Unit 15 - The endocrine and Nervous system - Illustrated Report - Distinction PDF

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Emma O’Casey 07/04/2021 Unit 15 – Illustrated Report The Human Endocrine and Nervous Systems

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Introduction

This report demonstrates a basic understanding of the human endocrine and nervous systems and the vital roles they play. It will: examine the importance of the spinal reflex arc, propagation of the neuron action potential, and synaptic transmission; and the role hormones plays in maintaining homeostasis.

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CONTENTS PAGE

Title Page

Introduction

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Contents Page

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Findings

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Section 1

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Section 2

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Section 3

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Conclusion

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Recommendations

References

Bibliography

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Findings

Section 1 The Nervous System

Structure of the Nervous system

Figure 1 shows the Nervous System is divided into two parts, the Central Nervous System (CNS), and Peripheral Nervous System (PNS). The Nervous System is made of specialist cell fibres known as neurons. They are concentrated in the CNS, and also laid out on specific routes within the body: to send and receive electrical messages, either sensory (afferent), or motor (efferent). The brain and spinal cord are the key features of the CNS, which is filled with cerebrospinal fluid and protected by surrounding membranes known as the meninges.

Figure 1: CNS and PNS (The Nervous System: Linking Neurons, 2011)

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Figure 2: The Brain. (Lumenlearning.com, 2019)

The Brain is the largest and most complex organ in the body. It is divided into three main regions: The forebrain is the largest, and includes the two hemispheres of the cerebral cortex. This is subdivided into four lobes, each associated with different functions. Conducting thought and memory. The midbrain connects the forebrain to the hindbrain. It’s the top part of the brain stem, which connects the brain to the spinal cord. Associated with motor functions and audio-visual information. The hindbrain contains the rest of the brainstem. The two main structures are the medulla oblongata and the cerebellum. Manages automatic functions, e.g., breathing, coordination.

The Spinal Cord is the motorway network. Running the length of the back, it carries information between the brain and body. From the brainstem, 31 spinal-nerve pairs enter it, and connect with the PNS. They fork into two when close to the spinal cord: the uppermost (dorsal root) carrying sensory neurons, the lower (ventral root) motor neurons. In a one-way-system, information arrives from the sensory tissues, e.g., skin, and travels up to the brain; and motor commands travel down the spine to the muscles and glands. It is an ‘intelligent’ network: hosting hundreds of life-preserving Reflex Arcs.

The PNS contains countless afferent and efferent nerves that connect the organs, muscles, and indeed every part of the body to the spinal cord.

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The Autonomic Nervous System controls muscles of internal organs and glands. It has two branches which are antagonistic to each other.

Figure 3: The Autonomic Nervous System (The Nervous System: Linking Neurons, 2011)

The sympathetic system is rapid-acting and activates the “fight-or-flight” response. The body speeds up, tenses and becomes super-alert. Functions that are not essential for immediate survival are shut down. The parasympathetic system is slow-acting and counterbalances this, resetting the default “rest and digest” mode: and restoring the body to a state of calm.

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Spinal Reflex Arc A receptor sends an impulse along an afferent neuron, which makes either multiple, or single synapses inside the spinal cord leading to motor stimulation, e.g., the knee jerk reflex is monosynaptic. Jana Vasković (2020) summarises the importance of spinal reflexes: “The spinal cord is phylogenetically older than most structures of the brain, which means reflexes are in charge of functions that the organism needs most. It is more important to remove your hand from a heat source in order to avoid getting burnt than to be able to speak. Many protective functions necessary for survival are embedded within the spinal reflexes.” An example of a polysynaptic spinal reflex arc is the Crossed-Extensor Reflex:

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Stepping on broken glass causes right-foot pain receptors to excite afferent neuron.

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Sensory neuron activates multiple interneurons.

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Motor neurons to flexor cause contraction of right leg.

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Motor neurons to left-leg extensor cause simultaneous extension – maintaining balance.

Figure 4: The components in the Crossed-Extensor Reflex (Cenveo,2020)

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Propagation of a Nerve Impulse In the Nervous System, information is conducted in the form of nerve action potentials. Prior to a nerve impulse, the inside of a neuron is polarised by the action of K+ ions: with a resting potential of -70mV relative to the outside. Figure 5: Resting potential (highbury.epearl.co.uk, n.d.) When a stimulus arrives at the resting neuron, the Na+ channels open. Ions move in slowly making the resting potential less negative. At a threshold of -50mv, rapid Na+ flow occurs, leading to a positive action potential inside the membrane of +40mv. This propagates along the axon away from the cell body in a wave of “depolarisation” moving section by section

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In all vertebrates, myelinated axons enable the action potentials to propagate at very high speeds. Conduction occurring in a series of ‘saltatory jumps,’ from one ‘node of Ranvier’ to another.

