The Nervous System: Structure and Function
Human body is made up of different systems and one of the most important systems is the nervous system that transmitted signals between different parts of the system in order to coordinate different actions. The nervous system is consisted of two parts – Central Nervous System or CNS and Peripheral Nervous System or PNS (Woolsey, Hanaway and Gado 2017). The unit of nervous system is known as Neuron. The main function of t he neuron is to transfer information throughout the body by the help of synapses. Some inhibitory neurons are present in the CNS, which play an important role in maintaining homeostasis in order to manage the excitation and inhibition activities. Neuron is made up of three major parts; cell body, dendrite and axon. According to function, there are three types of neuron – sensory neuron, motor neuron and interneuron (Gurney 2014).
Sensory neuron is the neurons of nerve cells.. Sensory neuron is activated through some sensory inputs. Such sensory inputs can be classified into two types; physical inputs and chemical inputs. Physical inputs include heat, light, sound and sight. The main function of the sensory neuron is to carry signals to the brain and spinal cord from sense organs. Sensory neuron accepts external stimuli from the environment through the receptors and converts them into internal electrical impulse.
Motor neuron is present in the spinal cord and they are the part of central nervous system. Motor neuron is connected to the glands, muscles and other organs of the human body. It transmits impulse to the skeletal muscle and smooth muscle from spinal cord to control the movements of the muscles.
Interneuron is the connector of the sensory neuron and the motor neuron. The main function of the interneuron is to relay the signal between sensory neuron and the motor neuron. Thus, it is known as relay neuron.
After being stimulated by external stimuli, the receptors send signal to the central nervous system and the signal is coordinated by the brain. Sometimes, the response is very quick so that there is no need of involvement of the brain. Such action is known as reflex action. For example, a person moves away his or her hand after touching any hot object. It is a reflex action, as the brain needs not to think about moving away the hand from the hot object. It happens because, receptors of the sensory neuron feel the sudden change in the external environment and the sensory neuron sends the signal to the motor neuron through the interneuron, the motor neuron then transmits the signal to the effecter and the effecter produces instantaneous response to the signal.
Types of Neurons: Sensory, Motor, and Interneurons
Figure 2: Diagram of reflex action (reflex arc)
Source: (Willemze et al., 2015)
Human endocrine system is constructed of ductless glands that secrets hormones to control the functions of other organs. For example, hormone secreted by the hypothalamus maintains the homeostasis by regulating the eating and drinking habits, reproduction, metabolism and blood pressure of a human (Sherwood 2015). The major endocrine glands include – pituitary gland, pineal gland, thyroid gland, parathyroid gland, thymus gland, adrenal gland, gonad gland and pancreas.
Figure 3: diagram of endocrine system showing different endocrine glands in the human.
Source: (Nussey and Whitehead 2013)
Pituitary is located at the base of brain, near the hypothalamus. It is known as the master gland of the body. It has three major parts; anterior, middle and posterior.
Hormones Released from Anterior Part:
The anterior part of the pituitary gland releases many essential hormones such as, growth hormone, Adrenocorticotrophin or ACTH, Thyroid stimulating hormone or TSH, Prolactin, Luteinizing hormone or LH and Follicle-stimulating hormone or FSH.
- Growth hormone in children helps to construct healthy body and in adults, it helps to maintain the body mass and the bone mass by distributing the body fats (Chaves, Júnior and Bertolini 2013).
- ACTH is the stress hormone that helps to maintain the stress level and blood pressure level in the human being.
- The main function of the TSH is to stimulate thyroid gland to release hormone in order to maintain the metabolism, growth, energy level and activities of the nervous system.
- Prolactin is responsible for stimulating the production of breast milk after the birth of a child. It also affects fertility by stimulating the hormones released from ovaries and testis.
- Luteinizing hormone is responsible for stimulating ovulation and testosterone production.
- FSH stimulates the production of sperm in the men and helps the ovaries in the women to develop eggs and produce estrogen.
The middle part of the pituitary gland releases Melanin stimulating hormone or MSH. It is responsible for stimulating the production of melanin, which is responsible for the color of skin eyes and hair.
The Antidiuretic hormone or ADH and Oxytosin are released by the posterior part of the pituitary gland. ADH helps to reabsorb water from renal tubules, thus, reduces the water loss through urine and maintains the water balance in the body. Oxytocin is known as the birth hormone. It helps in breastfeeding and progressing of labor during pregnancy.
Pineal gland is the smallest endocrine gland present in the epithalamus, centre part of the brain. Melatonin, a hormone derived from serotonin, is released by the pineal gland. Melatonin is responsible for stimulating the sleeping process in circadian and seasonal cycle as well.
