Human Nervous System II: Brain and Cranial Nerves

Human Nervous System II: Brain and Cranial Nerves

• Diencephalon is not part of the brainstem.
• Cranial nerve VIII is a pure sensory nerve. There's no compelling evidence for a motor function.
• Cranial nerve IX is involved in the gag reflex. It sends sensation to the medulla, and CN X sends a motor command to the pharyngeal muscles.

Lecture II: Neuroanatomy of the Brain and Cranial Nerve

• Topics include overview of the brain, brainstem and cerebellum, forebrain, and cranial nerves.

II.1: Overview of the Brain

• Brain weight: 1.45 - 1.6 kg in humans.
• Three major portions: cerebrum, cerebellum, brainstem.
• Cerebrum: 83% of brain volume, comprised of hemispheres separated by a longitudinal fissure, connected by the corpus callosum. Cerebral tissue is folded into gyri (elevations) and sulci (grooves/depressions).
• Cerebellum: 10% of brain volume, inferior and posterior to the cerebrum, separated by a transverse cerebral fissure.
• Brainstem: crucial for survival, consists of sub-regions and is continuous with the spinal cord inferiorly.
• Gray matter: found in the cortex of the cerebrum and deeper masses (nuclei) composed of neuronal cell bodies
• White matter: bundles of axons that connect the brain to the spinal cord or areas within parts of the cerebrum.

II.1.2 Meninges

• Three protective membranes: dura mater, arachnoid mater, pia mater encasing the brain.
• Dura mater has two layers (periosteal and meningeal).
• Arachnoid and pia mater are similar to the spinal cord.
• Dura mater folds to separate brain parts (e.g., falx cerebri, falx cerebelli).

II.1.3 Ventricles and Cerebrospinal Fluid (CSF)

• Hollow spaces within the brain are filled with CSF.
• Four ventricles (lateral ventricles, third ventricle, cerebral aqueduct, fourth ventricle)
• CSF is produced by the choroid plexus, a spongy mass of blood capillaries in the ventricles.
• CSF Functions: buoyancy, protection, metabolic stability.

II.1.4 Brain Barrier System (BBS)

• Regulates substances from the bloodstream entering the brain.
• Blood-Brain Barrier (BBB): astrocytes contact blood capillaries. The astrocytes stimulate tight junctions between endothelial cells.
• Blood-CSF barrier: formed at choroid plexuses by ependymal cells.

II.2: Brainstem and Cerebellum

• Brainstem consists of the diencephalon, midbrain, pons, and medulla oblongata.
• Medulla oblongata: contains nerve fibers connecting brain and spinal cord; contains crucial nuclei (e.g., cardiac and vasomotor centers) for physiological functions; includes structures on the surface such as pyramids and internal structures such as ascending and descending tracts (e.g., tectospinal and corticospinal) and nuclei (e.g., inferior olivary nucleus).
• Pons: position is rostral to the medulla; has cerebellar peduncles; continuation of ascending/descending tracts such as the tectospinal tract.
• Midbrain: contains the cerebral aqueduct and colliculi (superior and inferior); involved in visual and auditory reflexes. Internal structures include the central/periaqueductal gray and substantia nigra (important for motor control and suppressing unwanted movements).

II.2.5 Cerebellum

• Motor coordination center, composed of two hemispheres connected by the vermis.
• Internal structures include the arbor vitae (white matter), and specific neuron types (granule and Purkinje cells).
• Functions include motor coordination, learning motor skills, posture maintenance, coordinating body and eye movements, and coordinating complex actions, by comparing planned and actual movement, sending feedback to the cerebrum.

II.2.6 Reticular Formation

• Loosely organized gray matter in the brainstem.
• Functions: somatic motor control (muscle tone, balance, posture), cardiovascular control, pain modulation, sleep/consciousness, and habituation.

II.3: Forebrain

• Part of the brain encompassing the diencephalon and the cerebral hemispheres.

II.3.1 Diencephalon

• Most rostral portion of the brainstem; part of the forebrain.
• Consists of the thalamus, hypothalamus, and epithalamus

II.3.1 Thalamus

• Oval shape, separated into right and left parts.
• Consists of 23 nuclei.
• Acts as a gateway to the cerebral cortex.
• Relays signals from the cerebellum and other areas to the cerebrum.
• Involved in memory and emotion.

II.3.1 Hypothalamus

• Walls and floor of the third ventricle, connected to the thalamus by the infundibulum.
• Very important in regulating the endocrine system and autonomic nervous system.
• Functions: hormone production/secretion, regulation of growth, metabolism, and reproduction; autonomic effects on parts of the brainstem (heart rate, blood pressure, etc.); thermoregulation, regulating appetite, emotion, reproductive response, memory.

II.3.1 Epithalamus

• Consists of the pineal gland and habenula.
• Pineal gland secrets melatonin, regulating sleep and consciousness..

II.3.2 Gross Anatomy of the Cerebrum

• Extensive folding (gyri) increases surface area which houses neurons.
• Divided into five lobes: frontal, parietal, occipital, temporal and insula..
• Characteristics of each lobe: frontal (anterior, higher cognitive functions), parietal (sensory), occipital (vision), temporal (hearing, smell, taste, visceral sensation), insula (taste, visceral sensation).

II.3.1 White Matter

• Projection tracts: connect areas vertically between the brain and spinal cord; examples: corticospinal tract.
• Commissural tracts: connect one side of the brain to the other; an example is the corpus callosum.
• Association tracts: connect different regions on the same side of the brain.

II.3.3 Sub-systems in the Cerebrum

• Limbic system: center of emotion and learning, comprised of hippocampus, amygdala, cingulate gyrus; involved in receiving sensory input (vision, taste, smell), integrating sensation to emotion.
• Basal nuclei: later to thalamus, also known as basal ganglia; comprised of caudate nucleus, putamen. and globus pallidus; involved in motor control, planning, and execution of movement.

II.3.4 Higher Functions and Structures of the Cerebrum

• Primary cortex: cerebrum regions connect directly with sensory or motor neurons; (i.e. with third-order neurons or upper motor neurons.
• Association cortex: regions involved in higher functions e.g. cognitive processes and integrating sensory input.
• Special senses: vision, hearing, equilibrium, taste, and smell. Special sensory neurons relay signals to sensory cortex then to nearby association area to integrate sensation, memory and interpretation.
• General senses (somatosensory): general senses such as touch, pain etc.
• Motor control: intention to move begins in motor association area; neurons compile a program for muscle movement; program transmitted to precentral gyrus (primary motor area); motor commands sent to the brainstem and spinal cord.
• Language: integration of sensory and motor functions; Wernicke area receives auditory and visual info (speech), recognizes spoken/written language; Formulates phrases, sends it to Broca's area; Broca's area (motor association area) generates motor programs for muscles of tongue etc., transduces signal to primary motor cortex for execution.

II.4 Cranial Nerves

• The brain communicates with other body parts via spinal cord and spinal nerves.
• Cranial nerves directly arise from brain and brainstem, there are 12 pairs.
• Classification: sensory, motor, or mixed (sensory and motor functions).
• Mnemonic to remember cranial nerves: "Oh, Once One Takes the Anatomy Final, Very Good Vacation Ahead".
• Key characteristics for each cranial nerve, including name, function (sensory, motor, mixed), and connections (origin/termination) are given.

This quiz explores the human nervous system, focusing on the brain and cranial nerves. Key topics include the structure of the brain, the functions of cranial nerves, and their involvement in reflex actions. Test your knowledge of neuroanatomy with this engaging assessment.