The Essay On Gonadal Steroid Hormones And The Hypothalamus Functions.

Nervous System

Discuss about the Gonadal Steroid Hormones And The Hypothalamo.

The endocrine and nervous system are among the most important of body system with tight regulation involved. They control important aspects of human function and maintenance of an internal environment. The current paper is a discussion of the functions of the nervous system, the endocrine system, and their interrelation. The functions of the neurons in the nervous system and functions of hormones in the endocrine system will be discussed to show how they coordinate the functions of different parts of the body. Feedback regulation mechanisms will also be discussed.

Stimulation and communications in the nervous system are more immediate unlike the hormonal stimulation in the endocrine system that is slower and more prolonged (Waugh and Grant 2010).

The communication method in the nervous system is through an action potential or a nerve impulse which is basically a propagated electrical impulse (Barrett, Barman, Boitano, and Brooks, 2009). There is always more than one neuron involved in the transmission of a nerve impulse from its origin to the target organ. However, no contact is there between these neurons and the impulse passes from neuron to the other via a gap called a synapse (Hall 2015).

The neurons are divided into three categories; motor, sensory, and interneuron (Hall 2015). A basic reflex arc which uses all three neurons will be used to demonstrate this function (Barrett, Barman, Boitano, and Brooks, 2009).

A reflex arc is a fast response to a stimulus that does not involve the brain but passes through the spinal cord involving all three neuron types (Barrett, Barman, Boitano, and Brooks, 2009). It starts with a stimulus for example heat applied to the hand. The sensory nerves in the hand through sensory receptors initiate an action potential and propagates it to the spinal cord. At the level of the spinal cord, there are sensory pathways to the brain, but an interneuron loops back to the grey matter of the spinal cord and sends an impulse to the motor neurons supplying muscle groups in the hand leading to a motor response in this case withdrawal of the hand from the heat source (Barrett, Barman, Boitano, and Brooks, 2009).

The neural action is possible through generation and propagation of nerve impulses. It is initiated by a sensory nerve or when transmitted from one nerve to another (Barrett, Barman, Boitano, and Brooks, 2009). It is by movement of charged ions across the nerve membranes. The inside and outside of the membrane have different charges, called the resting membrane potential. Sodium ions are extracellular while potassium ions are intracellular (Barrett, Barman, Boitano, and Brooks, 2009). When stimulated, the permeability of the membrane to these ions changes. Sodium floods into the cell creating a depolarizing action potential which moves along the entire nerve from point of stimulation towards the resting potential (Barrett, Barman, Boitano, and Brooks, 2009).

The Endocrine System

For a nerve impulse to pass a synapse, neurotransmitters have to be involved. The arrival of an impulse at a presynaptic neuron releases neurotransmitter which is usually synthesized by the neuron and stored in vesicles at the presynaptic membrane. The neurotransmitters move across the synaptic cleft to act on the receptors on the post-synaptic membrane. The impulse is thus propagated when an action potential is initiated in the post-synaptic neuron.

The endocrine system is another system that relies on communication and signaling to control different body functions (Nussey and Whitehead 2013). The signaling and communication, however, is through hormones. It comprises the following main organs; the hypothalamus, pituitary gland, pineal gland, thyroid gland, parathyroid gland, adrenals, reproductive glands and the endocrine pancreas among others (Melmed 2016)

The signaling hormones are either made from amino acids (water soluble) or from cholesterol-based lipids (steroids) (Nussey and Whitehead 2013). They are produced by the endocrine organ and transported to the target organ by blood. Steroid hormones include thyroid hormones, glucocorticoids, mineralocorticoids among others (Nussey and Whitehead 2013). Water soluble hormones include glucagon, insulin, adrenaline among others (Nussey and Whitehead 2013).

The thyroid produces thyroxine and triiodothyronine which have receptors on various body organs.  The main functions of thyroid hormones include regulation of metabolic rate, with varied effects on the heart, digestive system, mood, bone chemistry and neural development in embryos (Mendoza and Hollenberg 2017).

It is a hormone produced by the adrenal gland but acts on various body systems (Pivonello, De Leo, Cozzolino, and Colao, 2015). It is the bodies main stress hormones and is released following stress and low blood glucose. It has effects on metabolism, glucose control, inflammation, mood and memory, sleep and electrolyte balance.

Insulin is produced by the beta cells of the endocrine pancreas and is the main glucose controlling hormone in the body (Rutter et al. 2015). It diffuses in blood to exert effects on the liver and fat cells following increased blood glucose levels. It acts on liver and fat cells to take up glucose and store (Melmed 2016).

Adrenaline is both a hormone and a neurotransmitter. It is produced by the adrenals in response to sympathetic stimulation (Barrett, Barman, Boitano, and Brooks, 2009). It functions to modulate the fight or flight response by increasing heart rate, blood pressure, vasoconstriction that diverts blood from non-essential organs to vital organs, increasing metabolic rate and pupillary dilatation (Barrett, Barman, Boitano, and Brooks, 2009).

Thyroid Hormones

Steroid hormones are produced by their respective organs and transported by the blood to their target organs. At the target organ, due to their lipid solubility, they easily penetrate the lipid membranes (Hall 2015). Their target receptors are intracellular either in the cytoplasm or in the nucleus and act as ligand-dependent transcription factors (Handa and Weiser 2014). The hormone -receptor binding leads to the target genes inhibition or stimulation. In short, they modulate gene expression through binding to intracellular receptors (Hall 2015).

These hormones are made from protein or peptides and are produced by endocrine organs to act on distant organs through their binding on specific cell sauce receptors (Hall 2015). Binding of the hormone to receptors triggers a series of intracellular events that lead to creating of second messengers (Hall 2015). The second messengers are molecules that will trigger intracellular events and exert the effects of the hormone in a process termed signal transduction.