Figure 6: The passage of depolarization ( Kujawski, S. 2017)

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Synaptic Transmission

Neurons communicate with each other by means of Synaptic Transmission, a complex series of electrochemical processes.

Figure 7: Synaptic transmission (Julien, R. M. 2005)

The diagram illustrates the key features. In essence, the electrical action potential in the incoming axon converts to chemical neurotransmitters at the presynaptic membrane of the Axon Terminal. Neurotransmitters, e.g., acetylcholine, cross synaptic clefts and trigger the creation of a new electrical impulse in the dendrites of the further neuron. Synapses have additional functions: ensuring one-way conduction; allowing inhibition (with different neurotransmitters) as well as excitation; protecting efferent organs from overstimulation; filtering out lowlevel stimuli; information processing by summation of incoming signals, and (in modified form) forming the basis of memory.

Section 2 The Endocrine System

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The endocrine system is a group of glands that produce and secrete hormones (chemical messengers) traveling to target organs to maintain homeostasis, respond to stress, and control growth and sexual development. Hormones are instrumental in maintaining correct physiological parameters for the optimal functioning of the organs.

Major Endocrine Glands

Figure 8: Major Endocrine Glands (Arpan Bhattacharyya, 2020) There are eight major glands: The Hypothalamus works closely with the Pituitary gland, and controls the release of other hormones. The Pituitary is called the ‘master gland’ due to its role instructing other glands as well as secreting its own. It is divided into two: Posterior and Anterior. They produce many hormones such as; growth hormones and Adrenocorticotrophic hormone to maintain healthy blood pressure and blood sugar levels and many more. The Pineal gland produces and regulates hormones such as melatonin and helps maintain biorhythms. The Thyroid gland is responsible for the maintenance of metabolism, and calcium levels in the blood.

The Thymus makes thymosin that helps develop T cells to fight infection. The Adrenal gland has two parts: the medulla and cortex. The medulla produces stress hormones such as cortisol and adrenalin. The cortex produces hormones such as aldosterone. The adrenal gland helps regulate blood pressure, metabolism, immune system, response to stress, etc. The Pancreas secretes insulin and glucagon which control blood glucose. The Gonads are sex organs including the male testes and female ovaries. Their role is to produce steroid hormones governing gamete production and development of secondary sexual characteristics. The Ovaries produce hormones such as oestrogen and progesterone, while the Testes produce testosterone. (courses.lumenlearning.com, n.d.)

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Regulation of Endocrine Glands

Hormone levels are primarily controlled through Negative Feedback, in which rising levels of a hormone inhibit its further release. Negative Feedback prevents over-activity of hormone systems ensuring the optimum level of hormone activity at the target tissue. After release of the hormone, the outcome of action of the hormone prevents further release. A Negative Feedback control system responds when conditions change from the optimum.

Thyroid regulation for example:

Low thyroxine levels stimulate the hypothalamus to secrete thyrotropin-releasing hormone (TRH). TRH stimulates the pituitary gland to produce thyroid-stimulating hormone (TSH). TSH, stimulates the thyroid gland to secrete its thyroxine. When thyroxine reaches normal levels, the hormones feedback to stop the hypothalamus from secreting TRH and the pituitary from secreting TSH. Without the stimulation of TSH, the thyroid stops secreting hormones returning conditions to its set point.

NOTE: ⊕ = stimulates; ⊖ = inhibits. Figure 9: Negative feedback (Hiller-Sturmhöfel and Bartke, 1998 )

There is a continuous cycle of events in Negative Feedback. Malfunction of the feedback loop can result in a variety of disorders, e.g., diabetes can be caused by too little or too much insulin.

Positive feedback is rare. One example is childbirth contractions which cause the hypothalamus to release oxytocin. Oxytocin travels to the uterus and stimulates more contractions. The contractions signal the body to release more oxytocin which creates more contractions. The feedback loop continues until the child is born.

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Section 3

Thermoregulation Normal body temperature is 37oC. Thermoregulation senses outside stimuli, and when temperature begins to change the CNS sends messages to the hypothalamus which reacts accordingly. If the body temperature falls below 37oC, the nervous system triggers physical responses to combat 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 skins surface. Body temperature rising above 37oC causes the opposite physical reaction: vasodilation, skin reddening, body hairs lying flat, and sweat pores opening to release water and cool by evaporation.

Figure 10: Thermoregulation (tamcross, 2017)

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Fight-or-Flight Response

During a stressful event, the brain processes the signals at the amygdala. This sends a nerve impulse to the hypothalamus. As command centre, the hypothalamus communicates with the rest of the body through the nervous system so that the person has the energy to fight or flee. (Harvard Health Publishing, 2018)

Figure 11: The Fight-or-Flight Response (Moodmetric, 2018)

The hypothalamus activates both the endocrine and sympathetic nervous system. Sympathetic neurons shut down systems which aren’t immediately essential. The adrenal medulla releases adrenalin and noradrenalin. Blood-flow and energy are diverted away from the digestive organs and the skin’s surface to increase heartbeat, breathing and brain-function-capacity; to dilate the pupils, tense muscles, and enhance blood clotting ability. Charles Darwin (1872) argued that “expressions of emotion resembled those of lower animals and that emotions are adaptive because they prompt action responses that are beneficial to the organism. Negative emotions, activated by threat, invoke defence responses.” If fear continues, when the initial adrenalin surge subsides, the hypothalamus activates the second stress response: the HPA axis (hypothalamus, pituitary, and adrenal glands). It triggers the release of the steroid cortisol, as well as more adrenalin, keeping the body on high alert. When the threat passes, the levels fall. The parasympathetic system then dampens down the stress response. (Harvard Health Publishing, 2018)

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Blood Glucose Regulation

In blood glucose regulation both nervous and endocrine systems play an integral role in maintaining blood sugar within the normal range. The body gets glucose from food eaten, which is broken down and absorbed into the blood stream. Hyperglycaemia, whereby glucose is too highly concentrated in the blood, stimulates the pancreas to secrete insulin which is created by beta cells. Insulin decreases the blood glucose level by causing liver cells to convert glucose into glycogen for storage.

The opposite effect is that hypoglycaemia stimulates the pancreas to secrete glucagon made by alpha cells. This causes the liver to release glucose to raise levels back to normal.

Figure 12: Insulin and glucagon (Norman, 2015)

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Conclusions This report shows the importance of both the nervous and endocrine systems and their vital roles in regulating various involuntary functions. Furthermore, the endocrine system plays a vital role in maintaining homeostasis. The role of various hormones is to maintain normal physiological parameters for organs to work efficiently. However, both systems rely on the correct function of the previous step. The slightest malfunction can throw out the delicate balance of hormones and can cause many hormone related diseases or disorders.

Recommendations I would like to study the endocrine system in more detail and, in particular, the working of positive feedback systems in the body. It would be interesting to read research papers for further research into the endocrine system.

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References Cenveo. (n.d.). Reflexes | Anatomy and Physiology I. [online]. [Accessed 06 April.2021]. Available at: https://courses.lumenlearning.com/austinccap1/chapter/reflexes/. courses.lumenlearning.com. (n.d.). Endocrine Glands | Boundless Biology. [online]. [Accessed 06 April.2021]. Available at: https://courses.lumenlearning.com/boundlessbiology/chapter/endocrine-glands/#:~:text=of%20biological%20rhythms Darwin, C. (1872). The Expression of the Emotions in Man and Animals, cited in Lang PJ. et al. (1997). Attention and orienting: sensory and motivational processes, cited in Kozlowska, K. et al. (2015). Fear and the Defense Cascade. Harvard Review of Psychiatry, [online]. [Accessed 06 April.2021]. 23(4), pp.263–287. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4495877/ Dr Arpan Bhattacharyya. (2020). What prompted us to write Endocrinology? [online] [Accessed 3 Apr. 2021].Available at: https://www.diabetesendocrinology.in/endocrine/what-prompted-us-to-writeendocrinology/ Harvard Health Publishing (2018). Understanding the stress response. [online]. [Accessed 06 April.2021]. Harvard Health. Available at: https://www.health.harvard.edu/staying-healthy/understanding-the-stress-response. highbury.epearl.co.uk. (n.d.). Unit 15 Highbury College. [online] [Accessed 06 April.2021]. Available at: https://highbury.epearl.co.uk/study-material/4514 [Accessed 7 Apr. 2021]. Hiller-Sturmhöfel, S. and Bartke, A. (1998). The endocrine system: an overview. Alcohol health and research world, [online]. [Accessed 06 April.2021]. 22(3), pp.153– 64. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6761896/. Julien, R. M. (2005). The neuron, synaptic transmission, and neurotransmitters. In R. M. Julien, A primer of drug action: A comprehensive guide to the actions, uses, and side effects of psychoactive drugs (pp. 60-88). New York, NY, USA: Worth Publishers. Kujawski, S. (2017). The Complete (but Practical) Guide to Phenytoin Dosing. [online]. [Accessed 06 April.2021]. Available at: https://www.tldrpharmacy.com/content/thecomplete-but-practical-guide-to-phenytoin-dosing Lumenlearning.com. (2019). Lobes of the Brain | Introduction to Psychology. [online]. [Accessed 06 April.2021]. Available at: https://courses.lumenlearning.com/waymakerpsychology/chapter/reading-parts-of-the-brain/. Moodmetric. (2018). Fight or flight response is stressful for our bodies. [online]. [Accessed 06 April.2021]. Available at: https://moodmetric.com/fight-flight-response/. Norman, J. (2015). Normal Regulation of Blood Glucose. [online]. [Accessed 06 April.2021]. EndocrineWeb. Available at: https://www.endocrineweb.com/conditions/diabetes/normal-regulation-blood-glucose. tamcross (2017). Thermoregulation Basics in CrossFit Athletes. [online] Tamalpais Crossfit. Available at: https://tamcrossfit.com/thermoregulation-basics-crossfitathletes/.

