Brain Anatomy

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Introduction

The brain, contained in and protected by the skull and suspended in cerebrospinal fluid, is one of the most important and complex organs in the body. It is the central organ of the nervous system, and with the spinal cord makes up the central nervous system, which controls most of the activities of the body, processing, integrating, and coordinating the information it receives from the sense organs, and determining the signals or instructions sent back to the rest of the body. [1]

At birth, the average brain weighs about 350 - 400grams, approximately 25% of the final adult brain weight of 1.4 - 1.45 kg and accounting for only 2% of overall body mass, which is reached between 10 and 15 years of age. Fastest growth occurs during the first 3 years of life, with almost 90% of the adult value reached by the age of 5 years. Its average width is about 140 mm, average length is about 167 mm, and average height about 93 mm. While the brain continues to change throughout our life span, changes in brain morphology during childhood, adolescence and adulthood are much more subtle than those in the first 4 years of life.[2][3][4] Rate and amount of growth that occurs in the brain after birth is neither constant nor pre-determined, nor is it protected from outside influences, both positive and negative, and as such can be speeded up and increased, or slowed down and decreased. [3]

Gross Anatomy

The brain consists of three main structural divisions; the cerebrum or cerebral cortex, the cerebellum, and the brain stem at the base of the brain, which extends from the upper cervical spinal cord to the diencephalon of the cerebrum. [5][6]

Cerebrum

The cerebrum is the largest part of the brain. The surface of the cerebrum is composed of depressions or grooves (sulci) and ridges or raised areas (gyri), which increase the surface area of the cerebrum without an increase in the size of the brain. Gray matter, approximately 2 to 4 mm thick, forms the outer surface of the cerebrum, which processes and integrates information from white matter fibre tracts , which forms the inner surface cerebrum. [7] The cerebrum consists of two cerebral hemispheres, the right hemisphere and the left hemisphere, connected by the corpus callous which facilitates communication between both sides of the brain, with each hemisphere in the main connected to the contralateral side of the body i.e. the left hemisphere of the cerebrum receives information from the right side of the body resulting in motor control of the right side of the body and vice versa. The Hemispheres are then further divided into four lobes; [1][7][5]

Frontal Lobe

The frontal lobe is located at the front of the brain, occupying the area anterior to the central sulcus and superior to the lateral sulcus. It is associated with reasoning, motor skills, higher level cognition, and expressive language. At the back of the frontal lobe, near the central sulcus, lies the motor cortex. This area of the brain receives information from various lobes of the brain and utilises this information to carry out body movements. Damage to the frontal lobe can lead to changes in sexual habits, socialisation, and attention as well as increased risk-taking. [1][5][6]

Area Function Dysfunction
Primary Motor Cortex Voluntary Control of Movement Altered Muscle Tone

Poor Motor Control

Pre Motor Area Selection of movement based on external events Dyspraxia
Supplementary Motor Area Selection of movement based on stored plans specified by internal cues. Involved in planning of motor actions.
Pre-Supplementary Motor Area Acquiring new sequences Dyspraxia
Broca's Area Left Hemisphere - Expression of Speech Expressive Dysphasia
Right Hemisphere - Non Verbal Communication
Pre-frontal Cortex Personality and Behaviour Changes in Character
Inappropriate Behaviour
Higher executive function - problem solving, initiation, moderation and termination of bevaiour. Dysexecutive Syndrome

Parietal Lobe

The parietal lobe is located in the middle section of the brain , occupying area posterior to the central sulcus and superior to the lateral sulcus, extending posteriorly as far as the parieto-occipitaq sulcus. It is associated with processing tactile sensory information such as pressure, touch, and pain. A portion of the brain known as the somatosensory cortex is located in this lobe and is essential to the processing of the body's senses. [1][5][6]

Area Function Dysfunction
Primary Somatosensory Cortex Receives Sensory Information from Whole Body Altered Sensation
Homunculus Representation
Sensory Association Area Integrates Sensory Information based on sensory inputs
  • Inferior Parietal Lobe
Left Hemisphere - Attention to Right Side Only Inability to Perceive and Attend to Objects, Space or Own Body despite vision, somatosensation and motor ability being intact.
Right Hemisphere - Attention to both Left and Right
Contralateral Neglect
  • Visual Association Area
Processing of Visual Information for perception of motor and spatial relationships Visuospatial Dysfnction

