Neuroanatomy and Neurophysiology

Neuroanatomy and Neurophysiology: Review of the Nervous System

• The nervous system is essential for sensory detection, integration, and motor responses.

Central Nervous System (CNS)

• The CNS includes the brain and spinal cord.
• The peripheral nervous system includes all neural tissue outside the CNS.
• The brain is the basic functional unit, containing approximately 86 billion neurons.

Brain

• The brain averages about 1400g in young adults and 1200g in older adults.
• The brain consists of the cerebrum, brainstem, and cerebellum.
• The cerebrum has 2 hemispheres, a thalamus, hypothalamus and basal ganglia
• The brainstem has a midbrain, pons, and medulla.
• The cerebellum is located under the cerebrum and behind the brainstem

Brainstem

• The brainstem connects the spinal cord to the cerebrum.
• It consists of the medulla oblongata, pons, and midbrain, with the reticular formation scattered throughout the three regions.
• The brainstem is the location of cranial nerve nuclei

Cerebellum

• The cerebellum controls muscle movement and tone and governs balance.
• It regulates the extent of intentional movement and is involved in learning motor skills.

Diencephalon

• The diencephalon connects the brainstem to the cerebrum.
• It has many relay and homeostatic functions, as listed under each subdivision
• Thalamus - major sensory relay center that influences mood and movement.
• Subthalamus - contains nerve tracts and nuclei.
• Epithalamus - contains nuclei responding to olfactory stimulation and contains the pineal gland.
• Hypothalamus - It's the major control center for maintaining homeostasis and regulating endocrine function.

Cerebrum

• The cerebrum controls conscious perception, thought, and conscious motor activity.
• It can override most other systems.

Four Lobes of the Cerebrum

• Frontal Lobe: involved in motor control (premotor cortex), problem-solving (prefrontal area), and speech production (Broca's area)
• Temporal Lobe: handles auditory processing (hearing), language comprehension (Wernicke's area), and memory/information retrieval
• Parietal Lobe: manages touch perception (somatosensory cortex), body orientation, and sensory discrimination
• Occipital Lobe: dedicated to sight (visual cortex), visual reception, and visual interpretation

Protective Structures of the Brain

• Skull: A bone structure that protects the brain from injury.
• Meninges: Fibrous connective tissues covering the brain and spinal cord.
  • They provide protection, support, and nourishment.
  • Three layers: Dura Mater, Arachnoid, and Pia Mater.
• Cerebrospinal Fluid (CSF): Clear and colorless fluid produced by the choroid plexus.
  • It has immune and metabolic functions.
• Cerebral Circulation:
  • Arteries: Aneurysms are outpouchings of the blood vessel due to weakness of the vessel wall and can lead to hemorrhagic stroke.
  • Veins
• Blood-Brain Barrier:
  • Has a protective function that filters substance circulation in the brain.
  • It can be altered by cerebral trauma, vertebral edema, and cerebral hypoxemia
  • It has implications for treatment and selection of medication for CNS disorders
• Herniation: Occurs when excess pressure in the cranial cavity causes brain tissue to be compressed or displaced.
  • It is potentially deadly as it can cut off blood supply.

Divisions of the Nervous System

• The nervous system has central and peripheral divisions
• The central nervous system (brain and spinal cord) processes and analyzes information.
• The peripheral nervous system connects to the central nervous system to give sensory and motor output
• The autonomic nervous system communicates with internal organs and glands.
  • Its sympathetic division is arousing (fight or flight)
  • Its parasympathetic division is calming (rest and digest).
• The somatic nervous system communicates with sense organs and voluntary muscles, with sensory (afferent) and motor (efferent) components.

Cells of the Nervous System

• Neuron: The basic functional unit of the brain.
• Dendrites: Receive electrochemical responses (afferent nerve fibers).
• Axon: Carries electrochemical responses away from the cell body (efferent nerve fibers).
• Cell body: Center of the neuron.
• Nucleus: Contains the genetic material of the neuron.
• Synapses: Facilitate communication pathways.
• Myelin sheath: Protects and insulates nerve fibers, increasing the speed of nerve conduction.
• Schwann cells: Myelinate the axon.
• Nodes of Ranvier: Facilitate rapid conduction.

Neurotransmitters

• Transmitters activate or inhibit chemicals and also function as hormones.
• Adrenaline (Epinephrine): Released in response to stress, increases heart rate, affects blood pressure and glucose metabolism.
• Noradrenaline (Norepinephrine): Affects brain activity, contracts blood vessels, and controls blood flow.
• Endorphin: Released during exercise, excitement, and sex, producing a feeling of well-being and is involved in pain management.
• Acetylcholine: Linked with thought, learning, and memory; stimulates muscle function
• Histamine: Controls vasodilation, inflammation, and immune response, regulates metabolism, body temperature, and sleep-wake cycle.
• Dopamine: Provides pleasure and motivation.
• GABA: High levels improve focus, and low levels cause anxiety, and calms the firing nerves.
• Serotonin: Helps in sleep, memory, mood, and relaxation; is affected by exercise and light exposure
• Neurotransmitters potentiate, terminate, or modulate a specific action and either excite or inhibit the target cell's activity.
• Enzymes either destroy or reabsorb neurotransmitters for future use.
• Imbalances of neurotransmitters are linked to many disorders.
  • Parkinson's Disease: Decreased dopamine.
  • Myasthenia Gravis & Alzheimer's Disease: Impaired acetylcholine binding to muscle cells.
• Research evaluates diagnostic tests that can detect abnormal levels of neurotransmitters.
  • Positron Emission Tomography (PET): Can detect dopamine, serotonin, and acetylcholine.
  • Single-Photon Emission Computed Tomography (SPECT): Can detect some neurotransmitters, such as dopamine in Parkinson's disease.

Spinal Cord

• The spinal cord connects the brain and the periphery.
  • It has an H-shaped central core of nerve cell bodies (gray matter) surrounded by ascending and descending tracts (white matter)
  • Protected by the vertebral column consisting of 7 cervical, 12 thoracic, and 5 lumbar vertebrae, and the sacrum

Cranial Nerves

• There are 12 cranial nerves.
  • I. Olfactory (Sensory) - Smell
  • II. Optic (Sensory) - Vision
  • III. Oculomotor (Motor) - Eye Muscles
  • IV. Trochlear (Motor) - Eye Muscles
  • V. Trigeminal (Both) - Touch, Muscles
  • VI. Abducens (Motor) - Eye Muscles
  • VII. Facial (Both) - Taste, Face Muscles
  • VIII. Vestibulocochlear (Sensory) - Hearing and Balance
  • IX. Glossopharyngeal (Both) - Taste and Throat
  • X. Vagus (Both) - Brain
  • XI. Accessory (Motor) - Muscles
  • XII. Hypoglossal (Motor) - Tongue Muscles