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The Spinal Cord
The spinal cord is a cylindrical mass of nerve tissues extending from the foramen magnum in the skull to the area between the first and second lumbar vertebrae at the conus medullaris. It is divided into three major sections (Figure 31) namely the cervical (C1-C8 spinal segments), thoracic (T1- T12 segments), lumbar (L1- L5), and sacral (S1- S5 segments).
Enlargements can be seen at the cervical and lumbosacral junctions of the spinal cord. These enlargements are sites where the brachial plexus and the lumbosacral plexus originate. These groups of peripheral nerves (brachial and lumboscral) control the upper and lower extremity functions.
The groups of spinal nerve roots from the conus medullaris to the most inferior aspect of the spine is collectively known as the cauda equina.
Figure 30. The end portion of the spinal cord
Looking at the transverse section of the spinal cord we shall see two major layers: an inner and an outer layer (Figure 31).
Figure 31. The layers of the spinal cord
The inner is composed of gray matter, seen as an H-shaped pillar with anterior and posterior gray columns or horns, joined by a thin gray commissure containing the central canal.
The anterior horn of the spinal cord is where impulses for muscular contraction from the brain exit in order to reach the muscles. The posterior gray column on the other hand, receives sensory stimuli from the sensory nerves in order for the stimuli to be passed on to the brain.
However lateral horns can also be found in the thoracolumbar junction that give rise to the preganglionic fibers of the autonomic nervous system.
The outer layer of the spinal cord is composed of white matter. It is also where the ascending and descending fiber systems for signal transmission are found.
The tables below show the ascending and descending tracts of the spinal cord. These also show their contributions to overall nervous system function.
Table 4. Ascending Tracts (sensory functions)
Table 5. Descending Tracts (Motor functions)
Myotomes (www.apparelyzed.com)
Each muscle in the body is innervated by a particular level or segment of the spinal cord and by its corresponding spinal nerve.
C3,4 and 5 supply the diaphragm (the large muscle between the chest and the belly that we use to breath).
C5 also supplies the shoulder muscles and the muscle that we use to bend our elbow .
C6 is for bending the wrist back.
C7 is for straightening the elbow.
C8 bends the fingers.
T1 spreads the fingers.
T1 T12 supplies the chest wall & abdominal muscles.
L2 bends the hip.
L3 straightens the knee.
L4 pulls the foot up.
L5 wiggles the toes.
S1 pulls the foot down.
S3,4 and 5 supply the bladder. bowel and sex organs and the anal and other pelvic muscles
Dermatomes
In the same manner any part of the body may be cutaneously innervated by a spinal segment and its corresponding spinal nerve (Figure 32).
Figure 32. Dermatome
The Diencephalon
The diencephalon (Figure 21) is composed of the thalamus, subthalamus, hypothalamus, and the epithalamus (O Sullivan and Siegelman, 2006).
Figure 21
The thalamus (Figure 22) is a large ovoid mass of gray matter which serves as a cell station to all main sensory systems (except the olfactory pathway). The thalamus contains two groups of nuclei: the sensory and the motor nuclei. The sensory nuclei relay sensory information from the body, face, retina, cochlea, and taste receptors to cerebral cortex and subcortical regions. The motor nuclei relay movement information from cerebellum and globus pallidus to precentral motor cortex.
Figure 22. The thalamus
The subthalamus (Figure 23) lies inferior to the thalamus. The structure of the subthalamus is extremely complex and contains the cranial ends of the red nuclei and the substantia nigra. These nuclei have important connections with the corpus striatum as a result, it is involved in controlling muscular activities. The subthalamus also contains important tracts that pass up from the tegmentum to the thalamic nuclei.
Figure 23. The substantia nigra
The hypothalamus (Figure 24) is the part of the diencephalon that extends from the ptic chiasma to the caudal border of the mammillary bodies. It is responsible for the integration and control of the autonomic nervous system and neuroendocrine system functions. It also maintains body homeostasis by regulating body temperature, eating, water balance, emotions, sexual behavior, and anterior pituitary functions.