Temporal Lobe

The temporal lobe is located on the bottom section of the brain, occupying the area inferior to the lateral sulcus. This lobe is also the location of the primary auditory cortex, which is important for interpreting sounds and the language we hear. The hippocampus is also located in the temporal lobe, which is why this portion of the brain is also heavily associated with the formation of memories. Damage to the temporal lobe can lead to problems with memory, speech perception, and language skills. [1][5][6]

Area Function Dysfunction
Primary Auditory Cortex Loudness Deafness
Pitch
Localisation of Basic Sound
Temporal Association Areas: Recognition and Identification fo Stimuli Agnosia - Acknowledge existence of a stimulus but unable to recognize what it is
  • Right Inferior Temporal Lobe
Face and Object Recognition Prosopagnosia
  • Superior Temporal Lobe - Auditory Association Area
Processing Complex Sound
  • Left Inferior Temporal Lobe - Wernicke's Area
Recognize Spoken Word Receptive Dysphasia
Interpret meaning of Speech
  • Visual Association Area
Process Visual Information related to form recognition and object representation Visuoperceptual Dysfuncton
Linked to storage of Long Term Memory
Pyriform Cortex Processing Olfactory Sensations Olfactory Dysfunction
Insula Cortex Process Taste Sensation Taste Dysfunction

Occipital Lobe

The occipital lobe is located at the back portion of the brain, occupying the small area behind the parietal-occipital sulcus. It is associated with interpreting visual stimuli and information. The primary visual cortex, which receives and interprets information from the retinas of the eyes, is located in the occipital lobe. Damage to this lobe can cause visual problems such as difficulty recognizing objects, an inability to identify colors, and trouble recognizing words. [1][5][6]

Area Function Dysfunction
Primary Visual Cortex Determines basic attributes of Vision
  • Light
  • Shape
  • Colour
  • Soze
  • Direction
  • Binocular Vision
Diplopia (Double Vision)
Blindness
Visual Association Area Respond to Visual Stimuli within Receptive Fields

Modulated by Attention & Working Memory

Cerebellum

The cerebellum, sometimes referred to as the "Little Brain", is found inferior to the tentorium cerebella or tentorial membrane in the posterior cranial fossa, posterior to the fourth ventricle, the pons and the medulla oblongata. The cerebellum accounts for approximately 10 percent of the brain's total size, but has for more than 50 percent of the total number of neurones located in the entire brain. It is the largest part of the hind brain forming the Deep Cerebellar Nuclei, which consists of an outer layer cortical region with an inner subcortical mass of cells. It consists of two cerebellar hemispheres joined by a narrow median vermis and is connected to the posterior aspect of the brainstem by 3 cerebellar peduncles, which are symmetrical bundles of nerve fibres. The cerebellum is divided into three small lobes; anterior, middle and flocculonodular lobes, which receive information from the balance system of the inner ear, sensory nerves, and the auditory and visual systems. [1][6]

The cerebellum is primarily involved in the coordination of movements as well as the learning of movements (motor learning) and has ipsilateral control of movement i.e. the left hemisphere of the cerebellum controls the left side of the body and vice versa. It regulates initiation, timing, sequencing, and force generation of muscle contractions, sequencing the order of muscle firing when a group of muscles work together to perform a movement and assists with balance and posture maintenance. While the cerebellum is associated with motor movement and control, motor commands do not originate here, rather the cerebellum serves to modify these motor commands it receives from sensory systems of the spinal cord and from other parts of the brain, and integrates these inputs to fine-tune motor activity in order to make motor movements accurate and useful. The cerebellum compares the intended movement originating from the motor cortex areas with the actual movement relayed back by the afferent systems and interneurons in the spinal cord. The main functions of the cerebellum are to; [1][5]

To act as a comparator: comparing descending supraspinal signals with ascending afferent feedback. If there is any discrepancy, this is then fine tuned to produce the actual movement desired via descending pathways. This helps to achieve smoothness and accuracy in movement.

