Brain Anatomy

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Introduction

The brain, contained in and protected by the skull, 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.

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. 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.

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. The surface of the cerebrum or cerebral cortex is composed of depressions (sulci) and ridges (gyri). These convolutions increase the surface area of the cerebrum without requiring an increase in the size of the brain. The outer surface of the cerebrum is composed of gray matter approximately 2 to 4 mm thick, whereas the inner surface is composed of white matter fiber tracts (Fitzgerald et al., 2012). Information is conveyed by the white matter and is pro- cessed and integrated within the gray matter, although there are also several nuclei within the cerebral hemispheres that interconnect with the cortex and/or each other.

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.

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 side of the brain 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;

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 utilizes this information to carry out body movements. Damage to the frontal lobe can lead to changes in sexual habits, socialization, and attention as well as increased risk-taking

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.

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.

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.

Cerebellum

The cerebellum is found below the tentorial membrane in the posterior fossa. It is connected to the brainstem by 3 cerebellar peduncles. Its primary role is that of coordination and learning of movements. Sometimes referred to as the ​"Little Brain," the cerebellum lies on top of the pons behind the brain stem. The cerebellum is comprised of small lobes and receives information from the balance system of the inner ear, sensory nerves, and the auditory and visual systems. It is involved in the coordination of movements as well as motor learning.

The cerebellum makes up approximately 10 percent of the brain's total size, but it accounts for more than 50 percent of the total number of neurons located in the entire brain.4 This structure is associated with motor movement and control, but this is not because the motor commands originate here. Instead, the cerebellum serves to modify these signals and make motor movements accurate and useful.

There are three main systems.

1. Spinocerebellum - involved in the control of axial musculature and posture.
2. Pontocerebellum - coordination and planning of intended limb movement.
3. Vestibulocerebellum - involved in posture and eye movements.

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. It also gains input from the vestibular systems.

Main Functions:

• 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).
• Initiating and Storing 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.

Dysfunction causes:

• Hypotonia (reduced muscle tone)
• Ataxia/incoordination
• Dysarthria (the inability to articulate words properly)
• Nystagmus (rapid jerky eye movements)
• Palatal tremor/ myoclunus (tremor of the palate)
  

Brainstem

The brainstem is made up of the part of the brain that begins at the foramen magnum. It extends to the cerebral peduncles and thalamus and consists of;

1. Medulla; located directly above the spinal cord in the lower part of the brain stem and controls many vital autonomic functions such as heart rate, breathing, and blood pressure.
2. Pons; connects the cerebral cortex to the medulla and to the cerebellum and serves a number of important functions including playing a role in several autonomic functions such as stimulating breathing and controlling sleep cycles.
3. Midbrain; often considered the smallest region of the brain. It acts as a sort of relay station for auditory and visual information. The midbrain controls many important functions such as the visual and auditory systems as well as eye movement. Portions of the midbrain called the red nucleus and the substantia nigra are involved in the control of body movement. The darkly pigmented substantia nigra contains a large number of dopamine-producing neurons are located.

It contains the following:

• Nuclei for 10 of 12 Pairs of Cranial Nerves (All except the Olfactory and Optic Nerves)
• Apparatus for Controlling Eye Movements (3rd, 4th, 6th Cranial Nerves)
• 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

Limbic System

This refers to a number of areas within the brain lying mainly on the medial side of the temporal lobe. It includes a number of structures as seen in the diagram below

The limbic system 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.

Long term potentiation LTP is the increase in the strength of a synaptic transmission with repetitive use, it can be seen to be effected in the hippocampus and is thought to be important for memory acquisition.

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.

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.

Basal Ganglia

This consists of:

1. Neostratum (Caudate and Putamen)
2. Internal and External segments of Globus Pallidus
3. Pars Reticulata
4. Compacta of Substantia Nigra
5. Subthalamic nucleus

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).

Subcortical Fibre Tracts

Brainstem

The brainstem is made up of the part of the brain that begins at the foramen magnum. It extends to the cerebral peduncles and thalamus. It consists of;

1. Medulla
2. Pons
3. Midbrain

It contains the following:

• Nuclei for 10 of 12 pairs of cranial nerves (not olfactory or optic nerves)
• Apparatus for controlling eye movements.(3rd, 4th, 6th cranial nerves)
• Monoaminergic nuclei that project widely in the CNS
• Vital respiration and cardiovascular centres.
• Autonomic centres
• Areas important for consciousness
• Ascending and descending pathways, linking spinal cord to brain.

Visual Pathways

Blood Supply

Arterial blood supply to the brain comes from four vessels;

• Right and Left Internal Carotid 
• Right and Left Vertebral Arteries

Vertebral Arteries: 

Arising from 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 PICA supplies the lateral medulla and cerebellum.

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 Anterior and Posterior circulation anastomose at the base of the brain. This is called the Circle of Willis.

Basilar Artery:

This has a number of branches: anterior and inferior cerebellar artery, artery to the labyrinth, pontine branches and superior cerebellar artery.

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.

Venous Drainage

Venous drainage of the brainstem and cerebellum is straight into the dural venous sinuses which are adjacent to the posterior cranial fossa.

The cerebral hemispheres have external and internal veins.

• External veins drain the cortex and drain into the transverse sinus. This then drains into the lateral sinus and empties into the internal jugular vein.
• Internal cerebral veins drain the deep structures of the cerebral hemisphere into the great vein of Galen, and then into the sinus.

If occlusion of either of these venous systems then raised intracranial pressure can develop.

Function

Resources

References