The Human brain
See also: List of regions in the human brain and Outline of the human brain
Gross anatomyEdit
The adult human brain weighs on average about 1.2–1.4 kg (2.6–3.1 lb) which is about 2% of the total body weight, with a volume of around 1260 cm3 in men and 1130 cm3 in women, although there is substantial individual variation. Neurological differences between the sexes have not been shown to correlate in any simple way with IQor other measures of cognitive performance.
The cerebrum, consisting of the cerebral hemispheres, forms the largest part of the brain and is situated above the other brain structures. The outer region of the hemispheres, the cerebral cortex, is grey matter, consisting of cortical layers of neurons. Each hemisphere is divided into four main lobes.
The brainstem, resembling a stalk, attaches to and leaves the cerebrum at the start of the midbrain area. The brainstem includes the midbrain, the pons, and the medulla oblongata. Behind the brainstem is the cerebellum (Latin: little brain).
The cerebrum, brainstem, cerebellum, and spinal cord are covered by three membranes called meninges. The membranes are the tough dura mater; the middle arachnoid materand the more delicate inner pia mater. Between the arachnoid mater and the pia mater is the subarachnoid space, which contains the cerebrospinal fluid.In the cerebral cortex, close to the basement membrane of the pia mater, is a limiting membrane called the glia limitans; this is the outermost membrane of the cortex. The living brain is very soft, having a gel-like consistency similar to soft tofu.The cortical layers of neurons constitute much of the brain's grey matter, while the deeper subcortical regions of myelinated axons, make up the white matter.
Structural and functional areas of the human brain
Human brain bisected in the sagittal plane, showing the white matter of the corpus callosum
Functional areas of the human brain. Dashed areas shown are commonly left hemisphere dominant
CerebrumEdit
The cerebrum is the largest part of the human brain, and is divided into nearly symmetricalleft and right hemispheres by a deep groove, the longitudinal fissure. The outer part of the cerebrum is the cerebral cortex, made up of grey matter arranged in layers. It is 2 to 4 millimetres (0.079 to 0.157 in) thick, and deeply folded to give a convoluted appearance. Beneath the cortex is the white matter of the brain. The largest part of the cerebral cortex is the neocortex, which has six neuronal layers. The rest of the cortex is of allocortex, which has three or four layers. The hemispheres are connected by five commissures that span the longitudinal fissure, the largest of these is the corpus callosum. The surface of the brain is foldedinto ridges (gyri) and grooves (sulci), many of which are named, usually according to their position, such as the frontal gyrus of the frontal lobe or the central sulcus separating the central regions of the hemispheres. There are many small variations in the secondary and tertiary folds. Each hemisphere is conventionally divided into four lobes; the frontal lobe, parietal lobe, temporal lobe, and occipital lobe, named according to the skull bones that overlie them. Each lobe is associated with one or two specialised functions though there is some functional overlap between them.
Major gyri and sulci on the lateral surface of the cortex
Lobes of the brain
The cerebrum is the largest part of the human brain, and is divided into nearly symmetricalleft and right hemispheres by a deep groove, the longitudinal fissure. The outer part of the cerebrum is the cerebral cortex, made up of grey matter arranged in layers. It is 2 to 4 millimetres (0.079 to 0.157 in) thick, and deeply folded to give a convoluted appearance. Beneath the cortex is the white matter of the brain. The largest part of the cerebral cortex is the neocortex, which has six neuronal layers. The rest of the cortex is of allocortex, which has three or four layers. The hemispheres are connected by five commissures that span the longitudinal fissure, the largest of these is the corpus callosum.The surface of the brain is foldedinto ridges (gyri) and grooves (sulci), many of which are named, usually according to their position, such as the frontal gyrus of the frontal lobe or the central sulcus separating the central regions of the hemispheres. There are many small variations in the secondary and tertiary folds.Each hemisphere is conventionally divided into four lobes; the frontal lobe, parietal lobe, temporal lobe, and occipital lobe, named according to the skull bones that overlie them. Each lobe is associated with one or two specialised functions though there is some functional overlap between them.CerebellumEdit
Main article: Cerebellum
Human brain viewed from below, showing cerebellum and brainstem
The cerebellum is divided into an anterior lobe, a posterior lobe, and the flocculonodular lobe. The anterior and posterior lobes are connected in the middle by the vermis. The cerebellum has a much thinner outer cortex that is narrowly furrowed horizontally.[28]Viewed from underneath between the two lobes is the third lobe the flocculonodular lobe.
The cerebellum rests at the back of the cranial cavity, lying beneath the occipital lobes, and is separated from these by the cerebellar tentorium, a sheet of fibre.
