Human Brain Anatomy and Development

The Human Brain

• The human brain weighs approximately 3 pounds
• It continually functions 24 hours a day, from birth until an exam
• All bodily processes, including emotions, problem-solving, reading and breathing, are controlled within the brain
• The brain is made of 100 billion neurons and 10 trillion neuroglia

Brain Development

• In the first three weeks of pregnancy, the neural tube of the embryo undergoes flexing, forming three primary brain vesicles
• The three vesicles are: Forebrain (Prosencephalon), Midbrain (Mesencephalon), and Hindbrain (Rhomencephalon)
• The Forebrain (Prosencephalon) consists of the Telencephalon (cerebrum and lateral ventricles) and the Diencephalon (thalamus, hypothalamus, epithalamus and third ventricle)
• The Midbrain (Mesencephalon) consists of the midbrain and the cerebral aqueduct
• The Hindbrain (Rhomencephalon) contains the Metencephalon (pons and cerebellum) and the Myelencephalon (medulla oblongata)
• Brain development is rapid, with up to 50,000 neurons added per second
• By birth, the neonatal brain resembles an adult brain, already containing virtually all of the neurons it will eventually have

Major Parts of the Brain

• The brain stem is the continuous extension of the spinal cord, including the medulla oblongata, pons, and midbrain
• The cerebellum is the second largest part of the brain
• The diencephalon forms the thalamus and hypothalamus
• The cerebrum is the largest and most recently evolved part of the brain, responsible for functions like perception, thought, imagination, judgment and decision making

Brain Organization

• The cerebral cortex, the outer layer of the cerebrum, processes sensory information from the body and sends corresponding signals to the muscles
• The basal ganglia initiate and terminate movements
• The thalamus relays sensory information from other parts of the brain to the cerebral cortex
• The hypothalamus controls homeostasis (bodily functions) such as body temperature, respiration, blood pressure and heartbeat, and it also controls endocrine system activity
• The cerebellum coordinates subconscious movements involved in muscle tone and balance
• The brain stem controls essential life functions via the cardiovascular and respiratory centers

Protective Coverings

• The cranial meninges are protective layers of tissue that wrap around the brain
• Meninges are continuous with spinal meninges
• The three layers (external to internal) are: Dura mater (tough outer layer), Arachnoid mater (spidery middle layer) and Pia mater (thin inner layer)
• The dura mater has extensions that form specific membranes (e.g. falx cerebri separating the R/L brain hemispheres, falx cerebelli separating the cerebellum R/L hemispheres, and the tentorium cerebelli separating the brain from the cerebellum)

Brain Tumors

• Brain tumors may be supratentorial (above the tentorium cerebelli) or infratentorial (below the tentorium cerebelli)
• Brain swelling can lead to brain herniation into the tentorium cerebelli, which is life threatening

Brain Blood Flow

• The brain is approximately 2% of total body weight, but uses 20% of the body's oxygen and glucose
• Internal carotid arteries supply blood to the anterior part of the brain, while vertebral arteries supply blood to the posterior part
• The Circle of Willis connects the carotid and vertebral arteries, ensuring a continuous blood supply
• The internal jugular veins provide the venous return from the brain

Blood Brain Barrier (BBB)

• The BBB protects the brain from harmful substances
• Tight junctions between the epithelial cells in capillaries in the CNS limit the passage of large molecules like proteins and medications
• Only substances with a specific structure (e.g., small molecules, water soluble substances, fats) and in specific amounts may pass through the BBB
• Harmful pathogens likewise cannot pass the barrier

Cerebral Spinal Fluid (CSF)

• CSF is a fluid that surrounds the brain, approximately 80 to 150ml, and it cushions the brain and helps maintain its stability
• CSF circulates through the brain ventricles and subarachnoid space
• CSF is produced, and subsequently reabsorbed, through special structures called the choroid plexuses located within the brain ventricles
• Obstructions to or abnormal production of CSF may result in hydrocephalus

Choroid Plexuses

• Choroid plexuses are networks of blood vessels that protrude into the brain ventricles, producing CSF.
• Hydrocephalus is an excess of CSF, which can be due to abnormal conditions, injuries, or infections.

