Structural Divisions of the Nervous System

The nervous system is complex and primarily associated with the brain and spinal cord. It features various areas within the brain, each performing unique functions. Unlike the digestive system's clearly visible organs, the parts of the nervous system often require tools like microscopes to distinguish.

Two Main Parts of the Nervous System

1. Central Nervous System (CNS):

   • Components: Includes the brain and spinal cord, both encased within protective structures; the brain is protected by the skull, while the spinal cord is safeguarded by the vertebral column.

   • Functions: Responsible for processing sensory information, controlling motor functions, and facilitating cognitive processes such as thinking, memory, and emotions. The brain is divided into various regions, including the cerebrum, cerebellum, and brainstem, each with specialized functions.

2. Peripheral Nervous System (PNS):

   • Components: Encompasses all other parts of the nervous system that extend beyond the CNS, including sensory and motor neurons. Some PNS components can be found within the head and spinal regions.

   • The term "peripheral" indicates its location at the outer edges, away from the brain and spinal cord.

   • Subdivisions: The PNS is further subdivided into the somatic nervous system (controls voluntary movements and reflexes) and the autonomic nervous system (regulates involuntary physiological functions, like heart rate and digestion), which can be further divided into the sympathetic (fight or flight response) and parasympathetic (rest and digest) systems.

Nervous Tissue Composition

Nervous tissue consists of two primary cell types:

• Glial Cells:

  • Function: Provide essential support to neurons, including nutritional support, maintenance, and repair of the nervous system. They also play roles in protecting the nervous system from pathogens, insulating neurons, and facilitating signal transmission.

  • Types of glial cells include astrocytes, oligodendrocytes, microglia, and Schwann cells.

• Neurons:

  • Function: Crucial for communication across the nervous system. Each neuron comprises a central cell body (soma) and processes:

    • Axon: The main fiber responsible for transmitting electrical impulses away from the cell body to target cells. Axons can be very long, facilitating communication over significant distances.

    • Dendrites: Extensions that receive input from other neurons, converting signals into electrical impulses that travel toward the soma.

Divisions in Nervous Tissue

Examination reveals regions characterized by their cell composition:

• Gray Matter:

  • Rich in neuron cell bodies and dendrites. Appears pinkish due to blood content or slightly tan from preservation processes. It is primarily involved in processing and integrating information.

• White Matter:

  • Comprised mainly of axons insulated by myelin, a lipid-rich substance that imparts a white appearance. White matter facilitates communication between different parts of the nervous system. The differentiation in appearance helps identify areas of the brain and spinal cord, with gray matter typically found on the outer layers of the brain and in central areas of the spinal cord.

Terminology in the Nervous System

The terminology for structures in the nervous system varies based on location:

• In the CNS:

  • A group of neuron cell bodies is referred to as a nucleus.

• In the PNS:

  • A group of neuron cell bodies is referred to as a ganglion.

• Bundles of Axons:

  • In the CNS, called a tract.

  • In the PNS, referred to as a nerve.

  • Example: The optic nerve travels from the eye to the optic chiasm, where it transitions to an optic tract within the head.

Key Structures Table

Terms to Know

• Optic Chiasm: The point where the optic nerve transitions to the optic tract, signifying the boundary between PNS and CNS components.

• Myelin: A lipid-rich substance that insulates axons, imperative for maintaining the integrity and speed of electrical signal transmission in white matter.

• Neurotransmitters: Chemical messengers released from neuron terminals to communicate with other neurons or target cells, contributing to the overall function of the nervous system.