Human Brain: Structure and Functions of different parts

Anatomy of Human brain:

• Human brain weighs about 1.5 kilograms (3.3 lbs) and is larger in relation to body size.
• It is the most complex part of the body in human that is responsible to control thoughts, memory and speech, legs and arm movements along with the many functions within the body.
• The brain is located inside the cranium.
• Cranium is the bony covering that protects the brain from external injury.
• The Nervous system:
  • The nervous system in human is divided into central nervous system and peripheral nervous system.
  • The central nervous system comprises of brain, its cranial nerves and the spinal cord.
  • The peripheral nervous system comprises of autonomous nervous system (divisible into the sympathetic and parasympathetic nervous system) and the spinal nerves that branch from the spinal cord.

Cells of the Human brain:

• The brain is composed of two types of cells i.e. nerve cells (neurons) and glial cells also termed as neuroglia or glia.

1. Nerve cells (Neurons):

• Neurons vary in shape and size, however they all consist of cell body, dendrites and axon.
• The neuron transmits the information via the electrical and chemical signals.
• The transmission of information is mediated by a gap called as synapse.
• Dendrites are the arms that plays role as antennae picking up messages from other nerve cells.
• The neurotransmitter after crossing the synapse, fit into special receptors on the receiving nerve cell, that stimulates that cell to convey the message.

2. Glial cells:

• These are about 10-50 times more glial cells than the neurons.
• Glial cells are responsible for nourishing the neurons. It also aids for the protection as well as structural support to neurons.

What are the types of glial cells?

• Astroglia or astrocytes: They are the nurse cells and aids in regulating the blood brain barrier, permitting the nutrients to make contact with neurons.
• They regulate homeostasis, and also have impact on electrical impulses.
• They are also engaged in defense and repair of neurons.
• Oligodendroglia cells forms myelin that aids in the fast transport of electrical impulses.
• Ependymal cells secrete cerebrospinal fluid (CSF) and line the ventricles.
• Microglia cells are the macrophages of brain that are responsible for invading and cleaning up debris.

How is the brain protected in the body?

• The brain is contained in the skull, where a layer of fluid called cerebrospinal fluid is suspended.
• CSF is a transparent watery substance and is produced within the channels in the brain called as ventricles.
• Major portion of CSF is produced by the choroid plexus.
• Choroid plexus is a specialized structure within each ventricle.
• It protects the brain from mechanical shocks and jolts that are mild.
• In addition, it also plays minor immunological functions and provides the brain’s necessary nutrients.

Ventricles in brain:

• There are 4 ventricles of brain that are connected to each other by foramen, and tubes.
• Lateral ventricles (first and second): These are two ventricles that are enclosed in the cerebral hemispheres.
• Third ventricle: It is located at the center of the brain and its wall are made up of the thalamus and hypothalamus. There is a pineal gland behind the third ventricle. It helps control the inner clock and circadian rhythms of the body through the secretion of melatonin. In sexual growth, it has some function.
• Fourth ventricle:
• It is located posterior or dorsal to the pons and medulla oblongata and is anterior to the cerebellum.
• The first and second ventricle connects with the third ventricle through a distinct opening termed as the Foramen of Munro.
• The communication between the third and the fourth ventricle takes place via the Aqueduct of Silvius that is a long tube.

How is blood supplied to the brain?

• Two paired arteries, the internal carotid arteries and the vertebral arteries, bring blood to the brain.
• Most of the cerebrum is supplied by the internal carotid arteries.
• The cerebellum, brainstem, and the cerebrum’s underside are supplied by the vertebral arteries.
• The right and left vertebral arteries join together after passing through the skull to form the basilar artery.
• At the base of the brain, called the Circle of Willis, the basilar artery and the internal carotid arteries interact with each other.
• A significant safety function of the brain is the communication between the internal carotid and vertebral-basilar structures.
• It is possible for collateral blood flow to come through the Circle of Willis and avoid brain damage if one of the main arteries is blocked.
• The brain’s venous circulation varies significantly from that of the rest of the body.
• As they supply and drain particular areas of the body, arteries and veins usually run together.
• Hence one would assume that a pair of vertebral veins and internal carotid veins would be there.
• This is not the case in the brain, however.
• The main vein collectors are integrated into the dura to form venous sinuses.
• Blood from the brain is gathered by the venous sinuses and transferred to the internal jugular veins.
• The superior and inferior sagittal sinuses drain the cerebrum and the anterior skull base is drained by the cavernous sinuses.
• Eventually, all sinuses drain to the sigmoid sinuses, which leave the skull and form the veins of the jugular.
• In reality, these two jugular veins are the sole drainage of the brain.

