PPT 3 Brain and Brain
The cerebrum is the largest part of the brain.
The surface of the cerebrum is composed of depressions or grooves called sulci, and ridges or raised areas called gyri, which increase the surface area of the cerebrum without an increase in the size of the brain. If the cerebrum were smooth it would have to be about the size of a beach ball to have the same amount of surface area!
Grey matter, approximately 2 to 4 mm thick, forms the outer surface of the cerebrum, and is referred to as the cerebral cortex.
The cerebrum consists of two cerebral hemispheres, the right hemisphere and the left hemisphere, connected by the corpus callosum which facilitates communication between both sides of the brain, with each hemisphere 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 two cerebral hemispheres are separated by the longitudinal fissure and each hemisphere is further divided into four lobes, named according to the corresponding cranial bone that approximately overlies each part. Each hemisphere has a frontal, temporal, parietal and occipital lobe.
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Gastrulation cells migrate to the interior of the embryo, forming the three germ layers: the endoderm, which is the deepest layer, the mesoderm, or the middle layer, and the ectoderm or the surface layer. It is from these three layers that all tissues and organs will arise in a process called organogenesis.
Essentially, the ectoderm gives rise to skin and the nervous system, the mesoderm gives rise to the muscle cells and connective tissue in the body, and the endoderm gives rise to the digestive system and other internal organs.
The first organ system that is produced during organogenesis is the nervous system and it is formed from the ectoderm germ layer via a process called Neurulation.
Before the nervous system can be formed however, the mesodermal layer must form a very important structure called the Notochord. The notochord is a rod-like collection of cells that runs along the entire length of the developing embryo, and which is essential in the production of the nervous system.
If we look at this cross section through the developing embryo, we can see the notochord in the centre here.
So, what’s so special about the notochord?
1. During the third week of gestation, around 18 days after fertilization, the notochord sends out chemical signals that interact with the ectoderm overlying it. These signals induce a thickening in the ectodermal cells overlying the notochord and cause these cuboidal epithelial cells to differentiate into a columnar epithelium that we call the Neural Plate. The neural plate is also called the Neural Ectoderm or neuroectoderm.
2. Besides inducing the differentiation of the neural plate, the notochord also has another important role. It defines the longitudinal axis of the embryo and determines the orientation of the vertebral column.
Following gastrulation, formation of the notochord, and inductive signaling of the neural plate, the next major stage of embryogenesis is Neurulation. Neurulation is the formation of the neural tube from the ectoderm of the embryo. At this stage, the embryo is referred to as a Neurula.
So, how does this neural tube form?
During the third gestational week, a crease or fold appears in the neural plate. The crease rapidly deepens and becomes known as the neural groove. The entire embryo is lengthening as this happens. As the neural groove continues to deepen, the lateral edges of the plate begin to rise like two ocean waves coming together. These edges are called the neural folds, and the tips of the neural folds are called the neural crest cells. As the neural folds approach each other and start fusing, these neural crest cells just separate from the neural folds and form their own layer. Now the neural folds fuse in the midline to form a neural tube.
The fusion of the neural tube begins at about Day 21, near the future cervical region, which is the middle portion of the embryo, and progresses toward both its cephalic and caudal ends. Finally, by Day 26, the neural tube is almost completely enclosed, except for two openings at the ends of the neural tube which persist for a relatively short period of time and are called the anterior and posterior neuropores. They temporarily form open connections between the neural tube lumen and the amniotic cavity.
The Anterior or cranial neuropore closes about day 24.
The Posterior or caudal neuropore closes at about day 26.
As the neural tube grows and differentiates it enlarges to expand and constrict to form the three primary brain vesicles: Forebrain/Prosencephalon, Midbrain/Mesencephalon, and Hindbrain/Rhombencephalon. As development continues, the three primary vesicles give rise to five secondary brain vesicles: Telencephalon, Diencephalon, Mesencephalon, Metencephalon, and Myelencephalon.
Those five vesicles can be aligned with the four major regions of the adult brain, which are, the cerebral hemispheres, the diencephalon, the cerebellum and the brain stem.
The cerebrum is formed directly from the telencephalon. The diencephalon is the only region that keeps its embryonic name. The mesencephalon, metencephalon, and myelencephalon become the brain stem. The cerebellum also develops from the metencephalon and is a separate region of the adult brain.
The spinal cord develops out of the rest of the neural tube and retains the tube structure, with the nervous tissue thickening and the hollow center becoming a very small central canal through the cord. The rest of the hollow center of the neural tube corresponds to open spaces within the brain called the ventricles, where cerebrospinal fluid is found.
Neurulation also contributes to the cephalocaudal flexion of the embryo. The extensive proliferation of the nervous tissue causes curvature of the embryo on its long axis.
With the appearance of the vesicles, the neural tube bends ventrally to form 2 flexures:
1. A cervical flexure: at the junction of the spinal cord and hindbrain.
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2. A cephalic flexure: in the midbrain
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3. Later, between the above 2 major flexures, unequal growth in the hindbrain produces the pontine flexure, in the opposite direction.
