Neuroanatomy

Introduction

The brain, being the most complex organ of the body, controls every action and thought. It is divided into various functional units formed from brain cells that work together to respond to particular stimuli. The brain cells connect with other cells of the body to create communication routes for brain signals. As such, scientists are working to understand the complex organization of the brain with the help of tools that can tag and trace the circuits of the brain as well as finding out how different types of brain cells function. Therefore, the brain and the entire nervous system are crucial in providing information on disease symptoms to physicians and thus assist in diagnosis. This paper aims to discuss the composition of the neuroanatomy and its importance in the diagnosis of diseases.

Neuroanatomy involves the study of body anatomy and how the nervous system is organized (Fisch, 2009). The nervous system differs depending on the animal’s symmetry and as such vertebrates have a segregated nervous system consisting of the internal structure of the brain and the spinal cord. This is connected to the rest of the body by a route of nerves known as the peripheral nervous system. The nervous system is divided into the central and peripheral whereby the central nervous system consists of the spinal cord, brain, and retina. The peripheral nervous system, on the other hand, consists of the all the nerves that connect the body to the central nervous system (Webster & Fay, 2013).

Further, nervous system consists of the somatic and the autonomic nervous system where the somatic nervous system is composed of the afferent neurons responsible for transmitting sensory information from the sense organs to the central nervous system and efferent neurons which transmit motor instructions to the muscles. The autonomic nervous system, on the other hand, consists of the parasympathetic and sympathetic nervous system responsible for regulating the functions of the internal organs of the body, such as salivating, breathing, heartbeat, and digestion (Fisch, 2009). Therefore, the autonomic nerves are made up of afferent and efferent fibers.

The Composition of the Central Nervous System

The nervous system is composed of the extracellular matrix, glial cells, and neurons at the tissue level. Neurons represent the cells that are responsible for processing information in the nervous system. They do this by sensing the environment and communicating with each other using neurotransmitters and electrical signals thereby producing body movements, thoughts, and memories. The glial cells are responsible for protecting and supporting the neurons of the brain, producing myelin, and maintaining homeostasis. However, some glial cells, such as the astrocytes circulate intercellular calcium waves thereby producing gliotransmitters (Patestas & Gartner, 2013). Moreover, on the molecular level, the extracellular matrix is responsible for providing support for the brain cells.

The other part of the nervous system consists of the brain regions, for instance, the hippocampus. This parts form the organ level of the nervous system and are often modular and serve various specific roles within the nervous system pathways, for example, the hippocampus forms the memory. The nerves contained in the nervous system, which are basically fibers, travel from the brain and spinal cord to all parts of the body and are made of neuron axons and membranes thereby forming nerve fascicles.

The neuron is a cell that is responsible for communicating with other cells. It is composed of a nucleus and other common organelles with cytoplasm extensions, namely the axon and the dendrites. The cytoplasmic process of the neuron is called neurite. The axon forms the cytoplasmic extension of uniform diameter responsible for impulses or neuronal signals. The neuron is made up of one axon which branches terminally although it may contain collateral branches at the base of the cell body. It is also composed of a myelin sheath which allows for a more rapid action of the neurons (Webster & Fay, 2013). The myelin, however, gives the white matter of the brain its pale color. Further, an axon combined with its myelin sheath and associated neuroglial cells form the nerve fiber.

A dendrite, on the other hand, forms the cytoplasmic process which widens with the neuronal cell body by narrows as the length traveled increases (Blumenfeld, 2014). They are responsible for increasing the surface area of the cell to create room for various afferent synaptic terminals. A synapse is the functional contact site between the cytoplasmic processes of neurons or the axon’s terminal branch. Therefore, presynaptic neurite release chemical neurotransmitters to act on receptors on the postsynaptic neurite which is inhibited or excited by the receptor changes. The summation of the inhibitory and excitatory synaptic inputs usually determines the functioning of the postsynaptic cell.

A receptor represents the molecule that responds to chemical signals at the cellular level, for instance, the synaptic transmitter. It also represents a sense organ, such as an eye. The nervous system also contains neuroglial or glial cells which are not neurons and usually outnumber the neurons at a ratio of 10 to 1. The typical peripheral glial cells surrounding the axons are referred to as Schwann cells and are useful in myelination. The microglia, oligodendrocytes, and astrocytes form the central glia and are associated with the neuronal cell bodies. The gray matter of the central nervous system contains synapses, axons, dendrites, and neuronal cell bodies.