Thyroid gland is present in the neck, below the larynx. Two hormones are released by this gland; Thyroxin or T4 and Triiodothyronine or T3. They are known as the calorigenic hormone as they are responsible for increasing the BMR level (Pitt-Rivers and Tata 2013). They stimulate the metabolism rate and the speed of work done by the cell of the body.
Parathyroid gland is made up with four small glands and located in the neck, within the thyroid gland. Each glands release parathyroid hormone that controls the calcium level of the blood. When calcium level is high in the blood, parathyroid hormone released to manage the blood calcium level and when the calcium level is low, the parathyroid gland stops releasing the hormone.
Reflex Action: Coordination without Brain Involvement
Thymus gland is located between lungs , behind the sternum. It is known s the clock of ageing, as it remains until puberty. The thymus gland releases thymosin. The hormone is responsible for stimulating the T cells in order to increase the immunity.
Adrenal gland is located to the upper part of the kidney. It has two parts; outer part is known as cortex and inner part is known as the medulla. Cortex releases minerelocorticoids, that is related to the synthesis of minerals and glucocorticoids that is responsible for the synthesis of glucose. Medulla of the adrenal gland releases adrenaline and nor-adrenaline. During the stress situation, the hormones produce rapid responses.
Pancreas is present between the stomach and the small intestine (proximal portion). It functions as both the endocrine and exocrine system. As an endocrine gland, it produces hormones like Insulin and Glucagon. They controls the sugar level in the blood stream.
Gonad gland is considered as additional endocrine gland. It includes the male testis and female ovaries. Testis releases Testosterone, which influence the development of primary and secondary sex organs, production of sperm cells, growth of hair on face and body and the voice pitch of male. Ovary releases Estrogen and Progesterone. Estrogen helps to develop the primary and secondary sex organs of female and prepare a female body for childbirth. Progesterone produces the lining of uterus in order to implement the fertilized egg within the uterus and helps to shed the endometrium during menstruation.
There are some steroids, which can act as hormones. Such steroid hormones are classified into two groups; corticosteroids and sex steroids. Corticosteroids are released by the cortex of adrenal gland, for example, glucocorticoids and mineralocorticoids. The gonad gland releases sex corticoids such as estrogen and progesterone. Water-soluble hormones are hydrophilic hormones, for example, thyroid hormones.
The steroid hormones use two different types of mechanism in order to affect the target cell; genomic mechanism and non-genomic mechanism. In the process of genomic mechanism, the fat-soluble steroid hormones pass through cell membrane and undergo enzymatic alteration. Then they bind to the receptors and then two such receptors join in order to from one DNA molecule to enter the nucleus. In the nucleus, the complex structure binds to the appropriate DNA molecule and transcript the target genes (Duarte-Guterman, Navarro-Martín and Trudeau 2014). In the non-genomic process, the hormones bind to the receptor on plasma membrane and affects membrane fluidity, transporters and the ion channels. Water-soluble hormones fail to pass through the lipid bi-layer of the cell membrane, thus, bind to the receptor to the surface of the cell. They do not affect the transcription directly; instead, a second messenger carries out the process.
There are so many examples of positive feedback regulation regarding nervous system and endocrine system, such as, breast milk secretion and contraction of uterine during childbirth. During the childbirth, increase in the release of oxytocin influence the contraction, increase the intensity of the contraction. After the delivery of the baby, pituitary gland stops releasing oxytocin.
Negative feedback includes, regulating blood sugar level, controlling blood pressure, regulating body temperature of human and many others. When blood pressure increases, the nervous system send signal to the brain from blood vessels and brain transmit the signal to the heart, then the heart rate slows down and become normal.
References
Chaves, V.E., Júnior, F.M. and Bertolini, G.L., 2013. The metabolic effects of growth hormone in adipose tissue. Endocrine, 44(2), pp.293-302.
Duarte-Guterman, P., Navarro-Martín, L. and Trudeau, V.L., 2014. Mechanisms of crosstalk between endocrine systems: regulation of sex steroid hormone synthesis and action by thyroid hormones. General and comparative endocrinology, 203, pp.69-85.
Gurney, K., 2014. An introduction to neural networks. CRC press.
Nussey, S.S. and Whitehead, S.A., 2013. Endocrinology: an integrated approach. CRC Press.
Pitt-Rivers, R. and Tata, J.R., 2013. The thyroid hormones. Elsevier.
Sherwood, L., 2015. Human physiology: from cells to systems. Cengage learning.
Willemze, R.A., Luyer, M.D., Buurman, W.A. and De Jonge, W.J., 2015. Neural reflex pathways in intestinal inflammation: hypotheses to viable therapy. Nature Reviews Gastroenterology and Hepatology, 12(6), p.353.
Woolsey, T.A., Hanaway, J. and Gado, M.H., 2017. The brain atlas: a visual guide to the human central nervous system. John Wiley & Sons.
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