UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function

UNIT 3 - CHAPTER 10: NERVOUS SYSTEM I, Basic Structure and Function

10-1 LEARNING OUTCOMES:

10.1 General Characteristics of the Nervous System

• Describe the general functions of the nervous system.

• Identify the two types of cells that compose nervous tissue.

• Identify the two major groups of nervous system organs.

• List the functions of sensory receptors.

• Describe how the nervous system responds to stimuli.

10.2 Nervous Tissue Cells: Neurons and Neuroglia

• Describe the parts of a neuron.

• Describe the relationships among myelin, the neurilemma, and nodes of Ranvier.

• Distinguish between the sources of white matter and gray matter.

• Identify structural and functional differences among neurons.

• Identify the types of neuroglia in the central nervous system and their functions.

• Describe the role of Schwann cells in the peripheral nervous system.

10.3 Cell Membrane Potential

• Explain how a cell membrane becomes polarized.

• Describe the events leading to the generation of an action potential.

• Explain how action potentials move down an axon.

• Compare impulse conduction in myelinated and unmyelinated neurons.

10.4 The Synapse

• Explain how information passes from a presynaptic neuron to a postsynaptic cell.

10.5 Synaptic Transmission

• Identify the changes in membrane potential associated with excitatory and inhibitory neurotransmitters.

• Explain what prevents a postsynaptic cell from being continuously stimulated.

10.6 Impulse Processing

• Describe the basic ways in which the nervous system processes information.

10.1 GENERAL CHARACTERISTICS OF THE NERVOUS SYSTEM

• The nervous system is a network of cells that sense and respond to stimuli in ways that maintain homeostasis.

• Composition of the nervous system includes:

  • Neural tissue, comprised of:

  • Neurons

  • Neuroglial cells

  • Blood vessels

  • Connective tissue.

• Functions of Neurons:

  • Processes that receive information (dendrites) and send bioelectric signals (axons) in the form of neurotransmitters across synapses.

• Major groups of organs in the nervous system are:

  • Central nervous system (CNS)

  • Peripheral nervous system (PNS)

10.2 NERVOUS TISSUE CELLS: NEURONS AND NEUROGLIA

• The main function of the nervous system is to coordinate all body systems through electrochemical signals from body parts to the brain and vice versa.

A. Division of Nervous System Organs

• Major groups:

  1. Central Nervous System (CNS)

  • Comprises the brain and spinal cord.

  1. Peripheral Nervous System (PNS)

  • Includes cranial nerves and spinal nerves extending from the brain and spinal cord.

B. Major Functions of the Nervous System

1. Sensory Input Function

   • Comprises the PNS and sensory receptors that detect environmental changes (stimuli).

   • Sensory receptors transmit sensory impulses to the CNS via sensory neurons.

2. Integrative Function

   • Occurs in the CNS (brain/spinal cord) involving interpretation of incoming sensory impulses.

   • Integration occurs through interneurons.

   • Initiates motor impulses based on sensory input.

3. Motor Function

   • Involves the PNS, leading to the response of body parts.

   • Motor impulses travel from the CNS to effectors via motor neurons.

   • Effectors are:

     • Muscles: that contract

     • Glands: including endocrine (hormone-secreting) & exocrine (substance-secreting) glands.

C. Levels of Organization of the Nervous System

• Divides into:

  • Central Nervous System: Brain & Spinal Cord

  • Peripheral Nervous System: Cranial Nerves & Spinal Nerves

  • Interneurons: Integrating signals in CNS

• Sensory (Input into CNS) - Motor (Output from CNS)

  • Afferent Neurons: Sensory

  • Efferent Neurons: Motor

• Somatic vs. Autonomic:

  • Somatic: Conscious control (Effectors: Skeletal Muscle)

  • Autonomic: Unconscious control (Effectors: Smooth Muscle, Cardiac Muscle, Glands)

• Divisions of Autonomic:

  • Parasympathetic: Homeostasis (NT: Acetylcholine)

  • Sympathetic: Fight-or-Flight (NT: Norepinephrine)

10.3 CELL MEMBRANE POTENTIAL

A. Neuron Structure

• A neuron is the structural and functional unit of the nervous system; it is a nerve cell composed of:

  • Cell Body: (soma or perikaryon)

  • Contains typical organelles excluding centrioles.

  • Notable components include:

    • Nucleus

    • Prominent nucleolus

    • Many Nissl bodies (chromatophilic substance/ RER).

  • Neurofibrils provide structural support extending into axons.

  • Neuron Processes/Nerve Fibers: Extensions from the cell body, categorized as:

  1. Dendrites:

     • Many per neuron

     • Short and branched

     • Receptive portion, carrying impulses toward the cell body.