Meninges of the brain:

• The three layers of tissue called as meninges cover and secure the brain and spinal cord.
• They are the dura mater, arachnoid mater, and pia mater from the outermost layer to inwards.
• Dura mater:
• It is a strong, thick membrane that lines the inside of the skull closely.
• It has two layers i.e. the periosteal and meningeal dura that are fused. They separate only to form venous sinuses.
• Little folds or compartments are formed by the dura.
• Two unique dural folds, the falx and the tentorium, are present.
• The falx divides the brain’s right and left hemispheres and the cerebrum is separated from the cerebellum by the tentorium.
• Arachnoid mater:
• It is a thin, web-like membrane covering the whole brain.
• The arachnoid is made of tissue that is elastic.
• The space between the dura and the arachnoid membranes is termed as the subdural space.
• Pia mater:
• It embraces the surface of the brain following its fold and grooves.
• There are several blood vessels in pia mater that reach far into the brain.
• The subarachnoid space is termed as the space between the arachnoid and the pia.
• The cerebrospinal fluid bathes and cushions the brain in pia mater.

Describe the structure and function of different parts of human brain

• Brain are divisible into the fore brain, the mid-brain and the hind brain.
• The forebrain is further divided into cerebrum, hypothalamus and thalamus.
• The midbrain includes tectum and tegmentum.
• The hind brain is composed of cerebellum, medulla and pons.
• However, the three main components of the brain are:
  • Cerebrum
  • Brain stem
  • Cerebellum

1. Cerebrum:

• It is the largest part of the brain and is divisible into right and left hemispheres.
• The right and left hemispheres is connected by a bundle of nerve fibres called as corpus callosum.
• The outer layer of the cerebrum is termed as cerebral cortex.
• Cerebrum is responsible for many of functions such as learning, emotions, problem-solving, motor control, interpretation of sensory information etc.
• Cerebrum can be classified into four different lobes: the frontal, parietal, temporal and occipital lobes.
• Frontal lobe:
  • It is located in the frontal part of the brain just over the eyes where the largest section of the brain is located.
  • Human frontal lobe is larger and more developed in comparison to any other organism.
  • It is responsible for executive functions such as language, reasons, emotions, personality characteristics and movement.
  • In the frontal lobe, the Broca’s area, one of the areas in speech is located.
  • The area of the brain responsible for controlling the voluntary movement is also present in it.
  • It is because of the frontal lobe that we are able to communicate and form rational thoughts.
• Parietal lobe:
  • It lies just behind the frontal lobe and is separated from frontal lobe by the central sulcus.
  • The two major functions of parietal lobes are:
  • Somatosensation: touch sensations such as pressure, pain, temperature, heat, cold.
  • Proprioception: the sense of how the body parts are oriented in the space.
  • It also regulates the ability to taste.
• Temporal lobe:
  • It lies posterior to the frontal lobe and is separated by lateral fissure.
  • In appropriate terms, it lies in the base of the brain.
  • Primarily, its function is to process and interpret sounds.
  • It is also a center for the cognitive processes for the memory formation and recognition of language.
  • In this lobe, primary auditory cortex is present that receives the sensations related to hearing.
  • Along with it, the processing of complex visual information, such as environments or images having various elements and large variety of colors takes place in the temporal lobe.
  • The key function of temporal lobe is the long-term memory.
  • Hippocampus lies in this lobe that processes memory formation.
• Occipital lobe:
  • It is situated at the back of the brain and is primarily engaged in vision.
  • It is the main area for visual processing.
  • It helps in recognition and identification of the visual world. 

