Fundamentals of the Nervous System and Nervous Tissue

Chapter 11: Fundamentals of the Nervous System and Nervous Tissue

11.1 Functions of the Nervous System

The nervous system is the master controlling and communicating system of the body. It uses electrical and chemical signals to coordinate rapid, specific, and almost immediate responses to internal and external changes.

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

• Integration: The nervous system processes and interprets sensory input and decides what should be done at each moment.

• Motor Output: The nervous system activates effector organs (muscles and glands) to produce a response.

Example: Seeing a glass of water (sensory input), your brain processes the information (integration), and you reach out to grab it (motor output).

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. It is the integration and control center, interpreting sensory input and dictating motor output.

• Peripheral Nervous System (PNS): Consists of nerves (bundles of axons) that extend from the brain and spinal cord. It includes:

  • Spinal nerves: Carry impulses to and from the spinal cord.

  • Cranial nerves: Carry impulses to and from the brain.

Functional Divisions of the PNS

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

  • Somatic sensory fibers: Carry impulses from skin, skeletal muscles, and joints.

  • Visceral sensory fibers: Carry impulses from visceral organs.

• Motor (Efferent) Division: Transmits impulses from the CNS to effector organs (muscles and glands).

  • Somatic Nervous System: Conducts impulses from the CNS to skeletal muscles; voluntary control.

  • Autonomic Nervous System (ANS): Regulates smooth muscle, cardiac muscle, and glands; involuntary control.

    • Sympathetic Division: Mobilizes body systems during activity (fight or flight).

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

11.2 Neuroglia

Nervous tissue consists of two principal cell types:

• Neuroglia (glial cells): Small cells that surround and wrap delicate neurons, providing support, nutrition, and protection.

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

Neuroglia of the CNS

• Astrocytes: Most abundant, highly branched. Functions include supporting neuronal activity, removing excess ions and neurotransmitters, providing nutrients, and mediating brain development.

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

• Ependymal Cells: Line the central cavities of the brain and spinal cord. Often ciliated, they help circulate cerebrospinal fluid (CSF).

• Oligodendrocytes: Branched cells that form myelin sheaths around CNS nerve fibers, increasing the speed of impulse transmission.

Neuroglia of the PNS

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

• Schwann Cells (Neurolemmocytes): Surround all peripheral nerve fibers and form myelin sheaths in thicker fibers; vital to regeneration of damaged peripheral nerve fibers.

11.3 Neurons

Neurons are the structural units of the nervous system. They are large, highly specialized cells that conduct impulses.

• Extreme longevity: Can function for a person's lifetime.

• Amitotic: With few exceptions, neurons do not divide after maturity.

• High metabolic rate: Require continuous supply of oxygen and glucose.

• All neurons have a cell body and one or more processes.

Neuron Cell Body (Soma or Perikaryon)

• Biosynthetic center: synthesizes proteins, membranes, and chemicals.

• Contains a spherical nucleus with a nucleolus.

• Most cell bodies are located in the CNS (nuclei); in the PNS, clusters are called ganglia.

Neuron Processes

• Dendrites: Short, tapering, diffusely branched processes. They are the receptive (input) region of the neuron and convey incoming messages as graded potentials.

• Axon: Each neuron has a single axon that arises from the axon hillock. Axons can be over 1 meter long and are called nerve fibers. They transmit nerve impulses away from the cell body.

• Axons may have branches (axon collaterals) and branch profusely at their end (axon terminals or terminal boutons).

Axon Functional Characteristics

• Conducting region of the neuron, generating and transmitting nerve impulses along the axolemma to the axon terminal.

• Axon terminals secrete neurotransmitters, which can excite or inhibit other neurons.

Myelination of Axons

• Myelin sheath: A whitish, protein-lipid substance that protects and electrically insulates axons, increasing the speed of nerve impulse transmission.

• Myelinated fibers: Conduct impulses faster than nonmyelinated fibers.

• In the PNS, myelin is formed by Schwann cells; in the CNS, by oligodendrocytes.

• Myelin sheath gaps (nodes of Ranvier): Gaps between adjacent Schwann cells where axon collaterals can emerge.

• White matter: Regions of the brain and spinal cord with dense collections of myelinated fibers.

• Gray matter: Mostly neuron cell bodies and nonmyelinated fibers.

Clusters of Cells vs. Long Fibers: Location in Body

• Nuclei: Clusters of neuron cell bodies in the CNS.

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

• Tracts: Bundles of neuron processes in the CNS.

• Nerves: Bundles of neuron processes in the PNS.

Classification of Neurons

Structural Classification

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

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

• Unipolar (pseudounipolar): One T-like process (two axons); found mainly in the PNS.

Functional Classification

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

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

• Interneurons: Shuttle signals through CNS pathways; most are entirely within the CNS and account for 99% of neurons in the body.

Table: Comparison of Structural Classes of Neurons

Neuron Type	Structure	Location	Function
Multipolar	Many processes (1 axon, many dendrites)	Major neuron type in CNS	Motor and interneurons
Bipolar	Two processes (1 axon, 1 dendrite)	Retina, olfactory mucosa	Sensory (special senses)
Unipolar (Pseudounipolar)	One T-like process (2 axons)	Mainly in PNS, dorsal root ganglia	Sensory neurons

Clinical Connection: Homeostatic Imbalance 11.1

• Certain viruses and bacterial toxins (e.g., polio, rabies, herpes simplex, tetanus toxin) can damage neural tissues by using retrograde axonal transport.

• Research is ongoing to use retrograde transport for gene therapy, such as delivering corrected genes or microRNA to suppress defective genes in neurons.