Fundamentals of the Nervous System and Nervous Tissue

Fundamentals of the Nervous System and Nervous Tissue

Functions of the Nervous System

The nervous system is the primary control and communication system of the body, utilizing electrical and chemical signals to produce rapid and specific responses. Its functions are essential for maintaining homeostasis and coordinating bodily activities.

• Sensory Input: The nervous system gathers information from internal and external environments through sensory receptors.

• Integration: The central nervous system interprets sensory input and decides on an appropriate response.

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

• Example: Touching a hot surface triggers sensory input (pain receptors), integration (spinal cord processes), and motor output (muscle contraction to withdraw hand).

Structural and Functional Divisions of the Nervous System

The nervous system is divided into structural and functional components, each with specialized roles.

• Central Nervous System (CNS): Consists of the brain and spinal cord; serves as the control center for processing information.

• Peripheral Nervous System (PNS): Composed of nerves extending from the CNS; connects the CNS to the rest of the body.

• Functional Divisions of PNS:

  • Sensory (Afferent) Division: Transmits information to the CNS.

    • Somatic Sensory: From skin, muscles, and joints.

    • Visceral Sensory: From internal organs.

  • Motor (Efferent) Division: Carries commands from the CNS to effectors.

    • Somatic Nervous System: Voluntary control of skeletal muscles.

    • Autonomic Nervous System (ANS): Involuntary control of smooth/cardiac muscle and glands.

      • Sympathetic Division: "Fight or flight" responses.

      • Parasympathetic Division: "Rest and digest" responses.

Types of Neuroglia and Their Functions

Neuroglia, or glial cells, are support cells in the nervous system that maintain the environment for neurons and provide various functions.

• CNS Glia:

  • Astrocytes: Support neurons, facilitate nutrient exchange, and regulate the chemical environment.

  • Microglia: Act as immune defense cells, performing phagocytosis of pathogens and debris.

  • Ependymal Cells: Line CNS cavities and help circulate cerebrospinal fluid (CSF).

  • Oligodendrocytes: Form myelin sheaths in the CNS, insulating axons and speeding up signal transmission.

• PNS Glia:

  • Satellite Cells: Support neuron cell bodies in ganglia.

  • Schwann Cells: Form myelin sheaths in the PNS and assist in axon regeneration.

• Example: Oligodendrocytes myelinate multiple CNS axons, while Schwann cells myelinate a single PNS axon.

Neurons: Structure and Function

Neurons are the functional units of the nervous system, specialized for transmitting electrical signals.

• Characteristics: Long-lived, mostly non-dividing, and have a high metabolic rate requiring continuous supply of oxygen and glucose.

• Cell Body (Soma): Contains the nucleus and organelles; site of metabolic activity.

• Processes:

  • Dendrites: Receive incoming signals and generate graded potentials.

  • Axons: Conduct action potentials away from the cell body; terminate in axon terminals that release neurotransmitters.

  • Myelin Sheath: Insulates axons, increasing the speed of electrical conduction.

    • PNS: Formed by Schwann cells.

    • CNS: Formed by oligodendrocytes.

    • Nodes of Ranvier: Gaps in myelin sheath where action potentials are regenerated.

• Example: In multiple sclerosis, myelin sheaths in the CNS are damaged, leading to impaired nerve conduction.

Transport Along the Axon

Axonal transport is essential for moving materials between the neuron cell body and axon terminals.

• Anterograde Transport: Moves substances away from the soma, such as mitochondria and vesicles.

• Retrograde Transport: Moves substances toward the soma, including waste products and signaling molecules.

• Example: Viruses can exploit retrograde transport to reach the neuron cell body.

Classification of Neurons

Neurons are classified based on their structure and function, reflecting their roles in the nervous system.

• Structural Classification:

  • Multipolar: One axon and multiple dendrites; most common type in CNS.

  • Bipolar: One axon and one dendrite; found in sensory organs like the retina and olfactory epithelium.

  • Unipolar: Single process; primarily sensory neurons in the PNS.

• Functional Classification:

  • Sensory (Afferent) Neurons: Transmit impulses toward the CNS.

  • Motor (Efferent) Neurons: Carry impulses from the CNS to effectors (muscles/glands).

  • Interneurons: Connect sensory and motor neurons; most abundant in the CNS.

Comparison: Nucleus vs. Ganglion and Nerve vs. Tract

Understanding terminology is important for distinguishing structures in the nervous system.

Term	Definition	Location
Nucleus	Cluster of neuron cell bodies	CNS
Ganglion	Cluster of neuron cell bodies	PNS
Nerve	Bundle of axons	PNS
Tract	Bundle of axons	CNS

Myelin Sheath: Formation and Clinical Importance

The myelin sheath is a multilayered lipid and protein covering that insulates axons, facilitating rapid transmission of electrical impulses.

• Formation in PNS: Schwann cells wrap around axons, forming the myelin sheath.

• Formation in CNS: Oligodendrocytes extend processes to multiple axons, forming myelin sheaths.

• Clinical Relevance: In multiple sclerosis, the immune system attacks CNS myelin, leading to neurological deficits.

• Example: Myelinated axons conduct impulses much faster than unmyelinated axons due to saltatory conduction.

Additional info:

• Saltatory conduction refers to the jumping of action potentials between nodes of Ranvier, greatly increasing conduction speed.

• Neuroglia outnumber neurons and are essential for maintaining the health and function of nervous tissue.