The ganglion is made up of various neuronal cell bodies, such as the sympathetic, parasympathetic as well as enteric ganglia that allow for the functioning of the sensory and cardiac muscle cells. The ganglion is also composed of the sensory ganglia of the cranial nerves and dorsal spinal roots. The basal ganglia, on the other hand, represent the large nuclei of the upper brain stem and forebrain while the retina cells with axons entering the optic nerve are referred to as ganglion cells (Webster & Fay, 2013). The optic nerve and retina form a crucial part of the nervous system.

The white matter of the nervous system is made up of myelinated axons which are organized into tracts, peduncles, capsules, and fasciculus. Any bundles of nerve fibers whether central or peripheral form the fasciculus while capsules represent the large, flattened parts of the white matter. The peduncle is the bundle that is responsible for joining together the two parts of the brain. The central nervous system has a region known as the tract in which axons with the same origin and destination occupy, for instance, the neuron axons with spinal cord cell bodies are contained in the spinothalamic tract whose fibers end in the thalamus (Patestas & Gartner, 2013).

The root represents a bundle of axons which is composed of supporting cells and connective tissues that traverse the subarachnoid space. The ventral and dorsal spinal roots join at each intervertebral foramen forming a mixed spinal nerve. The nerves form axon bundles that have connective tissues and supporting cells and are usually located outside the boundary of the central and peripheral nervous system (Blumenfeld, 2014). The rami form the branch of the nerve that is used for communications by carrying axons between the sympathetic ganglia and spinal nerves. The pathway forms a set of an interconnected group of neurons serving a specific function, such as the conduction of signals from the retina to the cerebral cortex is done by the visual pathway and is made up of different tracts and nuclei.

The largest cranial nerve is called trigeminal nerve and contains both motor and sensory fibers. The motor fibers emanate from the pons’ motor neurons to innervate the mastication muscles. The sensory fibers, on the other hand, relay information related to unconscious and conscious proprioception, touch, temperature, and pain (Paxinos, 2003). The sensory fibers that convey temperature and pain are associated with the tract and nucleus of the caudal medulla. The cell bodies of the trigeminal nerve fibers are located in the trigeminal ganglion and possess central and peripheral processes. Such peripheral processes distribute to temperature and pain receptors on the mucous membrane of the nose, portions of the cranial dura, teeth, oral cavity, nasal cavities, soft and hard palates, anterior two-thirds of the tongue, forehead, and the face.

At the levels of the pons, the central processes enter the brain. This is the same level where the trigeminal motor fibers leave the brain stem and the trigeminal sensory fibers enter the brain stem. The trigeminal ganglion neurons’ central processes transmitting temperature and pain flow into the brain stem and is made up of the spinal tract V, whose fibers end in the spinal nucleus V (Augustine, 2007). This cell group forms a swelling, tuberculum, on the caudal medulla’s lateral surface, where the temperature and pain fibers terminate. Further, these cells contain medially-curving axons that cross the midline near the medial lemniscus. The crossed fibers are known as the trigeminothalamic tract and ascend in the brain stem. The trigeminothalamic fibers terminate in the ventral posteromedial nucleus via the thalamus where the ventral posteromedial neurons project to the somatosensory cortex.

Importance of Neuroanatomy in Disease Diagnosis

There are diseases that affect different parts of both central and peripheral nervous system thereby causing multiple sensory or motor deficits, mental impairment, as well as reduced level of unconsciousness. To diagnose these diseases and disorders, a good knowledge of anatomy and nervous system connectivity is essential (Webster & Fay, 2013). This involves two approaches to checking the functionality of the nervous system pathways.

First, the physician would have to test whether the functions of the brain or the spinal cord are disordered. This is done to pinpoint the site of a destructive or irritating lesion where the physician works from the assumptions that specific functions are done in certain localized parts of the central nervous system. The other approach involves checking whether there has been an interruption of the functional pathways, which involves sensory or motor, by ascertaining the functioning of the memory or eloquence of speech. This method is commonly used to evaluate the diseases of the spinal cord, disorders that affect the peripheral nerves, ganglia, or roots.

The knowledge of the functioning of the peripheral nervous system and the sensory and motor dispositions discovery has been made possible by clinicopathology thereby enabling neuroradiologists to interpret images on the topographical anatomy of the brain. Scientists are able to diagnose complex diseases and disorders of the nervous system using imaging techniques, for instance, nuclear magnetic resonance imaging, angiography, as well as computer-assisted tomography (Patestas & Gartner, 2013). Physicians first consider the functioning of the nervous system on a regional basis and then assess the functioning of the pathways.

