Saturday, October 24, 2015

A Neuroanatomy Guide:

The human body is controlled by two complex systems called the Central Nervous System and Peripheral Nervous System. In a systematic breakdown of the Central Nervous System and Peripheral Nervous System this is paper will describe the associated features including basic structures, their location, and intended function. 

A Neuroanatomy Guide: The Central Nervous System and Peripheral Nervous System
The human body is controlled by the nervous system including the “central nervous system” (CNS) and the “peripheral nervous system” (PNS) (Carlson, 2009). The central nervous system consists of the brain and spinal cord, which make up the (CNS). The “peripheral nervous system” (PNS) consists of “cranial nerves, spinal nerves, and peripheral ganglia” (Carlson, 2009). These two systems work together to bring sensory and motor information from the body to the CNS and to bring sensory and motor information from the CNS to any location within the body (Carlson, 2009). The CNS interacts with the PNS through the use of the “cranial nerves, spinal nerves, and peripheral ganglia” (Carlson, 2009). Effectively the PNS works as a relay system that communicates all the information collected from the body to the CNS, so the CNS can send signals that regulate function throughout the whole body.  In very simple terms, the PNS can be seen as an interdependent system that connects the brain to the body so they work simultaneously.   
The CNS and the PNS are both well protected; in fact, both the CNS and PNS are in encased in bone and meninges. The brain, consisting of “neurons, glia, and other supporting cells”, is found floating in cerebrospinal fluid, “chemically guarded by the blood–brain barrier”, well protected by meninges and the skull (Carlson, 2009, p. 72). The spinal cord is encased within the vertebral column, which consists of the twenty-four individual vertebrae of the “cervical, thoracic, lumbar, sacral and coccygeal” regions that form the spine (Carlson, 2009, p. 94-95). The spinal nerves, cranial nerves and the peripheral ganglia that compose the PNS are protected by cerebrospinal fluid and two layers of meninges (dura mater and pia mater) (Carlson, 2009, p. 72). Meninges are a tough connective tissue that protects them from damage. Meninges has several layers: the outer tough flexible layer called the dura mater, the middle soft and spongy layer called the arachnoid membrane including a spall gap called the subarachnoid space which is filled with cerebrospinal fluid, and the pia mater which is closely attached to the brain and spinal cord (Carlson, 2009. p 72).
Central Nervous System Breakdown
A series of hollow unified chambers, called ventricles, full of cerebrospinal fluid (CSF) form the brain (Carlson, 2009, p. 74-75).  Located at the midline of the brain, the walls of the third ventricle divide the brain into symmetrical halves (Carlson, 2009). Connected to and located on each side of the third ventricle, are the largest chambers in the brain are called the “lateral ventricles” (Carlson, 2009). The “massa intermedia” a connective piece of neural tissue that crosses through the middle of the third ventricle, while the cerebral aqueduct is a long tube that connects the third ventricle to the fourth ventricle (Carlson, 2009, p. 74-75). Within these four chambers a special tissue called the choroid plexus protrudes and is responsible for the production of CSF (Carlson, 2009, p. 75). The CSF continually produced by the choroid plexus tissue located within the ventricles systematically floods the ventricles and subarachnoid space before passing the arachnoid granulations and reentering the blood stream through the “superior sagittal sinus” (Carlson, 2009, p. 75). Arachnoid granulations are pouch-shaped structures that extend into a blood vessel called the superior sagittal sinus that drains into the veins serving the brain. If there is an interruption in the flow of CSF ventricals may enlarge, a condition called obstructive hydrocephalus may result, causing intracerebral pressure, blood vessels to become occluded, and permanent or possibly fatal brain damage can occur (Carlson, 2009, p. 75).
The brain is divided into two hemispheres called left and right hemispheres. These hemispheres are connected by the corpus callosum, which is “a large band of axons that connects corresponding parts of the cerebral cortex of the left and right hemispheres” (Carlson, 2009, p 87). Typically, but not always, the left hemisphere analyzes and performs functions that include verbal activities, such as talking, understanding the speech of other people, reading, and writing, while the right hemisphere synthesis information, specializes in seeing the global picture and putting things together to make a whole as done in activities like as drawing, read maps, and constructing complex objects out of smaller pieces (Carlson, 2009, p. 87). However, the forebrain, the midbrain, and the hindbrain are the three major subdivisions of the brain.
The front part of the brain, called the forebrain, surrounds the lateral and third ventricles and includes the two subdivision called the telencephalon and diencephalon (Carlson, 2009, p 82).  The midbrain, also called the mesencephalon, is the middle section of the brain, surrounds the cerebral aqueduct and includes the “tectum and the tegmentum”. (Carlson, 2009, p. 91). The hindbrain, located at the rear of the brain, found surrounding the fourth ventricle contains the subdivisions called the metencephalon and myelencephalon (Carlson, 2009, p. 93).
The telencephalon describes the two equal “cerebral hemispheres”, covered by the “cerebral cortex” containing the “limbic system and the basal ganglia”, which form the majority of cerebrum located in the forebrain (Carlson, 2009, p. 83). The cerebral cortex is organized into the frontal lobe, parietal, temporal, and occipital lobes, with the central sulcus dividing the frontal lobe from the other three (Carlson, 2009. p. 95). 

The “central sulcus” deals with movement and the planning of movement, while the other three lobes deal principally with perception and “learning” (Carlson, 2009. p. 95).The limbic system, includes brain structures involved in “emotion, motivation, and learning”, such as the frontal “thalamic nuclei, amygdala, hippocampus, limbic cortex, parts of the hypothalamus” called mammillary bodies, and their interwoven fiber bundles called the fornix (Carlson, 2009, p. 88).

 The limbic cortex is located near the middle edge of the cerebral hemispheres, while the “cingulate gyrus” part of the limbic cortex lies just along the sidewalls of a channel “separating the cerebral hemispheres”, just above the corpus callosum (Carlson, 2009, p. 88).  The hippocampus and the parts of the limbic cortex that surround it are associated with learning and memory, while the amygdala and other parts of limbic cortex are specifically involved in “feelings and expressions of emotions, emotional memories, and recognition of the signs of emotions in other people” (Carlson, 2009, p. 88). The basal ganglia is a group of subcortical nuclei including the caudate nucleus, the globus pallidus, and the putamen, that play an important role in the motor system (Carlson, 2009, p. 89). Mammillary bodies refer to a protrusion of the bottom of the brain at the back end of the hypothalamus and contain some hypothalamic nuclei (Carlson, 2009, p. 88).
The diencephalon, located between the telencephalon and the mesencephalon surrounding the third ventrical, includes the thalamus and the hypothalamus (Carlson, 2009, p. 89). The thalamus is located within the dorsal section of the diencephalon, near the middle of the cerebral hemispheres, toward the mid-line and back of the basal ganglia, and above the hypothalamus (Carlson, 2009, p. 89). The thalamus contains nuclei that send information to different regions in the cerebral cortex and receive information from it (Carlson, 2009, p. 89). The hypothalamus is the group of nuclei that lies at the base of the brain under the thalamus, which governs the endocrine system, the regulation of the autonomic nervous system, controls the anterior and posterior pituitary glands, and integration of species-typical behaviors (Carlson, 2009, p. 90).

 The midbrain section, located between the forebrain and hindbrain includes the tectum and the tegmentum. Tectum is the part of the brain concerned with “audition and the control of visual reflexes and reactions to moving stimuli” (Carlson, 2009, p. 96).The tegmentum contains the reticular formation vital to sleep, arousal, and movement; the periaqueductal gray matter that controls various species-typical behaviors; and the red nucleus and the substantia nigra parts of the motor system (Carlson, 2009, p. 96). The hindbrain is located at the back of the brain surrounding the fourth ventricle and containing the cerebellum, the pons, and the medulla (Carlson, 2009, p. 97). The cerebellum contributes to integrating and coordinating movements, while the pons contains various nuclei that are important in sleep and arousal (Carlson, 2009). The medulla oblongata also is involved with the regulation of sleep and arousal, but also plays a significant role in the control of movement and regulating vital functions such as heart rate, breathing, and blood pressure (Carlson, 2009, p. 96).

The spinal cord is a long tapering structure, about as thick as the pinky finger and has various reflexive control circuits (Carlson, 2009, p. 95). The spinal cord extends only to about two-thirds the length of the vertebral column and the rest of the space contains a mass of spinal roots composing the cauda equina (Carlson, 2009, p. 95).The spinal cord contains white matter and gray matter, like the brain but on the spinal cord, unlike in the brain, the white matter is on the outside and gray matter is on the inside (Carlson, 2009, p. 95). White matter consists mostly of ascending and descending bundles of myelinated axons, while the gray matter consists of neural cell bodies and short unmyelinated axons (Carlson, 2009, p. 95). The primary function of the spinal cord is to provide motor fibers to the organs of the body including glands and muscles and collect somatosensory information to share with brain (Carlson, 2009, p. 94).

