Unit 17 - Human Endocrine and Nervous Systems PDF

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The Human Endocrine and Nervous Systems Kate Seeley 23/10/2019

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Introduction The nervous and endocrine systems work together to coordinate bodily functions and appropriate biological responses. Whereas the nervous system sends electrical impulses in emergency situations making split-second responses, the endocrine system plays a longer game distributing chemical signals to negotiate growth and development through hormones. Sources, diagrams and tables are from online articles, online revision hubs and educational videos, as well as study materials provided via ePearl, to present my understanding of the human nervous and endocrine systems and roles they play within the body.

Contents

Title Page Introduction Contents Section 1 – The Nervous System 1.1 Structure 1.2 Spinal reflex arc mechanism 1.3 Nerve impulse and synaptic transmission

Section 2 – The Endocrine System 2.1 Major endocrine glands 2.2 Endocrine regulation

Section 3 – Nervous and Endocrine Roles in Self-Preservation 3.1 Homeostasis

Conclusion Recommendations References list Bibliography

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The Nervous System 1.1 Structure The nervous system is a complex system of nerves and specialist cells and fibres, better known as neurones. They are mapped out at specific points within the body to send and receive electrical messages regarding needs and sensation. Figure 1 shows how it’s structured, but also that the nervous system is subdivided into two parts, the central nervous system (CNS, in red) and the peripheral nervous system (PNS, in Figure 1 - Human nervous system (Mandal, 2019) blue). CNS refers to the brain and spinal cord and PNS is neurones distributed across the body which relays messages. The spinal cord is an extension of the brain and reaches down to the second lumbar vertebrae (Chung, 2019). The PNS is wired to the CNS sectionally by spinal nerves, providing function to different areas and organs. For example, having a cold can affect nerves attached to the C1-3 vertebrae at the top of the spine and the bladder is controlled through nerves attached to the S2-4 and L1 and L2 vertebrae towards the bottom. The CNS is filled with cerebrospinal fluid and protected by the meninges. The brain is divided into three regions:   

The forebrain processes thought and memory The midbrain is associated with motor function as well as audio-visual information The hindbrain manages automatic functions, such as breathing, and coordination (ThoughtCo)

The forebrain region is home to the hypothalamus, which controls set points regarding our needs; hunger, body temperature, tiredness etc. Should an abnormal condition arise, the hypothalamus communicates with various glands to secrete hormones or physical action to correct a balance. The hypothalamus uses two sets of nerves relating to uses of bodily energy and resources. The parasympathetic nerves are used more during states of rest and the sympathetic used during times of stress (figure 1). 1.2 Spinal reflex arc mechanism Reflex arcs are immediate, unconscious reactions to stimulus as a form of self-preservation. If I was to accidentally set my hand down on a hotplate, my body automatically knows to jerk my hand away as soon as the heat because intense enough to cause injury. As signals can travel through neurones

4 at a rate of up to 431 km/h, or 268 mph (Zimmermann), the reaction time is very quick. This is an inherent behaviour all humans have. Whilst neurones are nerve cells, they are more complex than the basic-units-of-life cells. They have a nucleus floating in axoplasm in the cell body (soma) but they don’t reproduce, so damage to nerve cells is often permanent leading to loss of use. Neurones (figure 2) use dendrites, branching away from the soma, creating synapses, like a neural junction. This allows specialised electrical signals to transmit to target cells. Axons receive and conduct impulses away from the soma up to metres in distance. Most neurones have parts of coating known as myelin sheath, Figure 2 – Motor neurone (HealthDirect) allowing signals to travel fast without losing much energy. The myelin sheath is supported by glial cells to supply nutrients and get rid of waste. Nodes of Ranvier appear between myelin sheathes allowing for diffusion of ions in and out, fuelling the rate of message relay. This is known as salutatory conduction. There are several types of neurones having different purposes. They can be unipolar, bipolar or multipolar, denoting how many channels of communication they use. Motor neurones send information to effector organs such as muscles to communicate that the body is moving. Sensory neurones have receptor cells to identify stimuli and changes in environment. These receptor cells react to different forms of stimulus, such as photoreceptors to light in the eyes and baroreceptors to air pressure. 1.3 Nerve impulse and synaptic transmission In order to receive an electrical signal, the neurone membrane must be in a polarised state, also known as resting potential. Being polarised indicates that the outside of the membrane is positivelycharged due to a field of sodium ions (Na+). The inside of the membrane is negatively-charged and surrounded by potassium ions (K+). When stimulus is received, the neurone depolarises; switching the electrical charges over and releasing K+ out of the membrane to take Na+ in fuelled by adenosine triphosphate (ATP). This has created action potential. The neurone membrane is selectively-permeable allowing proteins, sodiumpotassium pumps (figure 3) to channel the ions which actions these changes. Figure - Sodium-Potassium Pump (BioNinja)

5 The depolarised electrical signal then travels along the length of the axon until it meets the next neurone via synaptic bulbs. Resting potential must be restored for 1 millisecond before the next stimulus can be received to stop signals from overlapping and losing their way. The speed on which action potential can travel can vary depending on how wide an axon is, whether it’s myelinated and number of synapses used. Synapses have criteria by which they can receive signal; a chemical transmitter (acetylcholine), must be present on both synaptic bulbs in order to cross a synaptic cleft, to pass on the action potential.

