Anatomy of the Nervous System

Anatomy of the Nervous System

• The nervous system has two main divisions: the central nervous system (CNS) and the peripheral nervous system.
• The CNS consists of the brain and spinal cord.
• The peripheral nervous system includes cranial nerves, spinal nerves, and the autonomic nervous system.
• The neuron is the basic functional unit of the brain.
• Neurotransmitters facilitate communication between neurons or between a neuron and a target cell, like a muscle or endocrine cell.
• Brain functions are modulated via neurotransmitter receptor site activity.
• Acetylcholine is a major neurotransmitter in the parasympathetic nervous system, with excitatory effects.
• Norepinephrine is a major neurotransmitter in the sympathetic nervous system, also typically excitatory.
• The brain is divided into the cerebrum, brainstem, and cerebellum.
• The cerebrum includes two hemispheres, the thalamus, hypothalamus, and basal ganglia.
• The brainstem includes the midbrain.

Structures Protecting the Brain

• Cerebrospinal fluid (CSF) is a clear, colorless fluid produced in the choroid plexus of the ventricles, circulating around the brain and spinal cord.
• CSF blockage can cause obstructive hydrocephalus.
• CSF is vital for immune and metabolic functions.
• CSF is produced at approximately 500 mL/day.
• CSF composition is similar to other extracellular fluids, with differing constituent concentrations.
• Cerebral circulation involves the Circle of Willis.
• The blood-brain barrier restricts access to many substances in blood plasma to protect the CNS.
• Endothelial cells in the brain's capillaries form continuous tight junctions, creating this barrier.
• This barrier can be compromised by trauma, cerebral edema, and cerebral hypoxemia.

The Spinal Cord

• The spinal cord is continuous with the medulla, linking the brain and the periphery.
• The spinal cord extends from the foramen magnum to the lower border of L1, tapering into the conus medullaris.
• Nerve roots that extend past the conus medullaris below the second lumbar space form the cauda equina.
• The meninges surround the spinal cord.

Peripheral Nervous System

• Twelve pairs of cranial nerves emerge from the brain's lower surface, passing through skull openings.
  • Three cranial nerves are sensory (I, II, VIII)
  • Five are motor (III, IV, VI, XI, XII)
  • Four are mixed sensory and motor (V, VII, IX, X).
• Cranial nerves are numbered by their order of origin from the brain.
• Cranial nerves innervate the head, neck, and special sense structures.
• The spinal cord includes 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal.
• Each spinal nerve has a ventral and dorsal root.
• Dorsal roots are sensory, transmitting impulses from dermatomes to dorsal horn ganglia.
• Ventral roots are motor, carrying impulses from the spinal cord to the body.

Autonomic Nervous System

• The autonomic nervous system regulates the functions of internal organs such as the heart, lungs, blood vessels, digestive organs, and glands.
• The sympathetic nervous system primarily produces excitatory responses, such as the "fight-or-flight" response.
• Epinephrine and norepinephrine are the main sympathetic neurotransmitters.
• The parasympathetic nervous system mainly controls visceral functions.

Physiology of the Nervous System

• The nervous system is responsible for controlling motor, sensory, autonomic, cognitive, and behavioral activities.
• It monitors the body's processes.
• It responds to the internal and external environments.
• It maintains homeostasis.
• It uses chemical and electrical messages to direct all psychological, biologic, and physical activity.

Assessment of the Nervous System

• Common symptoms include pain, seizures, dizziness, vertigo, visual disturbances, muscle weakness, and abnormal sensations.
• Physical assessment includes assessing consciousness and cognition, and balance & coordination.
• It also includes examining cranial nerves, the motor system, and the sensory system
• Assessing consciousness and cognition involves evaluating mental status, intellectual function, thought content, emotional status, language ability, lifestyle impact, and the level of consciousness.
• Consciousness involves wakefulness and the ability to respond to the environment.
• Assessment of the level of consciousness includes observing alertness and ability to follow commands, eye opening, verbal response, and motor response to stimuli.
• Cranial nerves are assessed when there is a decreased level of consciousness, brain stem pathology, or peripheral nervous system disease.
• Right and left cranial nerve functions are compared throughout the examination

Examining Reflexes

• Motor Ability is an assessment of muscle size and tone, strength, coordination, and balance.
• Muscle Strength is assessed via a 5-point scale:
  • 5: Normal strength
  • 4: Slight weakness
  • 3: Moderate weakness
  • 2: Severe weakness
  • 1: Very severe weakness
  • 0: No movement
• Sensory system assessment includes tests for tactile sensation, superficial pain, temperature, vibration, and proprioception.
• Sensory deficits from peripheral neuropathy or spinal cord injury follow anatomic dermatomes.
• Cerebellar and basal ganglia influence on the motor system affects balance and coordination.
• Coordination in the hands and upper extremities is tested via rapid alternating movements and point-to-point testing.
• Coordination in the lower extremities is tested by running the heel down the anterior tibia surface.
• Ataxia is incoordination of muscle action, particularly in activities like walking or reaching.
• Reflexes are involuntary muscle contractions in response to stimuli.
• Reflexes are classified as:
  • Deep tendon reflexes
  • Superficial reflexes
  • Pathologic reflexes
• Deep tendon reflexes include the biceps, triceps, brachioradialis, patellar, and Achilles reflexes, as well as clonus.
• Deep tendon reflex responses are graded from 0 to 4+, with 2+ considered normal.