To act as a timing device: it converts descending motor signals into a sequence of motor activation. This helps the movement achieve smoothness and coordination, maintaining posture and balance.(receiving input from the vestibular system).

To initiate and store movement: has ability to store and update motor information. there is a significant role played in accurate learned movement. This is due to a modifiable synapse at the purkinje cell.

Cerebellar damage produces disorders in fine movement, equilibrium, posture, and motor learning depending on the part of the cerebellum involved and how it is damaged. Dysfunction may result in; Hypotonia (reduced muscle tone), Ataxia, Dysarthria (the inability to articulate words properly), Nystagmus (rapid jerky eye movements) and Palatal Tremor / Myoclunus (tremor of the palate) [5][6]

Brainstem

The brainstem evolutionarily, is the most ancient part of the brain beginning at the foramen magnum occupying the posterior cranial fossa of the skull and is divided into three regions; the medulla oblongata, the pons and the midbrain. It connects the narrow spinal cord with the forebrain. [1][5][6]


Medulla Oblongata

Located directly above the spinal cord at the level of the foramen magnum forming the base of the brain stem, and connecting the pons superiorly to the spinal cord inferiorly. The medulla oblongata is oblong in shape, approximately 3cm long with the descending tracts passing through it from two pyramids on the ventral surface of the medulla between the anterior median fissure, which is disrupted where the tracts cross midline at the decussation of the pyramids. Cranial Nerves IX (Glossopharyngeal), X (Vagus), XI (Accessory) and XII (Hypoglossal) emerge form the Medulla Oblongata. [1][5][6]

The Medulla Oblongata controls many vital autonomic functions such as heart rate, breathing, and blood pressure, with the following three main functions;

  1. to serve as a conduit for the ascending and descending tracts to connect the spinal cord to the higher centers in the forebrain,
  2. to contains reflex centres associated with control of respiration, the cardiovascular system and consciousness,
  3. to contains nuclei of Cranial Nerves III and XII.

Pons

Located anterior to the cerebellum, the Pons connects the medulla oblongata to the midbrain. It is approximately 2.5cm long, composed of transverse fibres forming a ridge or bridge across the anterior surface area connecting the right and left cerebellar hemispheres, from which the pons gets its name. These fibres are the pontocerebellar fibers that are in turn projections from the corticopontine fibers. They cross to enter the contralateral middle cerebellar peduncle and thus enter the cerebellum. Cranial Nerves V (Trigeminal), VI (Abducens), VII (Facial) and VIII (Vestibulocochlear) emerge from the pons. [1][5][6]

The Pons serves a number of important functions including playing a role in several autonomic functions including stimulation of breathing and controlling sleep cycles.

Midbrain

Extending from the Pons to the Mamillary Body, the midbrain, also termed the mesencephalon is the superiormost aspect of the brainstem and considered the smallest region of the brain. The substantia nigra is closely associated with motor system pathways of the basal ganglia producing Dopamine which plays a role in movement, movement planning, excitation, motivation. Cranial Nerves II (Optic), III (Oculomotor) and IV (Trochlear) emerge form the midbrain. The midbrain is associated with vision, hearing, motor control, sleep / wake, arousal (alertness), and temperature regulation, acting as a sort of relay station for auditory and visual information. It contains the following: [1][5][6]

  • Nuclei for 10 of 12 Pairs of Cranial Nerves (All except the Olfactory and Optic Nerves)
  • Apparatus for Controlling Eye Movements (Cranial Nerves III, IV and VI )
  • Monoaminergic Nuclei that project widely in the CNS
  • Vital Respiration and Cardiovascular Centres
  • Autonomic Centres
  • Areas important for Consciousness
  • Ascending and Descending Pathways, linking Brain to the Spinal Cord

Diencephalon

The diencephalon is located at the base of the brain. It contains the: [1]

Thalamus

The thalamus, a large mass of gray matter in the dorsal part of the diencephalon, serves as a relay station for ascending input to the cortex and receives information from each of the cardinal senses, except smell. It is hypothesised that the thalamus serves a gating function in filtering information. The thalamus also plays an important role in regulating states of sleep and wakefulness [5][6]

