Chapter 11: The Nervous System – Structure, Function, and Cellular Organization

Nervous System Overview

Introduction to the Nervous System

The nervous system is the master controlling and communicating system of the body. It is responsible for regulating and coordinating all body activities by transmitting signals between different parts of the body.

• Sensory input: Monitoring internal and external stimuli.

• Integration: Interpretation and processing of sensory input.

• Motor output: Initiating a response to stimuli by activating effector organs (muscles or glands).

Organization of the Nervous System

Central Nervous System (CNS)

The CNS consists of the brain and spinal cord. It serves as the integration and command center, processing information and issuing instructions.

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

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

Peripheral Nervous System (PNS)

The PNS is composed of paired spinal and cranial nerves. It links all parts of the body to the CNS and carries messages to and from the CNS.

• Spinal nerves: Carry impulses to and from the spinal cord.

• Cranial nerves: Carry impulses to and from the brain.

• Ganglia: Collections of neuron cell bodies outside the CNS.

Functional Divisions of the Peripheral Nervous System

Sensory (Afferent) Division

The sensory division transmits impulses from sensory receptors to the CNS.

• Somatic afferent fibers: Carry impulses from skin, skeletal muscles, and joints.

• Visceral afferent fibers: Carry impulses from visceral organs (organs within the ventral body cavity).

Motor (Efferent) Division

The motor division transmits impulses from the CNS to effector organs (muscles and glands).

• Somatic nervous system (SNS): Known as the "voluntary nervous system." Composed of somatic motor nerve fibers that conduct impulses from the CNS to skeletal muscles, allowing conscious control of movement.

• Autonomic nervous system (ANS): Known as the "involuntary nervous system." Consists of visceral motor nerve fibers that regulate smooth muscle, cardiac muscle, and glands. The ANS is further subdivided into the sympathetic and parasympathetic branches, which generally have opposing effects on target organs.

Histology of Nerve Tissue

Principal Cell Types

• Neurons: Excitable cells that transmit electrical signals. They are the structural and functional units of the nervous system.

• Neuroglia (glial cells): Supporting cells that surround, insulate, and protect neurons. They also promote neuron health and growth.

Supporting Cells: Neuroglia

Neuroglia, or glial cells, are several types of cells that provide support for neurons:

• Insulate neurons to speed up action potential (AP) conduction.

• Promote health and growth of neurons.

Examples of glial cells:

• Astrocytes (CNS): Most abundant glial cells in the CNS; support and brace neurons, anchor them to nutrient supply lines.

• Microglia (CNS): Small cells that act as phagocytes, removing debris and pathogens.

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

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

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

• Satellite cells (PNS): Surround neuron cell bodies in the PNS; function is not fully understood.

Illustration: The Nervous System

The provided diagram shows the anatomical organization of the nervous system, highlighting the CNS (brain and spinal cord) and the PNS (cranial nerves, spinal nerves, and ganglia). The CNS is centrally located and protected by bone, while the PNS extends throughout the body to connect all regions to the CNS.

Summary Table: Organization of the Nervous System

Division	Main Components	Primary Function
Central Nervous System (CNS)	Brain, Spinal Cord	Integration, command, processing
Peripheral Nervous System (PNS)	Cranial nerves, Spinal nerves, Ganglia	Communication between CNS and body
Sensory (Afferent) Division	Sensory receptors, Afferent fibers	Transmit sensory input to CNS
Motor (Efferent) Division	Somatic and Autonomic nervous systems	Transmit motor output from CNS to effectors

Key Terms and Definitions

• Neuron: A nerve cell; the basic building block of the nervous system, specialized for transmitting electrical impulses.

• Neuroglia (Glial cells): Non-neuronal cells that provide support and protection for neurons.

• Afferent: Conducting or conducted inward or toward something (for nerves, toward the CNS).

• Efferent: Conducting or conducted outward or away from something (for nerves, away from the CNS).

• Somatic: Relating to the body, especially as distinct from the mind or from the viscera.

• Autonomic: Involuntary or unconscious; relating to the autonomic nervous system.

Additional info: Later sections of this chapter would typically cover neuron structure, action potentials, synaptic transmission, and integration, which are essential for a complete understanding of nervous system physiology.