Chapter 11: Nervous System and Nervous Tissue – Structure and Function

Chapter 11: Nervous System and Nervous Tissue

Overview of the Nervous System

The nervous system is the master controlling and communicating system of the body. It utilizes both electrical and chemical signals to regulate perception, thoughts, memory, emotions, actions, and the maintenance of homeostasis.

• Electrical signals: Rapid transmission of information via action potentials along neurons.

• Chemical signals: Communication between neurons and other cells through neurotransmitters.

• Functions: Sensory perception, integration of information, initiation of motor responses, and regulation of homeostasis.

Main Functions of the Nervous System

The nervous system performs three essential functions to maintain body coordination and response:

• Sensory input: Monitoring stimuli occurring both inside and outside the body through sensory receptors.

• Integration: Interpretation and processing of sensory input within the central nervous system (CNS).

• Motor output: Response to stimuli by activating effector organs such as muscles and glands.

Example: Touching a hot object triggers sensory input (pain receptors), integration (processing in the CNS), and motor output (muscle contraction to withdraw the hand).

Organization of the Nervous System

The nervous system is organized into two main divisions, each with specialized subdivisions:

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

• Peripheral Nervous System (PNS): Includes cranial and spinal nerves; connects the CNS to the rest of the body.

Subdivisions of the PNS

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

• Motor (efferent) division: Conducts impulses from the CNS to effectors (muscles and glands).

Motor Division Subdivisions

• Somatic Nervous System: Voluntary control of skeletal muscles.

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

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

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

Histology of Nerve Tissue: Cell Types

Nerve tissue is composed of two principal cell types, each with distinct roles in nervous system function:

• Neurons: Excitable cells that transmit electrical signals. They are responsible for communication within the nervous system.

• Supporting cells (neuroglia or glial cells): Non-excitable cells that provide structural and functional support to neurons.

Functions of Neuroglia (Glial Cells)

• Provide a supportive scaffolding for neurons.

• Segregate and insulate neurons from one another.

• Guide young neurons to proper connections during development.

• Promote health and growth of neurons.

Types of Neuroglia in the CNS and PNS

Neuroglia are classified based on their location and function:

• Astrocytes (CNS): Most abundant; support and brace neurons, regulate the chemical environment.

• Microglial cells (CNS): Defensive cells that act as phagocytes.

• Ependymal cells (CNS): Line cerebrospinal fluid-filled cavities; help circulate cerebrospinal fluid.

• Oligodendrocytes (CNS): Form myelin sheaths around CNS nerve fibers.

• Satellite cells (PNS): Surround neuron cell bodies in the PNS; function similar to astrocytes.

• Schwann cells (PNS): Form myelin sheaths around peripheral nerve fibers; aid in regeneration of damaged fibers.

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

Additional info: Myelin sheaths are essential for rapid signal conduction and insulation of nerve fibers. Damage to myelin (as in multiple sclerosis) impairs nervous system function.