CNS

CNS: Brain Overview of Brain Anatomy

  • The Human Brain
    • Composed of four major regions:
    • Cerebrum:
      • Comprised of two hemispheres, each having five lobes.
    • Diencephalon:
    • Brainstem:
    • Cerebellum:
      • The outer surface is folded.
      • Gyri: Ridges on the surface.
      • Sulci: Depressions between the ridges.
      • Fissures: Deep sulci.
      • Orientation terms: Anterior = rostral; Posterior = caudal.

Gray Matter and White Matter Distribution

  • CNS Composition: Both brain and spinal cord contain gray and white tissue.
    • Gray Matter:
      • Composed of neuron cell bodies, dendrites, and unmyelinated axons.
      • Cerebral Cortex: Gray matter surface of cerebrum.
      • Cerebral Nuclei: Regions of gray matter (clusters of cell bodies) deep in the cerebrum.
    • White Matter:
      • Consists of myelinated axons.

Cranial Meninges

  • Structure: Three connective tissue layers.
    • Functions: Separate and support the soft tissue of the brain, enclose, and protect blood vessels supplying the brain, help contain and circulate cerebrospinal fluid (CSF).
    • Layers from Deep to Superficial:
      • Pia Mater:
        • Innermost layer, adheres to brain surface, thin layer of areolar connective tissue.
      • Arachnoid Mater:
        • Web of collagen and elastic fibers, lies deep to dura mater.
        • Contains the subdural space, which may fill with blood if a vein ruptures.
      • Dura Mater:
        • Tough outer membrane, made of dense irregular connective tissue (2 layers):
          • Meningeal Layer: Deeper layer of dura.
          • Periosteal Layer: More superficial layer.
        • Forms the periosteum on the internal surface of cranial bones.
        • Layers typically fused but separate in areas forming dural venous sinuses to drain blood from the brain.
        • Epidural Space: Potential space between dura and skull containing arteries and veins.

Clinical View: Hematomas

  • Epidural Hematoma:
    • Pool of blood in the epidural space, usually due to severe head trauma.
    • Distorts and compresses adjacent brain tissue.
    • Can lead to severe neurological injury or death unless the bleeding is stopped and blood removed.
  • Subdural Hematoma:
    • Hemorrhage in the subdural space from ruptured veins due to rapid head movement.
    • Compression of brain tissue occurs more slowly compared to epidural hematoma.

Clinical View: Meningitis

  • Meningitis:
    • Inflammation of the meninges, typically caused by viral or bacterial infections.
    • Symptoms include fever, headache, vomiting, and a stiff neck, with pain referred to the posterior neck.
    • Untreated meningitis can lead to brain damage or death.

Brain Ventricles

  • Ventricles:
    • Cavities within the brain lined with ependymal cells.
    • Contain cerebrospinal fluid (CSF).
    • Connect with each other and with the spinal cord’s central canal.

Cerebrospinal Fluid (CSF)

  • CSF: Clear, colorless liquid surrounding the CNS.
    • Circulates in ventricles and subarachnoid space.
    • Provides buoyancy, reducing the brain’s apparent weight by 95%.
    • Protects CNS by providing a liquid cushion.
    • Maintains a stable CNS environment, helping transport nutrients and waste and protecting against chemical fluctuations.

Blood-Brain Barrier (BBB)

  • Functions:
    • Regulates substances entering the brain's interstitial fluid.
    • Prevents exposure of neurons to harmful substances (e.g., drugs, wastes).
    • Some drugs can pass and affect the brain (e.g., alcohol).
  • Composition:
    • Composed of specialized capillaries:
      • Endothelial cells connected by tight junctions.
      • Thick basement membrane.
      • Wrapped by perivascular feet (astrocyte extensions).
  • Location Specific Functionality:
    • BBB is reduced in locations like the choroid plexus (to produce CSF) and hypothalamus/pineal gland (for hormone secretion).

