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Brain

The brain (Latin: cerebrum, Greek: enképhalos) is the central anatomical part of the nervous system responsible for integrating most sensory information and coordinating body functions, both consciously and unconsciously, and providing complex functions such as thinking and feeling. The brain is an organ of neural tissue grossly composed of the brainstem, the cerebellum, and two cerebral hemispheres. 

The brain is contained within the cranial cavity and thus mechanically protected by the bones of the skull. Within the skull, three connective tissue layers called meninges cover the brain, and these are the dura mater, pia mater, and arachnoid mater. 

The human brain weighs about 1.2 to 1.4 kg on average, composes about 2% of the total body weight, and the brain tissue uses approximately 20% of the total energy expenditure of the human body. The brain tissue is constructed mainly by two types of cells: neurons – the functional units of the neural tissue, and the supportive glial cells. 

Parts of the brain

Major anatomical parts of the brain include the medulla oblongata, pons, midbrain, cerebellum, subcortical structures - diencephalon (including thalamus and hypothalamus), pituitary gland, limbic structures, basal ganglia - and cerebrum. The medulla oblongata, pons, and midbrain together form the portion of the brain known as the brainstem, which continues rostrally with the spinal cord. The brain structures contain the ventricles and cerebrospinal fluid.

Brainstem

The brainstem is the distal part of the brain. The brainstem works as a bridge between the cerebrum and the spinal cord and between the cerebrum and cerebellum, and between the cerebellum and the spinal cord. The brainstem consists of the midbrain, pons, and medulla oblongata. All parts of the brainstem regulate heart rate, respiration, blood pressure, and other essential functions. The brainstem contains many cranial nerve nuclei, tracts, and neural pathways traveling through. Between the brainstem’s tracts and cranial nuclei is the reticular formation consisting of neurons.

Medulla oblongata

The medulla oblongata is the lower part of the brain that appears as a swelling at the upper end of the spinal cord. Besides being a conduit for fibers running between the spinal cord and higher brain regions, it contains control centers for involuntary functions such as blood pressure, breathing, swallowing, and vomiting.

The medulla oblongata is the end part of the brainstem. The medulla oblongata lies in the posterior cranial fossa. The upper part of the medulla oblongata continues as the pons, while the lower part – as the spinal cord. The medulla oblongata has two surfaces – ventricular (anterior) and dorsal (posterior) surface. 

The ventricular surface faces the basilar part of the occipital bone and the dens of the axis. The ventricular surface houses the anterior median fissure. On both sides of the fissure is the medullary pyramid. The anterior median fissure divides the medulla oblongata into two halves. 

Each half has two sulci: anterolateral and posterolateral sulcus. Between sulci sits an oval structure – the olive. The olive contains the inferior olivary nucleus. The sulci act as an exit for some cranial nerves, like the hypoglossal nerve, glossopharyngeal nerve, vagus nerve, and accessory nerve.

The dorsal surface has the dorsal median sulcus in the middle. Parallel to the sulcus travel two prominences: the gracile fasciculus and cuneate fasciculus. Both fasciculi contain nuclei. The dorsal surface also has the trigeminal tubercle consisting of the spinal nucleus of the trigeminal nerve. The dorsal surface houses the inferior part of the rhomboid fossa.

Throughout the medulla oblongata and its parts, many nuclei and tracts are distributed or pass through. The nuclei are: 

  • the cranial nerve nuclei
  • relay nuclei
  • reticular nuclei
  • raphe nuclei
  • perihypoglossal nucleus
  • hypoglossal nucleus
  • dorsal nucleus of the vagus nerve
  • medial vestibular nucleus
  • cuneate nucleus
  • spinal trigeminal nucleus
  • nucleus ambiguus
  • lateral reticular nucleus
  • olivary nuclei. 

The tracts passing through are motor and sensory tracts: the pyramidal tract, medial lemniscus, medial longitudinal fasciculus, inferior cerebellar peduncle, the spinal tract of the trigeminal nerve, spinocerebellar tract, and spinothalamic tract.

The medulla oblongata is supplied by the branches of the vertebral and basilar arteries. The veins responsible for the drainage drain into the occipital sinus, the basilar plexus of veins, and the inferior petrosal sinus.

Pons

The pons is the middle segment of the brainstem located above the medulla oblongata. The pons lies in the posterior cranial fossa. Like medulla oblongata, also the pons houses nuclei and has tracts passing through them. Tracts within the pons not only travel to the medulla oblongata but also connect the cerebrum with the cerebellum. 

An angle between the medulla oblongata and pons is known as the pontomedullary junction. Another angle between the pons, cerebellum, and medulla oblongata is seen – the cerebellopontine angle. The functions of the pons are different, like sleep, swallowing, respiration, hearing, taste.

