Neural Function and Nervous System Physiology Study Guide

Unit Objectives for Neural Function

• Foundational Knowledge Review: Describe previously learned content concerning the anatomy and physiology of the neurovascular and cerebrovascular systems (CO1).
• Tissue Perfusion: Describe the importance of neural tissue perfusion to maintaining tissue function (CO1, CO3).
• Dementia Profile:     * Describe the common etiologies of dementia (CO5).     * Describe the progressive clinical manifestations of dementia (CO5).     * Identify specific nursing considerations related to safety in the care of people diagnosed with dementia (CO7).
• Strokes (Ischemic vs. Hemorrhagic):     * Differentiate between the pathophysiology of ischemic and hemorrhagic strokes (CO5).     * Identify the clinical manifestations associated with ischemic and hemorrhagic strokes (CO5).     * Identify risk factors for both types of strokes (CO5).     * Identify strategies for primary, secondary, and tertiary prevention of strokes (CO7).
• Seizures:     * Describe the relationship between electrical activity in the brain and the occurrence of seizures (CO1, CO5).     * Differentiate between focal onset and generalized onset seizures (CO5).     * Discuss the effects of seizures on an individual's awareness and motor function (CO5).     * Outline the phases and characteristics of a seizure, specifically the aura, ictal, and postictal phases (CO5).     * Describe safety strategies and the rationale utilized in nursing for patients experiencing seizures (CO7).

Overview of the Nervous System

• General Function: The nervous system receives and reacts to environmental stimuli on both a physiologic and cognitive level.
• Main Components: The system consists of three main components: the brain, the spinal cord, and the nerves.
• Structural Divisions:     * Central Nervous System (CNS): Comprised of the brain and the spinal cord.     * Peripheral Nervous System (PNS): Comprised of the nerves throughout the rest of the body.

Anatomy and Protective Structures of the Central Nervous System

• Housing and Protection:     * The skull houses and protects the brain.     * The vertebral column houses and protects the spinal cord.
• Meninges: Membranes that encase the CNS.     * Dura mater: The outer and toughest layer of the meninges.     * Arachnoid layer: The middle layer, characterized by a spider web–like vascular system.     * Pia mater: The innermost layer, closest to the brain and spinal tissue.
• Cerebrospinal Fluid (CSF): A plasmalike liquid that fills the space between the arachnoid and the pia mater layers to provide additional cushion and support.
• Ventricles: Interconnected, hollow areas within the brain where CSF is produced, filled, and flows.

Cellular Components: Neurons and Neuroglia

• Neuroglia Cells: Provide support and perform several critical functions:     * Scaffold neural tissue.     * Isolate and protect neuron cell membranes.     * Regulate interstitial fluid.     * Defend the neuron against pathogens.     * Assist with neural repair.
• Neurons: Cells responsible for generating and transmitting bioelectrical impulses.     * Regenerative Limitations:         * Neurons do not have the ability to divide.         * Losses due to aging or injury cannot be replaced, though not all cell death results in a total loss of functioning.         * Undamaged neurons in the brain may assume the functions of damaged neurons.         * Severed peripheral nerves can regenerate to a certain point to reestablish connections.         * Severed brain and spinal cord axons cannot be repaired.         * Severed spinal cord nerves result in paralysis and loss of sensation below the area of damage.     * Metabolic Requirements: Neurons require a constant supply of oxygen and glucose, making them highly vulnerable to hypoxia and hypoglycemia.

Detailed Structure of a Neuron

• Axons: Projections that transmit impulses away from the cell body.     * Collateral branches may occur along the length of an axon.     * In motor neurons, when the axon terminates, it branches many times, ending in individual muscle fibers.
• Dendrites: Projections that transmit impulses toward the cell body.
• Terminal Boutons: Tiny bulges at the end of the axon that communicate with other neurons, muscle fibers, or glands.
• Myelin Sheath: Surrounds some axons; it increases the rate of impulse transmission and allows impulses to “jump” from node to node.
• Schwann Cells: These cells produce the myelin sheath.
• Nodes of Ranvier: Specialized sites of nutrient exchange located where myelin is not present.
• White Matter: Consists of bundles of myelinated nerves.
• Synapse: The gap between neurons through which communication occurs.     * Presynaptic Terminal: Typically a terminal bouton or similar structure.     * Synaptic Cleft: The specific space between neurons.     * Postsynaptic Cell Membrane: The end opposite to the presynaptic terminal.     * Neurological disorders can arise from problems with the synapse, the receptor site, or the neurotransmitter itself.

