Secondary Vesicles:
- Telencephalon: develops into the cerebrum
- Diencephalon: becomes the thalamus and hypothalamus
- Metencephalon: forms the pons and cerebellum
- Myelencephalon: forms the medulla oblongata
Developmental Stages of the Embryo
Vesicles are critical in establishing major functional regions of the brain.
Some connections become clearer in the context of development:
- The retina connects to the diencephalon.
- The cerebellum originates from the metencephalon and connects to the pons.
Open spaces where Cerebrospinal Fluid (CSF) accumulates are referred to as ventricles, which are remnants of the original neural tube.
The Adult Brain: Major Regions
The adult brain is divided into four major regions:
- Cerebrum (cerebral hemispheres)
- Diencephalon (thalamus, hypothalamus, epithalamus)
- Brain Stem (midbrain, pons, medulla)
- Cerebellum
The Cerebrum
The cerebrum forms the superior part of the brain.
Surface markings include:
- Gyrus: ridge on the brain surface (plural: gyri)
- Sulcus: shallow groove (plural: sulci)
- Fissure: deep grooves
The cerebrum has five lobes separated by specific sulci:
- Frontal Lobe
- Parietal Lobe
- Temporal Lobe
- Occipital Lobe
- Insula: located deep to the temporal lobe.
Cerebral Hemispheres
Longitudinal Fissure: separates the two cerebral hemispheres.
Transverse Cerebral Fissure: separates the cerebrum from the cerebellum.
Central Sulcus: separates the frontal lobe from the parietal lobe.
Notable structures:
- Precentral Gyrus: anterior to the central sulcus; involved in motor function.
- Postcentral Gyrus: posterior to the central sulcus; involved in sensory function.
Functional Areas of the Cerebral Cortex
There are three types of functional areas in the cerebral cortex:
- Motor Areas: control voluntary movement.
- Sensory Areas: responsible for conscious awareness of sensation.
- Association Areas: integrate diverse information from sensory modalities.
Conscious behavior involves interaction across the entire cortex; no functional area operates in isolation.
Motor Areas: Frontal Lobe
Precentral Gyri contains large pyramidal cells.
Long axons form pyramidal (corticospinal) tracts, allowing conscious control of precise, skilled, voluntary movements.
Motor Homunculi: represents the motor innervation of various body regions in an upside-down caricature format.
Subcortical Structures
Corpus Callosum: facilitates communication between the two hemispheres.
Basal Nuclei: compare cortical processing with neural stimuli to regulate movement. Composed of:
- Caudate
- Putamen
- Globus Pallidus
- Striatum: formed by the caudate and putamen together.
Sensory Areas: Primary and Associative
Primary Somatosensory Cortex: receives sensory information from skin, skeletal muscles, and joints; involved in spatial discrimination and location detection; located in the postcentral gyrus.
Somatosensory Association Cortex: located posterior to the primary somatosensory cortex; integrates sensory information and helps in object identification.
Sensory Areas: Auditory and Visual
Visual Areas:
- Primary Visual Cortex: processes visual information from retinas; located at the extreme posterior tip of the occipital lobe.
- Visual Association Area: interprets visual stimuli; surrounds primary visual cortex.
Auditory Areas:
- Primary Auditory Cortex: interprets information from the inner ear (pitch, volume, location); located on the superior margin of the temporal lobes.
- Auditory Association Area: interprets various sound types; located posterior to primary auditory cortex.
Sensory Areas: Olfactory, Gustatory, and Balance
Olfactory Cortex: responsible for the conscious awareness and discernment of odors; located on the deep or medial aspect of the temporal lobes.
Gustatory Cortex: responsible for taste perception; located in the insula.
Visceral Sensory Area: involved in the perception of visceral sensations; located posterior to the gustatory cortex.
Vestibular Cortex: involved in equilibrium and balance; thought to be located in the posterior insula.
Multimodal Association Areas
Comprises the majority of the cortex and is more complex.
Integrates information from multiple sensory areas, allowing for memory storage and decision-making.
