The Nervous System: Structure, Function, and Organization

The Nervous System

Overview and Major Functions

The nervous system is the master control and communication system in the body. It is responsible for regulating and coordinating all body activities through electrical and chemical signals. The nervous system has three major, overlapping functions:

• Sensory input: The process of receiving information from sensory receptors about internal and external changes.

• Integration: The interpretation and processing of sensory input, allowing the nervous system to make decisions.

• Motor output: The activation of effector organs (muscles and glands) to produce a response.

Example: Touching a hot surface triggers sensory input (pain receptors), integration (processing in the brain/spinal cord), and motor output (moving your hand away).

Organization of the Nervous System

Divisions of the Nervous System

The nervous system is divided into two main parts:

• Central Nervous System (CNS):

  • Consists of the brain and spinal cord.

  • Acts as the integration and command center.

  • Important for processing information and making decisions.

• Peripheral Nervous System (PNS):

  • Includes all nervous tissue outside the CNS.

  • Spinal nerves connect the CNS to the body via the spinal cord.

  • Cranial nerves connect the CNS to the head and neck via the brain.

  • The PNS has two main functional divisions:

    • Sensory (afferent) division: Conveys impulses to the CNS from sensory receptors.

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

      • Visceral sensory fibers: Carry information from internal organs (e.g., lungs, heart).

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

      • Somatic nervous system: Controls voluntary movements of skeletal muscles.

      • Autonomic nervous system: Regulates involuntary functions (e.g., heart rate, digestion).

        • Sympathetic division: Prepares the body for "fight or flight" responses.

        • Parasympathetic division: Promotes "rest and digest" activities.

Nervous Tissue

Cell Types in Nervous Tissue

Nervous tissue is composed of two main cell types:

• Neurons: The excitable cells that transmit electrical signals.

• Neuroglia (glial cells): Support, nourish, and protect neurons.

Neuroglia (Glial Cells)

Neuroglia are non-excitable cells that provide structural and functional support to neurons. There are six types of neuroglia:

• CNS Neuroglia:

  1. Astrocytes

  2. Oligodendrocytes

  3. Microglia

  4. Ependymal cells

• PNS Neuroglia:

  1. Schwann cells

  2. Satellite cells

Functions of Neuroglia:

• Provide structural support for neurons

• Insulate neuronal axons (myelination)

• Maintain the extracellular environment

• Participate in repair and defense (e.g., microglia act as immune cells)

Neuron Structure and Function

Basic Anatomy of a Neuron

Neurons are highly specialized cells with unique structures for transmitting electrical impulses. They have lost their ability to divide and require glucose as their sole energy source.

• Cell Body (Soma):

  • Anatomy: Contains the nucleus and most organelles.

  • Function: Integrates incoming signals and maintains cell health.

• Dendrites:

  • Anatomy: Short, branched extensions from the cell body.

  • Function: Receive incoming signals from other neurons.

• Axon:

  • Anatomy: Long, singular projection from the cell body.

  • Function: Conducts electrical impulses away from the cell body toward other neurons or effectors.

• Myelin Sheath:

  • Anatomy: Fatty insulating layer formed by oligodendrocytes (CNS) or Schwann cells (PNS).

  • Function: Increases the speed of impulse transmission along the axon.

Functional Classification of Neurons

• Sensory (afferent) neurons: Transmit impulses from sensory receptors to the CNS.

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

• Interneurons: Connect sensory and motor neurons within the CNS for integration.

Sample Questions and Applications

• When you reach for a beverage, the somatic nervous system is activated to control voluntary muscle movement.

• The logical sequence of nervous system functions is: Sensory input → Integration → Motor output.

• Analogy: Electrical wire insulation is to oligodendrocytes as peripheral neuron insulation is to Schwann cells.

Neuron Diagram and Labeling

Key parts of a neuron to label and understand:

• Cell body (soma): Site of signal integration and metabolic activity.

• Dendrites: Receive incoming signals.

• Axon: Conducts action potentials away from the cell body.

• Axon terminal: Releases neurotransmitters into the synaptic cleft.

• Myelin sheath: Insulates axon, increasing conduction speed.

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

Example Application: At the axon hillock (initial segment), an action potential is initiated. The signal travels down the axon to the axon terminal, where neurotransmitters (red dots in the synapse) are released to communicate with the next cell (which could be another neuron, muscle, or gland cell).

Table: Comparison of CNS and PNS Neuroglia

Neuroglia Type	Location	Main Function
Astrocytes	CNS	Support neurons, maintain blood-brain barrier
Oligodendrocytes	CNS	Form myelin sheath in CNS
Microglia	CNS	Immune defense, phagocytosis
Ependymal cells	CNS	Line ventricles, produce cerebrospinal fluid
Schwann cells	PNS	Form myelin sheath in PNS
Satellite cells	PNS	Support neuron cell bodies in ganglia

Key Equations and Concepts

• Resting Membrane Potential: The difference in electrical charge across the neuron's membrane at rest, typically about $-70\ \text{mV}$.

• Action Potential: A rapid change in membrane potential that travels along the axon. The basic sequence is:

  • Depolarization: $\text{Na}^+$ influx

  • Repolarization: $\text{K}^+$ efflux

• Ohm's Law (applied to neurons): $V = IR$ Where $V$ is voltage, $I$ is current, and $R$ is resistance.

Additional info: This study guide covers foundational concepts in the anatomy and physiology of the nervous system, including structure, function, and cellular components. Understanding these basics is essential for further study in neurobiology and related health sciences.