Central nervous system (2024)

Grey and white matter

The chief cells of the brain and spinal cord are the neurons, which receive and transmit neural impulses. Each neuron has a body which is its micro-command center, and it has a gray color when observed microscopically. The neurons possess two or more neural processes that stem from the body and relay the neural information. The neural processes are divided into short (dendrites) and long (axons). Most of the axons are ensheathed by a substance called myelin, which gives them a distinguishable white color.

The parts of neurons comprise what we call gray and white matter. The gray matter is made up of clusters of neuronal bodies, while the white matter is composed of their myelinated axons. The axons are not just randomly interwoven through the neural tissue, but rather are organized into bundles that connect certain parts of gray matter and carry the relevant impulses. In the CNS, these bundles are called pathways and tracts, while in the PNS they comprise nerves.

The distribution of gray and white matter is highly specific throughout the brain and spinal cord;

• In the brain, the majority of gray matter is found superficially comprising the cerebral cortex, while the white matter composes its inner part. Smaller clusters of gray matter are found deep within the white matter comprising the subcortical structures, such as basal ganglia and diencephalon. Every unit of gray matter in the brain which is outside of the cortex is called the nucleus (plural - nuclei).
• In the spinal cord, the gray matter comprises its inner part and has a characteristic butterfly shape when observed on cross-section. The white matter, consisting of spinal cord pathways, is located externally surrounding the gray matter

Overwhelmed by the task of learning all the parts of the brain? Help yourself with ourbrain anatomy quizzes and diagrams!

Cortex and brain lobes

The cerebrum, or forebrain, is the most prominent part of the brain. It consists of two cerebral hemispheres interconnected by the corpus callosum. The surface of the cerebrum is highly irregular, being composed of sulci (ridges) and gyri (folds). The sulci and gyri increase the surface area of the cerebrum, providing it with the highest processing power and cognitive ability in the entire nervous system. Each hemisphere is composed of five regions called cerebral lobes: frontal, parietal, temporal, occipital and insular.

Each brain lobe carries a specific set of functions. This is why the entire cortex is mapped and divided into functional areas, such as motor cortex and sensory cortex. These areas of the cortex are further divided based on their level of function into primary, secondary and associative areas. These hierarchical subdivisions closely communicate with each other in order to process information and generate adequate body responses, either motor or sensory. Each lobe is outlined by specific sulci and is responsible for distinct functions. The most prominent sulcus is the central sulcus (of Rolando) which separates the precentral and postcentral gyri. They house the primary motor and primary somatosensory cortices, respectively. These are the regions where motor functions are initiated and sensations are detected. The grey matter surrounds the white matter of the cerebrum, which forms the bulk of the deep cerebral structures.

For more details about the brain lobes and cerebrum, take a look below.

Subcortical structures

The subcortical structures are a neural group of structures embedded deep within the brain. They include the diencephalon, basal ganglia, limbic system and pituitary gland.

Brainstem

The brainstem is the inferior-most part of the brain. It sits in the posterior cranial fossa and consists of three parts: midbrain, pons and medulla oblongata. Internally, it is divided into the basal area, tegmentum and tectum. The brainstem has three main important functions;

• It contains the nuclei of the majority of the cranial nerves.
• Facilitates the trajectory of neural pathways traveling between the spinal cord and brain.
• Regulates vegetative functions such as heart rate, blood pressure, breathing and gastrointestinal functions.

Cerebellum

The cerebellum is located in the posterior cranial fossa, posterior to the brainstem and fourth ventricle. It is separated from the cerebrum by the tentorium cerebelli. In addition, the cerebellum is anchored to and communicates with the brainstem via the superior, middle and inferior cerebellar peduncles. The functions of the cerebellum include coordination and precision of motor activities, as well as motor learning. Blood supply to the cerebellum is derived from theposterior inferior cerebellar (PICA), superior (SCA), and anterior inferior (AICA) cerebellar arteries.

The cerebellum consists of three parts; a middle vermis flanked by two hemispheres. The superior (tentorial) surface points superiorly and the inferior (occipital) surface faces inferiorly. The cerebellum is divided horizontally into three lobes (anterior, posterior, flocculonodular) and approximately ten lobules (I-X).

Test your knowledge on the cerebellum with this quiz.

Learn more about the cerebellum using videos, quizzes, illustrations and articles provided in the following study unit.

Spinal cord

The spinal cord continues inferiorly from the medulla oblongata. It extends from the foramen magnum of the skull to the level of the L1/L2 vertebrae. The spinal cord consists of five segments (cervical, thoracic, lumbar, sacral, coccygeal) and a total of 31 pairs of spinal nerves emerging out of them.

The spinal cord has four surfaces, one anterior fissure and three sulci. Internally, it consists of a central grey matter surrounded by white matter. Blood supply to the spinal cord originates from the vertebral and segmental arteries.