The pathway includes:
- Sensory Receptors → appropriate sensory cortex → sensory association cortex → multimodal association cortex → premotor cortex.
Multimodal Association Area: gives meaning to the information received, compares it to past experiences, and decides on actions to be taken.
Anterior / Prefrontal Cortex
Responsible for intellect, cognition, memory recall, and personality.
Most complex area; linked with the limbic system and is involved in emotional regulation.
Contains working memory required for judgment, reasoning, persistence, conscience, and impulse control.
Damage can lead to agnosia (loss of knowledge) and contralateral neglect.
Develops slowly, guided by social feedback; last area of the brain to mature, explaining impulsive adolescent behavior.
Located anterior to the frontal eye field.
Limbic Association Area
Part of the limbic system, centrally located in the brain.
Provides emotional impact, enhancing memory establishment when associated with the prefrontal area.
Posterior Association Area
A large region found in the temporal, parietal, and occipital lobes.
Involved in pattern and face recognition, localization in space, and understanding written and spoken language (Wernicke’s area).
Cerebral White Matter
Responsible for communication between the hemispheres and between the cerebral cortex and lower CNS centers.
Myelinated fibers bundled into large tracts classified based on direction:
- Commissures: connect corresponding gray areas of both hemispheres; horizontally oriented.
- Corpus Callosum: the largest commissure located superior to the lateral ventricles within the longitudinal fissure.
- Association Fibers: connect different parts of the same hemisphere; horizontally oriented.
- Projection Fibers: connect the hemispheres to the lower brain or spinal cord; vertically oriented. Sensory information enters the cerebral cortex via these fibers, whereas motor signals exit through motor fibers.
Basal Nuclei of the Cerebrum
Located as subcortical nuclei, which are gray matter regions within the white matter of the cerebrum.
Also known as the corpus striatum due to the appearance of fibers coursing through and around them.
Involved in motor control and regulation of attention and cognition; their functions remain partly understood.
The Diencephalon
Central core of the forebrain, composed of a set of paired gray matter areas deep within the cerebral hemispheres:
- Thalamus
- Hypothalamus
- Epithalamus
Encloses the third ventricle and acts as a relay center for information from the brain and peripheral nervous system to the cerebrum.
Thalamus (80% of diencephalon):
- Forms the superolateral walls of the third ventricle and serves as the gateway to the cerebral cortex, with most sensory inputs passing through the thalamic nuclei.
Hypothalamus:
- Located below the thalamus; contains multiple nuclei, including mammillary bodies which are olfactory relay stations.
- The infundibulum stalk connects it to the pituitary gland.
Hypothalamic Function
Serves as the body's main visceral control center, crucial for homeostasis.
Acts as an autonomic control center for various visceral functions including:
- Blood pressure regulation
- Heart rate control
- Digestive tract motility
- Glandular function regulation
- Body temperature maintenance.
Addresses emotional responses, biological rhythms, and drives, serving as the heart of the limbic system; initiates physical expressions of emotion via the ANS.
Also regulates temperature, hunger, thirst, water balance, sleep cycles, and hormonal balance.
The Diencephalon: Epithalamus
The most dorsal portion of the diencephalon, forming the roof of the third ventricle.
Contains the pineal gland, which secretes melatonin, regulating sleep-wake cycles.
The Brain Stem
Similar structure to the spinal cord, containing embedded nuclei and controlling automatic behaviors necessary for survival.
Houses fiber tracts connecting higher brain areas with lower neural centers.
Composed of three regions:
- Midbrain: the uppermost portion.
- Pons: characterized by its central ventral bulge which connects the cerebrum with the cerebellum.
- Medulla Oblongata: joins the spinal cord at the foramen magnum, containing various autonomic reflex centers regulating vital functions such as:
- Cardiovascular rate.
- Respiratory rate.
- Regulatory centers for hiccups, vomiting, swallowing, coughing, and sneezing.
The Cerebellum
Receives descending input from the cerebellum through the large white matter structure of the pons.
Ascending input from the body and spinal cord enters via fibers from the inferior olive.
Outputs to the midbrain, sending descending signals to the spinal cord.
