Structures and Functions of Nervous System

Chapter 60 - Assessment: Nervous System

Structures and Functions of Nervous System (Central and Peripheral)

  • Central Nervous System (CNS)

    • Comprises the brain and spinal cord.

    • Contains cranial nerves I and II.

  • Peripheral Nervous System (PNS)

    • Consists of cranial nerves III to XII and spinal nerves.

    • Includes the autonomic nervous system (ANS).

Neurons

  • Definition: Neurons are the primary functional units of the nervous system.

  • Characteristics:

    • Excitability: Ability to respond to stimuli.

    • Conductivity: Capacity to transmit nerve impulses.

    • Influence: Affect other neurons or target tissues.

Neuron Structure

  • Components:

    • Cell Body (Soma): Contains the nucleus and organelles.

    • Dendrites: Receives signals from other neurons.

    • Axon: Transmits impulses away from the cell body.

  • Axon Function: Carries nerve impulses.

  • Myelin Sheath: A white, lipid-protein substance that insulates the axon and enhances impulse conduction.

    • Types: Myelinated or unmyelinated axons.

  • Node of Ranvier: Gaps in the myelin sheath that facilitate rapid conduction of nerve impulses through saltatory conduction.

Glial Cells

  • Function: Provide support, nourishment, and protection to neurons.

  • Composition: Constitute almost half the mass of the brain and spinal cord.

  • Types of Glial Cells:

    • Microglia: Specialized macrophages that protect neurons, are mobile within the brain, and multiply when brain is damaged.

    • Macroglia:

    • Astrocytes: Provide structural support; form the blood-brain barrier; play a role in synaptic transmission; act as phagocytes during brain injury; contribute to gliosis.

    • Oligodendrocytes: Produce myelin sheaths in the CNS, primarily found in white matter.

    • Ependymal Cells: Line brain ventricles and assist in cerebrospinal fluid (CSF) secretion.

Neuroglia

  • Properties:

    • Mitotic: Can replicate and replace destroyed neurons.

    • Comprise most primary CNS tumors.

Nerve Regeneration (Repair)

  • Process: Damaged nerve cells attempt to regenerate.

    • Success: More successful in the PNS than CNS.

    • Astrocyte Proliferation: Recent research shows that astrocytes proliferate following CNS injuries.

    • Historically, neurons were deemed non-mitotic.

Nerve Impulse Transmission

  • Action Potential: A brief reversal of membrane potential (depolarization) that propagates along the neuron.

  • Synapse: A junction between two neurons that includes:

    • Presynaptic Terminal: End of the neuron transmitting the impulse.

    • Synaptic Cleft: The small gap between neurons.

    • Postsynaptic Cell: Contains receptor sites for neurotransmitters.

  • Neurotransmitters: Chemicals that facilitate impulse transmission across the synaptic cleft.

    • Excitatory Neurotransmitters: Examples include epinephrine, norepinephrine, and glutamate.

    • Inhibitory Neurotransmitters: Examples include serotonin, GABA (gamma-aminobutyric acid), and dopamine.

Spinal Cord Anatomy

  • Gray Matter: Centrally located; contains cell bodies of various neurons.

  • White Matter: Surrounds gray matter; contains axons of ascending sensory and descending motor fibers, characterized by myelination.

Spinal Pathways (Tracts)

  • Naming Convention: Named based on their point of origin and destination.

    • Ascending Tracts: Carry sensory information to higher CNS levels (e.g., spinocerebellar tract, spinothalamic tract).

    • Descending Tracts: Convey motor impulses for muscle movement (e.g., pyramidal tract, extrapyramidal system).

Motor Neurons

  • Upper Motor Neurons (UMNs):

    • Originate in the cerebral cortex and project downward, influencing skeletal muscle tone (spasticity).

  • Lower Motor Neurons (LMNs):

    • The final pathway through which descending motor tracts impact skeletal muscle.

Brain Anatomy and Functions

  • Cerebrum: Comprised of right and left hemispheres.

    • Lobes: Frontal, temporal, parietal, and occipital.

    • Basal Ganglia: Involved in initiation, execution, and completion of voluntary movements; learning, emotional response, and automatic movements associated with skeletal muscle.