Figure 24. Hypothalamus
The epithalamus in turn, has the habenular nuclei that integrate olfactory, visceral, and somatic afferent pathways. It also houses the pineal gland that influences the pituitary gland and several organs, as well as our circadian rhythm (Melatonin).
The Midbrain
The midbrain (mesencephalon) belongs to a group of brain structures known as the brainstem. Other parts of the brainstem include the pons and the medulla oblongata.
Figure 25. The midbrain
Figure 26.
The midbrain connects the pons to the cerebrum. The superior peduncle in turn connects the midbrain to the cerebellum.
Parts of the crus cerebri, substantia nigra, tegmentum, and the red nuclei are inclusive of the midbrain. These parts are important for movement control and coordination, and contain cranial nerve nuclei (oculomotor and trochlear nerves).
The superior colliculus is an important relay station for vision and visual reflexes. The inferior colliculus, on the other hand, relays to the cortex stimuli for hearing and auditory reflexes.
The midbrain also contains endorphin-producing cells that are used by the body in suppressing pain (opiate system).
The Hindbrain
The hindbrain (Figure 27) is composed of the pons, the medulla oblongata, and the cerebellum.
Figure 27
The Pons
The Pons is named after the Latin word for bridge (Encyclopedia of Neurological Disorders, 2007). The pons in fact bridges the two hemispheres and connects the midbrain to the medulla oblongata. It is also connected to the cerebellum via the middle peduncle.
The pons is involved in motor control, sensory analysis, and levels of consciousness and sleep (reticular formation). Some structures within pons are linked to the cerebellum, involving them in movement and posture.
In addition the pons is also important in modulating pain. It also contains severl cranial nerve nuclei: abducens, trigeminal, facial, and vestibulocochlear.
The Medulla Oblongata
The medulla oblongata connects the spinal cord to the pons. It contains important centers for vital functions like cardiac, respiratory, and vasomotor centers. It also contains relay nuclei of the dorsal columns of the spinal cord, fibers of which cross to contralateral side to give rise to medial lemniscus. Its inferior cerebellar peduncle relays the dorsal spinocerebellar tract to the cerebellum. The corticospinal tracts also cross in this area of the brain at the cervicomedullary junction. It also holds structures that are important in head movement control and gaze stabilization.
The Cerebellum
The cerebellum is located behind dorsal pons and medulla. It is joined to brainstem by three pairs of peduncles: superior, middle, and inferior. It is also comprised of two hemispheres divided in between by the vermis.
The three major parts of the cerebellum include the archicerebellum (flocculonodular lobe), the paleocerebellum (rostral cerebellum), and the neocerebellum (posterior lobe).
The archicerebellum connects with the vestibular system and is concerned with equilibrium and regulation of muscle tone. It also aids in processing the vestibule-ocular reflex. The paleocerebellum on the other hand, controls the synergistic movement of the muscles that is important in maintaining posture and fluidity in functional movements. Lastly the neocerebellum ensures that the bodys movements are accurate in terms of timing, distance, and force, which is important in movement coordination.
The White Matter of the Cerebrum
The white matter of the cerebrum as stated earlier is composed of myelinated nerve fibers located centrally. Varieties of such are the transverse commissural fibers, the projection fibers, and the association fibers.
The transverse commissural fibers interconnect the two hemispheres. Examples are the corpus callosum, anterior commissure, and hippocampal commissure.
The projection fibers connect cerebral hemispheres with other portions of the brain and spinal cord.
The association fibers connect different portions of the cerebral hemispheres, allowing cortex to function as an integrated whole.
The Basal Ganglia
The term basal nuclei is applied to a collection of masses of gray matter situated deep within each cerebral hemisphere. They are the corpus striatum, the amygdaloid nucleus, and the claustrum.
Generally the basal ganglia are association sites for motor control, complimented by other brain nuclei located in the subthalamus and the midbrain (extrapyramidal structures).