The frontal cortex communicates intentions for voluntary movement to the cerebellum, which collects proprioceptive sensory information to optimize muscle activity coordination back to the cerebral cortex.
Blood Circulation in the Brain
Composed of various arteries supplying the brain with blood:
- Anterior Communicating Artery
- Middle Cerebral Artery
- Internal Carotid Artery
- Posterior Cerebral Artery
- Pontine Arteries
- Cerebellar Arteries
Venous drainage is facilitated through various sinuses returning blood to the jugular vein via the sigmoid sinus, including:
- Superior sagittal sinus.
- Inferior sagittal sinus.
- Straight sinus.
- Transverse sinus.
- Confluence of sinuses.
- Occipital sinus.
CSF and Meninges: Protection for the Brain
The skull and meninges provide protective layers for the CNS, protect blood vessels, and house venous sinuses. They also contain cerebrospinal fluid (CSF) which serves to:
- Cushion the brain.
- Form partitions in the skull.
CSF is produced in the four ventricles and circulated throughout the ventricular system.
CSF Circulation
Choroid plexuses in the ventricles produce CSF, which circulates and enters the subarachnoid space via the median and lateral apertures. CSF is ultimately reabsorbed into the blood via arachnoid granulations.
Homeostatic Imbalances of the Brain: Traumatic Injuries
Different types of traumatic brain injuries include:
- Concussion: temporary alteration in brain function.
- Contusion: permanent damage to brain tissue.
- Subdural or Subarachnoid Hemorrhage: increases intracranial pressure and may lead to brainstem displacement, risking death.
- Cerebral Edema: brain swelling associated with head injury.
Homeostatic Imbalances of the Brain: CVA
Cerebrovascular Accidents (CVAs) (strokes) occur due to blocked blood circulation, causing brain tissue death. Consequences can include hemiplegia, sensory deficits, and motor speed deficits.
Transient Ischemic Attacks (TIAs): temporary periods of reversible cerebral ischemia; hemorrhagic strokes tend to be more fatal.
Homeostatic Imbalances of the Brain: Degenerative Diseases
Alzheimer’s Disease (AD): a progressive degenerative disease leading to dementia.
Parkinson’s Disease: involves degeneration of dopamine-releasing neurons in the substantia nigra of the midbrain.
Huntington’s Disease: a hereditary disorder characterized by the accumulation of the huntingtin protein, which degenerates basal nuclei and cerebral cortex.
Other Disorders:
- Epilepsy: seizure disorder.
- Hydrocephalus: disorder caused by disrupted CSF flow leading to fluid accumulation in ventricles.
Spinal Cord Anatomy
The spinal cord begins at the foramen magnum and ends at the conus medullaris near L1 or L2 vertebra.
Functions include:
- Providing two-way communication between the brain and body (afferent/efferent);
- Containing centers for spinal reflexes.
Spinal Cord Protection
The spinal cord is cushioned by bone, meninges, and CSF, with a cushion of fatty tissue and veins in the epidural space.
Denticulate Ligaments: extensions of pia mater securing the spinal cord to the dura mater.
Filum Terminale: fibrous extension from the conus medullaris anchoring the spinal cord to the coccyx.
Spinal Nerves
Comprised of ventral (motor) and dorsal (sensory) roots that fuse together to form spinal nerves, which exit the spinal cord via intervertebral foramina.
Cauda Equina: collection of nerve roots at the inferior end of the vertebral canal.
Spinal Cord Disorders
Paresthesias: sensory loss due to nerve damage.
Paralysis: loss of motor function due to injury.
Transection: complete severing of the spinal cord can cause total motor and sensory loss below the site of injury.
Paraplegia: affects lower limbs due to injuries between T1 and L1.
Quadriplegia: affects all four limbs due to cervical spinal cord injuries.
Spinal Cord Diseases
Poliomyelitis: destruction of ventral horn motor neurons via the poliovirus, resulting in muscle atrophy.
Amyotrophic Lateral Sclerosis (ALS): progressive destruction of motor neurons leading to loss of motor function and can result in death within five years due to respiratory failure.