The pons has two surfaces: anterior and posterior surface. The anterior surface has horizontal lines traveling through it. The corticopontocerebellar fibers make the lines. The anterior surface is an exit point for the trigeminal nerve, abducens nerve, facial nerve, and vestibulocochlear nerve. The pons’ anterior surface has a groove for the basilary artery. 

The posterior surface is connected to the fourth ventricle and the cerebellum. The posterior surface of the pons is largely made by the superior part of the rhomboid fossa. Large peduncles go from the pons to the cerebellum containing pontocerebellar fibers. These peduncles are called the middle cerebellar peduncles. Important features of the posterior surface are the medial eminences, facial colliculus, and stria medullaris.

Looking at the cross-section, the pons has ventral or basal and dorsal or tegmental portions. The basal portion is made of corticospinal and corticobulbar tracts. In contrast, the dorsal portion contains the nuclei of the cranial nerves CN V – CN VIII, the fourth ventricle, the medial longitudinal fasciculus, medial and lateral lemnisci. Tracts associated with the pons are the corticospinal tracts, corticobulbar tracts, and spinothalamic tracts.

The branches of the basilar artery supply the pons. The venous drainage happens through the anterior pontomesencephalic veins that drain into the basal vein and then into the cerebral veins. Inferiorly, blood drains into the internal jugular veins via the inferior petrosal sinus.

Midbrain

The midbrain or mesencephalon is the uppermost part of the brainstem. The midbrain lies between the pons below and the thalamus above. The midbrain takes part in regulating eye movement, visual and auditory functions, temperature regulation. The midbrain has two parts separated by the cerebral aqueduct. These parts are the anterior part and the posterior part.

The anterior part has the red nucleus and cerebral peduncles. The nucleus is where some of the tracts start, and some tracts end. The tracts associated with the red nucleus are the corticobulbar, cerebellorubral, rubrospinal, and rubro-olivary tracts. The oculomotor nerve exits from the anterior part. 

The posterior part contains the quadrigeminal plate and four colliculi. The colliculi are divided into two superior and two inferior. The separation site between the colliculi is the exit for the trochlear nerve.

In the cross-section, the midbrain has the tectum, tegmentum, and crus cerebri. The tectum is found dorsal to the cerebral aqueduct of Sylvius, the tegmentum – ventral to the cerebral aqueduct, while the crus cerebri are collections of tracts. 

The substantia nigra separates the crus from the tegmentum. The red nucleus can be seen on the cross-section. The major nuclei found in the midbrain are the superior colliculi, the nucleus of the oculomotor nerve, red nucleus, inferior colliculi, nucleus of the trochlear nerve, the nucleus of the trigeminal nerve.

The branches of the basilary artery supply the midbrain. The veins from the midbrain drain into the great cerebral vein via the basal veins.

Reticular formation

The reticular formation is made of neurons spread between the spinal cord and the thalamus and branching connections also to the brainstem. The nuclei of the reticular formation can be found in the brainstem. These nuclei are divided into three groups: lateral, medial, and median. The lateral nuclei are located in the lateral region. This region’s borders are the inferior colliculus and the spinal cord. The medial region stretches between the mesencephalic midbrain and the fourth ventricle. The median nuclei are located in the median region associated with the dorsal median sulcus.

Cerebellum

The cerebellum lies in the posterior cranial fossa. It is located dorsally to the pons and medulla oblongata. The fourth ventricle is separating the cerebellum from those other structures. The cerebellum is responsible for coordination, motor functions, precision, and motor learning. Like the cerebrum, the cerebellum also consists of two hemispheres. A junction between the hemispheres is called the vermis. The cerebellum is divided into three lobes and many lobules.

The cerebellum is made of the cerebellar cortex, a medullary core, and four pairs of intrinsic nuclei. The cerebellar cortex contains many laminae, and the medullary core consists of white matter. The four nuclei are fastigial, globose, emboliform, and dentate.

The structures connecting the cerebellum to the brainstem are three pairs of cerebellar peduncles, with each pair of peduncles connecting to the different parts of the brainstem. The superior cerebellar peduncles connect the cerebellum to the midbrain, the middle cerebellar peduncles – to the pons, the inferior cerebellar peduncles – to the medulla oblongata. 

The cerebellum has the superior or tentorial surface and inferior or occipital surface. Many fissures are found on both surfaces. Horizontal fissure travels around the cerebellum on its lateral and posterior margins until the posterolateral fissure. A postlunate fissure goes from left to right across the superior surface of the cerebellum. The separation between the anterior and posterior lobes is marked by the primary fissure. The last fissure is the retrotonsilar fissure lying behind the cerebellar tonsil.

