GAP Ch12 The Nervous System and Nervous Tissue

Overview of the Nervous System

• Divisions:

• Central Nervous System (CNS): Composed of the brain and spinal cord. It is the primary processing center for the entire nervous system, integrating sensory information and coordinating responses.

• Peripheral Nervous System (PNS): Consists of all nervous tissue outside the CNS, including cranial and spinal nerves, as well as sensory and motor neurons that connect the CNS to limbs and organs. The PNS is further divided into the somatic nervous system and the autonomic nervous system.

Nervous Tissue

• Types of Cells:

• Neurons: The fundamental units of the nervous system, responsible for communication and signal transmission via electrical impulses, known as action potentials. Neurons are classified into sensory neurons, motor neurons, and interneurons based on their functions.

• Glial Cells (neuroglial cells): Supportive cells that outnumber neurons and provide crucial support functions, including maintenance of homeostasis, forming myelin, and providing support and protection for neurons. Glial cells also play roles in the repair of nervous tissue and neurotransmitter regulation.

Gray and White Matter

• Gray Matter:

• Primarily composed of neuronal cell bodies, dendrites, and synapses. It is involved in muscle control and sensory perception such as seeing and hearing. Areas enriched in gray matter are crucial for processing information.

• White Matter:

• Composed largely of myelinated axons that form connections between different brain regions. Myelin, which gives white matter its color, aids in rapid signal transmission. White matter is essential for communication between different parts of the CNS.

Anatomical Terminology

• Central Nervous System:

• Nucleus: A collection of neuron cell bodies located within the CNS that performs specific functions.

• Peripheral Nervous System:

• Ganglion: A collection of neuron cell bodies located outside the CNS.

• Nerve: A bundle of axons that transmit information to and from the CNS.

• Tract: A bundle of axons within the CNS that connects various brain regions or spinal cord segments.

Functional Divisions

• Basic Functions of the Nervous System:

1. Sensation: Receiving and processing environmental stimuli through specialized receptor cells. This includes a variety of sensory modalities such as touch, taste, sight, hearing, and smell.

2. Integration: The brain integrates incoming sensory information, processing it to form perceptions and memories, facilitating decision-making.

3. Response: Producing specific actions or reactions based on integrated sensory input, which may result in voluntary or involuntary muscle movements.

• Responses:

• Voluntary (Somatic Nervous System): Governs conscious movements by controlling the contraction of skeletal muscle.

• Involuntary (Autonomic Nervous System): Controls involuntary functions such as the contraction of smooth muscle, heart rate regulation, gland activation, and digestive processes.

Enteric Nervous System is often considered part of the autonomic nervous system, specifically regulating gastrointestinal function and communicating with the CNS.

Neuron Structure and Function

• Key Features of Neurons:

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

• Dendrites: Short, branching extensions that receive signals from other neurons, transmitting them towards the cell body.

• Axon: A long, slender projection that carries action potentials away from the cell body to other neurons or muscles.

• Action Potential: An electrical signal that propagates along the axon through a series of depolarization and repolarization of the membrane potential. This rapid change is crucial for communication.

• Myelin Sheath: Insulates the axons and increases the speed of signal transmission; produced by oligodendrocytes in the CNS and Schwann cells in the PNS, which wrap around axons forming a segmented sheath.

Types of Glial Cells

• Central Nervous System:

1. Oligodendrocytes: Glial cells that produce myelin for multiple axons in the CNS, crucial for efficient transmission of impulses.

2. Astrocytes: Star-shaped cells that maintain the chemical environment of neurons, contribute to the blood-brain barrier, and regulate blood flow in the brain.

3. Microglia: The immune cells of the CNS, actively removing damaged neurons and glial cells through phagocytosis, thus contributing to the maintenance of homeostasis.

4. Ependymal Cells: Line the ventricles of the brain and the central canal of the spinal cord, involved in the production and circulation of cerebrospinal fluid (CSF) that cushions and protects the CNS.

• Peripheral Nervous System:

1. Satellite Cells: Functionally similar to astrocytes, they surround neuron cell bodies in ganglia, providing structural support and regulating the microenvironment.

2. Schwann Cells: Play a crucial role in myelination of individual axons in the PNS and also aid in the repair of injured nerves.

Example of Nervous Function

• Scenario: Adjusting Shower Temperature

1. Sensation: Temperature receptors in the skin detect the water temperature, highlighting environmental stimuli.

2. Action Potential: The sensory neuron generates an action potential, traveling along the neural pathways to the spinal cord (dorsal root).

3. Integration: The signal is processed in the brain, particularly in the thalamus, which compares current information with past experiences and memories regarding temperature.

4. Response: An upper motor neuron sends signals through descending tracts to a lower motor neuron in the spinal cord, leading to skeletal muscle contraction as the individual withdraws from the hot water.

• Neural Pathways:

• Ascending Tracts: Carry sensory information from the body up to higher brain regions for processing.

• Descending Tracts: Transmit motor commands from brain areas to the spinal cord, ultimately leading to effector responses in muscle tissues.

Summary

• The nervous system is a complex and intricate network responsible for processing sensory information, integrating this data, and generating appropriate responses. A comprehensive understanding of its anatomy and functions is essential for grasping how the body interacts dynamically with its environment, enabling coordinated activities and responses to various stimuli.