The function of the spinal cord is to convey information between the brain and the rest of the body. In addition, the spinal cord regulates lower bodily functions independently from the brain, such as reflexes.

The anatomy of the spinal cord is described in more detail below:

Meninges

The meninges represent three membranes that envelop the brain and spinal cord. The meninges of the brain are referred to as the cranial meninges, while the meninges of the spinal cord are called the spinal meninges. The cranial and spinal meninges are continuous with each other through the foramen magnum. The function of the meninges is to protect the CNS, support blood vessels and hold the cerebrospinal fluid (CSF). From superficial to deep, the meninges include:

• Dura mater - it is a two-layered sheath consisting of the periosteal and meningeal dura mater. Dura mater of the brain is firmly attached to the skull by its periosteal layer. The spinal dura (theca) envelops the spinal cord and is separated from the vetebral column by the epidural space. The layers of the cranial dura diverge from each other in several sites, forming a space that contains the dural venous sinuses.
• Arachnoid mater - the cranial and spinal arachnoid lie underneath the dura mater. The space between the dura and the arachnoid is called the subdural space.
• Pia mater - the cranial and spinal pia mater tightly adhere to the surfaces of the brain and spinal cord, respectively. Pia mater is highly vascular, containing numerous blood vessels that nourish the surfaces of the CNS. The cranial pia mater envelops the whole brain, and is inferiorly continued by the spinal pia. The spinal pia overlies the spinal cord and terminates as the filum terminale past the S2 vertebra. The space between the arachnoid and pia mater is called the subarachnoid space. It contains the CSF and superficial blood vessels of the brain and spinal cord. Moreover, it shows mushroom-like protrusions through the overlying dura mater called the arachnoid granulations, whose purpose is to enable the main route for CSF elimination.

Take a look at the following study units to simplify the anatomy of the cranial and spinal meninges:

Brain ventricles and cerebrospinal fluid (CSF)

The cerebrospinal fluid (CSF) is a colorless fluid that bathes the brain and spinal cord. It is produced by the specialized tissue called the choroid plexus which is found within the walls of the brain ventricles. The CSF circulates sequentially through the ventricles and subarachnoid cisterns to finally be reabsorbed into the venous system through the subarachnoid granulations. The function of the CSF is to absorb mechanical force, protect the CNS and supply nutrients to it.

The brain ventricles are the CSF filled cavities embedded deep within the brain parenchyma.Within the brain, there are four ventricles: the two lateral ventricles within the lobes of the cerebrum, a single third ventricle in the midline between the hemispheres, and a fourth ventricle situated posterior to the brainstem.

The ventricles communicate via five foramina, which ensure that the CSF can circulate through the ventricular system:

• Interventricular foramen (of Monro) between the lateral ventricles and the third ventricle.
• Cerebral aqueduct (of Sylvius) between third and fourth ventricles.
• Two lateral apertures (of Luschka) between the fourth ventricle and cerebellopontine cistern.
• Median aperture (of Magendie) between the fourth ventricle and cisterna magna of the subarachnoid space

Learn more about the cerebrospinal fluid and ventricular system by exploring the following study unit:

Neural pathways and spinal cord tracts

The neural pathways are the bundles of axons that interconnect different neurons. The pathways can be found exclusively within the brain connecting its different parts, or they can connect the brain and the spinal cord.

The pathways within the brain are referred to as the tracts. The purpose of tracts is to hand over the information between the various parts of the brain during the process of analyzing and integrating some stimulus. The tracts are usually termed according to the structures that they connect. For example, a tract that connects the cerebral cortex with the cranial nerve nuclei is called the corticonuclear tract.

Looking for ways on how to learn and understand the CNS anatomy faster? Try out our 8 steps to create your own anatomy flashcards!

Neural pathways that connect the brain and the spinal cord are called the ascending (sensory) and descending (motor) tracts. They convey sensory and motor information between the CNS and the periphery. Spinal cord tracts travel within the white matter of the spinal cord. They consist of three order neurons which sequentially synapse one onto another. Spinal cord tracts are classified as follows:

• Ascending tracts, consisting of dorsal column (fine touch, proprioception), spinothalamic (crude touch, pressure, pain, temperature), spinocerebellar (proprioception, coordination, posture), spino-tectal (spinovisual reflex), spino-reticular (consciousness) and spino-olivary (cutaneous, proprioception) pathways.
• Descending tracts, including the corticospinal (voluntary movements), corticobulbar (influences activity of cranial nerves), reticulospinal (facilitation or inhibition of voluntary and reflex actions), tectospinal (auditory and visual reflexes), rubrospinal (fine involuntary movement) and vestibulospinal (balance) pathways.

As you can see, there are many convoluted neural pathways and spinal cord tracts. Clarify their anatomy using the resources provided below: