Central nervous system
The central nervous system (CNS) is a division of the nervous system whose function is to analyze and integrate various intra- and extrapersonal information, as well as to generate a coordinated response to these stimuli. Put simply, the CNS is the supreme command center of the body.
The CNS consists of two organs which are continuous with each other; the brain and spinal cord. They are enveloped and protected by three layers of meninges, and encased within two bony structures; the skull and vertebral column, respectively. The brain consists of the cerebrum, subcortical structures, brainstem and cerebellum. The spinal cord continues inferiorly from the brainstem and extends through the vertebral canal.
While analyzing the information and preparing adequate body responses, the parts of the brain and spinal cord communicate with each other via many neural pathways. Once the final output is ready, they convey it to the rest of the body via nerves of the peripheral nervous system (PNS), which stem directly from them. More specifically, the brain gives off 12 cranial nerves that innervate the head, neck and thoracic and abdominal viscera, while the spinal cord gives off 31 pairs of spinal nerves. The spinal nerves complement the innervation of the viscera, as well as the remainder of the body that is not supplied by the cranial nerves (upper and lower limbs).
This article will introduce you to the anatomy and functions of the central nervous system.
|Definition and function||The supreme command center of the body that receives, integrates, processes, and generates neural impulses that control all the bodily functions|
- Grey and white matter
- Cortex and brain lobes
- Subcortical structures
- Spinal cord
- Brain ventricles and cerebrospinal fluid (CSF)
- Neural pathways and spinal cord tracts
- Clinical notes
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 our brain 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.
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.
The diencephalon represents the posterior part of the cerebrum. It is a collection of four cerebral structures located on each side of the midline, bilaterally to the third ventricle of the brain. These include:
- Thalamus, which is an ovoid nuclear mass that consists of four groups of nuclei: anterior, medial, lateral and intralaminar nuclei of thalamus. The thalamus relays sensory and motor information to and from the cerebral cortex.
- Epithalamus, which represents the most posterior part of the diencephalon. It consists of the pineal body, stria medullaris and habenular trigone. The epithalamus is involved in the control of the sleep-wake cycle (circadian rhythm) and movement initiation and control.
- Subthalamus, which is located ventral to the thalamus. It comprises the subthalamic nucleus, zona incerta (of Forel) and peripeduncular nucleus. The role of the subthalamus is in the control, integration and accuracy of motor activity.
- Hypothalamus, which is positioned inferoanterior to the thalamus. It is divided into three groups of nuclei (anterior, middle, posterior) and three zones (periventricular, medial, lateral). The hypothalamus regulates the stress response, metabolism and reproduction through various hypothalamic axes and the hypophyseal portal system.
More information about the diencephalon is provided below.
The basal ganglia are a group of nuclear masses of grey matter spread out throughout the lower part of cerebrum, diencephalon and midbrain. These nuclei include the caudate nucleus, putamen, globus pallidus, substantia nigra and subthalamic nucleus. The caudate nucleus and putamen collectively form the striatum. The putamen and globus pallidus form the lentiform nucleus.
To master the complex anatomy of the basal ganglia, take a look below:
The limbic system is a set of structures spanning the cerebrum, subcortex and brainstem. It consists of two divisions called the limbic cortex and deep limbic structures. The limbic cortex is formed by various cerebral sulci and gyri of the frontal, temporal and parietal lobes. The deep part of the limbic system is formed by the hippocampal formation, amygdala, diencephalon, olfactory cortex, basal ganglia, basal part of the cerebrum and brainstem.
The overall function of the limbic system is to control emotions, olfaction and homeostasis. One particular important structure of the limbic system is the hippocampal formation involved in long term memory and spatial navigation. In addition, the amygdala is the one responding to fear.
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 all twelve 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.
The midbrain is the most superior part of the brainstem. It is located between the thalamus superiorly and the pons inferiorly. The midbrain is involved in visual and auditory reflexes, alertness and temperature control. It is supplied by the pontine branches of basilar artery.
The anterior aspect of the midbrain contains several important structures; two cerebral peduncles, interpeduncular fossa, red nucleus, optic tracts and oculomotor nerve (CN III). In turn, the posterior aspect of the midbrain exhibits four prominences called colliculi (superior, inferior). These colliculi collectively form the quadrigeminal plate (corpora quadrigemina). The trochlear nerve (CN IV) also emerges from the posterior aspect.
The pons is the middle component of the brainstem, situated between the midbrain and medulla oblongata. The pons is involved in various functions like sleep, hearing, swallowing, taste, respiration, equilibrium and motor actions. Its blood supply is from the pontine branches of basilar artery.
The anterior aspect of the pons shows several important landmarks; the pontine sulci (superior, inferior), basilar groove, as well as the striations created by the fibers of the corticopontocerebellar tract. Four pairs of cranial nerves emerge from the anterior aspect of the pons: trigeminal (CN V), abducens (CN IV), facial (CN VII) and vestibulocochlear (CN VIII) nerves. On the opposite side, the posterior aspect of the pons contains the superior half of the rhomboid fossa, median eminence, posterior median sulcus, facial colliculus, striae medullares, locus coeruleus and vestibular areas.
The medulla oblongata is the most inferior part of the brainstem. It regulates autonomic activities related to cardiac, respiratory, reflex, and vasomotor functions. The medulla oblongata is supplied by the inferior cerebellar arteries and anterior spinal artery.
The anterior aspect of medulla oblongata contains various protuberances and emerging cranial nerves. These include the anterior median fissure, pyramids, olives, hypoglossal (CN XII), glossopharyngeal (CN IX) and vagus nerves (CN X). The posterior surface of medulla oblongata is also marked with many important anatomical structures, such as; posterior medial sulcus, cuneate fasciculus, gracile fasciculus, cuneate tubercle, gracile tubercle, trigeminal tubercle, lateral funiculus, inferior half of the rhomboid fossa and obex.
Don’t feel overwhelmed by the anatomy of the brainstem. Learn more about its concepts using the following study unit.
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 the superior and inferior 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).
Learn more about the cerebellum using videos, quizzes, illustrations and articles provided in the following study unit.
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:
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) attaches to the walls of the vertebral canal. The layers of the cranial dura diverge from each other in several sites, forming a space that contains the dural venous sinuses. There is a potential space between the dura mater and overlying bone called the epidural space.
- 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 cisterna magna.
- Median aperture (of Magendie) between the fourth ventricle and central canal of the spinal cord
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 (course 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:
A stroke is a vascular event disrupting blood flow to the brain. This may be ischaemic (blockage by an embolus), or haemmorhagic (a bleed in the cerebral arteries. A stroke affecting the blood vessel on the right side of the brain will cause hemiplegia on the left side of the body and vice-versa. This is due to the decussation of the corticospinal tract.
Degradation of the substantia nigra means that dopamine is not produced as much. This results in an inability of the dopamine to ‘disinhibit’ the inhibition of basal ganglia; hence a greater amount of effort is required to perform a movement. This results in:
- slow movements that are difficult to initiate
- masked facies
- shuffling gait
- lack of emotion
If Broca’s area in the inferior frontal gyrus of the brain is damaged by a stroke or otherwise, forming speech becomes difficult. The person will understand speech but will find it impossible to form it.
Sub-acute combined degeneration of the cord
This is a condition caused by vitamin B12 deficiency. Symptoms include gradually worsening weakness and numbness in the legs, arms and trunk, as well as visual and mental state changes in the late stage of the disease.