSF and tissue fluid bathing CNS cells.

• 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 and act like astrocytes.

• Schwann cells (neurolemmocytes): Surround all peripheral nerve fibers and form myelin sheaths in thicker nerve fibers, similar in function to oligodendrocytes. They are vital to the regeneration of damaged peripheral nerve fibers.

Neurons: Structure and Function

Neuron Cell Body

Neurons are the structural units of the nervous system. They are large, highly specialized cells that conduct impulses. The 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 that receives 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 that extend 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. There are two types of processes:

• Dendrites: Receptive (input/afferent) region of the neuron; convey incoming messages as graded potentials.

• Axon: Conducting region; generates and transmits nerve impulses along the axolemma to the axon terminal, which secretes neurotransmitters.

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 surrounding most long or large-diameter axons.

• Myelination in the PNS: Myelin sheath gaps (nodes of Ranvier) are gaps between adjacent Schwann cells. Nonmyelinated fibers are thin fibers not wrapped in myelin but surrounded by Schwann cells.

• Myelination in the CNS: Myelin sheaths are formed by oligodendrocyte processes. Each cell can wrap up to 60 axons at once. White matter consists of myelinated fibers, while gray matter is mostly neuron cell bodies and nonmyelinated fibers.

Key CNS versus PNS Terminology

Terminology Table

The following table summarizes key 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:

• 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. Peripheral process is associated with sensory receptors, and the proximal process enters the CNS.

Functional Classification

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

• Interneurons (association neurons): Lie between motor and sensory neurons; shuttle signals through CNS pathways and are mostly within the CNS.

Membrane Potentials

Resting Membrane Potential and Changes

Neurons have a resting membrane potential (RMP), which can rapidly change. Changes in membrane potential occur when ion concentrations across the membrane change or 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.

Depolarization is a decrease in membrane potential (moves toward zero), making the inside less negative and increasing the probability of producing an action potential. Hyperpolarization is an increase in membrane potential (moves away from zero), making the inside more negative and decreasing the probability of producing an action potential.

Graded Potentials

Characteristics of 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 sometimes hyperpolarization. Graded potentials are named according to their location and function:

• Receptor potential (generator potential): Graded potentials in receptors of sensory neurons.

• Postsynaptic potential: Graded potentials in neurons.

Once a gated ion channel opens, depolarization spreads from one area of the membrane to the next. However, current flows dissipate quickly and decay, so graded potentials are signals only over short distances.

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