Written by Maired L. Baguinaon
Reviewed by Dr. Reuben J C. los Baños, Ph.D.
One organ stands out in the human body – it is the brain. Now, let us talk about your brain. Your brain is like a working supercomputer inside your head. It is the source of your creativity. It guards your identity by shaping your personality and guiding decisions.
The nervous system comprises the brain, spinal cord, and complex network of nerves. The nervous system has two parts. The central part includes the brain and spinal cord within the meninges. The other part is the peripheral system. It contains the nerves that run throughout the body.
The nervous system’s main job is to send messages between the brain and spinal cord to the whole body. It does this using billions of tiny cells called neurons. They work together to create a communication system. There are three main types of neurons. Sensory neurons handle sensation. Motor neurons connect the brain to the rest of your muscles and allow you to move. Receptor neurons sense the environment and convert it into energy. Interneurons connect to your other neurons.
The brain is the most complex organ in the human body. It is the command center of the central nervous system. It consumes twenty percent of the oxygen you breathe. Your brain weighs 1.4 kilograms and contributes about 2 percent of your body weight.
It comprises four lobes: temporal, parietal, occipital, and frontal. The temporal lobe handles processing sensory information and inputs emotional meaning. Some aspects of language perception live in it and play a role in long-term memory. The parietal lobe also handles your sensory data. This includes navigation, touch, and space sense. The occipital lobe houses the visual cortex of your brain. The frontal lobe houses most dopamine-sensitive neurons. They play a role in short-term memory, attention, motivation, and planning.
The frontal lobe is the largest in the cortex. It contains the precentral gyrus and the superior, middle, and inferior frontal gyri. The precentral gyrus contains the main motor cortex. It handles combining signals from the brain’s regions. This helps with voluntary movement. The front part of the precentral sulcus holds the superior frontal gyrus. The middle frontal gyrus lies between the superior and inferior sulci. The inferior frontal gyrus separates from the middle frontal gyrus. It is divided into three parts: pars opercularis, pars triangularis, and pars orbitalis.
Did you know that your brain also has four ventricles? These ventricles make space for your cerebrospinal fluid or CSF. The CSF is a fluid that flows around your brain and spinal cord. It functions to protect your brain from shocks, bring nutrients, and remove waste.
What is the histological classification of neurons?
Neurons are the brain’s messaging system. They are in charge of processing and sending information to where it needs to go in your body.
Each neuron has three main parts: the cell body, dendrites, and axon. The cell body is where all the action happens. Dendrites branch out from the cell body. They act like antennas, grabbing messages from other neurons. Then, the axon carries outgoing messages to other neurons.
Neurons in your body can communicate with each other through the neurotransmitter. Neurotransmitters are tiny molecules. They act as messengers and carry signals between neurons. One can classify it as an amino acid.
The brain has an outer cortex of neurons. It processes and interprets information. It also has an inner white matter with myelin-wrapped axons. This white matter transmits information to other parts of your nervous system.
There are four types of neurons. The number of processes from the soma decides how researchers classify neurons. The four types are multipolar, bipolar, unipolar, and pseudounipolar. The multipolar cells have a single axon. It extends from one cell body and has dendrites that branch from the other side of the cell body. This cell is the most dominant and it appears as fusiform or polygonal. The bipolar cells also have a single axon and dendrite, one from each side. This cell can join afferent impulses. It is in the hearing, olfactory, and ocular systems. The unipolar cells have one axon that projects from the spherical body.
Other cell membrane regions have no dendrites. Your peripheral nerves and sensory ganglia contain them. Scientists use the term pseudounipolar cells to describe unipolar cells. These cells are in the dorsal root ganglia. Joint position and proprioception are linked to them.
Glial cells, called neuroglia, are in the central and peripheral nervous systems. These types of cells support neurons. There are four types of cells. They are astrocytes, oligodendrocytes, ependymal, and radial glia. Astrocytes anchor neurons to their blood supply. They have many projections. These include regulating their local environment. Oligodendrocytes create myelin sheaths. The sheaths allow them to send signals. Ependymal cells are responsible for the creation and secretion of cerebrospinal fluid. Radial glia scaffolds new nerve cells in an embryo’s creation of a nervous system.
Where are cell bodies of neurons found in the brain and spinal cord?
In the brain and spinal cord, the cell bodies of neurons gather into clusters. These clusters are in the central nervous system (CNS). These clusters are known as nuclei. They can also be arranged into layers called laminae. This happens especially in structures like the cerebral cortex.
In the brain, nuclei are clusters of cell bodies located deep within the brain tissue. They play various roles in processing and relaying information. In the spinal cord, cell bodies are in groups called dorsal root ganglia and ventral root ganglia. They are next to the spinal cord.
The placement of cell bodies in the brain and spinal cord reflects different functions. It also helps with the complex processing. It also helps with sending information to the nervous system.
Your cell body is sometimes called soma or perikaryon. It provides space for your nucleus and other organelles. They are in the cytoplasmic membrane, like in non-neuronal cell bodies.
Cell bodies have many dendrites. The dendrites connect with other neurons. This lets the cells receive signals and process information. Dendritic spines cover these dendrites. The dendritic spines connect with the axon and form synapses. Specialized areas called synapses are where an action potential begins.