Peripheral Nervous System Breakdown
All communication from the organs, glands, muscles and extremities is transmitted to the CNS from the PNS nerves called spinal and cranial nerves. First the nerves gather sensory information then convey this information to the central nervous system and then the CNS conveys messages from the central nervous system to the body’s parts (Carlson, 2009, p. 95).  Any cell body that takes information to the CNS (spine or brain) is called an afferent axon, while any cell body that takes information away from the CNS is referred to as efferent (Carlson, 2009, p. 95). The transfer of sensory information is part of the somatic nervous system; this system governs the information from the sensory organs and those organs that control movements of the skeletal muscles (Carlson, 2009, p. 97). The autonomic nervous system (ANS) is also part of the PNS, but is concerned with regulation of smooth muscle, cardiac muscle, and glands which control regulation of “vegetative processes” in the body (Carlson, 2009, p. 97). Autonomic nervous system uses a pathway that contains preganglionic axons from the brain or spinal cord to the sympathetic or parasympathetic ganglia, and postganglionic axons from the ganglia to the target organ (Carlson, 2009, p. 101). The autonomic nervous system is further broken down into two anatomically separate systems: the sympathetic division and the parasympathetic division (Carlson, 2009, p. 97).

With the exception of the retina, all cell bodies of axons that convey sensory information into the brain and spinal cord are located outside the CNS and called afferent axons (Carlson, 2009, p. 97). Dorsal roots and ventral roots are small bundles of fibers that emerge from each side of the spinal cord in two straight lines along its front and back surfaces (Carlson, 2009, p. 95). At the point when the dorsal and ventral roots join together passing through the intervertebral foramens, they become spinal nerves (Carlson, 2009, p. 95). A spinal nerve is a peripheral nerve attached to the spinal cord that branches out along that path it travels to the organ it supplies (Carlson, 2009, p. 97). A cranial nerve is part of the peripheral nervous system that connects with the brain directly (Carlson, 2009). The twelve cranial nerves that are affixed directly to the front bottom surface of the brain provide sensory and motor functions to the head and neck regions (Carlson, 2009, p. 97). For example: The tenth and largest cranial nerve is called the vagus nerve and regulates the functions of organs in the thoracic and abdominal cavities by conveying efferent fibers of the parasympathetic division of the autonomic nervous system (Carlson, 2009, p.97).
The term peripheral ganglia describes a group of cells found in the Peripheral Nervous System outside the spinal cord and brain that are not protected (Carlson, 2009). Peripheral ganglia function is to connect the central nervous system to the different parts of the body, and they are found near the organs in the upper area of the body, specifically the head, abdomen, thorax, stomach, spleen, liver, kidneys, and along the pelvis (Carlson, 2009, p. 95). Dorsal root ganglia is a cell body on the dorsal root that takes somatosensory information to the spinal cord (Carlson, 2009, p. 97). The term sympathetic ganglia refers to nodules containing synapses between preganglionic and postganglionic neurons of the sympathetic nervous system (Carlson, 2009, p. 98)
The sympathetic divisions preganglionic cells are located in the thoracic and first two lumbar segments of the spinal cord, while the parasympathetic division preganglionic neurons are located in the brain stem and in sacral segments of the spinal cord (Henson, 2013). The sympathetic division also controls the adrenal medulla. The Adrenal Medulla a set of cells located in the center of the adrenal gland, just above of the kidney and is similar in nature to the sympathetic ganglion. The adrenal medulla and is controlled by sympathetic nerve fibers and secretes epinephrine and norepinephrine (Carlson, 2009, p. 100).  The secretion of these hormones controls functions like increase blood flow to the muscles, the breakdown of stored nutrients within skeletal muscle cells into glucose and increase energy available to these cells (Carlson, 2009, p. 100)
Parasympathetic Divisions ganglia are located right next to the intended organs (Carlson, 2009, p. 95). The nuclei that give rise to preganglionic axons in the parasympathetic nervous system are located in the nuclei of the “cranial nerves and the intermediate horn of the gray matter in the sacral region of the spinal cord” (Carlson, 2009, p. 100). The Parasympathetic Division works to increase the body’s supply of stored energy including primary functions like salivation, gastric and intestinal motility, secretion of digestive juices, and increased blood flow to the gastrointestinal system (Carlson, 2009, p. 100).

Typically speaking, the Sympathetic Division and the Parasympathetic Division both interact with the organs they affect, just in opposite ways. For example : the  Parasympathetic Division of the autonomic nervous system will constrict the pupil of the eye and slow the heart, while the Sympathetic Division will dilate the eye and speed the heart (Carlson, 2009, p. 97). The sympathetic division of autonomic nervous system controls functions like arousal and expenditure of energy, while the Parasympathetic division of autonomic nervous system controls the functions that occur during a relaxed state (Carlson, 2009, p. 98- 100).

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