Organisation Of The Nervous System

Organisation of the nervous system  There are two major divisions: o Central nervous system (CNS). o Peripheral nervous system (PNS). Caudal Rostra l Rostra l Caudal    Nerve cells and their processes are either afferent or efferent, making up the somatic nervous system. Afferent division subdivided into three main categories: o Somatic afferents. o Special afferents. o Visceral afferents. Efferent division subdivided into two main categories: o Somatic efferents. o Autonomic system – divided into sympathetic and parasympathetic. Brain structure and size  The relative size of a section of a brain is dependent on function. o Opossum and coyote have a relatively large olfactory lobe compared to other animals, such as monkeys and chimpanzees due to their reliance on smell.  Brain size relative to body weight is important.  Another important aspect is the way evolution has organised the brain structure.  Humans have similarities in brain structure, however there are some variations in terms of size and shape. Organisation of the brain  There are 4 major lobes: frontal lobe, parietal lobe, occipital lobe and temporal lobe.  In a medial view, there is a limbic lobe in the center.   Line through parietoccipital sulcus to occipital notch marks the junction between parietal lobe and occipital lobe. Line through lateral sulcus (Sylvian fissure) to the occipital border separates parietal lobe from temporal lobe. Meninges  A series of structures that separate the CNS and bony structures surrounding it.  Dura mater. o Most exterior of the meningeal layers. o Toughest and hardest layer. o It has two layers that are not discernable. o Outer layer is the periosteal layer, which is in contact with the inside of the skull. o Inside layer is the meningeal layer.  Arachnoid. o Directly opposed to the meningeal layer of the dura.  Pia mater. o Surrounds the cortical tissue itself. o Very fine and tender layer. o Follows all the convolutions (sulci and gyri) of the brain.  Subarachnoid space. o In between the inner arachnoid layer and the pia mater. o Occupied by blood vessels and a trabecular system (arachnoid trabeculae). o The arachnoid trabeculae projects from the arachnoid to the pia mater, and is a meshwork of connective tissue. Sinuses  Receive blood from internal and external veins of the brain; receive CSF from the subarachnoid space and ultimately empty into the internal jugular vein.  Formed by splitting of the dural layers (between periosteal and meningeal dura) in certain areas. Folds  Arched folds of the dura mater.  Falx cerebri. o Sickle-shaped fold. o Runs down the middle, in between the two hemispheres.  Tentorium cerebelli. o Sits on the base of the occipital lobe. o Separates the cerebrum and the cerebellum. Ventricles       Large spaces in the brain. Acts as an area for CSF production and storage. Extensive coverage of the brain, running from the forward region to the posterior horn, and also branching off into lateral regions. There are two lateral ventricles, one on each hemisphere. These are connected to the 3rd ventricle via the interventricular foramen. o 3rd ventricle sits between the two regions of the diencephalon. From the 3rd ventricle, there is an aqueduct that runs to the 4th ventricle. o 4th ventricle spans the space between the cerebellum and the brainstem. Cerebrospinal fluid (CSF)  Once thought to be an ultrafiltrate of blood, but is now understood to be more elaborate.  The rate of production is ~500mL/day.  At any one time within the brain and spinal cord, there is 200mL of CSF  therefore, turnover of 2 – 3 times a day.  Ventricles store 25mL of CSF.  Majority is made by the choroid plexus, a specialised region in the ventricles. o Two components: choroid capillary and choroid epithelium. o When blood runs through the blood vessels of the subarachnoid space, it passes through choroid plexus. o Here, the pia mater alters its organisation, allowing for a greater space in between successive cells, which allow diffusion of substances from the blood vessel. o Epithelium contains tight junction to prevent diffusion of large substances into CSF.  Drainage of CSF. o Done through arachnoid villi that project out from subarachnoid space. o There is a larger pressure in the subarachnoid space compared to the venous system. o Therefore, CSF is drained into sinuses that are in between the dura layers.