Brain and Cranial Nerves Review

Introduction to the Brain and Cranial Nerves

• General Characteristics of the Adult Human Brain

  • Nervous Tissue Proportion: The adult human brain contains approximately $97\%$ of the total nervous tissue in the body.

  • Average Weight: The brain weighs roughly $1.4\,kg$ ($3\,lb$).

  • Volume Specifications:

    • The typical volume is $1200\,mL$.

    • Functional range in normal individuals varies from $750\,mL$ to $2100\,mL$.

  • Sex Differences: The brains of males are approximately $10\%$ larger than those of females; however, there is no established correlation between brain size and intelligence.

Major Regions of the Brain

• Cerebrum:

  • The largest portion of the adult brain.

  • Responsible for higher mental functions, including conscious thought, intellect, and memory storage/processing.

  • Controls conscious and subconscious regulation of skeletal muscle contractions.

  • Divided into left and right cerebral hemispheres.

  • Cerebral Cortex: The surface layer of gray matter characterized by:

    • Gyri: Rounded elevations that increase surface area.

    • Sulci: Shallow depressions separating the gyri.

    • Fissures: Deep grooves.

• Cerebellum:

  • The second-largest part of the brain.

  • Coordinates complex somatic motor patterns and repetitive body movements.

  • Adjusts the output of other somatic motor centers in the brain and spinal cord.

  • Consists of two hemispheres covered by a layer of gray matter called the cerebellar cortex.

• Diencephalon:

  • Located underneath the cerebrum and cerebellum.

  • Thalamus: Relays and processes sensory information.

  • Hypothalamus: Involved in emotions, autonomic function, and hormone production.

  • Pituitary Gland: A major endocrine gland connected to the hypothalamus via the infundibulum (stalk); it integrates the nervous and endocrine systems.

• Brainstem:

  • Relays information between the spinal cord and the cerebrum or cerebellum.

  • Composed of three sections:

    • Midbrain: Processes sight and sound data, manages associated reflexes, and maintains consciousness.

    • Pons: Connects the cerebellum to the brainstem; contains tracts (CNS axon collections), relay centers, and nuclei for somatic and visceral motor control.

    • Medulla Oblongata: Connects the brain to the spinal cord; relays sensory information and regulates autonomic functions such as heart rate, blood pressure, and digestion.

Embryonic Development of the Brain

• The Neural Tube: Develops into the Central Nervous System (CNS).

• Primary Brain Vesicles (at 3 weeks):

  • Prosencephalon: Known as the "forebrain."

  • Mesencephalon: Known as the "midbrain."

  • Rhombencephalon: Known as the "hindbrain."

• Secondary Brain Vesicles (at 6 weeks):

  • From the Prosencephalon: Telencephalon and Diencephalon.

  • From the Mesencephalon: Mesencephalon stays as such.

  • From the Rhombencephalon: Metencephalon and Myelencephalon.

• Adult Brain Structures and Ventricular Origins:

  • Telencephalon becomes the Cerebrum (associated with the lateral ventricles).

  • Diencephalon remains the Diencephalon (associated with the third ventricle).

  • Mesencephalon becomes the Midbrain (associated with the cerebral aqueduct).

  • Metencephalon becomes the Cerebellum and Pons (associated with the fourth ventricle).

  • Myelencephalon becomes the Medulla oblongata (associated with the fourth ventricle).

Ventricular System

• Neurocoel: The expand chambers in the embryo neural tube that form ventricles lined with ependymal cells.

• Lateral Ventricles: One large ventricle in each cerebral hemisphere, separated by the septum pellucidum.

• Third Ventricle: Located in the diencephalon; communicates with lateral ventricles via the interventricular foramen.

• Fourth Ventricle:

  • Extends into the medulla oblongata and joins the central canal of the spinal cord.

  • Connects to the third ventricle via the cerebral aqueduct in the midbrain.

Protection and Support of the Brain

• Physical Protection: Provided by the bones of the cranium, the cranial meninges, and cerebrospinal fluid.

• Biochemical Isolation: Maintained by the blood-brain barrier.

• Cranial Meninges: Three layers continuous with spinal meninges:

  1. Dura Mater:

     • Inner Fibrous Layer: Meningeal cranial dura.

     • Outer Fibrous Layer: Periosteal cranial dura (fused to periosteum).

     • Dural Folds: Inward extensions of the meningeal layer that stabilize the brain. They contain dural venous sinuses (collecting veins). Key folds include the Falx cerebri (between hemispheres, contains superior/inferior sagittal sinuses), Tentorium cerebelli (separates cerebrum from cerebellum, contains transverse sinus), and Falx cerebelli (divides cerebellar hemispheres).

  2. Arachnoid Mater: Covers the brain; separated from the dura by the subdural space and from the pia by the subarachnoid space.

  3. Pia Mater: Attached to the brain surface by astrocytes.

• Cerebrospinal Fluid (CSF):

  • Functions: Supports the brain's weight, cushions neural structures, and transports nutrients, messengers, and wastes.

  • Production: The choroid plexus (specialized ependymal cells surrounding capillaries) produces approximately $500\,mL/day$.

  • Circulation Path: Choroid plexus → ventricles → central canal of spinal cord → subarachnoid space (via two lateral apertures and one median aperture in the roof of the fourth ventricle).

  • Absorption: Reabsorbed into venous circulation through arachnoid granulations (clusters of arachnoid villi that penetrate the dura into the superior sagittal sinus).

• Blood Supply: Delivered by internal carotid and vertebral arteries; removed by internal jugular veins which drain the dural venous sinuses.

