Chapter 13: Organization and Control of Neural Function

Chapter 13: Organization and Control of Neural Function

Functions of the Nervous System

  • The nervous system plays several critical roles, including:

    • Control of Muscles: Directly controls skeletal muscle movement.

    • Regulation: Helps regulate cardiac and visceral smooth muscle activity.

    • Sensory Function: Enables the reception, integration, and perception of sensory information.

    • Cognitive Functions: Provides the substratum necessary for intelligence, anticipation, and judgment.

    • Environmental Adjustments: Facilitates adjustment to an ever-changing external environment.

Major Components of the Nervous System

  • The nervous system is divided into two primary parts:

    • Central Nervous System (CNS):

    • Comprises the brain and spinal cord.

    • Peripheral Nervous System (PNS):

    • Comprises cranial nerves originating from the brain and spinal cord.

    • Pathway Functions:

    • Both systems form incoming (afferent) sensory pathways and outgoing (efferent) motor pathways.

Nervous Tissue Cells

Types of Nervous Tissue Cells

  • Neurons:

  • Functional information-processing cells.

  • Interneurons:

    • Interspersed between afferent and efferent neurons.

    • Modulate and control the body's response to sensory input from both internal and external environments.

  • Neuroglial Cells:

  • Protect the nervous system and provide metabolic support.

Structure of Neurons

  • Neurons consist of three distinct parts:

  • Cell Body or Soma:

    • Contains a large, vesicular nucleus with one or more distinct nucleoli.

    • Holds the same deoxyribonucleic acid (DNA) and genetic code content present in other cells of the body.

    • Contains a well-developed rough endoplasmic reticulum which produces ribonucleic acid (RNA) necessary for protein synthesis.

    • The cytoplasm is filled with ribosomes (RNA masses).

  • Dendrites:

    • Multiple, short, branched extensions conducting information toward the cell body.

    • Main source of information for the neuron.

  • Axons:

    • Long efferent process projecting from the cell body.

    • Provides a physical conduit for transporting materials between the cell body and the synaptic terminals of the axon.

    • Utilizes motor proteins, specifically kinesins and dyneins.

Neuroglial Cells in Detail

  • Central Nervous System (CNS):

  • Astrocytes:

    • Associated with the blood–brain barrier.

  • Oligodendrocytes:

    • Produce myelin, which insulates nerve cells and increases the velocity of nerve impulse conduction (white matter).

  • Microglia:

    • Small phagocytic cells.

  • Peripheral Nervous System (PNS):

  • Schwann Cells:

    • Produce myelin to insulate nerve cells and increase the velocity of nerve impulse conduction (white matter).

  • Satellite Cells:

    • Produce basement membrane.

  • ### Metabolic Requirements of Nervous Tissue

    • The nervous system demands a high proportion of metabolic energy.

    • Glucose:

    • The major fuel for the nervous system.

    • The brain constitutes only 2% of body weight but receives 15% to 20% of the resting cardiac output.

Neurophysiology of Neurons

  • Action Potentials:

    • Neurons communicate using electrical impulses known as action potentials.

    • Mechanism of Action Potentials:

    • Involves the movement of electrical charge along the axon membrane where frequency conveys information.

    • Sodium Influx:

      • Na+ influx in the soma causes depolarization.

      • Triggers voltage-gated Na+ and K+ channels in the axon.

      • K+ efflux repolarizes the membrane.

    • Hypopolarization:

      • Increases the excitability of the postsynaptic neuron to the threshold potential, requiring a smaller subsequent stimulus for the neuron to fire.

    • Hyperpolarization:

      • Brings the membrane potential further from the threshold, decreasing the likelihood of generating an action potential.

Phases of Action Potentials

  • Three Phases:

  • Polarized State:

    • Resting potential; membrane is polarized due to large charge separation.

  • Depolarization:

    • Membrane becomes suddenly permeable to sodium ions.

  • Repolarization:

    • Polarity of resting membrane potential is reestablished via sodium–potassium adenosine triphosphatase (Na+–K+ ATPase) pump.

Synaptic Transmission

  • Neurons communicate through synapses:

  • Electrical Synapses:

    • Allow the passage of current-carrying ions through gap junctions, enabling current to travel bidirectionally.

  • Chemical Synapses:

    • Involve presynaptic and postsynaptic membrane structures separated by a synaptic cleft.

Types of Postsynaptic Potentials

  • Excitatory Postsynaptic Potential (EPSP):

  • Occurs when a neurotransmitter combines with a receptor site, causing partial depolarization of the postsynaptic membrane.

  • Inhibitory Postsynaptic Potential (IPSP):

  • Results when a neurotransmitter combines with a receptor site, leading to hyperpolarization of the nerve membrane and decreased excitability.

Synthesis and Release of Transmitters

  • Synthesis may require several enzyme-catalyzed steps.

  • After synthesis, neurotransmitter molecules are stored in synaptic vesicles at the axon terminal.

  • An impulse arrival at the nerve terminal prompts vesicles to move to the cell membrane and release neurotransmitter molecules into the synaptic space.

  • A released neurotransmitter can:

  • Be broken down into inactive substances by enzymes.

  • Be taken back into the presynaptic neuron (reuptake).

