The central nervous system (CNS) consists of the brain and spinal cord and the peripheral nervous system (PNS) consists of the spinal nerves and ganglia.
The brain occupies the cranial cavity and can be divided into three main parts. The forebrain, midbrain and hindbrain. The midbrain and hindbrain are collectively know as the brain stem and contain the nuclei from which the cranial nerves originate.
The cerebrum is the largest part of the brain and is divided into left and right hemispheres by a longitudinal fissure that runs along the median sagittal plane. Inferiorly the hemispheres are connected together by a band of white matter called the corpus collosum. The outer layer of the cerebrum is composed of grey matter and called the cerebral cortex. It is responsible for the analysis of sensory input, memory, learning and cognitive thought. Each hemisphere is greatly folded forming gyri (folds) and sulci (grooves) which increases the surface area of the cerebral cortex. Although the exact location of the sulci and gyri varies between different individuals, there are a number of large gyri and deep sulci which can be identified as constant landmarks. The main ones have been listed below;
Each hemisphere can be further divided into lobes, their names of which correlate with the surrounding bones that protect them.
The diencephalon consists of two thalami, two hypothalami and a single pineal body.
The thalami are the largest parts of the diencephalon and are located in the centre of the brain in the outer walls of the third ventricle. They are often connected to each other across the third ventricle by a small interthalamic adhesion.
The thalamus receives sensory and motor input as well as influences mood. It receives mostly sensory input including auditory and visual input and relays the signals to the cerebral cortex.
This is a small pine-cone shaped gland projecting from the posterior of the third ventricle by a mid line stalk. Its role is not fully understood but it is thought to be involved in the sleep-wake cycle and the onset of puberty.
The hypothalamus is located at the very bottom of the diencephalon below the thalamus and behind the optic chiasma. It is very important and is often referred to as the 'master gland' as it controls a large number of bodily functions, one of the most important being that of homeostasis. Homeostasis is the maintenance of the bodies physiology, i.e. the maintenance of blood pressure, body temperature, weight and the chemical composition of the body's fluids. Other regulatory roles of the hypothalamus are control of our mood and emotions, autonomic functions, food and water intake, sleep wake cycle and endocrine function.
The smallest part of the brainstem measuring 1.5 cms it consists of the tectum, tegmentum, cerebral peduncles and the substantia nigra. It is responsible for the visual and gustatory response as well as the coordination of movement.
Roof of the midbrain, consisting of four nuclei which form 4 mounds, collectively know as quadrigeminal bodies, on the dorsal surface of the brain stem.
The 2 superior nuclei are called the superior colliculi the 2 inferior nuclei are called the inferior colliculi. The superior colliculi control the visual response and the inferior colliculi control the auditory response.
The midbrain and hindbrain are collectively know as the brain stem. It is the lowest part of the brain and is continuous inferiorly with the cervical spinal cord at the foramen magnum. Its fibres connect the peripheral nervous system (spinal nerves and cranial nerves) to the central nervous system (brain and spinal cord).
The brain stem is extremely important because it contains the nuclei from which most cranial nerves originate as well as the vital centres necessary for survival; breathing, digestion, heart rate, blood pressure and for consciousness (being awake and alert).
The retinacular formation is a series of important nuclei that span the brainstem and receive the majority of the sensory information from the body and the motor signals from the cerebrum. The nuclei also play an integral role in the maintenance of the conscious state.
The hindbrain consists of the pons superiorly, the cerebellum posteriorly and the medulla oblongata inferiorly. The medulla oblongata is continuous inferiorly with the spinal cord.
Located in front of the cerebellum, the pons is only 2.5 cm in length and bulges anteriorly. It consists of descending fibres travelling to the spinal cord and ascending fibres to the cerebellum. It also contains the nuclei of four of the cranial nerves and the respiratory centre which controls expiration.
The medulla oblongata is only 3 cm in length and is the most inferior portion of the brainstem being continuous with the spinal cord inferiorly. It consists of the pyramids and olives and contains ascending and descending nerve tracts, several nuclei and importantly the 'vital centres', which regulate heart rate, respiration and blood vessel diameter. It also contains some non-vital centres involved in swallowing, vomiting, sneezing and coughing.
The cerebellum is the lobe of the brain situated in the posterior cranial fossa. Its surface is folded into folia and consists of two hemispheres connected in the mid line by the vermis. It is separated from the pons and medulla oblongata anteriorly by the fourth ventricle. The cerebellum is responsible for coordination of movement and sends information to the thalamus and cortex.
The brain and spinal cord contain both grey and white matter.
The grey matter can be found in the cerebral cortex, the basal ganglia and the limbic system. It is made up of the cell bodies, dendrites and synapses of the neurons and are grouped into functionally important nuclei.