The Endocrine System 2.1 Major endocrine glands The endocrine system is a group of major glands responsible for production of hormones to develop and regulate the body. Hormones are more than sex hormones associated with teenage angst, but more than fifty different chemical messages travelling to target organs to maintain homeostasis, respond to stress and control growth and sexual development. Hormones are produced by endocrine cells in the main glands; hypothalamus, pineal, pituitary, thyroid and parathyroid, thymus, adrenals, pancreas and ovaries or testes. The hypothalamus is the base of operations and monitors set points and levels. It instructs other glands via chemical messages to the correct what’s unbalanced where necessary and works in conjunction with the nervous system to stay prepared. Cells receive hormones through signal receptors and precursor molecules make a difference to how they bind. 

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Figure - Major endocrine gland (BasicMedicalKey)

Steroids are made from cholesterol and can easily pass through lipid-based cell membranes to get through the cytoplasm to receptor proteins in the nucleus. Amino-acid water soluble hormones are not permeable to cell membranes. They must use receptors on the cell’s surface to communicate with an internal messenger, cyclic adenosine monophosphate (cAMP). A derivative of ATP, cAMP can activate or reduce enzyme activity and open ion channels where needed. Once a receptor is bound, the target cell is activated and the hormone alters its activity to maintain homeostasis. A hormone will only bind with receptors in target organs due to them having complimentary shapes. Activity is altered according to the hormone’s content:   

Figure - Signal receptor (Wikipedia)

Transcription of genetic information e.g. oestrogen Protein synthesis e.g. growth hormone Enzyme activity e.g. adrenaline

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Exchange of materials across a cell membrane e.g. insulin (ePearl, 2.1)

2.2 Endocrine regulation There are two mechanisms that the pituitary gland uses to secrete the correct hormone to produce the appropriate response, negative feedback loops or positive feedback loops. The negative feedback loop responds to an adverse reaction in the body. For example, consuming a large amount of carbohydrates increases the glucose level in the body. The hypothalamus recognises this can have negative consequences, and triggers the pancreas to release the correct amount of insulin needed to bring body back down to the set point. The chain reaction ends and the set point is maintained until the next time carbohydrates are eaten and the loop begins again. The positive feedback loop follows a similar pattern, but encourages activity to see it Figure - Negative feedback loop (BBC Bitesize) through to completion, such as childbirth and blood clotting. The hormone oxytocin is made in the hypothalamus and released into the blood stream via the posterior pituitary to incite contractions in childbirth, throughout labour and postpartum.

Nervous and Endocrine Roles in Self-Preservation 3.1 Homeostasis Normal human body temperature is 37°C and blood glucose levels are typically between 4-7 mmol/l. Either set point drifting outside of these figures are cause for the hypothalamus to initiate action through the hormonal negative feedback loop. Thermoregulation senses outside stimulus and reacts accordingly. If the body temperature falls below 37°C, mechanisms ruled by the nervous system trigger physical responses to try to combat the cold. This causes vasoconstriction, blanching the skin and sending blood flow deeper into skin tissues to warm up internally and skeletal muscle tenses to cause shivering. On the outside of the body, sweat pores close and hairs stand on end to try to keep heat on the skin’s surface. Body temperature rising above 37°C inevitably causes the opposite physical reaction; vasodilation, skin reddening, body hairs lying flat and sweat pores opening to release water to cool down. Blood glucose regulation also has responses for high or low levels. The body gets glucose from food eaten, which is broken down and absorbed into the blood stream through the small intestine. Hyperglycaemia follows, whereby the blood is highly concentrated with glucose. The pancreas secretes insulin created by b-cells to travel through the blood targeting glucose cells to convert it into glycogen to be stored in the liver. Hypoglycaemia has the opposite effect and alpha cells use the stored glycogen to convert it back to glucose and raise the blood glucose to a homeostatic level. The fight or flight response is the body’s reaction to severe stress, enough to trigger a survival mode. Both nervous and endocrine

sy ation by putting the hypothalamus into ov he adrenal-cortical system. The hypothalamus Figure - Fight or flight response (Layton) emits corticotropin-releasing factor to the pituitary which in-turn secretes adrenocorticotropic hormone (Layton, 2005). This is travels through the blood to the adrenal-cortical system, which releases around thirty hormones, including glucose, epinephrine and noradrenaline. These hormones create a cascade and are responsible for the physical reactions experienced by the sympathetic nerves. Referring back to figure 1, the sympathetic nerves essentially shut down systems which aren’t essential, such as the immune and digestive systems. That energy is diverted to increase the heartbeat and breathing, dilating the pupils and tensing the muscles.

