Neuroscience/Cellular Neurobiology/Nervous System Structure

The nervous system is divided between the peripheral nervous system, or PNS, and the central nervous system, or CNS. The central nervous system consists of the brain and the spinal cord, while the peripheral nervous system is made up of the spinal nerves, the autonomic nervous system and the cranial nerves

Terminology
To refer to various positions in the nervous system, a set of spatial terms are used. In the spinal cord, dorsal is closer to the back of a person, and ventral is closer to the person's chest, or front. Closer to the brain is referred to as rostral, and closer to the end of the spinal cord is caudal. In the brain, superior and dorsal refer to a position closer to the top of the head, and inferior and ventral refer to a position closer to the bottom or base of the head. Also, in the brain, to refer to something closer to the face, it is called rostral or anterior, and something closer to the back of the head, it is called posterior or caudal. Throughout the nervous system, concentric structures are found, and to refer a structure closer to the outside of the head, the term lateral is used, and closer to the center of the brain is called medial. In addition, slices called sections of structures can show three dimensions of the structure. A section that would cut ear to ear and nose to back of the head is called a horizontal section. A section parallel to the face is a coronal section, and a section that could separate the two hemispheres of the brain is a sagittal section.

Neuralation
In the beginning of neural development (neurulation) in a growing embryo, there are three layers: the endoderm, which becomes the viscera; the mesoderm, which becomes the bones and muscle; and the ectoderm, which becomes the skin and nervous system. The disctinction of the nervous system appears after three weeks. The neural plate becomes a groove, which then differentiates to a tube, which becomes the CNS, and a crest, which becomes the PNS. In the mesoderm, the are somites (masses of tissue) that become the spinal cord vertebrae and the somatic muscles.

First Phase: Rostral
The first phase of neural differentiation occurs on the rostral end of the neural tube (the tube becomes the brain) which forms three separate vesicles: the prosencephalon becomes the forebrain; the mesencephalon becomes the midbrain, and the rhombencephalon becomes the hindbrain. The forebrain, or prosencephalon, develops into two pairs of vesicles, the optic vesicles and the telencephalic vesicles, and an unpaired diencephalon. The optic vesicles, which are parts of the brain, form into optic stalks and optic cups, which become the optic nerve and retina.

Telencephalon
The telencephalon differentiation occurs in 4 stages: First, the telencephalon grows over and around the diencephalon. Next, vesicles begin to form ventrally to become the olfactory bulbs, and then the gray matter differentiates into the different structures of the telencephalon. Finally, the white matter, the fatty tissue or myelin, forms around axonal inputs and outputs of the cells in the telencephalon. The telencephalon contains the cerebral cortex and the basal telencephalon while the diencephalon contains the thalamus and the hypothalamus. In the paired telencephalon are the lateral ventricles as well. The diencephalon contains the third ventricle. The white matter that develops is involved with three different systems: the cortical white matter connects different neurons in the cortex. The corpus collosum connects the two hemispheres, and the internal capsule connects the cortex to the diencephalon.

Forebrain
The forebrain can be seen to have many different structures. The differentiation of the telecephalon results in a pair of cerebral hemispheres that contain the cerebral cortex, olfactory bulbs, basal ganglia, hippocampus and amygdala. the cerebral cortex is associated with cognition and motor and sensory functions. The olfactory bulbs relay olfactory information to the cortex. the basal ganglia contributes to the fine tuning of voluntary motor control. The hippocampus is associated with the formation of short term memories, and the amygdala is associated with fear and emotion. In the diencephalon, there is the thalamus and hypothalamus. The thalamus is a sensory relay to cortex, and the hypothalamus maintains control of the autonomic nervous system and endocrine systems.

Midbrain
The midbrain (mesencephalon) differentiates into the tectum, which contains the superior and inferior colliculus, the tegmentum, which contains the dopamine reward system (contributes to motor control), and the cerebral aqueduct, which connects two ventricles and contains cererbro-spinal fluid.

Hindbrain
The hindbrain (rhombencephalon) differentiates into two structures on its rostral end, and one on its caudal end. In the rostral end, the rhombic lips swell and fuse to become the cerebellum. The ventral rhombencephalon becomes the pons. The caudal end of the ventral rhombencephalon becomes the medulla. In the hindbrain, the fourth ventricle is also formed, connecting to the cerbral aqueduct. The hindbrain acts as a bidirectional relay between the forebrain and the spinal cord. The cerebellum controls movement and coordination, and it is where inputs from the spinal cord and cortex converge to coordinate motions. The pons acts as relay to the cerebellum: 90% of axons from the cerebral cortex synapse in the pons. The medulla is a relay area for the somatic sensory, autonomic sensory, and the motor nuclei.

Spinal Cord
The spinal cord is formed from the caudal end of the neural tube, and it is characterized by two colors. The gray matter contains the dorsal horns, which contains sensory axons and terminals, an intermediolateral column, and the ventral horns, which carry motor axons and neurons. The white matter is made of three different columns carrying ascending sensory signals and descending motor signals.

Four Regions of Dermatomes
The somatic motor system in the spine is divided into four regions. Starting from the top of the spine, the cervical division (C1 through C8), the thoracic division (T1-T12), the lumbar division (L1-L5), and the sacral division (S1-S5, Coc1). There are two spinal nerves coming out of each side of each spinal column, and the dorsal nerve carries sensory signals to the brain, and the ventral nerve carries motor signals from the brain.

Autonomic Nervous System
The autonomic nervous system in the spine has two major divisions that act in opposite ways. The sympathetic division prepares the body for action (fighting, fleeing etc...) while the parasympathetic division causes the body to relax.