The white matter is made up of the myelinated fibres (axons) which connect the different parts of the brain to each other as well as to the spinal cord.
The spinal cord is oval in cross section and consists of white and grey matter. The grey matter lies centrally and is arranged into ventral, dorsal and lateral grey horns (anterior and posterior horns). It consists of neurons and neurites, neuroglia and blood vessels. It appears grey because of the abundance of neuronal cell bodies.
The white matter surrounds the grey mater and is white in colour due to the presence of myelin, which insulates the nerve fibres.
Inside the brain are four interconnected cavities filled with cerebral spinal fluid; two lateral ventricles, a single third ventricle and a single fourth ventricle. The two lateral ventricles are the largest ventricles and lie one in each cerebral hemisphere. They are approximately C-shaped (wish bone), each communicating with the thin mid line third ventricle via an intraventricular foramen. The third ventricle communicates inferiorly with the fourth ventricle via the cerebral aqueduct and descends in the mid plane through the midbrain. The fourth ventricle is a small, triangular chamber found between the pons in front and the cerebellum behind. Inferiorly it narrows to form the central canal which descends though the medulla oblongata and spinal cord.
Each ventricle contains a choroid plexus which secretes cerebral spinal fluid (CSF) into the ventricles. The third ventricle contains the thalamus and hypothalamus in its lateral walls and the infundibulum, tuber cinereum and the mammillary bodies in its floor. The corpus collosum forms the roof of the lateral and third ventricles.
The CSF is a clear fluid produced by the choroid plexuses of the ventricles. It circulates within the ventricles as well as in the subarachnoid space between the pia mater and arachnoid mater surrounding the brain and spinal cord. CSF baths the brain and spinal cord in a chemically stable environment and provides it with nutrients. It also allows the brain to be buoyant and protects the brain from jolting into the cranium.
Surrounding the brain and spinal cord are three membranous layers;
Around the brain these three layers are collectively known as the meninges;
The outermost layer, the dura mater is dense and consists of two layers, a periosteal layer and a meningeal layer. The periosteal layer adheres to the internal surface of the cranium and for the majority of its course lies directly touching the meningeal layer. At certain locations the periosteal and meningeal layers are pulled away from each other to create a space, a dural sinus. The sinuses are filled with venous blood from the brain via the cerebral veins. They drain blood into the internal jugular vein.
The meningeal layer folds inwards to form two double thickness sheets which help to hold the brain in place. The falx cerebri is the fold of dura which projects vertically into the longitudinal fissure between the cerebral hemispheres. The tentorum cerebelli is the fold of dura which projects horizontally between the cerebellum below and the cerebrum above.
The middle layer, the arachnoid mater, is thin and transparent and lines the inner surface of the dura mater. It possesses arachnoid trabeculae (granulations) which project into the pia mater and villi which project into the dura mata.
The innermost layer, the pia mater lies directly on the surface of the brain. This layer is very thin and transparent and closely follows all of the gyri and sulci. Between the pia mater and the arachnoid mater is the subarachnoid space in which the CSF is circulated. CSF is returned to the blood via the arachnoid trabeculae (granulations).
The spinal cord meninges are a continuation of the cranial meninges and are connected to the foramen magnum. Like the cranial meninges the fibrous dura mater is the thick outermost layer and is connected posteriorly to the posterior longitudinal ligament. The arachnoid mater lines the inner surface of the dura mater and the pia mater lies directly on the spinal cord itself. The pia mater attaches to the dura mater via the denticulate ligament.
The spinal cord terminates at the level of L2, but the dura continues to the level of S2 creating a cistern into which the the lower spinal roots hang.
The spinal cord is continuous with the medulla oblongata at the foramen magnum, and descends in the vertebral canal. It consists of 31 segments corresponding to the 31 spinal nerves; 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal.
At the level of the second lumbar vertebrae the spinal cord terminates by tapering to a conus medullaris. From the conus medullaris is a long thin filament called the filum terminale. The vertebral canal below the second lumbar vertebra is filled with the nerve roots from the lumbosacral spine; this bunch of nerve roots resembles a horses tail and so is known as the cauda equina.
Along its course the spinal cord has two enlargements, the cervical enlargement and the lumbosacral enlargement, in the cervical and lumbar regions respectively. These swellings are due to the large spinal nerves which emerge from these parts of the cord to supply the upper and lower limbs.
The spinal cord is made up of a column of grey matter (contains cell bodies) surrounded by a cylinder of white matter (myelinated neurons). The neurons of the grey matter are arranged into ventral, dorsal and lateral horns. The fibres of the white matter travel longitudinally along the spinal cord in designated columns.