Conclusion The nervous and endocrine systems are only as efficient as they are due to thousands of years of evolution. In terms of the human fight or flight response, it is on level to that of another wild animal, like an owl puffing up its feathers: it’s giving a perceived form to a potential threat. Both systems are intricate and rely on the previous step in a perspective chain to fulfil its obligation. As such, there are many health conditions that can befall any of us. With a dysfunctional pancreas, one lacks insulin and develops diabetes mellitus. Imbalances in acetylcholine can lead to neurodegenerative conditions, such as Alzheimer’s disease. It is best to live preventatively in order to avoid these complications.

Recommendations I recommend further reading into how endocrine conditions can lead to development of neurological and nervous conditions. It is also worth dipping into research papers behind related subjects to broaden scope.

References list  

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BBC Bitesize 2, Coordination and control – The human endocrine system [Online] Available at: https://www.bbc.co.uk/bitesize/guides/z8t47p3/revision/ (Accessed October 2019) BioNinja, Resting Potential [Online] Available at: https://ib.bioninja.com.au/standardlevel/topic-6-human-physiology/65-neurons-and-synapses/resting-potential.html (Accessed October 2019) Chung, A. (2019), Spinal Cord Anatomy [Online] Available at: https://www.healthpages.org/anatomy-function/spinal-cord-anatomy (Accessed October 2019) En.wikipedia.org (2019), Hormone receptor [Online] Available at: https://en.wikipedia.org/wiki/Hormone_receptor (Accessed October 2019) ePearl, The Human Nervous and Endocrine Systems [Online] Available at: https://www.epearl.co.uk/study-material/4583 (Accessed October 2019) HealthDirect, (2017), Motor neurone disease (MND) [Online] Available at: https://www.healthdirect.gov.au/motor-neurone-disease-mnd (Accessed October 2019) Layton, J. (2005), How Fear Works [Online] Available at: https://science.howstuffworks.com/life/inside-the-mind/emotions/fear2.htm (Accessed October 2019) Mandal, A. (2019), What is the Nervous System? [Online] Available at: https://www.newsmedical.net/health/What-is-the-Nervous-System.aspx (Accessed October 2019)

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Pathology & Laboratory Medicine (2016), Endocrine System Disorders [Online] Available at: https://basicmedicalkey.com/endocrine-system-disorders/ (Accessed October 2019) ThoughtCo, Anatomy of the Brain [Online] Available at: https://www.thoughtco.com/anatomy-of-the-brain-373479 (Accessed October 2019) Zimmermann, K. (2018), Nervous System: Fact Functions & Diseases [Online] Available at: https://www.livescience.com/22665-nervous-system.html (Accessed October 2019)

Bibliography

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BBC Bitesize 1, The nervous system, BBC.co.uk, viewed October 2019, https://www.bbc.co.uk/bitesize/guides/zkdnb9q/revision/1 BBC Bitesize 2, Coordination and control – The human endocrine system, BBC.co.uk, viewed October 2019, https://www.bbc.co.uk/bitesize/guides/z8t47p3/revision/1 BBC Bitesize 3, How does the nervous system help us respond? – OCR 21C, BBC.co.uk, viewed October 2019, https://www.bbc.co.uk/bitesize/guides/ztjrng8/revision/1 BioNinja, Resting Potential, BioNinja, viewed October 2019, https://ib.bioninja.com.au/standard-level/topic-6-human-physiology/65-neurons-andsynapses/resting-potential.html Chung, A., 2019, Spinal Cord Anatomy, Health Pages, viewed October 2019, https://www.healthpages.org/anatomy-function/spinal-cord-anatomy En.wikipedia.org, 2019, Hormone receptor, Wikipedia, viewed October 2019, https://en.wikipedia.org/wiki/Hormone_receptor

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ePearl, The Human Nervous and Endocrine Systems, ePearl, viewed October 2019, https://www.epearl.co.uk/study-material/4583

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HealthDirect, 2017, Motor Neurone Disease (MND), healthdirect.gov.au, viewed October 2019, https://www.healthdirect.gov.au/motor-neurone-disease-mnd

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Layton, J., 2005, How Fear Works, HowStuffWorks.com, viewed October 2019, https://science.howstuffworks.com/life/inside-the-mind/emotions/fear2.htm

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Mandal, A., 2019, What is the Nervous System?, News Medical Life Sciences, viewed October 2019, https://www.news-medical.net/health/What-is-the-Nervous-System.aspx

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News Medical Life Sciences, 2016, Neurons in hypothalamus help maintain blood glucose levels, study finds, News Medical Life Sciences, viewed October 2019, https://www.newsmedical.net/news/20160324/Neurons-in-hypothalamus-help-maintain-blood-glucose-levelsstudy-finds.aspx

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Pathology & Laboratory Medicine, 2016, Endocrine System Disorders, Basicmedical Key, viewed October 2019, https://basicmedicalkey.com/endocrine-system-disorders/

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ThoughtCo, Anatomy of the Brain, ThoughtCo, viewed October 2019, https://www.thoughtco.com/anatomy-of-the-brain-373479 Zimmermann, K., 2018, Nervous System: Facts, Function & Diseases, Live Science, viewed October 2019, https://www.livescience.com/22665-nervous-system.html

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