Anatomy of the Brainstem

• Medulla Oblongata:

  • Reticular Formation: Regulates autonomic functions.

  • Cardiovascular Centers: Manage heart rate and blood flow.

  • Respiratory Rhythmicity Centers: Set the pace for breathing.

  • Sensory and Motor Nuclei: Associated with cranial nerves VIII, IX, X, XI, and XII.

  • Relay Stations:

    • Gracile and Cuneate Nuclei: Pass somatic sensory info to the thalamus; cross at the decussation of pyramids.

    • Solitary Nuclei: Receive visceral sensory information.

    • Inferior Olivary Complex: Relays motor command information.

• Pons:

  • Contains nuclei for respiration (apneustic and pneumotaxic centers).

  • Links the cerebellum with other parts of the brain via transverse pontine fibers.

  • Hosts nuclei for cranial nerves V, VI, VII, and part of VIII.

• Midbrain:

  • Tectum (roof): Contains the corpora quadrigemina (Superior colliculi for visual reflexes; Inferior colliculi for auditory reflexes).

  • Tegmentum: Contains the red nucleus (highly vascular) and the substantia nigra (pigmented gray matter).

  • Cerebral Peduncles: Nerve fibers on the ventrolateral surface carrying descending motor commands.

Cerebellum Structure and Function

• Anatomy: Includes the cerebellar cortex (folia/folds), anterior and posterior lobes (separated by primary fissure), and the vermis (narrow central band).

• Functional Layers:

  • Purkinje Cell Layer: Large, branched neurons in the cortex receiving up to $200000$ synapses each.

  • Arbor Vitae: "Tree of life"; the internal white matter that relays information to Purkinje cells.

• Peduncles: Superior, middle, and inferior tracts linking the cerebellum to the brainstem and cerebrum.

• Functions: Adjusts postural muscles and programs fine-tuned conscious/subconscious movements.

• Ataxia: A coordination disorder caused by trauma, stroke, or intoxication.

Diencephalon

• Thalamus:

  • Separated by the third ventricle; linked by the interthalamic adhesion.

  • Anterior nuclei: Limbic system (emotions).

  • Medial nuclei: Awareness of emotional states.

  • Ventral nuclei: Relay sensory info to the cortex.

  • Geniculate bodies: Lateral (visual), Medial (auditory).

• Hypothalamus:

  • Functions: Hormone secretion (ADH and Oxytocin), temperature regulation (pre-optic area), autonomic control, coordination of nervous/endocrine systems, circadian rhythm regulation (suprachiasmatic nucleus), and behavioral drives (Feeding, Thirst, and Satiety centers).

  • Anatomy: Includes mammillary bodies (reflex eating), infundibulum (pituitary connection), and tuber cinereum (hormone production).

Limbic System and Cerebrum

• Limbic System: Establishes emotional states, links high-level intellect with autonomic function, and facilitates memory.

  • Components: Amygdaloid body, Hippocampus, Fornix (white matter tract), Cingulate gyrus, and Parahippocampal gyrus.

• Cerebrum:

  • Gyri and Sulci: Increase area. Longitudinal fissure separates hemispheres. The Central sulcus separates the frontal and parietal lobes.

  • Lobes: Frontal, Parietal, Temporal, Occipital, and the Insula (medial to the lateral sulcus).

  • White Matter:

    • Association Fibers: Connect areas within one hemisphere (Arcuate fibers/Longitudinal fasciculi).

    • Commissural Fibers: Link the two hemispheres (Corpus callosum, Anterior commissure).

    • Projection Fibers: Link cortex to lower centers (Internal capsule).

  • Basal Nuclei: Subconscious control of muscle tone and learned movement coordination. Dysfunction is associated with Parkinson’s disease.

• Functional Areas:

  • Primary Motor Cortex: Precentral gyrus (Pyramidal cells).

  • Primary Somatosensory Cortex: Postcentral gyrus.

  • Integrative Centers: Wernicke’s area (language comprehension, usually left side), Broca’s area (speech production, usually left side), and the Prefrontal Cortex (intellect and predicting consequences).

Brain Activity and EEG

• Electroencephalogram (EEG): Monitors electrical patterns (brain waves).

• Wave Types:

  • Alpha: Healthy resting adults (closed eyes).

  • Beta: Concentrating or stressed adults.

  • Theta: Children or frustrated adults; may indicate adult brain disorders.

  • Delta: Deep sleep or brain damage in awake adults.

• Seizure: Temporary cerebral disorder with abnormal electrical activity.

Cranial Nerves Summary

• I Olfactory: Special sensory; smell; pass via cribriform plate.

• II Optic: Special sensory; vision; pass via optic canals; converge at optic chiasm.

• III Oculomotor: Motor; eye movements (4 of 6 muscles); intrinsic eye muscles control.

• IV Trochlear: Motor; superior oblique muscle.

• V Trigeminal: Mixed; facial sensation/mastication; three branches: Ophthalmic ($V_1$), Maxillary ($V_2$), Mandibular ($V_3$). Associated with Tic douloureux.

• VI Abducens: Motor; lateral rectus muscle.

• VII Facial: Mixed; facial expression, taste (ant. 2/3), lacrimal/salivary glands. Associated with Bell’s palsy.

• VIII Vestibulocochlear: Special sensory; balance (vestibular) and hearing (cochlear).

• IX Glossopharyngeal: Mixed; taste (post. 1/3), swallowing, parotid gland.

• X Vagus: Mixed; widely distributed to thorax/abdomen (digestive, respiratory, cardiovascular).

• XI Accessory: Motor; neck/back muscles (sternocleidomastoid, trapezius).

• XII Hypoglossal: Motor; tongue movements.