The Nervous System and Nervous Tissue

Introduction to the Nervous System

The nervous system is a highly intricate network that integrates not just biological components but also increasingly incorporates technology to restore functions following traumatic events, such as brain injuries or spinal cord damage. Future advancements in neurotechnology hold the promise of further augmenting our nervous systems, potentially enhancing cognitive and physical abilities in individuals.

Chapter Objectives

• Understand the major anatomical and functional divisions of the nervous system.

• Distinguish between gray matter and white matter, outlining their structural and functional significance.

• Identify and describe the components of a multipolar neuron, focusing on polarity and function.

Overview of the Nervous System

The nervous system is one of the most complex organ systems known, responsible for processing sensory information, coordinating responses, and facilitating communication between different body parts. Current progress in neuroscience, fueled by advanced technologies and research methodologies, continues to unfold new insights at an impressive rate. Understanding the structure of the nervous system requires an emphasis on its larger divisions. At a fundamental level, nervous tissue comprises two essential types of cells: neurons and glial cells, each playing critical roles in system function.

12.1 Basic Structure and Function of the Nervous System

Divisions of the Nervous System

• Central Nervous System (CNS): Comprising the brain and spinal cord, the CNS is enclosed within the cranial cavity and vertebral column, respectively. It serves as the primary processing center for the entire nervous system, integrating sensory data and directing motor responses.

• Peripheral Nervous System (PNS): This system includes all neural elements outside the CNS, including ganglia and nerves. It connects the CNS to limbs and organs, facilitating communication between the brain and the peripheral body.

Gray Matter vs. White Matter

• Gray Matter: Contains the cell bodies and dendrites of neurons. This area is integral to processing and interpreting signals; it may appear pink or tan based not solely on cellular structure but also on preservation techniques used in preparation.

• White Matter: Composed primarily of myelinated axons, white matter appears white due to myelin insulation. This structure is crucial for signal transmission over long distances within the nervous system.

Functions of the Nervous System

The nervous system fulfills several vital functions, primarily categorized as:

• Sensation: Detection of environmental stimuli through sensory receptors.

• Integration: Combining sensory input with stored memories, emotional context, and cognitive processes to produce appropriate responses.

• Response: Execution of motor functions leading to bodily actions.

Controlling of the Body

• somatic nervous system: A division of the peripheral nervous system that controls voluntary movements of skeletal muscles and transmits sensory information to the central nervous system.

• autonomic nervous system: A division of the peripheral nervous system that regulates involuntary bodily functions, such as heart rate, digestion, and respiratory rate, without conscious control.

• enteric nervous system: A complex network of neurons that governs the function of the gastrointestinal system, often referred to as the "second brain" due to its ability to operate independently of the central nervous system.

The Structure of Neurons

Neuron Anatomy

Neurons, the fundamental units of the nervous system, consist of several key components:

• Soma (Cell Body): This part contains the nucleus and essential organelles, supporting cellular metabolism and integration of synaptic signals.

• Dendrites: Branch-like structures that receive signals from other neurons, increasing the neuron's surface area for potential synaptic contacts.

• Axon: The long projection that conducts electrical impulses away from the neuron's cell body toward other neurons or muscles.

Types of Neurons

• Unipolar Neurons: Feature a single process extending from the cell body, typically found in sensory neurons.

• Bipolar Neurons: Have two processes, common in sensory systems such as vision and olfaction.

• Multipolar Neurons: Possess one axon and multiple dendrites, constituting the majority of neurons in the CNS, allowing for the integration of vast amounts of information.

Glial Cells in the Nervous System

Types of Glial Cells

The nervous system also relies on glial cells, which support and protect neurons:

• CNS Glial Cells:

  • Astrocytes: Provide structural support, regulate the chemical balance of the extracellular fluid, and contribute to the blood-brain barrier (BBB), which protects the brain from harmful substances.

  • Oligodendrocytes: Form myelin sheaths around axons in the CNS, greatly enhancing signal transmission speeds.

  • Microglia: Act as immune defenders, responding to injury or disease within the CNS.

  • Ependymal Cells: Line the ventricles of the brain and produce cerebrospinal fluid (CSF), serving protective and cushioning functions.