Epithalamus

The epithalamus, the posterior segment of the diencephalon, is made up of the habenula, the habenular commissure, the posterior commissure, and the pineal gland. The function of the epithalamus is to connect the limbic system to other parts of the brain. Some functions of its components include the secretion of melatonin and secretion of hormones from pituitary gland by the pineal gland circadian rhythms), and regulation of motor pathways and emotions. [5][6]

Hypothalamus

The hypothalamus lies on either side of the 3rd ventricle, below the thalamus and between the optic chiasm and the midbrain. It receives a large input from limbic structures. It has a significantly large efferent output to the ANS and has a highly significant role in the control of pituitary endocrine function. [5][6]

The hypothalamus as well as inputting the ANS also has a large part to play in the homeostasis of many physiological systems such as hunger, thirst, water and sodium balance and temperature regulation. It also plays a role in memory and emotional responses, providing autonomic and endocrine responses. It also plays a role in the control of circadian rhythms via retinal input to suprachiasmatic nucleus. There may also be hypothalamus input into sexual and emotional behaviour independent of its endocrine role. [5]

Limbic System

Limbic2.jpg

The limbic system is not defined by strict anatomic boundaries but incorporates several important structures. The limbic structures conventionally include the amygdala, the hippocampus, the fornix, the mammillary bodies, the cingulate gyrus, and the parahippocampal gyrus, which lie mainly on the medial side of the temporal lobe. These structures form connections between the limbic system and the hypothalamus, thalamus and cerebral cortex. [1][6]

When limbic system has been traditionally associated with our emotional behaviour. The hippocampus is important in memory and learning, while the limbic system itself is central in the control of emotional responses and provides high level processing of sensory information. The main outflow of the limbic system is to the prefrontal cortex and the hypothalamus as well as to cortical areas. It appears to have a role in attaching behavioural significance and response to a given stimulus. Damage to this area has profound effects on emotional responses. [1][5][6]

Basal Ganglia

The basal ganglia are a group of large nuclei, which consist of the Neostratum (Caudate and Putamen), Internal and External segments of Globus Pallidus, the Pars Reticulata, the subthalamic nucleus and the compacta of Substantia Nigra, that partially surround the thalamus, which are important in the control of movement. The red nucleus and substantia nigra of the midbrain have connections with the basal ganglia.[6]

The basal ganglia are closely integrated with the motor cortex, premotor cortex, and motor nuclei of the thalamus and plays a crucial role in modulation of movements. The Neostratum is the main area of signal reception for the basal ganglia and receives information from the whole cortex in a somatotopic fashion. It relays signals to the thalamus, which projects to the premotor cortex, supplementary motor cortex and prefrontal cortex. There is also a projection to the brainstem (pedunclopontine nucleus- involved in locomotion, and superior colliculus - eye movement). [1][5]


Cranial Nerves

There are 12 pairs of cranial nerves, numbered according to position where they originate in the inferior surface of the brain, that function mainly to convey motor signals to and sensory information from the head and neck. The lower cranial nerves have somewhat more complex visceral functions that are not strictly limited to the head and neck. The names of the Cranial Nerves (CN) are: [1][5][6]

  1. CN I Olfactory
  2. CN II Optic
  3. CN III Oculomotor
  4. CN IV Trochlear
  5. CN V Trigeminal
  6. CN VI Abducens
  7. CN VII Facial
  8. CN VIII Vestibulocochlear
  9. CN IX Glossopharyngeal
  10. CN X Vagus
  11. CN XI Accessory
  12. CN XII Hypoglossal
  13. Ventricles and Cerebrospinal Fluid

The ventricles are a system of cavities / space within the cerebral hemispheres, which produce and circulate cerebrospinal fluid, which is continuously produced and absorbed. The cerebrospinal fluid is produced, at a rate of about 450 mL/day, by the choroid plexuses within the lateral ventricles, and circulates through the ventricular system ultimately entering the subarachnoid space, which surrounds the cerebrum, cerebellum, brain stem and spinal cord down to the level of the second sacral vertebrae. At any given time there is approximately 150 mL of cerebrospinal fluid, thus, the volume of CSF in most adults is turned over about 3 times per day. [1][6]