Development of the Nervous System

The neural tube, which consists of the neurons and neuroglial cells emanate from the ectoderm of the embryo, which in turn gives rise to the epidermis. 16 days after fertilization, the thickening of the ectoderm usually referred to as the neural plate takes place, which is further indented in the midline within 18 days (Fisch, 2009). The rostral ends of the neural folds become enlarged in 22 days to become the cerebral hemispheres and at this point, the fusion of the neural folds takes place to form the initial parts of the cervical segments of the spinal cord. The fusion continues caudally and rostrally forming the neural tube which closes on the 27th and 24th day respectively. If the neural tube does not close, then severe developmental abnormality is likely to occur. An equivalent condition of the spinal cord in which the posterior neuropore fails to close is known as myeloschisis.

The caudal part of the spinal cord starts from the caudal cell mass, which is a collection of pluripotential cells that are dorsal to the development of the coccyx. The motor neurons represent the cells generated in the ventral part of the neural tube and their axons elongate into the ventral roots of the spinal nerves thereby supplying the skeletal muscles. Additionally, the neural crest is formed by the primary sensory neurons while the alar lamina forms the central projections of the dorsal part of the neural tube (Blumenfeld, 2014). The ventral horn of gray matter in the spinal cord emanates from the basal lamina and usually functions as a motor. The alar lamina, on the other hand, forms the dorsal horn which functions to receive sensory input.

The roof plate changes to become the wide, thin roof of the fourth ventricle from the spinal cord to the upper medulla and the pons. This displaces the alar lamina from the dorsal in a lateral position. The system also contains the meninges, ventricles, and cerebrospinal fluid whereby the meninges represent the dura matter which is the outermost and strongest meninx attached to the cranium. In the parts where the dura splits forming spaces for carrying venous blood leaving the brain are known as dural venous sinuses. These include the superior sagittal sinus, traverse sinuses, and the straight sinuses (Waxman, 2013). The dura is reproduced into the cranial cavity to form membranes, such as the falx cerebri and the tentorium cerebella.

There is also the pia mater, which is a thin layer of connective tissue contouring through the spinal cord and brain’s surface. There is an arachnoid which is thicker which is contained in the inside of the dura, but with a subarachnoid space that separates it from the pia. There also are delicate trabeculae of connective tissue that traverse the subarachnoid space, reminiscent of the spider web. This also contains veins that supply the cerebral cortex in the subarachnoid space before going into the dura to later into the superior sagittal sinus (Patestas & Gartner, 2013). The cerebellomedullar cistern, however, forms the largest intracranial cistern which is located below the cerebellum but above the medulla. The afferent fibers of the nervous system are useful in the regulation of both respiratory and cardiovascular functions although they do not lead to conscious sensations. Moreover, the physiological functions of the afferent fibers are not disturbed by unilateral lesions affecting the central connection of such fibers or the nerves.

In the autonomic nervous system, the motor neurons with cell bodies in the brain stem and spinal cord are responsible for supplying the skeletal muscles while neurons in the ganglia of the autonomic system supply the internal organs, the smooth muscle of the blood vessels, cardiac muscle as well as the glands. The neurons in the ganglia contain unmyelinated axons, secretory cells, cardiac muscle, and the postganglionic fibers innervating the cardiac and smooth muscle. The autonomic system is divided into three parts, which include enteric, parasympathetic, and sympathetic. As such, the sympathetic system’s ganglia consist of chains of paravertebral ganglia lying on the lateral aspects of the body in addition to the collateral or preaortic ganglia (Fisch, 2009).

The postganglionic fibers use a gray ramus communicans to enter the nerve where they are distributed to blood vessels, the little muscles that move hairs as well as the sweat glands. The sympathetic supply makes the blood vessels in the skin to constrict while muscles dilate. The postganglionic fibers emanating from the large superior cervical ganglion move with the carotid artery as well as its branches. The parasympathetic ganglia, on the other hand, are located in the head and are connected with cranial nerves and as such associated with the pelvic viscera and thoracic walls. The preganglionic fibers leave the brain stem and end in the cranial parasympathetic ganglia.

The enteric nervous system is composed of tiny, interconnected ganglia located in the alimentary walls with associated structures, for instance, the pancreas and biliary system (Waxman, 2013). These ganglia consist of different types of neurons, which contain many varieties of neurotransmitters. The enteric system does most of the work independently although it is modulated by preganglionic fibers from the pelvic splanchnic and vagus nerves. The parasympathetic activity is responsible for stimulating the propulsion of gut contents. Moreover, the preganglionic supply is crucial for secretion of acid in addition to opening the pyloric sphincter. The sympathetic system’s activity produces the visceral blood vessels’ constriction in addition to retarding the propulsion of alimentary canal contents (Fisch, 2009).