Cerebrum

  • Cerebrum: Origin of all complex intellectual functions, comprising two large hemispheres on the superior aspect of the brain.
    • Center for:
      • Intelligence and reasoning.
      • Thought, memory, and judgment.
      • Voluntary motor, visual, and auditory activities.
  • Cerebral Hemispheres:
    • Consist of left and right hemispheres, separated by the longitudinal fissure.
    • White Matter Tracts: Connect the hemispheres, including the corpus callosum, the largest tract.
    • Connection pattern: Generally crossed, with the left hemisphere receiving sensory signals from the right side and sending motor signals similarly.
    • Lateralization: Some higher-order functions, like speech, primarily controlled by one hemisphere (often left).

Lobes of the Cerebrum

  • Five Lobes per Hemisphere:
    • Four named for the overlying cranial bones:
      • Frontal Lobe: Anterior part, posterior border marked by the deep central sulcus.
        • Precentral Gyrus: Controls voluntary movement.
        • Functions include motor control, concentration, verbal communication, decision making, planning, personality.
      • Parietal Lobe: Superoposterior part; anterior border defined by the central sulcus (postcentral gyrus ridge).
        • Functions include evaluating shape and texture of objects.
      • Temporal Lobe: Internal to temporal bone, inferior to lateral sulcus; responsible for hearing and smell.
      • Occipital Lobe: Posterior part; functions in vision and visual memories.
      • Insular Lobe: Small, deep to the lateral sulcus, involved in memory and taste sensations.

Functional Areas of the Cerebrum

  • Motor Areas within the Frontal Lobes:

    • Primary Motor Cortex: Located in the precentral gyrus, controls skeletal muscle activity on the opposite side of the body.
      • Projects contralaterally within brainstem or spinal cord, represented as a motor homunculus reflecting the cortex area dedicated to each body part.
    • Motor Speech Area (Broca Area): Located in the inferolateral portion of the left frontal lobe (in most people); controls vocalization movements.
    • Frontal Eye Field: On the superior surface of the middle frontal gyrus; regulates eye movements needed for reading and binocular vision.
    • Premotor Cortex: Coordinates learned, skilled activities and is located anterior to the primary motor cortex.

  • Sensory Areas:

    • Primary Somatosensory Cortex: Located in the postcentral gyrus of the parietal lobes; processes sensory information (proprioceptors, touch, etc.).
      • Areas of the body sending input can be mapped as a sensory homunculus.
    • Somatosensory Association Area: Integrates touch information to identify objects by feel. Located just posterior to the postcentral gyrus in the parietal lobe.
    • Primary Visual Cortex: Located in the occipital lobe.
    • Visual Association Area: Integrates color, form, memory around the primary visual cortex to allow identification of seen objects (e.g., faces).
    • Primary Auditory Cortex: Located in temporal lobe, with an associated auditory association area interpreting sounds and memories.
    • Primary Olfactory Cortex: Located in temporal lobe, responsible for conscious awareness of smells.
    • Primary Gustatory Cortex: Located in insula, processes taste information.

  • Functional Brain Regions:

    • Prefrontal Cortex: Responsible for complex thought, judgment, planning, and personality; situated rostral to the premotor cortex.
    • Wernicke Area: Typically located in the left hemisphere; involved in language comprehension.
    • Gnostic Area: Integrates sensory association information to create awareness of situations (e.g., lunchtime).

Anatomical Asymmetries & Lateralization

  • Petalias: Protrusions of a lobe on one side compared to the other.
  • Cerebral Lateralization:
    • Categorical Hemisphere: Usually the left, specialized for language abilities and analysis.
    • Representational Hemisphere: Usually the right, related to visuospatial relationships and imagination.
    • Communication occurs through the corpus callosum and various commissures.
    • Lateralization develops early in childhood, observable before 5-6 years of age.

