Study Notes on Nervous System Fundamentals

Human Anatomy and Physiology

Chapter 11: Fundamentals of the Nervous System and Nervous Tissue

Sources

  • PowerPoint® Lectures: Slides prepared by Karen Dunbar Kareiva, Ivy Tech Community College
  • Copyright: © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved

Introduction

  • Understanding neurotransmitter function is crucial for awareness of how drugs affect the nervous system.

11.1 Functions of the Nervous System

  • The nervous system acts as the master control and communication system of the body.
  • Communication Mechanisms:
    • Cells communicate through electrical and chemical signals, which are rapid, specific, and lead to almost immediate responses.
Three Overlapping Functions
  1. Sensory Input:
    • Information gathered by sensory receptors regarding internal and external changes.
  2. Integration:
    • Processing and interpretation of sensory input to produce an appropriate response.
  3. Motor Output:
    • Activation of effector organs (muscles and glands) to produce a response.
Divisions of the Nervous System
  • The nervous system is divided into two principal parts:
    • Central Nervous System (CNS):
    • Composed of the brain and spinal cord located within the dorsal body cavity.
    • Functions as the integration and control center, interpreting sensory input and dictating motor output.
    • Peripheral Nervous System (PNS):
    • Consists of all nervous tissue outside the CNS.
    • Comprises nerves extending from both the brain and spinal cord:
      • Spinal Nerves: To and from the spinal cord.
      • Cranial Nerves: To and from the brain.
Functional Divisions of the PNS
  • PNS is functionally divided into:
    1. Sensory (Afferent) Division:
    • Somatic Sensory Fibers: Convey impulses from skin, skeletal muscles, and joints to the CNS.
    • Visceral Sensory Fibers: Convey impulses from visceral organs to the CNS.
    1. Motor (Efferent) Division:
    • Transmits impulses from the CNS to effector organs (muscles and glands).
    • Divided into:
      • Somatic Nervous System: Voluntary control of skeletal muscles.
      • Autonomic Nervous System (ANS): Involuntary control, regulating smooth muscle, cardiac muscle, and glands.
        • Further divided into:
        • Sympathetic: Mobilizes body systems during activity.
        • Parasympathetic: Promotes resting and maintenance functions.

11.2 Neuroglia

  • Nervous Tissue: Composed of two principal cell types:
    • Neuroglia (Glial Cells): Small cells that support and protect delicate neurons.
    • Neurons (Nerve Cells): Excitable cells that transmit electrical signals.
Types of Neuroglia in the CNS
  1. Astrocytes:
    • Most abundant, highly branched glial cells.
    • Functions include supporting neurons, guiding young neuron migration, controlling chemical environments, and participating in information processing in the brain.
  2. Microglial Cells:
    • Small, ovoid cells that monitor health and can phagocytize damaged cells and debris.
  3. Ependymal Cells:
    • Line the central cavities of the brain and spinal column, producing and circulating cerebrospinal fluid (CSF).
  4. Oligodendrocytes:
    • Branched cells that form insulating myelin sheaths around CNS nerve fibers.
Neuroglia in the PNS
  1. Satellite Cells:
    • Surround neuron cell bodies, function similarly to astrocytes.
  2. Schwann Cells (Neurolemmocytes):
    • Envelop peripheral nerve fibers and form myelin sheaths, vital for regeneration of damaged peripheral nerves.

11.3 Neurons

  • Neurons are the structural and functional units of the nervous system; they are specialized to conduct impulses.
    • Features:
    • Extreme Longevity: Neurons can last a lifetime.
    • Amitotic: Most do not divide; exceptions exist.
    • High Metabolic Rate: Require a constant supply of oxygen and glucose.
Structure of Neurons
  • Composed of a cell body (perikaryon or soma) and one or more processes (dendrites and axon).
  • Cell Body: Contains the nucleus and is the biosynthetic center of the neuron.
  • Neuron Processes:
    • Dendrites: Short, tapering branches that receive signals and convey them towards the cell body.
    • Axon: The long process that transmits impulses away from the cell body.
      • Axons may have branches (axon collaterals).
      • Termination of an axon is at the axon terminals, which release neurotransmitters.

11.4 Membrane Potentials

  • Neurons have a resting membrane potential of approximately –70 mV, and can rapidly change this potential to generate nerve impulses.
  • Membrane potential is altered by:
    • Ions concentrations across the membrane (primarily K+ and Na+).
    • Number of gated ion channels and their states:
    • Chemically Gated Channels: Open in response to chemical signals.
    • Voltage-Gated Channels: Open based on membrane potential changes.
    • Mechanically Gated Channels: Open when the receptor is physically deformed.

11.5 Graded Potentials

  • Short-lived localized changes in membrane potential triggered by stimulus-induced opening of gated channels.
  • They are named based on their location and function and spread but decay rapidly.

11.6 Action Potentials

  • The principal means by which neurons send signals over long distances; characterized by a brief reversal of the membrane potential.
  • Involves a sequential opening and closing of voltage-gated Na+ and K+ channels.
  • Stages of Action Potential:
    1. Resting State: All gated channels are closed.
    2. Depolarization: Na+ influx due to channel opening raises the membrane potential.
    3. Repolarization: Na+ channels inactivate and K+ channels open, bringing the membrane potential back down.
    4. Hyperpolarization: Some K+ channels remain open, making the neuron more negative than at rest.

11.7 The Synapse

  • Neurons communicate through synapses—junctions that can be chemical or electrical.
    • Presynaptic Neuron: Sends the signal, while Postsynaptic Neuron receives it.
    • Types of Synapses:
    • Chemical Synapses: Most common, involving neurotransmitter release and receptor binding, leading to graded potentials.
    • Electrical Synapses: Less common, involve direct electrical coupling via gap junctions.

11.8 Postsynaptic Potentials

  • Two primary types of postsynaptic potentials:
    • Excitatory Postsynaptic Potential (EPSP): Brings the neuron closer to firing an action potential (often due to Na+ influx).
    • Inhibitory Postsynaptic Potential (IPSP): Makes it less likely for the neuron to fire (often due to K+ efflux or Cl− influx).

11.9 Neurotransmitters

  • The nervous system's “language” includes over 50 identified neurotransmitters classified by chemical structure and function.
    • Types of neurotransmitters include:
    • Acetylcholine (ACh): Key player at neuromuscular junctions.
    • Biogenic Amines: Such as dopamine, norepinephrine, and serotonin.
    • Amino Acids: Notable neurotransmitters include glutamate and GABA.
    • Neuropeptides: Strings of amino acids such as endorphins and substance P.
    • Endocannabinoids: Lipid-based neurotransmitters involved in a variety of physiological processes.

Summary

  • The nervous system is complex and integral for communication and control. Understanding the mechanistic details, from cellular components to functions and intercellular communication, is crucial for comprehending overall bodily function and responses.

Clinical Correlates

  • Discussions of impaired functions, such as in clinical conditions, illustrate the practical implications of the neurobiology concepts discussed in the chapter.