Regional Anatomy of the Central Nervous System 

The cerebral cortex is tasked with receiving sensory pathways, formulating commands and sending orders through the motor pathways to the muscles, and interpreting the sensations thereby being considered as the core of thinking and consciousness. The central nervous system is composed of cerebrum or the forebrain made up of the two cerebral hemispheres, the midbrain, the hindbrain, and the spinal cord. The hindbrain consists of the cerebellum, pons, and the medulla. The brain stem, on the other hand, is made up of the midbrain, pons, and medulla. However, paired peduncles of white matter join the cerebellum to the brain parts (Fisch, 2009). As such, there are six cerebellar peduncles in the nervous system.

The spinal cord’s neural components consist of ascending and descending tracts similar to the cell columns occurring only in the thoracic and upper lumbar segments. The spinal cord’s small central canal contains cerebrospinal fluid is usually a remnant of the embryonic neural tube lumen. Any case of a destructive lesion in the spinal cord is predictable through knowledge of the segmental level as well as the functions of the tracts. Injury of the spinal cord leads to paralysis of the abdominal and lower limb muscles on the left, proprioceptive sensation, and loss of discriminative touch (Blumenfeld, 2014). This can also be predicted with the loss of pain and temperature sensibility directly below the right side nipple.

The brain stem forms the central nervous system where the spinal cord’s central canal elongates through the caudal half of the medulla before widening to make a ventricle, which represents the pons and upper medulla cavity (Waxman, 2013). However, on the roof of the ventricle, the walls of the superior and inferior cerebellar peduncles are formed. This allows the cerebrospinal fluid to flow from the brain’s ventricular system into the subarachnoid space through the fourth ventricle’s apertures. Moreover, there is a substantial slab of gray matter which is made up of paired superior and inferior colliculi forming pathway parts responsible for hearing and eye movements.

The system also contains fiber tracts of the spinal cord that extend to all parts of the brain stem. The brain stem, therefore, contains cranial nerve nuclei. As such, the motor nuclei correspond to ventral horn cells from the spinal gray matter as well as sensory nuclei that correspond to the dorsal horn. The nuclei connected with the cerebellum as well as the cell groups that form the reticular formation are the other major group of brain stem neurons (Waxman, 2013). Moreover, the midbrain’s most ventral parts contain a large amount of fibers emanating from the cerebral cortex that performs motor functions.

 

Structure and Function of the Neuroanatomy

Pituitary and Pineal Glands

These glands are closely associated with the hypothalamus in their functionality. As such, the pituitary gland produces an array of hormones once it receives signals from the hypothalamus. These hormones are responsible for regulating the activities of the other glands, such as prolactin, which is involved in milk production, adrenocorticotropic hormone which responds to stress through a stimulation of the epinephrine, hormone responsible for the stimulation of thyroid, as well as the luteinizing hormone, which regulates ovulation timing and promotes sperm and egg development, follicle-stimulating hormone and sex hormone (Augustine, 2007).

Other hormones produced by the pituitary gland include; dopamine, which functions to inhibit prolactin releases, melanocyte-stimulating hormone useful in skin pigmentation, and the human growth hormone. The pineal gland, on the other hand, produces melatonin hormone, which is responsible for skin pigmentation. This gland is, however, controlled by light-sensitive neurons originating from the retina of each eye and stop at the hypothalamus (Paxinos, 2003). The pineal gland plays a different role in animals by guiding reproductive functions.

Thalamus and Hypothalamus

These parts of the neuroanatomy play an integral role in body’s vital functions, movement, and perception. The thalamus, for instance, contains two oval masses joined by a bridge. These masses have nerve cell bodies responsible for sensory information transmission into the cerebral cortex with the exception of the sense of smell, which relays signals directly to the cortex without going through the thalamus. The hypothalamus is relatively smaller in size but controls a large number of bodily functions. This part is responsible for stimulating smooth muscle lining the intestines, stomach, and blood vessels at the autonomic level in addition to receiving impulses from such areas. As a result, the hypothalamus controls the heart rate, bladder contraction, and food passage through the gut.