If the frontal lobe sends the commands and information for body movements, the extrapyramidal structures are the ones responsible in controlling the intensity, velocity, distance, and other aspects of movement.
The corpus striatum is located lateral to the thalamus. It is completely divided into two by the internal capsule. The two divisions of the corpus striatum are the caudate nucleus and the lentiform nucleus.
The lentiform nucleus is a wedge-shaped mass of gray matter whose base is oriented laterally and blade medially. The structure that separates it from the claustrum is the external capsule. The nucleus is divided into an outer, darker internal portion, the putamen and an inner, lighter portion, the globus pallidus.
The amygdaloid nucleus is within the temporal lobe close to the uncus.
On the other hand the claustrum is a thin sheet of gray matter, the function of which is unknown.
Nevertheless, the cerebral hemispheres possess similarities. Both hemispheres also possess similarities. Both hemispheres are made up of six lobes. They are the frontal, parietal, occipital, temporal, insular, and limbic lobes (O Sullivan and Siegleman, 2006).
The frontal lobe controls and activates the voluntary muscles of the body (precentral gyrus). It also controls an individuals emotions and judgments (prefrontal cortex). Furthermore, there is also a portion of the frontal lobe that controls the motor aspects of speech (Brocas area). The frontal lobe also stores long term memory, together with the temporal lobe, hippocampus, amygdala, and the diencephalon
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Glucose Utilization during Exercise- A Function of the Insulin
Insulin, though not mentioned in Table 2, is also important in both aerobic and anaerobic exercise. As discussed, during these two types of exercise, glucose is one of the fuels that energize the body. And so glucose production has been shown to be stimulated by the catecholamines (and at rest, the glucagon). But how can the glucose produced and delivered to the blood be consumed by the cells in the body? The answer to this is through insulin coming from the pancreas.
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The table featured last time shows a simple concept on hormone reactions during exercise. As seen in the table glucose and glycogen breakdown during exercise is mainly the role of the catecholamines- the epinephrine and the norepinephrine. They ensure that these energy sources are available in the blood during activities. However one of the catecholamines, the norepinephrine also contributes to lipolysis or fat metabolism, transforming it to another consumable energy. Free fatty acid (FFA) mobilization that leads to lipolysis is being triggered by the GH. Protein synthesis on the other hand, has been shown to be stimulated again by the GH and others like testosterone, cortisol, and IGF-1. These hormones create and break down protein to provide growth and energy as well (gluconeogenesis).
It is also notable from the table above that each hormone has its own stimulant for release. As mentioned earlier, hormone response depends on the type of activities an individual is performing. As with this discourse, we focus on the reactions of the hormones to the different forms of exercise. And we shall see that the catecholamines are stimulated by performance of moderate to intense exercise. But later on we shall see that the release of these hormones is more sensitive when the body does an intense type of activity (anaerobic). On the other hand, prolonged exercise triggers cortisol release while light to moderate exercises stimulate estrogen release (aerobic). In addition, GH, testosterone, and IGF-1 are activated with any type of exercise but more of the aerobic (Marks and Kravitz, 2000
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Effects of Exercise on Endocrine Secretions
Physical exercise is associated with marked metabolic changes and elicits a variety of neuroendocrine response (Scheen et al., 1998). These endocrine responses primarily aim to provide energy and growth needed by the body in order to keep up with the metabolic demands of a specific activity or exercise. And the endocrine system does this by controlling the metabolism of different energy sources like carbohydrates, fats, and proteins
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Introduction
Exercise, as we all know, refers to bodily exertion for the sake of training or improvement of health (de Lisa et al., 1998). This improvement has been brought about by various adaptations by the body, including increased muscular strength, better blood circulation and blood pressure management, and other systemic responses. Aside from this, the bodys ability to provide energy is also continuously modified with sustained training in order to keep up with the energy demands of the activities involved. And in order to do these, all of the systems must work synergistically. One of the systems that contribute in the process is the endocrine system
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