As mentioned earlier, the cavities within the vesicles develop into the ventricles during the developmental transformation of the brain. The ventricles of the brain are continuous with one another and with the central canal of the spinal cord. The first and second ventricles (lateral ventricles) develop in the forebrain, a narrow third ventricle develops in the diencephalon, and the fourth ventricle develops from the cavity of the hindbrain.
When you think of the nervous system, the picture that comes to your mind is probably that of the brain and spinal cord. However, the nervous tissue that reach out from the brain and spinal cord to the rest of the body, i.e., the nerves, are also part of the nervous system.
We can anatomically divide the nervous system into two major regions: the central nervous system (CNS) is the brain and spinal cord, and the peripheral nervous system (PNS) is the nerves. The peripheral nervous system is so named because it is in the periphery—meaning beyond the brain and spinal cord.
In addition to the anatomical divisions listed above, the nervous system can also be divided based on its functions. The nervous system is involved in receiving information about the environment around us or in sensation, and generating responses to that information, and coordinating the two, i.e., integration.
Sensory information travels towards the CNS through the PNS nerves in the specific division known as the afferent (sensory) branch of the PNS. It is composed of sensory neurons and conducts signals from the receptors to the CNS.
The nervous system produces a response in effector organs (such as muscles or glands) due to the sensory stimuli. The motor (efferent) branch of the PNS is composed of motor neurons and carries signals away from the CNS to the effector organs.
When the effector organ is a skeletal muscle, the neuron carrying the information is called a somatic motor neuron; when the effector organ is cardiac or smooth muscle or glandular tissue, the neuron carrying the information is called an autonomic motor neuron. Voluntary responses are governed by somatic motor neurons and involuntary responses are governed by the autonomic motor neurons.
Finally, the autonomic division can be split into sympathetic (‘fight or flight’) or parasympathetic (‘rest and digest’) responses.
The brain is made up of two types of cells: neurons and glial cells, also known as neuroglia or glia.
•The neuron is responsible for sending and receiving nerve impulses or signals.
•Glial cells are non-neuronal cells that provide support and nutrition, maintain homeostasis, form myelin and facilitate signal transmission in the nervous system.
The meninges refer to the membranous coverings of the brain and spinal cord. There are three layers of meninges, known as the dura mater, arachnoid mater and pia mater.
These coverings have two major functions:
Provide a supportive framework for the cerebral and cranial vasculature.
Acting with cerebrospinal fluid (CSF) to protect the CNS from mechanical damage. We will learn about CSF together with the ventricles in the next lesson.
The central nervous system is made up of gray matter and white matter. About 40% of the human brain is made up of gray matter and the other 60% is white matter.
However, gray matter plays the most significant part in allowing humans to function normally daily. Gray matter makes up the outer most layer of the brain.
The gray matter contains most of the neuron cell bodies, making it appear tan with circulation but gray when prepared for examination outside of the body. This brain tissue is abundant in the cerebellum, cerebrum, and brain stem. It also forms a butterfly-shaped portion of the central spinal cord.
The gray matter is the area where the actual "processing" is done.
The white matter is so-called because it is composed of bundles of axons that are sheathed in the white fatty insulating protein called myelin. It is responsible for communication between the various gray matter regions and between the gray matter and the rest of the body.
In essence, the gray matter is where the processing is done, and the white matter is the channels of communication.
There are various ways of dividing the brain to study its component parts. One way we have seen is by the embryogenic development of the brain, where we divided the brain into forebrain, midbrain and hindbrain based on its origin from the three primary vesicles. Another way we have mentioned is by the four major regions of the adult brain, which are, the cerebral hemispheres, the diencephalon, the cerebellum and the brain stem, all of which develop from the five secondary vesicles.
More information about the different naming systems of the brain can be found at this link.
For the purpose of this lesson, we will study the parts of the brain under the headings, forebrain, midbrain and hindbrain.
The forebrain is the largest major part of the brain. It includes the cerebrum, the thalamus, the hypothalamus, the pituitary gland, and the limbic system.
Let’s discuss each of these parts a little before moving on to the midbrain.
The cerebrum is the largest part of the brain. The surface of the cerebrum is composed of depressions or grooves called sulci, and ridges or raised areas called gyri, which increase the surface area of the cerebrum without an increase in the size of the brain. If the cerebrum were smooth it would have to be about the size of a beach ball to have the same amount of surface area!
Grey matter, approximately 2 to 4 mm thick, forms the outer surface of the cerebrum, and is referred to as the cerebral cortex.
The cerebrum consists of two cerebral hemispheres, the right hemisphere and the left hemisphere, connected by the corpus callosum which facilitates communication between both sides of the brain, with each hemisphere 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 two cerebral hemispheres are separated by the longitudinal fissure and each hemisphere is further divided into four lobes, named according to the corresponding cranial bone that approximately overlies each part. Each hemisphere has a frontal, temporal, parietal and occipital lobe.
As mentioned in the previous slide, the names of the lobes correlate with the cranial bones that overlie them. The frontal bone forms the forehead. Two parietal bones form the upper sides of the skull, while two temporal bones form the lower sides. The occipital bone forms the back of the skull.
The skull is part of the Axial Skeleton.