Moreover, the hypothalamus acts as the interaction point for the endocrine system and the nervous system. Crucial changes in the body are detected by the hypothalamus and as such, respond to gland and organ stimulation by releasing hormones. The hypothalamus also translates emotion into physical response since it acts as the brain’s intermediary. The cerebral cortex relays impulses to the hypothalamus when strong feelings are generated in the mind through actions of thought or external stimuli (Paxinos, 2003). The hypothalamus, therefore, sends signals for physiological changes by the release of pituitary hormones.

The hypothalamus is also home to the neurons that control body temperature and blood flow. The neurons that lower the body temperature are located at the front part of the hypothalamus and perform this function by relaxing the blood vessels’ smooth muscles thereby causing a dilation of the vessels hence increasing the rate of heat loss from the body (Augustine, 2007). Moreover, the hypothalamus cools the body by increasing the perspiration rate and produces body heat by stimulating blood vessel contraction, which slows the rate of heat loss through shivering.

The hypothalamus thus controls the stimuli behind drinking and eating once stimulated to promote satiety feelings thereby inhibiting eating. This is observed in animals when they portray tendencies of excessive eating due to damage to that part of the hypothalamus. The hypothalamus plays an integral role in sleep induction since it forms a part of the reticular activating system, which is the physical basis for consciousness. Therefore, the hypothalamus is strategic to various connections in the body and acts as the mid post between feelings and thought as well as autonomic function and conscious act.

The Emotional Function of the Brain

This is connected to the limbic system, which controls drives and emotions as well as human involuntary behavior that is crucial for survival, for instance, affection, docility, sexual feelings, anger, fear, pleasure, and pain. The limbic system is also responsible for smell senses in animals triggered by nerves from the olfactory bulb. The limbic system is fed by the hypothalamus, thalamus, and the amygdala, the small almond-shaped nerve cells complex that is responsible for the reception of input from the cerebral cortex and the olfactory system (Paxinos, 2003). The limbic system is thought to be indirectly stimulated by brain’s electric discharge thereby setting off seizures.

Hippocampus

The hippocampus, which is shaped like a sea horse, forms a major part of the limbic system and is situated at the base of the temporal lobe adjacent to the sets of the association fibers. They form the nerve fiber bundles connecting both regions of the cerebral cortex to allow for information exchange between the entire cerebral cortex, limbic system, and the hippocampus (Augustine, 2007). The long-term memory of a human being is stored throughout the cerebral cortex.

Building Blocks of the Neuroanatomy

The nerve cell or the neuron forms the building block of the human brain. As such, the neuron relays signals through an axon, which extends outwards like a single long arm from the soma, or the body of the cell. The neurons contain numerous shorter arms called the dendrites, which send signals back to the soma. The axon’s ability is enhanced by the surrounding myelin sheath. The myelin sheath is the natural insulator protecting the axon from nearby nerve impulse interference. The speed of the electrical impulses can reach a high of 120 meters per second (Paxinos, 2003). The site of the communication is referred to as synapse and includes the dendrite and the button. There is also an electrical synapse, where the cell membrane of two neurons is linked by molecules of tubular protein.

As a result, the tubular protein bridge lets electrically-charged molecules and water to pass through with changes in electrical charges in one neuron being transmitted to the other. Additionally, there are messengers of choice, in the form of electrical synapses located outside of the nervous system. The gray matter of the brain forms cortex sheets on the cerebral hemisphere surface. The white matter, on the other hand, is named after the appearance of the myelin that encloses the axons’ elongated region. The brain is also made up of neuroglia, which forms the main matter providing structural support as well as metabolic energy for the billions of brain’s nerve cells (Augustine, 2007).

Chemical and Electrical Signals of the Neuroanatomy

The electrical and chemical signals in the brain are interdependent and as such meet at the synapse, where the chemical substances have the ability to change the electrical conditions outside and within the cell membrane. The cell membrane is polarized when the nerve cell at rest holds a slight negative charge as compared to the exterior (Paxinos, 2003). When the cell is at rest, it is less permeable to positively-charged sodium ions. However, when it is stimulated, the charges can freely pass through once they are attracted by the negative charges inside thereby triggering the membrane to reverse polarity. This sees a movement of sodium charges within a millisecond, which allows for potassium charges through the membrane, thus reversing the flow of positively-charged ions.

Therefore, molecules of neurotransmitter are allowed into the synaptic cleft and bound to particular receptor sites on the postsynaptic side through an alteration of the postsynaptic membrane’s ion channels. The neurotransmitters trigger the opening of sodium channels thereby allowing for an influx of sodium ions and thus neutralizing the negative charges inside the cell membrane through a transmission of the nerve impulse.

 

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