The cerebellum has three lobes: anterior, posterior, and flocculonodular lobe. The cerebellum is also divided into central vermal lobules and horizontal lobules. 

The cerebellum is supplied by the superior cerebellar, anterior inferior cerebellar, and posterior inferior cerebellar arteries. The blood from the cerebellum is drained by the superior and inferior cerebellar veins into the superior, petrosal, transverse, and straight dural venous sinuses.

Subcortical structures

The subcortical structures are different neural formations in the brain. These structures are the diencephalon, pituitary gland, limbic system, and basal ganglia. These structures take part in emotion, memory, and pleasure control, as well as hormone production. The subcortical structures change the information and send it to different areas in the brain.

Diencephalon 

Above the midbrain lies the diencephalon, which includes two major brain regions: the thalamus and hypothalamus. The thalamus processes and integrates all sensory information going to the higher regions of the brain, while the hypothalamus is critical for homeostasis, the maintenance of the body`s internal environment. 

The hypothalamus influences nervous control of all internal organs and also serves as the master regulator of endocrine functions by its control over the pituitary gland via the hypophyseal-pituitary axis. Apart from the major regions, the diencephalon also has the epithalamus, subthalamus, and metathalamus

The epithalamus is associated with the roof of the third ventricle. The epithalamus has five parts: stria medullaris, posterior commissure, Habenular nuclei, pineal body, and paraventricular nuclei. The subthalamus is below the posterior part of the thalamus. It contains the reticular nucleus and perigeniculate nucleus. The methathalmus has two geniculate bodies. The geniculate bodies work as relay stations for the auditory and optic systems. 

The thalamus consists of grey matter and is the largest mass in the diencephalon. The thalamus has an anterior and posterior pole. It has four surfaces: superior, inferior, medial, and lateral surfaces. The white matter layer covers the stratum zonale, while the external medullary lamina covers the lateral surface. The thalamus houses five groups of nuclei: anterior, lateral, medial, reticular, and intralaminar nuclei. 

The medial surface of the thalamus is made by the lateral wall of the third ventricle. The lateral surface separates the lateral surface from the basal ganglia. The inferior surface is associated with the hypothalamus, while the superior surface – with the stria terminalis.

The hypothalamus is located below the thalamus. Through the infundibulum, the hypothalamus is connected to the pituitary gland. The hypothalamus has three zones: medial, lateral, and periventricular. Apart from three zones, the hypothalamus also has three regions: anterior, middle, and posterior. Like the thalamus, the hypothalamus has four nuclei groups: preoptic, supraoptic, infundibular, and mammillary. 

The diencephalon is supplied by the posterior cerebral artery and the posterior communicating artery.

Pituitary gland

The pituitary gland lies in the sella turcica in the sphenoid bone. The pituitary gland has an anterior and posterior lobe. The gland secretes hormones that further affect other organ systems. While the anterior lobe secretes most of the hormones (growth hormone, prolactin, follicle-stimulating hormone, luteinizing hormone, thyroid-stimulating hormone, adrenocorticotropic hormone), the posterior lobe is responsible for only two hormones (oxytocin, vasopressin). 

The pituitary gland is supplied by the superior and inferior hypophyseal arteries, infundibular artery. The hypophyseal vein drains the blood.

Limbic structures

The limbic structures together are known as the limbic system. The limbic system is responsible for emotions and behavior. The system comprises the amygdala, olfactory bulb, hypothalamus, septal nuclei, and anterior and dorsomedial nuclei of the thalamus. 

Although the limbic system is considered a part of the subcortical structures, some components are cortical, like the orbital frontal cortex, insular cortex, hippocampus, cingulate gyrus, and parahippocampal gyrus. Most of the limbic system is supplied by the anterior and posterior cerebral arteries, anterior choroidal artery, and branches from the circle of Willis.

Basal ganglia

The basal ganglia consist of nuclei within the telencephalon, midbrain, and diencephalon. The basal ganglia is a part of the extrapyramidal motor system. The nuclei are arranged in five pairs of nuclei: subthalamic nucleus, globus pallidus, putamen, substantia nigra, and caudate nucleus. 

The basal ganglia affect the work of the motor cortex via the thalamus. The basal ganglia are supplied by the middle cerebral artery and its branches, the branches of the posterior cerebral and posterior communicating arteries. The venous drainage happens through the branches of the internal cerebral vein.

Cerebrum

The highest region of the brain is the cerebrum, which is divided into two almost symmetric cerebral hemispheres. Neuronal cell bodies form the gray matter of the cerebrum. It includes both the cerebral cortex visible on the outside of the brain and several subcortical structures, including the basal nuclei and hippocampus. 

