The central nervous system consists of the brain and spinal cord. Nerve fibers emerging from the brain and spinal cord and nerve nodes in other parts of the body form the peripheral nervous system. The peripheral nervous system is divided into the somatic system, which carries conscious movement and sensory signals, and the autonomic nervous system, which regulates involuntary activities such as heartbeat, respiration, digestion, sweating, and blood pressure.
Anatomically, the brain consists of 3 main parts: the cerebrum (the largest, main part of the brain), the brain stem and the cerebellum (cerebellum). The brain stem is located between the cerebrum and the spinal cord; The medulla is divided into three parts, the pons and the midbrain (mesencephalon). The cerebellum is located behind the brain stem, below the cerebrum.
The cerebrum is the largest component of the brain. It consists of the right and left hemispheres. A structure called the corpus callosum connects the two hemispheres. The corpus callosum consists of axon fibers that provide signal transmission of nerve cells.
Each hemisphere of the brain is divided into 4 subdivisions. These are called lobes. The frontal lobe, parietal lobe, temporal lobe, and occipital lobe are anatomically defined subregions of the cerebrum. The inner part of the temporal lobe is also called the limbic lobe by some scientists.
The frontal lobe forms the front part of the brain. It is separated from the posterior parietal lobe by a slit called the central sulcus. The parietal and frontal lobes are separated from the lower temporal lobe by the lateral sulcus. The parieto-occipital sulcus forms the border between the parietal lobe and the posterior occipital lobe.
When examining the cerebrum, it is divided into telencephalon and diencephalon. The telencephalon contains the cerebral cortex, subcortical fibers, and basal nucleus. The diencephalon consists mainly of the thalamus and hypothalamus. The most obvious difference between the brains of humans and other mammals is that the telencephalon part of the cerebrum in humans is much more developed.
Cortex and Subcortical Fibers
The outermost part of the cerebrum is the cortex or, in Turkish, the cerebral cortex. Because the cerebral cortex is gray in color, it is also called “gray matter” or “gray matter”. In the nervous system, areas where the bodies of nerve cells called neurons are concentrated appear gray. The brain shell has a curved structure like a walnut. The folds are called the “gyrus”, while the slits between the folds are called the “sulcus”.
The areas where the cable-like axons of neurons are concentrated in signal transmission are white in color. The reason for this is the myelin sheath, which mostly consists of fat and provides insulation in the axons. Areas of the brain where axons are found are called “white matter” or “white matter”. White matter is located deep in the cerebral cortex.
The limbic system is a system that consists of a part of the cerebral cortex and some subcortical structures, responsible for memory formation and emotions. The limbic system allows for complex interactions between the cortex, thalamus, hypothalamus, and brainstem. Anatomically, the boundaries are not very precise, but certain functionally differentiated structures are included in the limbic system. Major structures in the limbic system are the amygdala, hippocampus, fornix, mammillary bodies, cingulate gyrus, and parahippocampal gyrus.
Functional connections in the limbic system are summarized by the Papez circuit. Signals from the hippocampus reach the mammillary bodies via the fornix and the anterior nucleus of the thalamus via the mamillothalamic route. The thalamocingulate projection completes the circuit by transmitting signals to the cingulate gyrus and hippocampus. The hippocampus is the primary output structure in the limbic system. The hippocampus is critical in the formation of memory, defined as declarative or explicit memory, such as what was eaten for breakfast or where the car was parked. In time, certain declarative memories of the distant past may also be recalled independently of the hippocampus. The hippocampus is involved in the formation of long-term memory in the cortex.
While the amygdala receives various sensory signals, the output signals reach the hypothalamus, thalamus, hippocampus, brain stem, and cortex. It is a structure that plays a role in the emotional aspects of memory and fear responses.
The basal ganglia are composed of the caudate nucleus, putamen, globus pallidus, subthalamic nuclei, and substantia nigra. Different groupings of these structures may have different names. Putamen and globus pallidus together form the lentiform nucleus. The putamen and caudate nucleus form the striatum. The basal nuclei are integrated with the motor cortex, premotor cortex, and motor nucleus of the thalamus. They take part in the regulation of movements. Parkinson’s disease is associated with a decrease in dopaminergic cells in the substantia nigra. In Huntington’s disease , degeneration of the striatum is seen.
Between the telencephalon and the brain stem is the diencephalon. The diencephalon consists of the thalamus, epithalamus, subthalamus, and hypothalamus. The thalamus functions as a relay station for other senses other than smell before they reach the cortex. The hypothalamus is the regulator of endocrine, autonomic and homeostatic functions.
The brain stem is divided into three parts: the medulla oblongata, the pons, and the mesencephalon. Many descending and ascending nerve pathways pass through here, providing communication between the spinal cord and the brain. There are also cranial nerve nuclei. Cranial nerves are 12 pairs in total, they are responsible for the sensation of the head and neck region, the control of the muscles and their autonomic functions.
Behind the pons and medulla oblongata, the cerebellum is located in a space called the posterior fossa. It makes movements fluent and coordinated. The surface of the cerebellum also contains ridges (folia) and pits (fissures). It has two hemispheres. They are connected by a structure called vermis in the middle.
The brain and spinal cord are surrounded by 3 layers of membranes. These are the pia mater, arachnoid and dura mater from the inside out. Pia mater follows all the indentations and protrusions of the brain, it is rich in blood vessels. The arachnoid mater is a web-like layer. Between the arachnoid and the pia mater is the subarachnoid space. This place is filled with cerebrospinal fluid (CSF). The main vessels feeding the brain are located in this space. Aneurysm rupture (blood bubble burst in the brain) causes bleeding into the subarachnoid space . The outermost membrane is the dura mater. Bleeding between dura and arachnoid is called subdural hematoma, bleeding between dura and skull is called epidural hematoma.
Ventricles and Cerebrospinal Fluid (CSF)
There is CSF all around the brain and spinal cord, that is, the central nervous system almost floats in CSF. There are also spaces in the brain that contain CSF, these are called ventricles. There are four ventricles: the lateral ventricles within the hemispheres, the third ventricle connecting them, and the fourth ventricle, which is located further below in front of the cerebellum. CSF is produced in structures called the choroid plexus inside the ventricles. CSF passes from the fourth ventricle into the subarachnoid space around the brain and spinal cord. From there it is reabsorbed into the bloodstream (venous system). There is 150 cc of CSF in the body at any given time. Within 24 hours, 450 cc of CSF is produced and reabsorbed.
The main vessels feeding the brain are the carotid arteries (carotid artery) running through the front of the neck, and the vertebral arteries running more posteriorly and deeper. The carotid arteries give off the anterior (anterior) cerebral artery and the middle cerebral artery terminal branches. The vertebral arteries unite first to form the basilar artery. The basilar artery forms the posterior cerebral artery and the superior cerebellar arteries. The basilar artery, posterior cerebral arteries, posterior communicating arteries, anterior cerebral arteries and anterior communicating artery form the vascular structure called the polygon of Willis at the base of the brain. This region is a common site for cerebral aneurysms.