Electrophysiology of Neural Impulses

• Impulse Generation: Neural impulses are generated by small ionic changes.
• Potentials:     * Resting Potential: The charge of the neuron at rest. The plasma side of the neuron membrane has a slight charge due to sodium ions ($Na^+$) concentrated on the outside of the cell.     * Action Potential: The ability to create a bioelectric charge/impulse.
• Process of Depolarization:     1. Stimulating the neuron creates a bioelectric impulse.     2. The resulting potential shifts from $270$ millivolts to $130$ millivolts.     3. The membrane becomes depolarized due to an influx of $Na^+$ ions.     4. The impulse travels along the membrane as a wave of depolarization.     5. The efflux of potassium ($K^+$) ions restores the resting potential, allowing the neuron to transmit additional impulses almost immediately.
• Synaptic Transmission:     * Impulses travel down the nerve, triggering the release of neurotransmitters from the presynaptic terminal.     * Neurotransmitters are held in synaptic vesicles in axon terminals.     * Neurotransmitters cross the synaptic cleft in only one direction to stimulate an electrical reaction in nearby neurons.     * At each transmission, neurotransmitters are either destroyed by enzymes or reabsorbed by the postsynaptic membrane (recycled).     * Some neurotransmitters are inhibitory and prevent action potentials.

Cranial Nerves

• Definition: Twelve pairs of cranial nerves branch directly from the base of the brain to accomplish various physiological and cognitive functions.
• Function Types: Some carry sensory fibers, some only motor fibers, and others carry both.
• Exit Route: Each nerve travels from the brain through the foramen to its destination.
• Nerve Groupings for Assessment:     * $I$ (Olfactory)     * $II$ & $III$ (Optic & Oculomotor)     * $III$, $IV$, $VI$ (Oculomotor, Trochlear, Abducens)     * $V$ (Trigeminal)     * $VII$ (Facial)     * $VIII$ (Vestibulocochlear)     * $IX$ & $X$ (Glossopharyngeal & Vagus)     * $XI$ (Accessory)     * $XII$ (Hypoglossal)

Major Regions of the Brain: The Cerebrum

• General Characteristics: The largest region of the brain, responsible for higher thought processes.
• Cerebral Cortex: A thin layer of gray matter surrounding the cerebrum.
• Internal Composition: White matter lies beneath the gray matter, containing bundles of axons that transmit impulses from the cortex to the spinal cord.
• Structure: Divided into right and left hemispheres. Surface area is increased by folds called gyri, separated by grooves called sulci.
• Lobes and Functions:     * Frontal Lobe: Facilitates voluntary motor activity and personality traits.     * Parietal Lobe: Receives and interprets sensory input (excluding smell, hearing, and vision).     * Occipital Lobe: Processes visual information.     * Temporal Lobe: Essential for hearing and memory.
• Functional Classifications:     * Motor: Stimulates muscle activity.     * Sensory: Receives sensory information.     * Association: Integrates information and initiates coordinated responses.

Major Regions of the Brain: Diencephalon, Cerebellum, Basal Ganglia, and Limbic System

• Diencephalon:     * Thalamus: Receives and relays most sensory input, affects mood, and initiates body movements.     * Subthalamus: Participates in motor activities.     * Epithalamus: Functions are unclear.     * Hypothalamus: The most inferior portion; regulates many bodily functions.
• Cerebellum: Communicates with other brain regions to coordinate synergistic muscle movement, balance, and cognition.
• Basal Ganglia: Located deep within the cerebrum, diencephalon, and midbrain; plays a pivotal role in coordination, motor movement, and posture.
• Limbic System: Includes portions of the cerebrum and diencephalon; works with the hypothalamus to influence instinctive behavior, emotions, motivation, mood, pain, and pleasure.

