Overview Of the Nervous System
The human nervous system is divided into the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS, in turn, is divided into the brain and the spinal cord, which lie in the cranial cavity of the skull and the vertebral canal, respectively. The CNS and the PNS, acting in concert, integrate sensory information and control motor and cognitive functions.
The adult human brain weighs between 1,200 to 1,500 g and contains about one trillion cells. It occupies a volume of about 1400 cc - approximately 2% of the total body weight, and receives 20% of the blood, oxygen, and calories supplied to the body. The adult spinal cord is approximately 40 to 50 cm long and occupies about 150 cc. The brain and the spinal cord arise in early development from the neural tube, which expands in the front of the embryo to form the three primary brain divisions: the prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain). These three vesicles further differentiate into five subdivisions: telencephalon, diencephalon, mesencephalon, metencephalon, and the myelencephalon. The mesencephalon, metencephalon, and the myelencephalon comprise the brain stem.
The telencephalon includes the cerebral cortex (cortex is the outer layer of the brain), which represents the highest level of neuronal organization and function. The cerebral cortex consists of various types of cortices as well as closely related subcortical structures such as the caudate nucleus, putamen, globus, amygdala and the hippocampal formation.
The diencephalon consists of a complex collection of nuclei lying symmetrically on either side of the midline. The diencephalon includes the thalamus, hypothalamus, epithalamus and subthalamus.
The mesencephalon (or midbrain) consists of several structures around the cerebral aqueduct such as the periaqueductal gray (or central gray), the mesencephalic reticular formation, the substantia nigra, the red nucleus, the superior and inferior colliculi, the cerebral peduncles, some cranial nerve nuclei, and the projection of sensory and motor pathways.
The metencephalon includes the pons and the cerebellum. The myelencephalon (spinal cord-like) includes the open and closed medulla, sensory and motor nuclei, projection of sensory and motor pathways, and some cranial nerve nuclei.
The caudal end of the myelencephalon develops into the spinal cord. The spinal cord is an elongated cylindrical structure lying within the vertebral canal, which includes the central canal and the surrounding gray matter. The gray matter is composed of neurons and their supporting cells and is enclosed by the white matter that is composed of a dense layer of ascending and descending nerve fibers. The spinal cord is an essential link between the peripheral nervous system and the brain; it conveys sensory information originating from different external and internal sites via 31 pairs of spinal nerves. These nerves make synaptic connections in the spinal cord or in the medulla oblongata and ascend to subcortical nuclei.
The meninges are composed by three fibrous connective tissues. The most external is a dense collagenous connective tissue envelope known as the dura mater (Latin for “hard mother”). The second, or the intermediated membrane, is a delicate non-vascular membrane of fine collagenous layer of reticular fibers forming a web-like membrane, known as the arachnoid (Greek for “spider”). It is separated from the inner pia layer by subarachnoid space, which is filled with cerebrospinal fluid. The inner most delicate connective tissue membrane of collagenous is the pia mater, a thin translucent elastic membrane adherent to the surface of the brain and the spinal cord. Blood vessels located on the surface of the brain and the spinal cord are found on top of the pia matter. The meninges are subject to viral and bacterial infection known as meningitis, a life-threatening condition that requires immediate medical treatment.
The space between the skull and the dura is known as the epidural space. The space between the dura and the arachnoid is known as the subdural space. The space between the arachnoid and the pia is known as the subarachnoid space. In this space, there is a clear liquid known as the Cerebrospinal Fluid(CSF). The CSF serves to support the CNS, and to cushion as well as protect it from physical shock and trauma. The CSF is produced by the choroid plexus, which is composed of a specialized secretory ependymal layer located in the ventricular system.
The ventricular system is a derivative of the primitive embryonic neural canal. This system is an interconnected series of spaces within the brain, which contains the CSF.
In general, the CNS can be divided into three main functional components: the sensory system, the motor system, and homeostatic and higher brain functions. The sensory system consists of the somatosensory, viscerosensory, auditory, vestibular, olfactory, gustatory, and visual systems. The motor system consists of motor units, and the somatic (skeletal muscle) system, the spinal reflexes, the visceral (autonomic) system, the cerebellum, several subcortical and cortical sites, as well as the brain stem ocular motor control system. The homeostatic and higher functional system includes the hypothalamus, cortical areas involved in motivation, insight, personality, language, memory, imagination, creativity, thinking, judgment, mental processing, and subcortical areas involved in learning, thought, consciousness, memory, attention, emotional state, sleep and arousal cycles.