Human Brain

The anatomy of the brain is complex due its intricate structure and function. This amazing organ acts as a control center by receiving, interpreting, and directing sensory information throughout the body. There are three major divisions of the brain. They are the forebrain, the midbrain, and the hindbrain.

Anatomy of the Brain: Brain Divisions

The forebrain is responsible for a variety of functions including receiving and processing sensory information, thinking, perceiving, producing and understanding language, and controlling motor function. There are two major divisions of forebrain: the diencephalon and the telencephalon. The diencephalon contains structures such as the thalamus and hypothalamus which are responsible for such functions as motor control, relaying sensory information, and controlling autonomic functions. The telencephalon contains the largest part of the brain, the cerebral cortex. Most of the actual information processing in the brain takes place in the cerebral cortex.

The midbrain and the hindbrain together make up the brainstem. The midbrain is the portion of the brainstem that connects the hindbrain and the forebrain. This region of the brain is involved in auditory and visual responses as well as motor function.
The hindbrain extends from the spinal cord and is composed of the metencephalon and myelencephalon. The metencephalon contains structures such as the pons and cerebellum. These regions assists in maintaining balance and equilibrium, movement coordination, and the conduction of sensory information. The myelencephalon is composed of the medulla oblongata which is responsible for controlling such autonomic functions as breathing, heart rate, and digestion. * Prosencephalon – Forebrain * Diencephalon * Telencephalon * Mesencephalon - Midbrain * Rhombencephalon - Hindbrain * Metencephalon * Myelencephalon

Anatomy of the Brain: Structures
The brain contains various structures that have a multitude of functions. Below is a list of major structures of the brain and some of their functions.

Basal Ganglia * Involved in cognition and voluntary movement * Diseases related to damages of this area are Parkinson's and Huntington's

Brainstem * Relays information between the peripheral nerves and spinal cord to the upper parts of the brain * Consists of the midbrain, medulla oblongata, and the pons

Broca's Area * Speech production * Understanding language

Central Sulcus (Fissure of Rolando) * Deep grove that separates the parietal and frontal lobes

Cerebellum * Controls movement coordination * Maintains balance and equilibrium

Cerebral Cortex * Outer portion (1.5mm to 5mm) of the cerebrum * Receives and processes sensory information * Divided into cerebral cortex lobes

Cerebral Cortex Lobes * Frontal Lobes -involved with decision-making, problem solving, and planning * Occipital Lobes -involved with vision and color recognition * Parietal Lobes - receives and processes sensory information * Temporal Lobes - involved with emotional responses, memory, and speech

Cerebrum * Largest portion of the brain * Consists of folded bulges called gyri that create deep furrows

Corpus Callosum * Thick band of fibers that connects the left and right brain hemispheres

Cranial Nerves * Twelve pairs of nerves that originate in the brain, exit the skull, and lead to the head, neck and torso

Fissure of Sylvius (Lateral Sulcus) * Deep grove that separates the parietal and temporal lobes

Limbic System Structures * Amygdala - involved in emotional responses, hormonal secretions, and memory * Cingulate Gyrus - a fold in the brain involved with sensory input concerning emotions and the regulation of aggressive behavior * Fornix - an arching, fibrous band of nerve fibers that connect the hippocampus to the hypothalamus * Hippocampus - sends memories out to the appropriate part of the cerebral hemisphere for long-term storage and retrievs them when necessary * Hypothalamus - directs a multitude of important functions such as body temperature, hunger, and homeostasis * Olfactory Cortex - receives sensory information from the olfactory bulb and is involved in the identification of odors * Thalamus - mass of grey matter cells that relay sensory signals to and from the spinal cord and the cerebrum

Medulla Oblongata * Lower part of the brainstem that helps to control autonomic functions
Meninges
* Membranes that cover and protect the brain and spinal cord

Olfactory Bulb * Bulb-shaped end of the olfactory lobe * Involved in the sense of smell

Pineal Gland * Endocrine gland involved in biological rhythms * Secretes the hormone melatonin

Pituitary Gland * Endocrine gland involved in homeostasis * Regulates other endocrine glands

Pons * Relays sensory information between the cerebrum and cerebellum
Reticular Formation * Nerve fibers located inside the brainstem * Regulates awareness and sleep

Substantia Nigra * Helps to control voluntary movement and regualtes mood

Tectum * The dorsal region of the mesencephalon (mid brain)
Tegmentum
* The ventral region of the mesencephalon (mid brain).
Ventricular System - connecting system of internal brain cavities filled with

Cerebrospinal fluid * Aqueduct of Sylvius - canal that is located between the third ventricle and the fourth ventricle * Choroid Plexus - produces cerebrospinal fluid * Fourth Ventricle - canal that runs between the pons, medulla oblongata, and the

Cerebellum * Lateral Ventricle - largest of the ventricles and located in both brain hemispheres * Third Ventricle - provides a pathway for cerebrospinal fluid to flow