Control of hormone secretion and action is through feedback mechanisms. They can either be negative or positive feedback (Dagklis et al. 2015). Negative feedback is when a stimulus causes release of a hormone whose action will reverse the stimulus or have an opposing effect. This can either be direct stimuli like blood levels of glucose effects on insulin or indirect by the release of controlling hormones by the hypothalamus and the anterior pituitary. Examples of negative feedback include blood sugar regulation, hypothalamic-pituitary-thyroid axis, and the hypothalamic-pituitary-adrenal axis.

Positive feedback mechanism is whereby the stimulus leads to secretion of the hormone in increasing amounts until a particular process is complete and the stimulus ceases. Examples of positive feedback include oxytocin release during labor, estrogen during menstruation and the milk ejection reflex (Hall 2015).

This axis determines the level of circulating thyroid hormones. The hypothalamus produces thyrotropin-releasing hormone (TRH) that acts on the pituitary to produce thyrotropin (TSH) (Ortiga?Carvalho et al. 2016). TSH will, in turn, stimulate the synthesis and release of thyroid hormones from the thyroid gland. The negative feedback loop works in that when levels of T3 and T4 rise beyond the required levels, the hypothalamus senses the circulating levels which inhibit it from producing TRH and also inhibit the pituitary from producing TSH (Ortiga?Carvalho et al. 2016). This has the effect of reducing circulating levels of thyroid hormones.

This is a good example of positive feedback whereby the hormone is released in increasing levels, amplifying the effects until the stimulus stops. During labor, the baby’s head triggers mechanoreceptors in the uterine cervix which are sensed by centers in the hypothalamus that initiate production of oxytocin from the pituitary gland (Vannuccini et al. 2016). The uterus being sensitive to oxytocin contracts to aid the movement of the baby through the birth canal. As more stretch occurs, more oxytocin is producing and more contraction occurs in a positive feedback loop. This stops after the baby is ejected as the stimulus Is no longer present (Vannuccini et al. 2016).

Cortisol

Conclusion.

The nervous and endocrine system are two of the most important systems for maintaining body function and homeostasis. The nervous system basic unit is the neuron and functions through nerve impulses or action potentials. Three types of neurons namely motor, sensory and interneuron exist and communicate to bring about desired effects in the effector organs for example in a reflex arc. They communicate through a synaptic cleft by way of neurotransmitters.

The endocrine system, on the other hand, uses hormones which diffuse into the blood to bring about effector changes elsewhere. The hormones can be steroid hormones synthesized from cholesterol or water-soluble hormones synthesized from amino acids. Water soluble hormones, for example, insulin and glucagon act via cell surface receptors using second messengers to bring about their effects. Steroid hormones act via intracellular receptors to bring about gene expression and transcription.

These systems are regulated by feedback mechanisms that maintain the circulating levels at a functional level. They include positive feedback mechanisms and negative feedback mechanisms. An example of a positive feedback loop is the control of labor by oxytocin while an example of a negative feedback mechanism is the hypothalamic-pituitary-thyroid axis.

References

Barrett, K.E., Barman, S.M., Boitano, S. and Brooks, H., 2009. Ganong’s review of medical physiology. 23. NY: McGraw-Hill Medical.

Boran, G. and Seheult, J., 2013. Drug effects on the hypothalamic-pituitary-thyroid axis and thyroid hormone concentration. CPD Clinical Biochemistry, 11(3), pp.92-98.

Dagklis, T., Ravanos, K., Makedou, K., Kourtis, A. and Rousso, D., 2015. Common features and differences of the hypothalamic–pituitary–gonadal axis in male and female. Gynecological Endocrinology, 31(1), pp.14-17.

Hall, J.E., 2015. Guyton and Hall textbook of medical physiology e-Book. Elsevier Health Sciences.

Handa, R.J. and Weiser, M.J., 2014. Gonadal steroid hormones and the hypothalamo–pituitary–adrenal axis. Frontiers in neuroendocrinology, 35(2), pp.197-220.

Melmed, S., 2016. Williams textbook of endocrinology. Elsevier Health Sciences.

Mendoza, A. and Hollenberg, A.N., 2017. New insights into thyroid hormone action. Pharmacology & therapeutics, 173, pp.135-145.

Nussey, S.S. and Whitehead, S.A., 2013. Endocrinology: an integrated approach. CRC Press.

Ortiga?Carvalho, T.M., Chiamolera, M.I., Pazos?Moura, C.C. and Wondisford, F.E., 2016. Hypothalamus?pituitary?thyroid axis. Comprehensive Physiology.

Pivonello, R., De Leo, M., Cozzolino, A. and Colao, A., 2015. The treatment of Cushing's disease. Endocrine reviews, 36(4), pp.385-486.

Rutter, G.A., Pullen, T.J., Hodson, D.J., Martinez-Sanchez, A., McLuskey, K., Mottram, J., Delorge, I., Figueroa, C.M., Feil, R., Lunn, J.E. and Van Dijck, P., 2015. Pancreatic β-cell identity, glucose sensing and the control of insulin secretion. Biochem J, 466(2), pp.203-218.

Vannuccini, S., Bocchi, C., Severi, F.M., Challis, J.R. and Petraglia, F., 2016, June. Endocrinology of human parturition. In Annales d'endocrinologie (Vol. 77, No. 2, pp. 105-113). Elsevier Masson.

Waugh, A. and Grant, A., 2010. Ross & Wilson Anatomy and Physiology in Health and Illness E-Book. Elsevier Health Sciences.