It is connected to the midbrain of the brainstem by the superior cerebellar peduncles, to the pons by the middle cerebellar peduncles, and to the medulla by the inferior cerebellar peduncles. The cerebellum consists of an inner medulla of white matter and an outer cortex of richly folded grey matter. The cerebellum's anterior and posterior lobes appear to play a role in the coordination and smoothing of complex motor movements, and the flocculonodular lobe in the maintenance of balance although debate exists as to its cognitive, behavioural and motor functions.
BrainstemEdit
Main article: Brainstem
The brainstem lies beneath the cerebrum and consists of the midbrain, pons and medulla. It lies in the back part of the skull, resting on the part of the base known as the clivus, and ends at the foramen magnum, a large opening in the occipital bone. The brainstem continues below this as the spinal cord,protected by the vertebral column.
Ten of the twelve pairs of cranial nerves[a]emerge directly from the brainstem. The brainstem also contains many cranial nerve nuclei and nuclei of peripheral nerves, as well as nuclei involved in the regulation of many essential processes including breathing, control of eye movements and balance. The reticular formation, a network of nuclei of ill-defined formation, is present within and along the length of the brainstem. Many nerve tracts, which transmit information to and from the cerebral cortex to the rest of the body, pass through the brainstem.MicroanatomyEdit
The human brain is primarily composed of neurons, glial cells, neural stem cells, and blood vessels. Types of neuron include interneurons, pyramidal cells including Betz cells, motor neurons (upper and lower motor neurons), and cerebellar Purkinje cells. Betz cells are the largest cells (by size of cell body) in the nervous system.
The adult human brain is estimated to contain 86±8 billion neurons, with a roughly equal number (85±10 billion) of non-neuronal cells.[36] Out of these neurons, 16 billion (19%) are located in the cerebral cortex, and 69 billion (80%) are in the cerebellum.
Types of glial cell are astrocytes (including Bergmann glia), oligodendrocytes, ependymal cells (including tanycytes), radial glial cellsand microglia. Astrocytes are the largest of the glial cells. They are stellate cells with many processes radiating from their cell bodies. Some of these processes end as perivascular end-feet on capillary walls.
The glia limitans of the cortex is made up of astrocyte foot processes that serve in part to contain the cells of the brain.
Mast cells are white blood cells that interact in the neuroimmune system in the brain.
Mast cells in the central nervous system are present in a number of brain structures and in the meninges; they mediate neuroimmune responses in inflammatory conditions and help to maintain the blood–brain barrier, particularly in brain regions where the barrier is absent.
Across systems, mast cells serve as the main effector cell through which pathogens can affect the gut–brain axis
Blood supplyEdit
Main article: Cerebral circulation
Two circulations joining at the circle of Willis
Diagram showing features of cerebral outer membranes and supply of blood vessels
The internal carotid arteries supply oxygenated blood to the front of the brain and the vertebral arteries supply blood to the back of the brain.These two circulations join together in the circle of Willis, a ring of connected arteries that lies in the interpeduncular cistern between the midbrain and pons.
The internal carotid arteries are branches of the common carotid arteries. They enter the cranium through the carotid canal, travel through the cavernous sinus and enter the subarachnoid space.They then enter the circle of Willis, with two branches, the anterior cerebral arteries emerging. These branches travel forward and then upward along the longitudinal fissure, and supply the front and midline parts of the brain.One or more small anterior communicating arteries join the two anterior cerebral arteries shortly after they emerge as branches. The internal carotid arteries continue forward as the middle cerebral arteries. They travel sideways along the sphenoid bone of the eye socket, then upwards through the insula cortex, where final branches arise. The middle cerebral arteries send branches along their length.
The vertebral arteries emerge as branches of the left and right subclavian arteries. They travel upward through transverse foramina – spaces in the cervical vertebrae and then emerge as two vessels, one on the left and one on the right of the medulla.They give off one of the three cerebellar branches. The vertebral arteries join in front of the middle part of the medulla to form the larger basilar artery, which sends multiple branches to supply the medulla and pons, and the two other anterior and superior
Blood drainageEdit
Cerebral veins drain deoxygenated blood from the brain. The brain has two main networks of veins: an exterior or superficial network, on the surface of the cerebrum that has three branches, and an interior network. These two networks communicate via anastomosing(joining) veins.[The veins of the brain drain into larger cavities the dural venous sinusesusually situated between the dura mater and the covering of the skull.Blood from the cerebellum and midbrain drains into the great cerebral vein. Blood from the medulla and pons of the brainstem have a variable pattern of drainage, either into the spinal veins or into adjacent cerebral veins.