  • Thalamus: Major relay center for sensory input from the body, face, retina, cochlear, and taste receptors; connects cerebellum and basal ganglia to the frontal cortex.

  • Limbic System: Associated with emotion, aggression, feeding behavior, and sexual response.

  • Hypothalamus: Regulates hormone release from the anterior pituitary gland; controls appetite, body temperature, water balance, circadian rhythms, and emotional expressions.

  • Brainstem: Comprises midbrain, pons, and medulla; contains ascending and descending fibers, cranial nerves III to XII, and vital centers responsible for respiratory, vasomotor, and cardiac functions.

  • Cerebellum: Coordinates voluntary movement and influences motor activity by processing information received.

  • Ventricles: Four interconnected fluid-filled cavities; the lower part of the fourth ventricle connects with the central canal; CSF circulation cushions and carries nutrients.

Peripheral Nervous System (PNS)

  • Components: Includes spinal nerves, cranial nerves, ganglia, and portions of the autonomic nervous system (ANS).

Autonomic Nervous System (ANS)

  • Divisions: The sympathetic division (SNS) and parasympathetic division (PSNS).

    • Sympathetic Division Function:

    • Eye: Dilates pupil

    • Salivary glands: Inhibits saliva production

    • Lung: Dilates bronchi

    • Heart: Increases heart rate and muscle contraction

    • Adrenal gland: Stimulates epinephrine and norepinephrine release

    • Liver: Stimulates glucose release

    • Kidney: Inhibits urine secretion

    • Digestive tract: Inhibits peristalsis and pancreatic secretion

    • Bladder: Inhibits urination

    • Genitalia: Inhibits erection

    • Parasympathetic Division Function:

    • Eye: Constricts pupil

    • Salivary glands: Stimulates saliva production

    • Lung: Constricts bronchi

    • Heart: Decreases heart rate and muscle contraction

    • Kidney: Stimulates urine secretion

    • Digestive tract: Stimulates peristalsis and pancreatic secretion

    • Bladder: Stimulates urination

    • Genitalia: Promotes erection

Blood-Brain Barrier

  • Function: Protects the brain from harmful agents.

    • Lipid-soluble Compounds: Enter easily through the barrier.

    • Water-soluble Agents: Enter slowly.

Gerontologic Considerations

  • Effects of Aging on Nervous System:

    • CNS Changes:

    • Loss of neurons, enlargement of ventricles, decreased brain weight, and decreased cerebral blood flow and CSF.

    • PNS Changes:

    • Decreased nerve conduction, coordinated neuromuscular activity, memory, and sensory/neuromuscular functions.

Assessment of Nervous System

  • Subjective Data:

    • Characteristics of present illness (e.g., mode of onset, course of illness, history of trauma).

    • Functional health patterns (e.g., health perception, metabolic patterns, activity, sleep, and self-perception).

Objective Data

  • Physical Assessment:

    • Mental Status: Examine general behavior, cognition, mood, and affect.

    • Cranial Nerve Function: Evaluate olfactory (CN I), optic (CN II), oculomotor (CN III), and others for their specific functions and identify impairments.

    • Motor Function: Assess strength, muscle tone, and balance; note conditions such as hypotonia (flaccidity) and hypertonia (spasticity).

    • Sensory Function: Test light touch, pain, temperature, vibration sense, and proprioception; conduct the Romberg test.

    • Reflexes: Assess biceps, triceps, brachioradialis, patellar, and Achilles tendon reflexes using a scale of 0-5.

Diagnostic Studies of Nervous System

  • Cerebrospinal Fluid Analysis: Involves lumbar puncture (LP) and requires pre-procedure preparation and post-procedure monitoring.

  • Imaging and Electrographic Studies:

    • Includes PET scans, SPECT, myelograms, EEG, EMG, electroneurography, and biopsies.

Audience Response Questions

  1. Mental Status Data Collection: The most relevant time to gather data is during the nursing health history.

  2. Spinothalamic Tract Lesion Assessment: Expected findings may include loss of peripheral sensitivity to pain.

  3. Cranial Nerves III, IV, and VI Assessment: Assessed through eye movements.