Your cell bodies produce a long, single projection known as an axon. The axon hillock connects to your cell body. It adds membrane potentials together before sending them to the axon. This happens in the last part of the cell body. The axon may have myelin sheaths or remain unmyelinated.
Myelinated fibers make up the brain’s white matter. Unmyelinated fibers and cell bodies make up the gray matter. Schwann cells make myelin. They do this in the peripheral nervous system. Its cytoplasm, nucleus, and outer membrane cover the axon at the Nodes of Ranvier. Unmyelinated regions exist between myelin segments.
How many neurons are in the brain and spinal cord?
A study published in the Proceedings of the National Academy of Sciences in 2012 found that there are 86 billion neurons in the human brain. Neurons do not generate new copies of themselves, unlike other cells, once they form around the time of birth. So, if a neuron dies, the body cannot replace it.
The spinal cord comprises fewer neurons than that of your brain. Estimates suggest the spinal cord has millions of neurons. These include sensory, motor, and interneurons.
What are the histologic layers of the cerebrum?
You know the front part of your brain as the cerebrum. It is the biggest part of your brain. It starts and coordinates movements and regulates temperature.
The cerebral cortex, also known as the gray matter, coats the outer layer of the cerebrum. The cerebral medulla, also known as the white matter, coats the inner layer. Nerve cells, fiber, blood, and glial cells make up the cerebral cortex.
The cerebral cortex consists of six types of neuronal cells. The main output neurons in the cerebral cortex are the pyramidal cells. Fusiform cells follow them. Then, there are granular (stellate) cells and Cajal-Retzius’s horizontal cells. Also, there are basket cells and the cells of Martinotti. There are also six layers of cerebral cortex based on neuronal bodies’ size and shape. The order of naming is from shallow to deep and Roman numerals mark them.
The molecular layer (I) consists of nerve axons and some Cajal-Retzius cells. The external granular layer (II) has varying densities of granular and pyramidal cells. The external pyramidal layer (III) consists of varying sizes of pyramidal cells. The internal granular layer (IV) is the narrowest. It has granular cells and some pyramidal cells. Layer V is internal. It has pyramidal cells. They range from medium to large. Layer VI is multiform. It has different types of neurons and fusiform cells. It also has some interneurons and pyramidal cells.
The cerebral medulla, or the white matter consists of fibers that pass in all directions. This includes the association fiber, commissural fibers, and projection fibers. It forms from the axon received from a superficial neuron. The neuron is in the molecular layer. The commissural fibers form from axons. The axons come from the cortex’s deepest neurons. The projection fibers form in the white matter. They come from the axon sent to the brainstem by the cerebral cortex’s two deepest layers.
What is the histological structure of the cerebral hemisphere?
The brain has a cortex and white matter. It also has the basal ganglia, limbic system, and the ventricles.
The cerebral cortex is the outer part of your cerebral hemisphere. Gray matter consists of cell bodies, dendrites, and axons. Its large surface is due to its folds called sulci. Gyri are the ridges that cover the cortex.
Underneath the cerebral cortex is the white matter. It consists of a myelinated axon. Axons form bundles called tracts. They connect areas of the cortex and to other parts of the brain and spinal cord.
The cerebrum divides into two hemispheres: the right and left hemispheres. The right hemisphere controls your body’s left side. The left hemisphere controls the right side of your body. The interhemispheric fissure connects these hemispheres. It runs from the front to the back of the head.
Deep within the cerebral hemisphere are structures known as the basal ganglia. They help regulate movement and contribute to various cognitive functions.
Another structure is deep in the cerebral hemisphere: the limbic system. It holds the hippocampus, amygdala, and hypothalamus.
There are also ventricles found in your cerebral hemispheres. These ventricles produce CSF that protects your brain and spinal cord.
What is the histological structure of the cerebellum?
The hindbrain has a main structure. It is the cerebellum, also called the little brain. It is rich in neurons containing 80% of your brain’s neurons. It looks like a cauliflower. This occurs because grey matter coats the white matter and contains a stem. It is behind the pons and medulla.
It is under the occipital and temporal lobes of the brain. Its major function is the transmission of sensory signals to the motor part of your brain. It also controls your body’s motor function. It does this by coordinating your muscles and balance.
The cerebellum contains an outer layer known as the cerebellar cortex. It consists of folded gray matter arranged in a series of folia (folds). Folia are leaflike gyri.
When you slice the cerebellum and view it under the microscope, you can see its lobes and folds. If stained, the pia matter is also likely to appear around the cerebellum.
The grey matter has three layers. It has an outer molecular layer. In the middle are Purkinje cells. Inside is the granular layer.
The outer molecular layers contain many axons and dendrites. The middle layer contains a single Purkinje cell. Their cell bodies are the largest and have a pear-shaped structure. The inner granular layer contains Golgi type II cells. Granule cells pack them. Scientists know that granule cells are the smallest neurons in your brain. Its axons branch in a T shape. They form parallel fibers. They connect with Purkinje’s dendrites, stellate cells, and basket cells.
Encompassing the cerebellar cortex is a layer of white matter comprising myelinated axons. These axons link many areas in the cerebellum and other brain regions. They help send signals and process information.
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