Wernicke's Area * Region of the brain where spoken language is understood

Motor Functions
The motor system of the brain and spinal cord is responsible for maintaining the body’s posture and balance; as well as moving the trunk, head, limbs, tongue, and eyes: and communicating through facial expressions and speech. Reflexes mediated through the spinal cord and brainstem is responsible for some body movements. They occur without conscious thought. Voluntary movements, on the other hand, are movements consciously activated to achieve a specific goal, such as walking or typing. Although consciously activated, the details of most voluntary movements occur automatically. After walking begins, it is not necessary to think about the moment-to-moment control of every muscle because neural circuits in the reticular formation automatically control the limbs. After learning how to perform complex tasks, such as typing, they can be performed relatively automatic.

Voluntary movements result from the stimulation of upper and lower motor neurons. Upper motor neurons have cell bodies in the cerebral cortex. The Axons of upper motor neurons from descending tracts that connects to lower motor neurons. Lower motor neurons have cell bodies in the anterior horn of the spinal cord gray matter or in cranial nerve nuclei. Their axons leave the central nervous system and extend through spinal or cranial nerves to skeletal muscles. Lower motor neurons are the neurons forming the motor units.

Motor areas of the cerebral cortex
The motor areas are located in both hemispheres of the cortex. They are shaped like a pair of headphones stretching from ear to ear. The motor areas are very closely related to the control of voluntary movements, especially fine fragmented movements performed by the hand. The right half of the motor area controls the left side of the body, and vice versa.

Two areas of the cortex are commonly referred to as motor: * Primary motor cortex , which executes voluntary movements * Supplementary motor areas and premotor cortex, which select voluntary movements. * In addition, motor functions have been described for: * Posterior parietal cortex , which guides voluntary movements in space * Dorsolateral prefrontal cortex , which decides which voluntary movements to * make according to higher-order instructions, rules, and self-generated thoughts.

Descending tracts

The most important descending spinal tract originates in the cerebral cortex and is called the corticospinal tract The other major descending spinal tracts worth mentioning are: the tectospinal tract arising from the superior colliculus, the rubrospinal tract arising from the red nucleus in the mid-brain, the vestibulospinal tract with its nuclei located in the floor of the fourth ventricle, and the reticulospinal tract arising from the reticular formation in the pons and the medulla. The cortico-bulbar tract which is associated with cranial nerves will not be described in this review of neuroanatomy as it is not prominently employed in the treatment of patients.

1. The corticospinal system (pyramidal system)

The corticospinal tract supplies impulses to most of the voluntary muscles. It originates in the precentral gyrus of the cerebral cortex (area 4). The axons pass through the internal capsule and descend to the mid-brain where they form the crus cerebri (basis pedunculi). In the medulla oblongata, 80 to 90 percent of the fibers decussate to the opposite side and descend in the spinal cord where they form the lateral corticospinal tract. In the spinal cord, the axons of the lateral corticospinal tract are located internal to the posterior spinocerebellar tract and posterior to the lateral spinothalamic tract.

The lateral corticospinal tract irradiates branches at all levels of the spinal cord. Thefibers enter the gray matter where they synapse in the ventral horn with second-orderneurons. The latter emerge from the spinal cord in the ventral spinal roots and supply the voluntary muscles through the peripheral nerves.

The remainder of the corticospinal tract which does not cross over in the medulla oblongata divides into two separate tracts: the anterior corticospinal tract and the anterolateral corticospinal tract. The axons of the anterior corticospinal tract descend uncrossed into the spinal cord. They occupy an antero-medial position in the anterior white commissure and are contiguous to the anterior median fissure. Most of the fibers of the anterior corticospinal tract descend to the upper cervical spine where they cross in the anterior white commissure. The fibers enter the gray matter where they synapse in the ventral horn with second-order neurons.

The anterolateral corticospinal tract is the smallest of the three descending tracts. The fibers descend in the lateral funiculus and remain uncrossed in the entire course of the tract. The axons of the anterolateral corticospinal tract synapse in the ventral horn with second-order neurons. It should be emphasized that the pyramidal or voluntary muscle system is made of a two-neuron system. The neurons of the corticospinal tracts leaving the precentral gyrus and descending in the spinal cord to terminate their course in the ventral horn are called upper motor neurons. The second-order neurons leaving the spinal cord to supply the voluntary muscles are called lower motor neurons. The distinction between upper and lower motor neurons paralysis is important in clinical neurology.

Basal nuclei

The basal nuclei are a group of functionally related nuclei. Two primary nuclei are the corpus striatum, located deep within the cerebrum, and the substantia nigra, a group of darkly pigmented cells located in the midbrain.

Anatomy of cerebral circulation

Arterial supply of oxygenated blood

Four major arteries and their branches supply the brain with blood. The four arteries are composed of two internal carotid arteries (left and right) and two vertebral arteries that ultimately join on the underside (inferior surface) of the brain to form the arterial circle of
Willis, or the circulus arteriosus.