The brain has 4 ventricles, which is a series interconnected spaces in the core of the brain. The lateral ventricles are within the cerebral hemispheres, which are connected to each other and to the third ventricle through a pathway called the Interventricular Foramen of Monro. The third ventricle lies in the midline, separating deeper brain structures such as the left and right thalami. The third ventricle communicates with the fourth ventricle through the Cerebral Aqueduct of Sylvius, which is a long narrow tube. [1][5]

Blood Supply

Adequate supply of oxygen and nutrients is vital for normal function of the brain, which is achieved through a dense network of blood vessels. Arterial blood supply to the brain comes from four vessels the right and left Internal Carotid and the right and left Vertebral Arteries, which join at the base of the brain to form the basilar artery. The Anterior Circulation is composed of the carotid arteries and their branches, while the Posterior Circulation is composed of the Vertobrobasilar arteries. [1][5][6]

[9]
[10]

Internal Carotid Arteries

Traverse the skull within the carotid canal and cavernous sinus. They then pierce the dura and enter the middle cranial fossa lateral to the optic chiasm. They then divide to supply the anterior and middle sections of the cerebral hemispheres, anterior and middle cerebral arteries. The internal carotid artery on each side terminates into the anterior cerebral artery, the middle cerebral artery, and the posterior communicating artery.[5]

Vertebral Arteries

Arising from the sub-clavian artery these two arteries enter skull through foramen magnum after passing through foramina in the transverse processes of the upper cervical vertebra. They unite in the brainstem to form the basilar artery which ascends to form the two posterior cerebral arteries at the superior border of the Pons. This is the posterior cerebral hemisphere blood supply. This network is termed the posterior circulation. On its journey to becoming the basilar artery, the vertebral arteries give off a number of branches, including the posterior spinal artery, posterior inferior cerebral artery and the anterior spinal artery. These constitute the blood supply to the upper cervical cord. The posterior inferior cerebral artery supplies the lateral medulla and cerebellum. Damage to these blood vessels can result in motor loss or sensory loss. [5][6]

Basilar Artery

Formed by the two vertebral arteries. This has a number of branches: anterior and inferior cerebellar artery, artery to the labyrinth, pontine branches and superior cerebellar artery which supply the brain stem and cerebellum. [5]

Anterior Cerebral Arteries

The anterior cerebral artery arises from the internal carotid, at the medial extremity of the lateral cerebral fissure. It passes forward and medialward across the anterior perforated substance, above the optic nerve, to the commencement of longitudinal fissure. Is one of a pair of arteries on the brain that supplies oxygenated blood to most midline portions of the frontal lobes and superior medial parietal lobes. [5][6]

Middle Cerebral Arteries

These supply parts of the frontal, temporal, occipital and parietal lobes bilaterally, with branches also supplying the basal ganglia and posterior limb of the internal capsule.

Posterior Cerebral Arteries

These supply blood to the posterior parietal cortex, occipital lobe and inferior temporal lobe. There are several branches of this artery that supply the midbrain, thalamus, subthalamus, posterior internal capsule, optic radiation and cerebral peduncle.

Circle of Willis

Brainblood.png The basilar artery, the posterior cerebral arteries, the posterior communicating arteries, and the anterior cerebral arteries, along with the anterior communication artery, form an important collateral circulation, the Anterior and Posterior Circulation anastomose at the base of the brain, termed the Circle of Willis. These vessels lie within the subarachnoid space and are a common location for cerebral aneurysms to form. [5][6]

Venous Drainage

The venous circulation of the brain is very different from that of the rest of the body. Usually arteries and veins run together as they supply and drain specific areas of the body, however, this is not the case in the brain. The major vein collectors are integrated into the dura to form venous sinuses, which are adjacent to the posterior cranial fossa. The venous sinuses collect the blood from the brain and pass it to the internal jugular veins. The superior and inferior sagittal sinuses drain the cerebrum, the cavernous sinuses drains the anterior skull base. All sinuses eventually drain to the sigmoid sinuses, which exit the skull and form the jugular veins. These two jugular veins are essentially the only drainage of the brain. If occlusion of either of these venous systems then raised intracranial pressure can develop. [1][5][6]

References

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