Pathway of CSF

• CSF travels from the lateral ventricles through the interventricular foramina to the third ventricle, then through the cerebral aqueduct to the fourth ventricle
• Next, it exits through the median aperture, and lateral apertures, into the subarachnoid space, and is subsequently reabsorbed into the bloodstream through arachnoid villi
• CSF is reabsorbed as quickly as it is made, thus providing constant pressure (~20ml/hr)

Parts of the Brain

• The brain main parts include: Cerebrum, Diencephalon in the upper part, and the Cerebellum posterior aspect
• Diencephalon contains the thalamus, hypothalamus, and epithalamus
• Brainstem elements include the midbrain, pons, and medulla oblongata

Medulla Oblongata

• The medulla oblongata is a component of the brainstem, situated inferior at the pons' edge
• It extends to the foramen magnum, a passageway at the bottom of the skull
• It houses ascending and descending fiber tracts
• The medulla oblongata performs several vital functions, including cardiovascular and respiratory regulation

The Pons

• The pons lies above the medulla and in front of the cerebellum
• It acts as a bridge between the spinal cord, cerebellum, midbrain areas
• The pons also plays a role in relaying sensory and motor information
• Controls aspects of respiration

Cranial Nerves (V-XII)

• A detailed description of each cranial nerve’s origin, target, and specific function is provided in supplemental documents
• Summary of each cranial nerve’s category (motor, sensory, special sense etc.) are included

The Midbrain

• The midbrain extends from the pons to the diencephalon
• The cerebral aqueduct passes through the midbrain, connecting the 3rd and 4th ventricles
• It contains sensory and motor tracts, as well as cranial nerve nuclei, primarily nerve pairs III, and IV,

The Reticular Formation

• The reticular formation, a network of neuronal cell bodies, controls levels of alertness (in particular the RAS)
• The RAS helps us stay awake and aware
• It consists of sensory axons that project to the cerebral cortex
• Damage to the RAS can result in coma

The Cerebellum

• The cerebellum is the second largest part of the brain located behind the pons and the medulla oblongata; it's slightly inferior to the cerebrum
• It plays a critical role in coordinating complex movements, maintaining muscle tone, and regulating posture and balance within the body

The Diencephalon

• The diencephalon, located above the midbrain, encloses the third ventricle
• It comprises the thalamus, hypothalamus, and epithalamus
• The thalamus is a key relay center for sensory information to the cerebral cortex, except for smell
• The hypothalamus is involved in regulating many aspects of homeostasis and endocrine function
• The epithalamus includes the pineal gland and is associated with biological rhythms and emotional responses

The Cerebrum

• The cerebral cortex is the seat of higher-level brain functions; it's divided into regions responsible for specific cognitive functions (motor and sensory)
• The cerebrum is split into specific lobes (frontal, parietal, temporal, and occipital) based on their positions in the skull, and separated by sulci and gyri.
• Specific regions within the cerebral cortex are associated with functions such as speech, conscious thought, voluntary movement, pain, sensory perception and taste, as well as primary auditory and visual perception.

Functional Organization of the Cerebrum

• Brodmann's areas are numbered regions within the cerebral cortex with correspondences to specific cognitive functions

Limbic System

• The limbic system, a ring of structures encircling the upper brain stem and the corpus callosum, is involved in emotional responses such as pleasure, pain, anger, fear
• It plays a significant role in memory

Hemispheric Lateralization

• Each cerebral hemisphere is specialized for distinct functions (e.g., language functions in the left hemisphere, spatial recognition in the right hemisphere)

Cranial Nerves

• Summary of the 12 pairs of cranial nerves, their functions, and associated pathways