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The Nervous System: Linking Neurons. (2011). [online]. [Accessed 06 April.2021]. Available at: https://www.distancelearningcentre.com/resources/The%20Nervous %20System%20and%20the%20Endocrine%20System-%20Communicating%20Within %20the%20Body.pdf. Vasković, J. (2020) Spinal reflex. [online]. [Accessed 06 April.2021]. Available at: https://www.kenhub.com/en/library/anatomy/spinal-reflex. BIBLIOGRAPHY

Cenveo. (n.d.). Reflexes | Anatomy and Physiology I. [online]. [Accessed 06 April.2021]. Available at: https://courses.lumenlearning.com/austinccap1/chapter/reflexes/. courses.lumenlearning.com. (n.d.). Endocrine Glands | Boundless Biology. [online]. [Accessed 06 April.2021]. Available at: https://courses.lumenlearning.com/boundlessbiology/chapter/endocrine-glands/#:~:text=of%20biological%20rhythms Darwin, C. (1872). The Expression of the Emotions in Man and Animals, cited in Lang PJ. et al. (1997). Attention and orienting: sensory and motivational processes, cited in Kozlowska, K. et al. (2015). Fear and the Defense Cascade. Harvard Review of Psychiatry, [online]. [Accessed 06 April.2021]. 23(4), pp.263–287. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4495877/ Dr Arpan Bhattacharyya. (2020). What prompted us to write Endocrinology? [online] [Accessed 3 Apr. 2021].Available at: https://www.diabetesendocrinology.in/endocrine/what-prompted-us-to-writeendocrinology/ Harvard Health Publishing (2018). Understanding the stress response. [online]. [Accessed 06 April.2021]. Harvard Health. Available at: https://www.health.harvard.edu/staying-healthy/understanding-the-stress-response. highbury.epearl.co.uk. (n.d.). Unit 15 Highbury College. [online] [Accessed 06 April.2021]. Available at: https://highbury.epearl.co.uk/study-material/4514 [Accessed 7 Apr. 2021]. Hiller-Sturmhöfel, S. and Bartke, A. (1998). The endocrine system: an overview. Alcohol health and research world, [online]. [Accessed 06 April.2021]. 22(3), pp.153– 64. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6761896/. Julien, R. M. (2005). The neuron, synaptic transmission, and neurotransmitters. In R. M. Julien, A primer of drug action: A comprehensive guide to the actions, uses, and side effects of psychoactive drugs (pp. 60-88). New York, NY, USA: Worth Publishers. Kujawski, S. (2017). The Complete (but Practical) Guide to Phenytoin Dosing. [online]. [Accessed 06 April.2021]. Available at: https://www.tldrpharmacy.com/content/thecomplete-but-practical-guide-to-phenytoin-dosing Lumenlearning.com. (2019). Lobes of the Brain | Introduction to Psychology. [online]. [Accessed 06 April.2021]. Available at: https://courses.lumenlearning.com/waymakerpsych...