Major Regions of the Brain: The Brainstem and Reticular Formation

• Brainstem Function: Connects the brain to the spinal cord; regulates basic body functions. Injury can result in death. 10 of the 12 cranial nerves exit here.     * Midbrain: The smallest region; acts as a relay station for auditory and visual information; controls eye movement.     * Pons: Contains nerves that regulate sleep and breathing.     * Medulla: Conduction pathway for nerve tracts; coordinates heart rate, peripheral vascular resistance, breathing, swallowing, vomiting, coughing, and sneezing.
• Reticular Formation:     * Acts as a "gatekeeper," receiving all incoming and outgoing information.     * Reticular Activation System (RAS): Sends impulses to the cerebral cortex; responsible for alertness during the day and can prevent sleeping at night.

The Spinal Cord (CNS)

• Path: Exits the skull through the foramen magnum. Extends through the vertebral canal to the level of $L2$.
• Cauda Equina: The transition into individual nerve roots at the $L2$ level.
• Spinal Gray and White Matter:     * Gray Matter: H-shaped central area containing nerve cell bodies.     * White Matter: Surrounds gray matter; composed of nerve fiber tracts or pathways.
• Fiber Tracts:     * Ascending Fibers (Afferent Tracts): Carry sensory action potentials from the periphery to the brain (parietal lobe).     * Descending Fibers (Efferent Tracts): Carry motor action potentials from the brain (Upper Motor Neurons) to the PNS.
• Specific Tract Names:     * Spinothalamic Tracts: Light touch, pressure, tickling, itching, pain, and temperature.     * Spinocerebellar Tracts: Establish body position in relation to the cerebellum.     * Corticospinal Tracts: Coordinate movements, especially in the hands.     * Vestibulospinal Tracts: Responsible for involuntary movements.     * Reticulospinal Tracts: Responsible for involuntary movements.
• Reflexes:     * Spinal Reflex Arcs: Process creating an unconscious response to stimuli.     * Flexor Reflex: A withdrawal reflex in response to unpleasant stimuli (e.g., touching a hot surface); moves the limb without conscious brain action.

The Peripheral Nervous System (PNS)

• Nerve Composition: Bundles of nerve fibers; 31 spinal nerve pairs: $8$ cervical, $12$ thoracic, $5$ lumbar, $5$ sacral, and $1$ coccygeal.
• Ganglia: Collections of nerve cell bodies outside the CNS.
• Nerve Origins: Spinal nerves arise from rootlets that form dorsal and ventral roots, which combine to form the spinal nerve.
• Dermatome: An area of skin innervated by a given pair of spinal sensory nerves.
• Interneurons: Connect sensory and motor neurons within the spinal cord.
• Nerve Plexuses: Collateral intersections of nerves:     * Cervical Plexus: $C1$ to $C4$.     * Brachial Plexus: $C5$ to $T1$.     * Lumbar Plexus: $L1$ to $L4$.     * Sacral Plexus: $L4$ to $S4$.

The Autonomic Nervous System (ANS)

• Function: Controls smooth muscles and unconscious responses (heart rate, blood pressure, intestinal motility).
• Subdivisions (Antagonistic relationship):     * Sympathetic Nervous System (SNS): "Fight-or-flight"; augmented by the adrenal medulla; stimulates adrenergic receptors.     * Parasympathetic Nervous System (PNS): "Rest-and-digest"; stimulates cholinergic receptors.

Comparison of Autonomic Functions

• Cardiovascular:     * Parasympathetic: Decreased cardiac output and heart rate.     * Sympathetic: Increased contraction, heart rate, and cardiac output.
• Pulmonary:     * Parasympathetic: Bronchial constriction.     * Sympathetic: Bronchial dilatation.
• Eyes:     * Parasympathetic: Constrict pupils.     * Sympathetic: Dilate pupils.
• Salivary Glands:     * Parasympathetic: Stimulates salivation.     * Sympathetic: Inhibit salivation.
• Gastrointestinal:     * Parasympathetic: Increased gastric motility, increased secretions, liver stimulates bile release, stimulates peristalsis.     * Sympathetic: Decreased motility, decreased secretions, liver stimulates glucose release, inhibits peristalsis.
• Urinary:     * Parasympathetic: Increased urinary output, sphincter relaxation, bladder contraction.     * Sympathetic: Decreased urinary output, sphincter contraction, bladder relaxation.
• Other Sympathetic Specifics:     * Adrenal Gland: Stimulates release of epinephrine and norepinephrine.     * Glycogen: Converts glycogen to glucose.     * Nasal Mucosa: Affected by SNS.     * Musculoskeletal: Muscular contraction (vs. relaxation in Parasympathetic).