The white matter is formed by nerve fibers (myelinated axons of neurons) that lie deep into the cortex. The cerebrum is responsible for such processes as conscious sensation and voluntary movement and advanced functions such as thinking, learning, and emotion. The hemispheres are separated by a deep longitudinal fissure consisting of the corpus callosum. 

The brain is divided into five lobes: frontal, parietal, temporal, occipital, and insular. Sometimes also, a limbic lobe can be mentioned, but it is not an actual lobe. The outermost layer of the cerebrum is the cerebral cortex. The cortex is made of grey matter and is in the form of sulci and gyri. The cerebral cortex has many areas responsible for different functions. The brain is surrounded by a membrane called the meninges. 

The frontal lobe is the anterior part of the cerebrum. The frontal lobe is responsible for muscle control, intellect, personality, mood, social behavior, and language. It is supplied by the anterior and middle cerebral arteries from the internal carotid artery. 

The parietal lobe is found between the frontal and occipital lobes. This lobe takes part in language, calculation, and perception of different sensations. The parietal lobe is supplied by the branches of the anterior, middle, and posterior cerebral arteries. 

The temporal lobe is found below the lateral sulcus. This lobe is vital for memory, language, and hearing. The middle and posterior cerebral arteries supply it. 

The occipital lobe is located in the posterior part of the cerebrum. Its primary function is being responsible for processing visual stimuli. The posterior cerebral artery supplies the occipital lobe. 

The insula can be found below other lobes. The functions of the lobe include processing sensations of taste, vestibular function, and visceral pain. The lobe receives its blood supply from the middle cerebral artery. 

The non-official lobe – the limbic lobe – is located above the corpus callosum. The limbic lobe is responsible for the cadency of hormonal, autonomic, and visceral functions, emotions, memory, and learning. 

The cerebral cortex is a layer of gray matter forming the outer part of the cerebral hemispheres. The surface of the cerebral cortex creates folded bulges called gyri and deep fissures called sulci. These cerebral folds increase the surface area of the cerebral cortex, thus increasing the amount of information that can be processed.

Functionally, the cerebral cortex may be divided into the motor, sensory, and association areas. Sensory areas receive sensory input, motor areas execute voluntary control over muscle movements, and association areas provide more complex functions such as thinking, learning, decision-making, and complex movements like writing and speaking.

Anatomically, a deep fissure called the central sulcus separates the precentral gyrus anteriorly and postcentral gyrus posteriorly. These two folds correspond to the primary motor and sensory areas. Other areas and anatomical landmarks of the cerebral cortex also correspond with different brain functions.

The main functional areas of the cerebral cortex are as following:

  • the prefrontal cortex - responsible for thinking, planning, problem-solving, decision making other and other complex cognitive processes, as well as personality expression and moderating social behavior;
  • somatomotor association area (or motor association cortex) - provides coordination of complex movements;
  • primary motor cortex - responsible for the initiation of voluntary movement;
  • primary somatosensory cortex - receives tactile information from the body;
  • somatosensory association area (or sensory association cortex) - processes multisensory information;
  • visual association area - provides complex processing of visual information;
  • primary visual cortex (or simply visual cortex) - responsible for the detection of simple visual stimuli;
  • Wernicke’s area - responsible for speech and language comprehension;
  • auditory association area - provides complex processing of auditory information;
  • primary auditory cortex (or simply auditory cortex) - responsible for the detection of auditory stimuli and of sound quality, including loudness and tone;
  • Broca’s area - the motor speech area that provides speech production and articulation, transforming thoughts into speech.

Meninges

The meninges consist of three membranous layers: dura mater, arachnoid, and pia mater. The meninges surround the brain and the spinal cord, protecting them. The spaces between the meninges are filled with cerebrospinal fluid. These spaces are the epidural, subdural, and subarachnoid

Dura mater is a superficial layer of the meninges immediately below the skull. The arachnoid mater is the layer below the dura mater. Pia mater is the deepest layer, closest to the cerebrum. This layer covers and accepts the form of grooves and fissures of the cerebrum. 

Ventricles and CSF

The ventricles are cavities in the brain. The ventricles are connected to each other and filled with cerebrospinal fluid. The cerebrospinal fluid takes nutrients to the brain, and the spinal cord removes waste products and protects the structures. Together are four ventricles: two lateral, third, and fourth ventricles. 

All ventricles are lined with the choroid plexus, and its cells are responsible for producing cerebrospinal fluid. The lateral ventricle is located centrally in the cerebrum and has three horns, and they project to the respective cerebrum lobe. The third ventricle is somewhat almost entirely surrounded by the lateral ventricles. The fourth ventricle is located inferiorly compared to the other ventricles and located within the brainstem.