The bones of the human body can be divided into two broad groups, the axial skeleton and the appendicular skeleton. The axial skeleton consists of the bones along the central axis of the body. This includes the skull, the vertebral column, and the rib cage. The appendicular skeleton consists of the limbs (arms and legs), also sometimes called appendages, and the bones that attach them to the trunk of the body (the pelvic and pectoral girdles).
The lobes of the brain do not function alone: they function through very complex relationships with one another.
The frontal lobe is the largest of the four lobes and is involved with higher cognitive functions, including problem-solving, decision-making, attention, intelligence, and voluntary behaviors. It is also responsible for planning and coordinating movements, and language production.
The parietal lobe is located near the back and top of the head. It receives and processes sensory information from across the body, such as touch, temperature, pressure and pain. The parietal lobes also tell us where our body is in relation to the objects around us. This allows us to move around without bumping into things. This function is known as Visuospatial Processing. The parietal lobes are also important for skills such as math, spelling, hand-eye coordination and fine motor movements such as tying shoelaces.
The temporal lobe sits behind the ear and is the second largest lobe. It is primarily responsible for processing auditory stimuli, understanding language, memory and information retrieval, face recognition, object recognition, and perception.
The occipital lobe is the smallest and rear-most of the lobes. It is responsible for interpreting incoming visual information.
The second major structure of the forebrain, the thalamus, is basically the brain’s switchboard or air traffic control system. It takes in information from various sensory systems throughout the body and then directs that information to the appropriate part of the brain. All our senses, except for smell, are routed through the thalamus before being directed to other areas of the brain for processing.
Directly underneath the thalamus is the hypothalamus, which controls several homeostatic processes, including the regulation of body temperature, appetite, and blood pressure. The hypothalamus connects the central nervous system to the endocrine system. It also has a highly significant role in the control of pituitary endocrine function.
The pituitary gland develops from an extension of the hypothalamus downwards. The pituitary is often called the "master gland" because it controls the secretion of hormones. The pituitary gland governs the function of other glands in the body, regulating the flow of hormones from the thyroid, adrenals, ovaries and testicles.
The limbic system is an interconnected group of structures, including the amygdala and hippocampus, that is generally responsible for emotions and memory.
•The amygdala, a small almond shaped structure, processes emotions and, in particular, fear.
A fascinating study comparing brain scans of people in prison for committing murder to brain scans of people of same age and background but not in prison found that this part of the brain was about 18% smaller for those convicted of murder. Another study of brain activity in people who had engaged in quite an extraordinary act of generosity, like donating a kidney to a total stranger, revealed nearly the opposite finding. The donors’ amygdala was found to be 8% larger than it is in most people.
•The hippocampus, another part of the limbic system, is a seahorse shaped structure, involved in forming, organizing, and storing memories. It’s also involved in connecting sensations and emotions to these memories, which is why a particular smell or song can often trigger a specific memory.
The second major part of the brain is the midbrain, and it is comprised of structures located deep within the brain, between the forebrain and the hindbrain.
The reticular formation is centered in the midbrain, but it actually extends up into the forebrain and down into the hindbrain. The reticular formation is important in regulating the sleep/wake cycle, arousal, alertness, and motor activity.
The substantia nigra (Latin for “black substance”) and the ventral tegmental area (VTA) are also located in the midbrain. Both regions contain cell bodies that produce the neurotransmitter dopamine, and both are critical for movement. Degeneration of the substantia nigra and VTA is involved in Parkinson’s disease. In addition, these structures are involved in mood, reward, and addiction.
The hindbrain is the part of the brain directly above the spinal cord. It contains the medulla, pons, and cerebellum.
•The medulla controls the automatic processes of the autonomic nervous system, such as breathing, blood pressure, and heart rate. It also causes reflex actions like vomiting, coughing and sneezing.
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•The word pons literally means “bridge,” and as the name suggests, the pons serves to connect the brain and spinal cord. It also is involved in regulating brain activity during sleep.
•The cerebellum controls fine muscle movement and balance. As we practice and learn complex movements like typing, dancing, piano playing etc., the cerebellum is crucial in fine tuning the movements.
There are 12 pairs of cranial nerves which connect the brain to different parts of the head, neck, and trunk.
Two sets of blood vessels supply blood and oxygen to the brain: the carotid arteries and the vertebral arteries.
The carotid arteries are located in the front of the neck and are what you feel when you take your pulse just under your jaw. The carotid arteries split into the external and internal arteries near the top of the neck with the external carotid arteries supplying blood to the face and the internal carotid arteries going into the skull. Inside the skull, the internal carotid arteries give out several branches that supply blood to the front two-thirds of the brain.
The vertebral arteries follow the spinal column into the skull, where they join together at the brainstem and form the basilar artery, which supplies blood to the posterior one third of the brain.
The basilar artery joins the blood supply of the internal carotid arteries in a ring at the base of the brain. This anastomosis of arteries is called the Circle of Willis.
From the Circle of Willis, major arteries arise and travel to all parts of the brain. The Circle of Willis acts to provide collateral blood flow between the anterior and posterior circulations of the brain, protecting against ischemia in the event of vessel disease or damage in one or more areas.