• PNS Glial Cells:

  • Satellite Cells: Offer structural support and regulate the environment around neurons in ganglia.

  • Schwann Cells: Responsible for myelinating axons in the PNS, crucial for rapid signal conduction.

Myelination and Action Potential

Myelin Function

Myelin sheaths, formed by glial cells, play a vital role in insulating axons, which facilitates faster transmission of electrical signals by allowing the action potential to jump between nodes of Ranvier, a process known as saltatory conduction.

Action Potential

• Generation: Action potentials are initiated when a neuron's membrane potential reaches a specific threshold voltage, leading to a rapid change in the membrane potential.

• Phases: The action potential consists of three main phases: depolarization (influx of sodium ions), repolarization (efflux of potassium ions), and hyperpolarization (overshoot leading to a more negative membrane potential), each dictated by the movement of specific ions across the membrane.

Communication Between Neurons

• graded potentials: These are changes in the membrane potential that vary in magnitude and can summate, influencing whether an action potential will be initiated.

  • generator potential: This is a type of graded potential that occurs in sensory neurons, initiated by a stimulus that causes a local depolarization, ultimately leading to the generation of an action potential if the threshold is reached.

  • receptor potential: This is another form of graded potential, occurring in response to specific stimuli at sensory receptors, which can also lead to the generation of an action potential if the depolarization reaches the necessary threshold.

  • postsynpathetic potantial: This refers to a graded potential that occurs in the postsynaptic neuron after neurotransmitter binding, which can vary in amplitude and contribute to the overall excitability of the neuron, potentially leading to the initiation of an action potential if the threshold is met.

    • excitatory postsynaptic potential: This is a type of postsynaptic potential that results from the binding of excitatory neurotransmitters, leading to a depolarization of the postsynaptic membrane and increasing the likelihood of an action potential being generated if the threshold is reached.

    • inhibitory postsynaptic potential: This is a type of postsynaptic potential that occurs when inhibitory neurotransmitters bind to receptors on the postsynaptic neuron, resulting in hyperpolarization of the membrane and decreasing the likelihood of an action potential being generated.

  • summate: This term refers to the process by which multiple postsynaptic potentials combine, either through temporal or spatial summation, to influence the overall membrane potential of the neuron, thus determining whether the neuron will reach the threshold to fire an action potential.

    • spatial summation: This occurs when multiple excitatory or inhibitory postsynaptic potentials from different presynaptic neurons arrive at the postsynaptic neuron simultaneously, allowing their effects to add together and potentially reach the threshold for action potential generation.

    • temporal summation: This occurs when a single presynaptic neuron fires multiple action potentials in rapid succession, leading to successive excitatory or inhibitory postsynaptic potentials that accumulate over time and can result in the depolarization of the postsynaptic membrane, increasing the chance of reaching the threshold for an action potential.

Synapses

• Chemical Synapses: These synapses involve the release of neurotransmitters from the presynaptic neuron that bind to receptors on the postsynaptic neuron, resulting in changes in its membrane potential and propagation of the signal.

• Electrical Synapses: These synapses allow for direct ionic flow between connected cells through gap junctions, facilitating rapid signal transmission.

Neurotransmitter Systems

• Cholinergic System: Involves acetylcholine, crucial for muscle activation and involved in various cognitive functions.

• Amino Acids: Includes main neurotransmitters like glutamate (excitatory) and GABA (inhibitory), which are essential for normal brain function.

• Biogenic Amines: Such as dopamine and serotonin, play diverse roles in mood regulation, arousal, and many other functionalities.

• Neuropeptides: Long chains of amino acids that modulate neurotransmission and can influence various physiological responses.

Neurological Disorders and Diseases

Neurological disorders, such as multiple sclerosis, arise from the demyelination of neurons. This deterioration of myelin affects signal transmission capabilities, resulting in symptoms that can include motor dysfunction, sensory loss, and cognitive impairments.

Conclusion

A comprehensive understanding of the structure, function, and communication mechanisms of the nervous system is foundational for grasping how this complex system controls and coordinates various bodily functions, influencing both physical health and cognitive abilities.