Diencephalon

  • Includes: Epithalamus, Thalamus, Hypothalamus.
    • Provides relays and switching centers for sensory, motor, and visceral pathways.
  • Epithalamus:
    • Contains the Pineal Gland, which secretes melatonin to help regulate circadian rhythms.
    • Habenular Nuclei: Relays signals from the limbic system to the midbrain; involved in emotional responses to odors.
  • Thalamus:
    • Receives signals from all conscious senses (excluding olfaction) and relays/filter signals to appropriate cortical areas.
  • Hypothalamus:
    • Infundibulum: Stalk extending from hypothalamus to the pituitary gland.
    • Functions:
      • Autonomic nervous system control (heart rate, blood pressure, digestion, respiration).
      • Endocrine system control (secretes hormones affecting anterior pituitary activities; produces antidiuretic hormone and oxytocin).
      • Regulation of body temperature through detection and signaling.
      • Emotional control as part of the limbic system, influencing responses like pleasure and fear.
      • Regulation of food and water intake through specialized nuclei monitoring nutrient levels and regulating thirst.
      • Influence over sleep-wake cycles via the suprachiasmatic nucleus directing melatonin secretion.

Brainstem

  • Serves as the connection: between the cerebrum, diencephalon, and cerebellum to the spinal cord.
    • Contains ascending and descending tracts, autonomic nuclei, cranial nerve nuclei, and reflex centers.
    • Components:
      • Midbrain:
        • Cerebral Peduncles: Motor tracts carrying voluntary commands from the primary motor cortex.
        • Superior Cerebellar Peduncles: Connects cerebellum to midbrain.
        • Medial Lemniscus: Bands of myelinated axons within the brainstem.
        • Substantia Nigra: Neurons producing dopamine; degeneration leads to Parkinson Disease.
      • Pons: Bulging region connecting brain and spinal cord, containing sensory and motor tracts. Includes middle cerebellar peduncles and pontine respiratory center.
      • Medulla Oblongata:
        • Caudal portion of the brainstem, continuous with the spinal cord.
        • Contains pyramids (corticospinal tracts for motor control) and olives (relay proprioceptive information to cerebellum).
        • Autonomic nuclei regulate heart output, blood vessel diameter, and breathing.

Cerebellum

  • Second largest brain area after the cerebrum.
    • Cerebellar Cortex: Convoluted surface with folia (folds) and left/right hemispheres.
    • Functions Include:
      • Coordinating and fine-tuning movements by ensuring muscle activity follows proper patterns.
      • Memory storage of previously learned movements.
      • Regulation along both voluntary and involuntary motor pathways.
      • Adjustment of movements initiated by the cerebrum to maintain smoothness and equilibrium.

Functional Brain Classification: Limbic System

  • Limbic System: Often referred to as the emotional brain, composed of both cerebral and diencephalic structures.
    • Components:
      • Cingulate Gyrus: Located above the corpus callosum.
      • Parahippocampal Gyrus: Associated with the hippocampus, located in the temporal lobe.
      • Hippocampus: Aids in the formation of long-term memories.
      • Amygdaloid Body: Involved in emotions and emotional memory, particularly fear.
      • Olfactory Structures: Process odors, which can provoke emotions.
      • Fornix: Connects hippocampus with other limbic structures.

Reticular Formation

  • Reticular Formation: Loosely organized gray matter of the brainstem.
    • Components:
      • Motor Component: Regulates muscle tone via spinal cord connections and assists in autonomic functions.
      • Sensory Component (RAS): Processes sensory information to enhance alertness and awareness necessary for higher states of consciousness.

