Fundamentals of the Nervous System and Nervous Tissue: Study Notes

Fundamentals of the Nervous System and Nervous Tissue

Main Functions of the Nervous System

The nervous system is the master control and communication system of the body. It is responsible for integrating sensory input, coordinating motor output, and regulating homeostasis.

• 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 cause a response.

• Homeostasis: Maintaining internal balance by regulating physiological processes.

• Mental Activity: Involved in consciousness, memory, and thought.

Structural and Functional Divisions of the Nervous System

The nervous system is divided both structurally and functionally to organize its complex activities.

• Structural Divisions:

  • Central Nervous System (CNS): Consists of the brain and spinal cord; responsible for integration and command.

  • Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS; connects the CNS to the body.

• Functional Divisions:

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

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

    • Somatic Nervous System: Controls voluntary movements of skeletal muscles.

    • Autonomic Nervous System (ANS): Regulates involuntary functions (e.g., cardiac and smooth muscle, glands).

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

      • Parasympathetic Division: Conserves energy and promotes "rest and digest" functions.

Neurons: Structure and Function

Neurons are the basic structural and functional units of the nervous system. They are specialized for rapid communication.

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

• Dendrites: Short, branched processes that receive signals from other neurons and convey them toward the cell body.

• Axon: Long process that transmits impulses away from the cell body to other neurons or effectors.

• Axon Terminals: Endings that form synapses with other cells.

Functional Role: Dendrites receive input, the cell body integrates signals, and the axon conducts impulses to targets.

Structure of a Synapse

A synapse is the junction between two neurons or between a neuron and an effector cell. It enables communication via chemical or electrical signals.

• Presynaptic Neuron: Sends the signal.

• Synaptic Cleft: Small gap between neurons.

• Postsynaptic Neuron: Receives the signal.

• Neurotransmitters: Chemical messengers released into the synaptic cleft.

Classification of Neurons

Neurons can be classified by structure and function.

• Structural Classification:

  • Multipolar: Many dendrites, one axon (most common in CNS).

  • Bipolar: One dendrite, one axon (e.g., retina, olfactory epithelium).

  • Unipolar (Pseudounipolar): Single process that splits into two branches (sensory neurons in PNS).

• Functional Classification:

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

  • Motor (Efferent) Neurons: Transmit impulses away from the CNS to effectors.

  • Interneurons (Association Neurons): Connect sensory and motor neurons within the CNS.

Neuroglia: Types, Location, and Function

Neuroglia (glial cells) support, protect, and nourish neurons. There are six main types:

Type	Location	Function
Astrocytes	CNS	Support neurons, regulate environment, form blood-brain barrier
Microglia	CNS	Phagocytize debris and pathogens
Ependymal Cells	CNS	Line ventricles, produce and circulate cerebrospinal fluid
Oligodendrocytes	CNS	Form myelin sheaths around CNS axons
Satellite Cells	PNS	Support neuron cell bodies in ganglia
Schwann Cells	PNS	Form myelin sheaths around PNS axons

Structure of Myelin Sheaths

Myelin sheaths are multilayered lipid coverings that insulate axons, increasing the speed of nerve impulse conduction.

• Formed by: Oligodendrocytes in the CNS, Schwann cells in the PNS.

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

• Saltatory Conduction: Impulses "jump" from node to node, speeding transmission.

Gray Matter vs. White Matter in the CNS

The CNS contains regions of gray and white matter, each with distinct functions.

• Gray Matter: Contains neuron cell bodies, dendrites, and unmyelinated axons; site of synaptic integration.

• White Matter: Composed mainly of myelinated axons; responsible for communication between different CNS regions.

Nerves: Structure and Components

A nerve is a bundle of axons in the PNS, surrounded by connective tissue layers.

• Endoneurium: Surrounds individual axons.

• Perineurium: Bundles groups of axons into fascicles.

• Epineurium: Encloses the entire nerve.

Structural Link Between PNS and CNS

The PNS connects the CNS to limbs and organs, serving as a communication relay. Sensory neurons carry information to the CNS, while motor neurons transmit commands from the CNS to effectors.

Reflexes and Reflex Arcs

A reflex is a rapid, automatic response to a stimulus. Reflex arcs are the neural pathways that mediate reflexes.

• Basic Components of a Reflex Arc:

  1. Receptor

  2. Sensory neuron

  3. Integration center (may involve interneurons)

  4. Motor neuron

  5. Effector

• Monosynaptic Reflex: Involves one synapse (e.g., knee-jerk reflex).

• Polysynaptic Reflex: Involves multiple synapses and interneurons (e.g., withdrawal reflex).

Roles of Interneurons in the CNS

Interneurons are found entirely within the CNS. They process information, form neural circuits, and play a critical role in reflexes, sensory integration, and higher functions such as learning and memory.