Functions of cerebrum:

• The cerebrum directs the body’s conscious or voluntary motor functions.
• Inside the primary motor cortex and other frontal lobe motor areas where actions are scheduled, these functions originate.
• Upper motor neurons in the primary motor cortex transmit their axons to the lower motor neurons, which innervate the muscles, to synapse into the brain stem and spinal cord.
• In some forms of motor neuron disease, damage to motor areas of the cortex can result.
• Instead of complete paralysis, this sort of harm results in loss of muscle strength and accuracy.
• The olfactory sensory system is unusual in that its axons are sent directly to the olfactory cortex by neurons in the olfactory bulb, rather than to the thalamus first.
• A deterioration of the sense of smell results in damage to the olfactory bulb.
• From such brain areas as the amygdala, neocortex, hippocampus, locus coeruleus, and substantia nigra, the olfactory bulb also receives ‘top-down’ knowledge.
• Its’ possible functions can be classified into four non-exclusive categories:
  • distinguishing between odors
  •  improving odor detection sensitivity
  • filtering out background odors
  • allowing higher areas of the brain involved in arousal and attention to alter odor detection or discrimination.
• Sections of the cerebral cortex are largely attributed to speech and language.
• In the frontal lobe, motor portions of language are attributed to Broca’s field.
• Wernicke’s area, at the temporal-parietal lobe junction, is due to speech comprehension.
• Damage to the area of Broca results in verbal aphasia (non-fluent aphasia), whereas receptive aphasia results in damage to the Wernicke’s area.

2. Amygdala:

• This is another aspect of the limbic system that is a functional portion of the cerebrum.
• In the temporal lobes, the amygdala lies and is engaged in several functions of the autonomic nervous system, including physiological responses to fear (the “Fight or Flight response) and hormone secretion.
• In each hemisphere, one amygdala is found (plural: amygdalae).
• In particular, this structure is concerned with controlling the emotions of fear, anger, and pleasure.
• The amygdala is the structure which decides what memories are stored and where they are stored in the brain.
• The amygdala is the mechanism which decides what memories are stored and where they are stored in the brain.

3. The thalamus:

• It is a gray matter which is located between the cerebral cortex and the midbrain.
• The thalamus, placed under the cerebral cortex, is involved in various sensory and motor functions.
• It also regulates the circadian rhythm partly by signaling to the brain during sleep to decrease those physiological functions.

4. Hypothalamus:

• It is a small but necessary region of the brain.
• It is situated at the base of the brain, in the proximity to the pituitary gland.
• It is located in the third ventricle’s floor and is the autonomic system’s master control.
• In regulating behaviors such as starvation, hunger, sleep, and sexual response, it plays a major role.
• It also controls the temperature of the body, blood pressure, emotions, and hormone secretion.

5. Basal ganglia:

• The basal ganglia are a collective term for a group of subcortal nuclei that are central to motor control, learning and executive functions as those regulated by the frontal lobe.
• The putamen, caudate, globus pallidus, subthalamic nucleus, and substantia nigra are the parts which make up the basal ganglia collectively.
• The caudate and putamen are referred to as the striatum together.
• These systems are best known for their role in movement, considering all they’re associated with.

6. Olfactory bulb:

• The olfactory bulb is a structure found in the anterior part of the brain, in the lower (bottom) part of the cerebral hemisphere.
• There is one olfactory bulb per hemisphere, and an elongated structure known as the olfactory stalk or the olfactory peduncle binds them to the cerebrum.
• The olfactory bulb is as the name implies, central to the sense of smell and is also partially involved in taste.

7. Brain stem:

• The distal portion of the brain that consists of the midbrain, pons, and medulla oblongata is the brain stem (brain stem).
• Each of the three parts has a particular structure and feature of its own.
• Together, they help to monitor breathing, heart rate, blood pressure, and a variety of other main functions.
• A stalk-like projection that stretches caudally from the base of the cerebrum is the brainstem.
• It favors the communication between the cerebrum, cerebellum, and the spinal cord.
• At its proximal end, the brainstem is broader and becomes smaller towards the distal end.
• Three parts of the brainstem exist:

i) Medulla oblongata:

• The narrowest and most distal section of the medulla is the medulla oblongata.
• The narrowest and most caudal portion of the brainstem is the medulla oblongata or medulla.
• It is a funnel-like structure that stretches from the decussation of the broad pyramids to the inferior pontine sulcus (pontomedullary groove) through the foramen magnum (which is the largest of all the foramina and fissures of the skull).
• The narrowest and most caudal portion of the brainstem is the medulla oblongata or medulla.
• It is a funnel-like structure that stretches from the decussation of the broad pyramids to the inferior pontine sulcus (pontomedullary groove) through the foramen magnum (which is the largest of all the foramina and fissures of the skull).
• As the medulla in the posterior cranial fossa proceeds upward it stops at the fourth ventricle inferior pontine sulcus (anteriorly) and medullary striae (posteriorly).

ii) Pons:

• To the anterior, pons lies in the middle segment of the brainstem.
• Another comparatively short part of the brainstem located in the posterior cranial fossa is the pons.
• The arrangement of approximately 2.5 cm rests against the skull clivus, below the tentorium cerebelli.
• The metencephalon, which is a secondary brain vesicle developed from the hindbrain (rhombencephalon), produces the pons.
• Caudal to the mesencephalon, and cranial to the myelencephalon (medulla), the metencephalon is located.

iii) Midbrain:

• The midbrain is the section that is broader and superior.
• The midbrain is the brainstem’s shortest section.
• It stretches caudally from the thalamus base to the fourth ventricle’s superior roof.
• In the tentorium cerebelli (an extension of the dura mater), it passes through an opening.
• The structure emerges from the mesencephalon, which is one of three primary brain vesicles that emerge (above the fourth pair of somites) from the cranial portion of the closed neural tube.
• Between the forebrain (prosencephalon) and the hindbrain (rhombencephalon) the mesencephalon is located.
• This part of the brainstem is split into tectum (the Latin word for roof), tegmentum (the Latin word for covering) and crus cerebri.
• The tectum is the dorsal component of the midbrain to the cerebral aqueduct of Sylvius (a conduit linking the third and fourth ventricles).
• In comparison, the tegmentum is ventral to the aqueduct of Sylvius.
• Wide arrays of ascending and descending tracts moving to and from the brain are the crus cerebri.
• The substantia nigra separates each crus from the tegmentum.
• This is a pigmented lamina that consists of neurons that are dopaminergic and GABAergic.
• These are cells that create unique neurotransmitters they are named for.
• It is important to note that the tegmentum is continuous throughout the midline of the midbrain, even though each crus cerebri is separate from each other.
• Some texts collectively refer to the crus cerebri and tegmentum as the cerebral peduncles.

Functions of brain stem:

• The brainstem has three essential functions:
• To act as a conduit for pathways to and from the brain to ascend and descend
• To house the nuclei of cranial nerves
• To merge the roles of many critical systems with each other

8. Cerebellum:

• It is a comparatively small region of the brain, about ten percent of the total weight, but it includes approximately half of the neurons of the brain, specialized cells that convey electrical signal information.
• The cerebellum is split into two lateral hemispheres, which are connected by a medial portion called the vermis.
• Each of the hemispheres is composed of a white matter central core and a gray matter surface cortex and is divided into three lobes.
• The flocculonodular lobe is the first portion of cerebellum to develop.
• The flocculonodular lobe receives sensory input from the vestibules of the ear.
• From the spinal cord, the anterior lobe receives sensory input.
• The posterior lobe is last to arise and receives nerve impulses from the cerebrum.
• Within the cerebellar cortex, all these nerve impulses are incorporated.
• The superior, middle, and inferior peduncles that link the cerebellum with the midbrain, pons, and medulla, respectively, transmit information to and from the cerebellum via three paired bundles of nerve fibres.
• The cerebellum has traditionally been regarded a motor structure as cerebellar damage contributes to impairments in motor function and posture and as the majority of the outputs of the cerebellum are part of the motor system.
• In the cerebellum, motor commands are not initiated; rather to make movements more adaptive and precise, the cerebellum modifies the motor commands of the descending pathways.

Functions of cerebellum:

• Balance and posture management:
  • In order to maintain equilibrium, the cerebellum is essential for making postural changes.
  • It modulates commands to motor neurons through its feedback from vestibular receptors and proprioceptors to compensate for changes in body position or changes in load on muscles.
  • Balance problems are suffered by patients with cerebellar injury, and they frequently develop stereotyped postural strategies to compensate for this issue (e.g. a broad-based stance).
• Voluntary movement synchronization:
  • Most movements consist of a variety of distinct muscle groups behaving in a temporarily organized manner together.
  • One of the cerebellum’s key functions is to coordinate the timing and force to produce fluid limb or body movements of these various muscle groups.
• Motor learning:
  • In adjusting and fine-tuning motor programs to make precise movements in a trial-and-error process e.g. learning to hit a baseball), the cerebellum plays a major role.
• Cognitive roles:
  • Though the cerebellum is most commonly understood in terms of its contribution to motor regulation, some cognitive functions, such as language, are also involved.
  • The cerebellum is therefore regarded historically as part of the motor system, like the basal ganglia, but its functions extend beyond motor control in ways that are not yet well understood.