  2. Axons:

     • One per neuron

     • Long and thin, transporting impulses away from the cell body.

     • Terminates in axonal terminals or synaptic knobs.

  • Myelination in CNS vs. PNS:

  • CNS:

    • Myelin produced by oligodendrocytes denotes white matter.

    • Gray matter consists of bundling cell bodies/unmyelinated fibers.

  • PNS:

    • Large axons have a myelin sheath from Schwann cells.

    • Nodes of Ranvier interrupt myelin sheaths; smaller axons lack this sheath but are also associated with Schwann cells.

10.4 THE SYNAPSE

• Synapses serve as junctions between neurons where nerve impulses are transferred.

  1. Synapses occur between:

  • Axon of presynaptic neuron

  • Dendrites or cell body of a postsynaptic neuron.

  1. The gap between the two neurons is designated as the synaptic cleft.

10.5 SYNAPTIC TRANSMISSION

A. Steps in Synaptic Transmission

1. The impulse reaches the axonal terminal of the presynaptic neuron, causing depolarization of the terminal.

2. Opening of Ca²⁺ channels allows calcium ions to influx, triggering the release of neurotransmitters via exocytosis into the synaptic cleft.

3. Neurotransmitters diffuse across the synaptic cleft, leading to:

   • Depolarization of the postsynaptic neuron's membrane.

4. Triggering an action potential (AP) in the postsynaptic neuron initiates an impulse.

B. Synaptic Potentials

1. Postsynaptic neuron responses to neurotransmitter binding include:

   • Excitatory Post-Synaptic Potential (EPSP): results in depolarization.

   • Inhibitory Post-Synaptic Potential (IPSP): results in hyperpolarization.

2. Summation: Occurs when multiple subthreshold stimuli allow the threshold potential to reach and triggers an action potential:

   • Example: +5, +5, +5 = +15 mV = threshold = action potential = impulse.

C. Neurotransmitters (NT)

1. Over 30 types identified in the CNS, with specific distribution among neurons:

   • Acetylcholine (ACh) is typical, functioning in:

     • All motor neurons (stimulating skeletal muscle)

     • Some CNS neurons.

   • Other neurotransmitter classes include:

     • Biogenic Amines (modified amino acids) influencing emotional behavior and circadian rhythm:

     • Examples: Epinephrine, norepinephrine, dopamine, serotonin, histamine.

     • Amino Acids (unmodified):

     • Examples: Glutamate, GABA (gamma-aminobutyric acid).

D. Events Leading to Neurotransmitter Release

• Processes involving enzymatic destruction (e.g., acetylcholinesterase breakdown of ACh) and reuptake of neurotransmitters prevent continuous postsynaptic stimulation.

• Monoamine Oxidase (MAO) inhibitors are prevalent in treating psychological disorders.

E. Neuropeptides and Gases

1. Neuropeptides: Serve as neurotransmitters/neuromodulators, altering neuronal response or blocking release.

   • Examples include enkephalins and endorphins (CNS), which alleviate pain, and Substance P (PNS).

2. Gases: Nitric Oxide influences memory in the CNS and vasodilation in the PNS.

F. Disorders Linked to Neurotransmitter Imbalances

• Various mental and neurologic conditions correlate with neurotransmitter levels. Examples:

  • Clinical Depression: Deficits in norepinephrine/serotonin

  • Parkinson's Disease: Dopamine deficiency

  • Schizophrenia: GABA deficiency leading to excess dopamine

  • Tardive Dyskinesia: Dopamine deficiency.

G. Drugs Modulating Neurotransmitter Levels

1. Various drugs affect neurotransmitter actions and levels:

   • Tryptophan stimulates serotonin synthesis, enhancing sleepiness.

   • Reserpine reduces norepinephrine packaging, lowering blood pressure.

   • Curare blocks ACh receptor binding, resulting in muscular paralysis.

   • Valium enhances GABA effects, decreasing anxiety.

   • Cocaine inhibits dopamine reuptake, leading to euphoria.

10.6 IMPULSE PROCESSING

A. Neuronal Pools

• Composed of interconnected neurons that synapse and facilitate processing through general excitation, making stimulation easier.

B. Convergence

• Multiple neurons converge on fewer neurons, allowing for summation, typically seen in motor pathways.

C. Divergence

• Fewer neurons extend to signal many, amplifying signals, often observed in sensory pathways, explaining varied responses to stimuli like odors.

NERVE PATHWAY SUMMARY

Overview

• Overview of sensory, integrative, and motor neuron pathways, specifying:

  • Central vs peripheral involvement

  • Types of neurons

  • Stimuli activating the receptive portions of neurons

  • Directions of impulse transmissions within neurons

  • Chemical transition of impulses between neurons/effectors via neurotransmitter activity.

KEYED SUMMARY TABLE