The vertebral arteries actually join to form a basilar artery. It is this basilar artery that joins with the two internal carotid arteries and their branches to form the circle of Willis.
Each vertebral artery arises from the first part of the subclavian artery and initially passes into the skull via holes (foramina) in the upper cervical vertebrae and the foramen magnum. Branches of the vertebral artery include the anterior and posterior spinal arteries, the meningeal branches, the posterior inferior cerebellar artery, and the medullary arteries that supply the medulla oblongata.

The basilar artery branches into the anterior inferior cerebellar artery, the superior cerebellar artery, the posterior cerebral artery, the potine arteries (that enter the pons), and the labyrinthine artery that supplies the internal ear.

The internal carotids arise from the common carotid arteries and pass into the skull via the carotid canal in the temporal bone. The internal carotid artery divides into the middle and anterior cerebral arteries. Ultimate branches of the internal carotid arteries include the ophthalmic artery that supplies the optic nerve and other structures associated with the eye and ethmoid and frontal sinuses. The internal carotid artery gives rise to a posterior communicating artery just before its final splitting or bifurcation. The posterior communicating artery joins the posterior cerebral artery to form part of the circle of Willis. Just before it divides (bifurcates), the internal carotid artery also gives rise to the choroidal artery (also supplies the eye, optic nerve, and surrounding structures). The internal carotid artery bifurcates into a smaller anterior cerebral artery and a larger middlecerebral artery.

The anterior cerebral artery joins the other anterior cerebral artery from the opposite side to form the anterior communicating artery. The cortical branches supply blood to the cerebral cortex. Cortical branches of the middle cerebral artery and the posterior cervical artery supply blood to their respective hemispheres of the brain.

The circle of Willis is composed of the right and left internal carotid arteries joined by the anterior communicating artery. The basilar artery (formed by the fusion of the vertebral arteries) divides into left and right posterior cerebral arteries that are connected (anastomsed) to the corresponding left or right internal carotid artery via the respective left or right posterior communicating artery. A number of arteries that supply the brain originates at the circle of Willis, including the anterior cerebral arteries that originate from the anterior communicating artery. In the embryo, the components of the circle of Willis develop from the embryonic dorsal aortae and the embryonic intersegmental arteries.

The circle of Willis provides multiple paths for oxygenated blood to supply the brain if any of the principal suppliers of oxygenated blood (i.e., the vertebral and internal carotid arteries) are constricted by physical pressure, occluded by disease, or interrupted by injury.

This redundancy of blood supply is generally termed collateral circulation. Arteries supply blood to specific areas of the brain. However, more than one arterial branch may support a region. For example, the cerebellum is supplied by the anterior inferior cerebellar artery, the superior cerebellar artery, and the posterior inferior cerebellar arteries. Venous return of deoxygenated blood from the brain Veins of the cerebral circulatory system are valve-less and have very thin walls. The veins pass through the subarachnoid space, through the arachnoid matter, the dura, and ultimately pool to form the cranial venous sinus.

There are external cerebral veins and internal cerebral veins. As with arteries, specific areas of the brain are drained by specific veins. For example, the cerebellum is drained of deoxygenated blood by veins that ultimately form the great cerebral vein. External cerebral veins include veins from the lateral surface of the cerebral hemispheres that join to form the superficial middle cerebral vein.
Narrative form

Arteriovenous malformation or AVM is an abnormal connection between veins and arteries, usually congenital. Arteries and veins are part of the human cardiovascular system. Normally, the arteries in the vascular system carry oxygen-rich blood, except in the case of the pulmonary artery. Structurally, arteries divide and sub-divide repeatedly, eventually forming a sponge-like capillary bed. Blood moves through the capillaries, giving up oxygen and taking up waste products, including CO2, from the surrounding cells. Capillaries in turn successively join together to form veins that carry blood away. The heart acts to pump blood through arteries and uptake the venous blood.
An AVM lacks the dampening effect of capillaries on the blood flow; it also causes the surrounding area to be deprived of the functions of the capillaries - removal of CO2 and delivery of nutrients to the cells. The resulting tangle of blood vessels, often called a nidus (Latin for "nest") has no capillaries and abnormally direct connections between high-pressure arteries and low-pressure veins. It can be extremely fragile and prone to bleeding.
Haemorrhagic stroke accounts for roughly 15% of all strokes. The stroke occurs when there is a larges accumulation of blood causing the surrounding brain tissue to be displaced and compressed, often causing blood to leak into the ventricles. There are large haemorrhages, which may be several centimeters, or small haemorrhages that may only be one to two centimeters in diameter. There may only be a slit, referred to as a petechial haemorrhage which is a very small pinhead size bleed. The main contributing factor to this type of stroke is hypertention