The blood in the deep part of the brain drains, through a venous plexus into the cavernous sinus at the front, and the superior and inferior petrosal sinuses at the sides, and the inferior sagittal sinus at the back. Blood drains from the outer brain into the large superior saggital sinus, which rests in the midline on top of the brain. Blood from here joins with blood from the straight sinus at the confluence of sinuses.
Blood from here drains into the left and right transverse sinuses. These then drain into the sigmoid sinuses, which receive blood from the cavernous sinus and superior and inferior petrosal sinuses. The sigmoid drains into the large internal jugular veins.
The blood–brain barrierEdit
The larger arteries throughout the brain supply blood to smaller capillaries. These smallest of blood vessels in the brain, are lined with cells joined by tight junctions and so fluids do not seep in or leak out to the same degree as they do in other capillaries, thereby creating the blood–brain barrier.
Pericytes play a major role in the formation of the tight junctions.The barrier is less pe
FunctionEdit
Motor and sensory regions of the brain
Motor controlEdit
The motor system of the brain is responsible for the generation and control of movement.Generated movements pass from the brain through nerves to motor neurons in the body, which control the action of muscles. The corticospinal tract carries movements from the brain, through the spinal cord, to the torso and limbs.The cranial nerves carry movements related to the eyes, mouth and face.
Gross movement – such as locomotion and the movement of arms and legs – is generated in the motor cortex, divided into three parts: the primary motor cortex, found in the prefrontal gyrus and has sections dedicated to the movement of different body parts. These movements are supported and regulated by two other areas, lying anterior to the primary motor cortex: the premotor areaand the supplementary motor area. The hands and mouth have a much larger area dedicated to them than other body parts, allowing finer movement; this has been visualised in a motor cortical homunculus.Impulses generated from the motor cortex travel along the corticospinal tract along the front of the medulla and cross over (decussate) at the medullary pyramids. These then travel down the spinal cord, with most connecting to interneurons, in turn connecting to lower motor neurons within the grey matterthat then transmit the impulse to move to muscles themselves.The cerebellum and basal ganglia, play a role in fine, complex and coordinated muscle movements.Connections between the cortex and the basal ganglia control muscle tone, posture and movement initiation, and are referred to as the extrapyramidal system.
SensoryEdit
Cortical areas
Routing of neural signals from the two eyes to the brain
The sensory nervous system is involved with the reception and processing of sensory information. This information is received through the cranial nerves, through tracts in the spinal cord, and directly at centres of the brain exposed to the blood. The brain also receives and interprets information from the special senses (vision, smell, hearing, and taste). Mixed motor and sensory signals are also integrated.
From the skin, the brain receives information about fine touch, pressure, pain, vibration and temperature. From the joints, the brain receives information about joint position.The sensory cortex is found just near the motor cortex, and, like the motor cortex, has areas related to sensation from different body parts. Sensation collected by a sensory receptor on the skin is changed to a nerve signal, that is passed up a series of neurons through tracts in the spinal cord. The dorsal column–medial lemniscus pathway contains information about fine touch, vibration and position of joints. Neurons travel up the back part of the spinal cord to the back part of the medulla, where they connect with "second order" neurons that immediately swap sides. These neurons then travel upwards into the ventrobasal complex in the thalamus where they connect with "third order" neurons, and travel up to the sensory cortex.The spinothalamic tract carries information about pain, temperature, and gross touch. Neurons travel up the spinal cord and connect with second-order neurons in the reticular formation of the brainstem for pain and temperature, and also at the ventrobasal complex of the medulla for gross touch.
Main article: Cerebral circulation
Two circulations joining at the circle of Willis
Diagram showing features of cerebral outer membranes and supply of blood vessels
The internal carotid arteries supply oxygenated blood to the front of the brain and the vertebral arteries supply blood to the back of the brain.These two circulations join together in the circle of Willis, a ring of connected arteries that lies in the interpeduncular cistern between the midbrain and pons.
The internal carotid arteries are branches of the common carotid arteries. They enter the cranium through the carotid canal, travel through the cavernous sinus and enter the subarachnoid space.They then enter the circle of Willis, with two branches, the anterior cerebral arteries emerging. These branches travel forward and then upward along the longitudinal fissure, and supply the front and midline parts of the brain.One or more small anterior communicating arteries join the two anterior cerebral arteries shortly after they emerge as branches. The internal carotid arteries continue forward as the middle cerebral arteries. They travel sideways along the sphenoid bone of the eye socket, then upwards through the insula cortex, where final branches arise. The middle cerebral arteries send branches along their length.
