Brain Anatomy and Cranial Meninges

Brain and Cranial Nerves

• The brain defines a person by recording experiences; the physical body carries out brain instructions.
• The brain defines life and death; it's the only organ not transplantable.
• The central nervous system (CNS) includes the brain and spinal cord, located in the dorsal cavity.
• The brain connects to the spinal cord at the foramen magnum of the occipital bone.

Protection System of the Brain

• The brain is protected by:

  • Cranial bones:

    • Compact bone (outer layer)
    • Spongy bone (inner layer).
    • Solid compact bone would make the cranium too heavy.

  • Cranial meninges:

    • Connective tissue coverings continuous with spinal cord.
    • Similar names to spinal meninges:
      • Dura mater (outermost):
        • Tough, inelastic, white fibrous connective tissue.
        • Lines the inside of cranial bones and vertebral canal.
        • Contains dural sinuses collecting venous blood to the cardiovascular system.
      • Arachnoid (middle):
        • Cobweb-like appearance
        • Contains villi that produce cerebrospinal fluid (CSF)
        • Choroid plexus also produces CSF
      • Pia mater (innermost):
        • Adheres to the brain; cannot be dissected away without injury.
        • Closely follows brain contours.

Cerebrospinal Fluid (CSF)

• A clear, viscous liquid.
• Contains sodium, potassium, calcium, magnesium, chloride, and white blood cells.
• Functions:
  • Circulation: Delivers nutrients, removes waste, and maintains optimal chemical environment.
  • Chemical protection: Provides optimal chemical environment for neuronal signaling.
  • Mechanical protection: Acts as a cushion, protecting the spinal cord.
• CSF is found in the ventricles of the brain, subarachnoid space, and the central canal of the spinal cord.

Ventricular System

• A series of spaces and canals deep inside the cerebrum.
• Specialized tissue in the ventricles is called choroid plexuses.
• Choroid plexuses are tiny masses of capillaries from the pia mater.
• 4 ventricles, with the two lateral ventricles being the largest.

Flow of CSF

• CSF is made in the choroid plexuses within the walls of the lateral ventricles.
• CSF passes from lateral ventricles to the third ventricle through the interventricular foramina.
• CSF travels to the fourth ventricle via the cerebral aqueduct.
• CSF enters the subarachnoid space through openings in the fourth ventricle's roof.
• CSF circulates in the central canal of the spinal cord and the subarachnoid space surrounding the brain and spinal cord.
• CSF is reabsorbed into the bloodstream through arachnoid villi that connect to the dural venous sinuses.

Hydrocephalus

• An abnormality in CSF drainage, potentially due to tumors, inflammation, head injuries, or meningitis.
• CSF pressure rises, compressing and damaging nervous tissue.
• Neurosurgeons may implant a shunt to drain CSF.

Blood Supply to the Brain and Cranial Nerves

• The brain has a backup system for blood supply.
• Main supply comes from common carotid arteries, dividing into internal and external carotid arteries.
• Internal carotid feeds brain structures.
• External carotid supplies face and neck.
• Vertebral arteries form the circle of Willis at the base of the brain.
• The brain has no oxygen, nutrient, or glucose reserve; constant blood supply is essential.
• Blood flow is crucial; impaired flow can cause unconsciousness, cell damage, or permanent injury.
• Glucose is the main energy source.
• The blood–brain barrier protects brain cells from harmful substances and pathogens.

Principle Parts of the Brain

• Brain Stem: Connects the brain to the spinal cord, with three parts:

  • Midbrain (most superior)
  • Pons (“bridge”) connected to the diencephalon.
    • Nuclei control respiration (pneumotaxic and apneustic areas).
  • Medulla Oblongata (aka Medulla)
    • Ascending and descending nerve fibers pass through.
    • Controls autonomic nervous system activities: cardiovascular center, respiration, etc.

• Diencephalon: Surround the third ventricle, composed of gray matter:

  • Thalamus
    • Sensory relay station.
    • Registers pain, temperature, light touch, and pressure.
  • Hypothalamus
    • Regulates homeostasis.
    • Regulates autonomic nervous system, pituitary gland, and many visceral activities.
    • Regulates hunger, thirst, body temperature, consciousness, and sleep/wake cycles.
    • Involved in producing hormones and regulating emotional behaviors.
  • Subthalamus
  • Epithalamus

• Cerebellum: The second largest part of the brain, located below the occipital lobes.

  • Smooths coordinated movements and regulates posture/balance.
  • Contains cerebellar cortex (gray matter layer) surrounded by white matter.

• Cerebrum: The largest part of the brain, located on top of the cranial cavity. Contains four surface lobes:

  • Frontal, temporal, parietal, and occipital lobes.
  • Controls mental functions: reading, writing, speech, calculations, memory, reasoning, etc.
  • Has a cerebral cortex.
  • The cerebrum contains two cerebral hemispheres (right and left) separated by a longitudinal fissure.

Brain Lateralization

• The two hemispheres connected by the corpus callosum.
• Left hemisphere typically dominant: - Right-handed control - Spoken & written language.
• Right hemisphere important for: - Left-handed control - Musical and artistic awareness - Spatial perception - Imagination and generating mental images.

Cranial Nerves

• Twelve pairs of cranial nerves extending from the brain.
• A mnemonic device (Oh Oh Oh To Touch And Feel Very Green Vegetables, Hold!) helps remember their order and functions.
• Some cranial nerves are sensory, while others are motor, and some are both.
• Functions include sensory perception (smell, sight), eye movement, facial expressions, hearing, balance, swallowing speech, etc.

Key Medical Terms

• Analgesia: Pain relief
• Anesthesia: Loss of feeling
• Babinski sign: Abnormal reflexes
• Coma: Abnormally deep unconsciousness.
• Dementia: Progressive loss of intellectual abilities.
• Lethargy: Sluggishness or torpor.
• Nerve block: Loss of sensation due to local anesthesia.
• Other terms related to neurological conditions and symptoms (neuralgia, paralysis, tinnitus, vertigo, etc).

Explore the anatomy of the brain, its connection to the spinal cord, and the protective structures like cranial bones and meninges. Understand the role of the brain in defining a person and its protection by cranial bones and the dura mater.