Diagnostic Evaluation: CT Scan

• Diagnostic evaluation includes the CT scan
• CT scans image differences in tissue densities within the skull, cortex, subcortical structures, and ventricles.
• An IV contrast agent may be used to highlight differences further.
• CT scans are typically performed first without contrast, then with IV contrast if necessary.
• Brain CT can detect tumors, masses, infarction, hemorrhage, ventricular displacement, and cortical atrophy.
• CT angiography allows visualization of blood vessels, sometimes eliminating the need for formal angiography.

Diagnostic Evaluation: MRI & PET

• MRI uses a powerful magnetic field to obtain images of different body areas.
• MRI scans can be performed with or without a contrast agent.
• MRI can identify cerebral abnormalities earlier and more clearly than other tests.
• MRI is useful in diagnosing brain tumors, stroke, and multiple sclerosis.
• PET is a computer-based nuclear imaging technique.
• PET requires the patient to inhale a radioactive gas or be injected with a radioactive substance.
• PET allows measurement of blood flow, tissue composition, and brain metabolism, thus indirectly evaluating brain function.
• PET can show metabolic changes in the brain, such as with Alzheimer's disease.
• PET can locate lesions, identify blood flow and oxygen use, and distinguish tumors from necrosis, while also revealing biochemical abnormalities.

Diagnostic Evaluation: Cerebral Angiography & EEG

• Cerebral angiography includes an X-ray study using a contrast agent injected into a selected artery.
• Cerebral angiography is used to determine vessel patency, identify collateral circulation, and detail vascular anomalies for intervention planning.
• Patient BUN and Cr levels should be checked prior to angiography.
• Patients should be well-hydrated before the angiography.
• If the femoral artery is selected for access, the groin area is clipped, prepared, and anesthetized locally.
• Neurologic assessment is conducted during and immediately after cerebral angiography.
• Nursing care post cerebral angiography involves observing the injection site for bleeding or hematoma formation.
• Assess color and temperature of the involved extremity to detect possible embolism.
• Encourage fluids to clear contrast through the kidneys.
• Monitoring is required for allergic reactions to the contrast agent and hypotension if vasodilatory medications are used for radial approaches.
• Electroencephalography (EEG) assesses cerebral electrical activity and takes 45-60 minutes.
• EEG is useful for diagnosing and evaluating seizure disorders, coma, or organic brain syndrome.
• Tumors, brain abscesses, blood clots, and infection can cause abnormal electrical activity patterns.
• EEG is also used to determine brain death.
• Anticonvulsants, tranquilizers, stimulants, and depressants should be withheld 24-48 hours before EEG, as they can alter or mask seizure wave patterns

Diagnostic Evaluation: Lumbar Puncture (LP)

• A lumbar puncture (spinal tap) involves inserting a needle into the lumbar subarachnoid space to withdraw CSF.
• The needle is inserted into the subarachnoid space between the second and third, third and fourth, or fourth and fifth lumbar vertebrae,.
• The test obtains CSF for examination.
• the test measures and reduces CSF pressure.
• the test determines the presence or absence of blood in the CSF.
• The test administers medications intrathecally (into the spinal canal).
• The CSF should be clear and colorless, and pink, blood-tinged, or grossly bloody CSF signifies subarachnoid hemorrhage.
• The CSF may be bloody initially from local trauma but becomes clearer as fluid is drained.
• Specimens are analysed for cell count, culture, glucose, protein, and other tests.
• The specimens should be sent to the laboratory inmediately because changes will take place and alter the result if the specimens are allowed to stand.
• Complications of LP include:
  • Post-lumbar puncture headache occurs a few hours to several days after the procedure, ranging from mild to severe; Caused by CSF leakage at the puncture site
  • Post-lumbar puncture headache may be avoided if a small-gauge needle (22 gauge) is used, is typically managed with analgesics, hydration, caffeine ingestion, and a lying supine position Additional complications of a LP may present
• Herniation of the intracranial contents
• Spinal epidural abscess
• Spinal epidural hematoma
• Meningitis
• Temporary voiding problems
• Slight elevation of temperature
• Backache or spasms
• Stiffness of the neck.