The vertebral arteries emerge as branches of the left and right subclavian arteries. They travel upward through transverse foramina – spaces in the cervical vertebrae and then emerge as two vessels, one on the left and one on the right of the medulla.They give off one of the three cerebellar branches. The vertebral arteries join in front of the middle part of the medulla to form the larger basilar artery, which sends multiple branches to supply the medulla and pons, and the two other anterior and superior
Blood drainageEdit
Cerebral veins drain deoxygenated blood from the brain. The brain has two main networks of veins: an exterior or superficial network, on the surface of the cerebrum that has three branches, and an interior network. These two networks communicate via anastomosing(joining) veins.[The veins of the brain drain into larger cavities the dural venous sinusesusually situated between the dura mater and the covering of the skull.Blood from the cerebellum and midbrain drains into the great cerebral vein. Blood from the medulla and pons of the brainstem have a variable pattern of drainage, either into the spinal veins or into adjacent cerebral veins.
The blood in the deep part of the brain drains, through a venous plexus into the cavernous sinus at the front, and the superior and inferior petrosal sinuses at the sides, and the inferior sagittal sinus at the back. Blood drains from the outer brain into the large superior saggital sinus, which rests in the midline on top of the brain. Blood from here joins with blood from the straight sinus at the confluence of sinuses.
Blood from here drains into the left and right transverse sinuses. These then drain into the sigmoid sinuses, which receive blood from the cavernous sinus and superior and inferior petrosal sinuses. The sigmoid drains into the large internal jugular veins.
The blood–brain barrierEdit
The larger arteries throughout the brain supply blood to smaller capillaries. These smallest of blood vessels in the brain, are lined with cells joined by tight junctions and so fluids do not seep in or leak out to the same degree as they do in other capillaries, thereby creating the blood–brain barrier.
Pericytes play a major role in the formation of the tight junctions.The barrier is less pe
FunctionEdit
Motor and sensory regions of the brain
Motor controlEdit
The motor system of the brain is responsible for the generation and control of movement.Generated movements pass from the brain through nerves to motor neurons in the body, which control the action of muscles. The corticospinal tract carries movements from the brain, through the spinal cord, to the torso and limbs.The cranial nerves carry movements related to the eyes, mouth and face.
Gross movement – such as locomotion and the movement of arms and legs – is generated in the motor cortex, divided into three parts: the primary motor cortex, found in the prefrontal gyrus and has sections dedicated to the movement of different body parts. These movements are supported and regulated by two other areas, lying anterior to the primary motor cortex: the premotor areaand the supplementary motor area. The hands and mouth have a much larger area dedicated to them than other body parts, allowing finer movement; this has been visualised in a motor cortical homunculus.Impulses generated from the motor cortex travel along the corticospinal tract along the front of the medulla and cross over (decussate) at the medullary pyramids. These then travel down the spinal cord, with most connecting to interneurons, in turn connecting to lower motor neurons within the grey matterthat then transmit the impulse to move to muscles themselves.The cerebellum and basal ganglia, play a role in fine, complex and coordinated muscle movements.Connections between the cortex and the basal ganglia control muscle tone, posture and movement initiation, and are referred to as the extrapyramidal system.
SensoryEdit
Cortical areas
Routing of neural signals from the two eyes to the brain
The sensory nervous system is involved with the reception and processing of sensory information. This information is received through the cranial nerves, through tracts in the spinal cord, and directly at centres of the brain exposed to the blood. The brain also receives and interprets information from the special senses (vision, smell, hearing, and taste). Mixed motor and sensory signals are also integrated.
From the skin, the brain receives information about fine touch, pressure, pain, vibration and temperature. From the joints, the brain receives information about joint position.The sensory cortex is found just near the motor cortex, and, like the motor cortex, has areas related to sensation from different body parts. Sensation collected by a sensory receptor on the skin is changed to a nerve signal, that is passed up a series of neurons through tracts in the spinal cord. The dorsal column–medial lemniscus pathway contains information about fine touch, vibration and position of joints. Neurons travel up the back part of the spinal cord to the back part of the medulla, where they connect with "second order" neurons that immediately swap sides. These neurons then travel upwards into the ventrobasal complex in the thalamus where they connect with "third order" neurons, and travel up to the sensory cortex.The spinothalamic tract carries information about pain, temperature, and gross touch. Neurons travel up the spinal cord and connect with second-order neurons in the reticular formation of the brainstem for pain and temperature, and also at the ventrobasal complex of the medulla for gross touch.
Comments
Post a Comment