  • Diffuse into intercellular fluid until its concentration is too low to influence postsynaptic excitability.

  • ### Neuromodulators

    • Neuromodulators are chemical messengers that interact with membrane receptors, causing slower and longer-acting membrane permeability changes.

    • Neurotrophic factors released from presynaptic terminals are essential for the long-term survival of postsynaptic neurons.

Developmental Organization of the Nervous System

  • Important aspects include:

    • Embryonic Development

    • Segmental Organization

    • Cell Columns

    • Peripheral Nerves

    • Longitudinal Tracts

    • Layers

Structure and Function of the Spinal Cord and Brain

Spinal Cord

  • Located in the upper two-thirds of the spinal canal; extends from the foramen magnum at the skull base to the conus medullaris, typically at L1 or L2 vertebrae in adults.

  • Gray Matter:

  • Comprises dorsal and ventral horns.

  • White Matter

  • Surrounded by the pia mater (specifically covering the spinal cord and its dorsal and ventral roots).

Anatomy of the Spinal Cord

  • Includes denticulate ligaments and is protected by the vertebral column:

  • Composed of pedicles, spinal processes, arches, foramina, and intervertebral foramina.

Horns of the Spinal Cord

  • Extensions of gray matter forming the letter “H”:

  • Dorsal Horns:

    • Extend posteriorly and contain IA neurons that receive afferent impulses through dorsal roots and other neurons.

  • Ventral Horns:

    • Extend anteriorly, containing OA neurons and efferent lower motor neurons (LMNs) exiting through ventral roots.

Spinal Nerves

  • Definition: Peripheral nerves carrying information to and from the spinal cord.

  • There are usually 32 or more pairs of spinal nerves:

  • 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 2 or more coccygeal.

  • Each pair is named based on the spinal cord segment from which it exits.

  • Comprised of both efferent and afferent neurons.

Reflex Types

  • Segmental Reflexes:

  • Involve neurons in a single cord segment.

  • Suprasegmental Reflexes:

  • Involve structures in the brain.

  • Types of Reflexes:

  • Myotatic or stretch reflex.

  • Inverse myotatic reflex.

  • Withdrawal reflex.

Structure of the Brain

  • Hindbrain:

  • Includes the medulla oblongata, the pons, and cerebellum.

  • Midbrain:

  • Comprised of two pairs of dorsal enlargements, the superior and inferior colliculi.

  • Forebrain:

  • Consists of two hemispheres with the cerebral cortex.

  • Contains basal ganglia and the diencephalon (thalamus and hypothalamus).

  • Brain Stem:

  • Composed of midbrain, pons, and medulla.

Lobes of the Brain

  • Frontal Lobe:

  • Extends from the frontal pole to central sulcus. Separated from the temporal lobe by the lateral sulcus.

  • Parietal Lobe:

  • Located posterior to central sulcus and above the lateral sulcus.

  • Temporal Lobe:

  • Lies below the lateral sulcus, merging with parietal and occipital lobes.

  • Occipital Lobe:

  • Positioned posterior to temporal and parietal lobes, arbitrarily separated from them.

Cranial Nerves

  • I. Olfactory

  • II. Optic

  • III. Oculomotor

  • IV. Trochlear

  • V. Trigeminal

  • VI. Abducens

  • VII. Facial

  • VIII. Auditory (vestibulocochlear)

  • IX. Glossopharyngeal

  • X. Vagus

  • XI. Spinal accessory

  • XII. Hypoglossal

Meninges

  • Comprises three layers: pia mater, arachnoid mater, and dura mater.

Ventricular System and Cerebrospinal Fluid (CSF)

  • CSF circulates in brain ventricles and is crucial for cushioning and protection.

  • ### Blood–Brain Barrier

    • Only allows water, carbon dioxide, and oxygen to enter the brain relatively easily; other substances are transported more slowly and controlled.

    • Blood–Brain Barrier:

    • Critical for maintaining homeostasis within the brain.

    • CSF–Brain Barrier:

    • Maintains a stable environment for neural function.

The Autonomic Nervous System

Overview

  • Regulates, adjusts, and coordinates vital visceral functions.

  • Divided into two major divisions:

  • Sympathetic System:

    • Excites visceral functions and is responsible for the "fight-or-flight" response.

  • Parasympathetic System:

    • Inhibits visceral function and focuses on conserving energy during periods of inactivity.

  • Efferent System:

  • Comprised of CNS and PNS components with a two-neuron pathway (preganglionic neuron in the CNS and postganglionic neuron outside of the CNS).

  • Sympathetic fibers exit the CNS at the thoracolumbar level; parasympathetic fibers exit at cranial and sacral levels.

  • The hypothalamus acts as the primary control center for autonomically mediated functions.

  • ### Main Neurotransmitters

    • Acetylcholine:

    • Acts as a neurotransmitter for all preganglionic neurons in both ANS divisions and for postganglionic neurons of the parasympathetic nervous system.

    • Norepinephrine and Epinephrine:

    • Catecholamines functioning as neurotransmitters for postganglionic neurons of the sympathetic nervous system.

    • Neurotransmitters exert their effects via specialized cell surface receptors, with different receptors for the same transmitter leading to varying tissue responses.