The Nervous System

Overview

The nervous system is a complex network responsible for coordinating the body's activities by transmitting signals to and from different parts of the body. It is essential for sensation, movement, cognition, and homeostasis.

Organization of the Nervous System

Main Divisions

• Central Nervous System (CNS):

  • Brain: The control center for processing information and directing responses.

  • Spinal Cord: Conducts signals to and from the brain and controls reflex activities.

• Peripheral Nervous System (PNS):

  • Sensory (Afferent) Division: Carries sensory information from receptors to the CNS.

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

    • Somatic System: Controls voluntary movements by innervating skeletal muscle.

    • Autonomic System: Controls involuntary responses by innervating smooth muscle, cardiac muscle, and glands.

Nervous Tissue

Cell Types

• Neuroglial (Glial) Cells:

  • Support, nourish, and protect neurons.

  • Do not transmit nerve impulses.

  • Include several types, such as Schwann cells (PNS) and Oligodendrocytes (CNS).

• Neurons:

  • Specialized for generating and transmitting nerve impulses (signals).

Neuron Structure

Major Components

• Dendrites: Branch-like processes that receive incoming messages and conduct them toward the cell body.

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

• Axon: Single, elongated process that conducts signals away from the cell body.

  • Axon Hillock: The region where the axon attaches to the soma; site of action potential initiation.

  • Axon Terminals (Synaptic Knobs): Contain neurotransmitters for communication with other cells.

Classification of Neurons

Functional Classification

• Sensory (Afferent) Neurons: Transmit sensory information from receptors in the PNS to the CNS.

• Interneurons: Connect neurons within the CNS; responsible for integration and processing.

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

Structural Classification

• Unipolar Neurons:

  • Have a single process that branches into two; axon originates from the dendrite, bypassing the cell body.

  • All unipolar neurons are sensory neurons.

• Multipolar Neurons:

  • Have multiple dendrites attached to the cell body; axon originates from the cell body.

  • Most common type; includes interneurons and motor neurons.

Neuroglial Cells

Functions and Types

• Outnumber neurons in nervous tissue.

• Do not transmit electrical signals.

• Provide structural support, insulation, and nutrients to neurons.

• Schwann Cells: Produce myelin in the PNS.

• Oligodendrocytes: Produce myelin in the CNS.

Myelin and Myelination

What is Myelin?

• Myelin is a protective, fatty sheath that wraps around axons.

• Functions as an insulator, increasing the speed of nerve impulse conduction.

• Produced by neuroglial cells (Schwann cells in PNS, oligodendrocytes in CNS).

Myelin Sheath Formation

• In the PNS: Schwann cells wrap their plasma membrane around a segment of an axon, forming the myelin sheath.

• In the CNS: Oligodendrocytes extend multiple processes to wrap around segments of several axons.

• Myelination begins during fetal development and continues rapidly in infancy.

Functions of Myelin

• Insulates axons, preventing electrical signal loss.

• Increases the speed of action potential propagation via saltatory conduction (jumping from node to node).

• Essential for proper nervous system function; demyelination leads to neurological diseases.

Summary Table: Neuroglial Cells and Myelin Production

Example: Myelin Sheath in the PNS

• Each Schwann cell myelinates a single segment of one axon.

• Nodes of Ranvier are gaps between myelinated segments, crucial for rapid signal transmission.

Additional info: Myelination is critical for efficient nervous system communication. Diseases such as multiple sclerosis involve the loss of myelin, leading to impaired nerve function.