Spinal Cord Gross Anatomy

  • Structure:
    • Extends inferiorly from the brain's medulla through the vertebral canal.
    • Ends at the L1 vertebra.
    • Enlargements:
      • Cervical Enlargement: Contains neurons for upper limb innervation.
      • Lumbar Enlargement: Contains neurons for lower limb innervation.
    • Spinal cord structure includes:
      • Cervical Part: 8 pairs of cervical spinal nerves.
      • Thoracic Part: 12 pairs of thoracic spinal nerves.
      • Lumbar Part: 5 pairs of lumbar spinal nerves.
      • Sacral Part: 5 pairs of sacral spinal nerves.
      • Coccygeal Part: 1 pair of coccygeal spinal nerves.
    • Alignments of spinal cord parts do not match vertebral names due to continued vertebral growth after spinal cord growth has been completed.

Protection and Support of the Spinal Cord

  • Meninges: Protect the spinal cord.
    • Pia Mater:
      • Delicate inner layer adheres to the spinal cord, made of elastic and collagen fibers.
      • Denticulate Ligaments: Lateral extensions help suspend spinal cord.
      • Filum Terminale: Pia anchoring the inferior end of spinal cord to the coccyx.
    • Arachnoid Mater: Web-like layer outside of pia with arachnoid trabeculae and the subarachnoid space through which CSF flows.
    • Dura Mater: Tough outer layer made of dense irregular connective tissue stabilizing the spinal cord.

Clinical View: Lumbar Puncture

  • Procedure: For obtaining CSF for medical diagnosis.
  • Pathway through tissue layers:
    • Skin, back muscles, ligamentum flavum, epidural space, dura mater, arachnoid mater into the subarachnoid space.
  • Location: Adult spinal cord ends at L1; lumbar punctures are performed below this, typically above or below L4.

Gray Matter and White Matter Organization in the Spinal Cord

  • Gray Matter:
    • Gray Commissure: Horizontal band surrounding central canal with unmyelinated axons connecting left and right gray matter parts.
    • Nuclei:
      • Sensory Nuclei: In posterior horn containing interneurons (somatic and visceral sensory nuclei).
      • Motor Nuclei: In anterior and lateral horns containing motor neurons (somatic and autonomic motor nuclei).

Distribution of White Matter in the Spinal Cord

  • Regions:
    • Posterior Funiculus: Contains sensory tracts.
    • Lateral Funiculus: Contains ascending and descending tracts
    • Anterior Funiculus: Connects anterior gray horns and anterior median fissure with ascending and descending tracts.

Overview of Conduction Pathways

  • Spinal Pathways Characteristics:
    • Types: Sensory (ascending) and Motor (descending).
    • Location: Axons are in spinal cord tracts; cell bodies are in ganglia, spinal cord gray horns, and brain gray matter.
    • Composed of a chain of two or more neurons supporting paired pathways (left and right) that often decussate (cross midline).

Sensory Pathways

  • Ascending Pathways: Convey signals for proprioception, touch, temperature, pressure, and pain from the body to the brain.
    • Pathway Structure:
      • Primary (1st order) neuron receives from peripherals and transmits to secondary neuron (interneuron).
      • Secondary (2nd order) neuron relays the primary input to the tertiary neuron or to the cerebellum.
      • Tertiary (3rd order) neuron proceeds to somatosensory cortex of the parietal lobe.

Motor Pathways

  • Descending Pathways: Elicit responses in effectors like skeletal muscles.
    • Structure:
      • Upper motor neuron in motor cortex or brainstem; contacts lower motor neuron.
      • Lower motor neuron in cranial nerve nucleus or spinal cord anterior horn; excites muscles.

Differences Between Sensory and Motor Pathways

  • Sensory Pathways:
    • Ascend in sensory pathways to the brain; primarily travel in posterior and lateral funiculi of the spinal cord.
  • Motor Pathways:
    • Descend from the brain, moving through anterior and lateral funiculi of the spinal cord.

Clinical View: Treating Spinal Cord Injuries

  • Challenges: Injuries can lead to paralysis and sensory loss.
    • Intervention Strategies:
      • Prompt use of steroids may help preserve muscle function.
      • Early antibiotics can reduce death from complications.
      • Neural stem cells may be a future therapy for regenerating CNS axons.