Body Systems 2: Nervous and Endocrine Systems - A Comparative Analysis

This essay delves into the intricate workings of the nervous and endocrine systems, two vital components of the human body responsible for maintaining homeostasis and coordinating bodily functions. The nervous system, with its rapid communication via neurons and action potentials, is contrasted with the endocrine system, which utilizes slower, hormone-mediated signaling. The essay discusses the different types of neurons (sensory, motor, and interneurons) and illustrates their roles within a reflex arc. It then explores the function of various hormones like thyroid hormones, cortisol, insulin, and adrenaline, detailing their mechanisms of action and target organs. Furthermore, the essay examines feedback mechanisms, both negative and positive, that regulate hormone secretion and action, providing examples such as the hypothalamic-pituitary-thyroid axis and the oxytocin-mediated control of labor. Through this comparative analysis, the essay highlights the essential roles of both systems in maintaining bodily functions and responding to internal and external stimuli. The essay concludes by emphasizing the importance of these two systems in maintaining body function and homeostasis.

BODY SYSTEMS 1
BODY SYSTEMS
Students name
Course
Institutional affiliation

BODY SYSTEMS 2
BODY SYSTEMS
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.
Nervous System
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).

BODY SYSTEMS 3
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

BODY SYSTEMS 4
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).
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.
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)

BODY SYSTEMS 5
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).

BODY SYSTEMS 6
Water soluble hormones include glucagon, insulin, adrenaline among others (Nussey and
Whitehead 2013).
Thyroid Hormones
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).
Cortisol.
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
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
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

BODY SYSTEMS 7
that diverts blood from non-essential organs to vital organs, increasing metabolic rate and
pupillary dilatation (Barrett, Barman, Boitano, and Brooks, 2009).
Mode of action of steroid 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).
Mode of action of water-soluble hormones
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.
Feedback mechanisms
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

BODY SYSTEMS 8
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).
The hypothalamic-pituitary-thyroid negative feedback loop

BODY SYSTEMS 9
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.
The oxytocin effect during labor
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).
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.

BODY SYSTEMS 10
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.

BODY SYSTEMS 11
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.

BODY SYSTEMS 12
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.