Reflexes in the nervous system
The Human Nervous System
A reflex is a simple,quick form of activity in the nervous system. It’s an involuntary response to the stimulus. The reflex arc is the neuron which forms a trail of the impulses of the reflex. Some of the reflex actions include eye blinking, breathing, plus other protective actions like drawing a finger out of a hot flame as shown below.
When a finger is drawn from a hot flame a withdrawal reflex is done. The stages of the reflex action are as presented below. First, the finger is a receptor which has the sensory neurons that transmit the impulses to the sensory nerves found in the dorsal root. Next the relay transmits the impulses from the vertebral column to the motor neurons found in the ventral root, where another neuron carries the impulse to the brain. A motor neuron carries the impulses to the muscle where the finger is drawn away. Here, the impulses reach the brain and a person feels the pain.
Types of Neurons.
A neuron is defined as a specialized cell within the cell body, the largest part which has cytoplasm and nucleus. A majority of metabolic activity of the cell such as the production of adenine triphosphate and production of proteins takes place in the cell body (Beloozerova, Sirota & Swadlow, 2003, p.1087). The dendrites plus the entire cell body surface which are the nerve cell receptive apparatus and also take impulses from the environment. The axon is protected by an insulating sheath of myelin in animals without vertebral column as well as some vertebrates and carries impulses from the cells body Neurons can take different shapes depending on their location as well as function (Beloozerova, Sirota & Swadlow, 2003, p.1087). The functional types of neurons are sensory neurons, motor neurons, and
Sensory/afferent neurons; are bipolar/multipolar/unipolar shaped cells used to conduct action potentials into the central nervous system (CNS). They have long dendrites plus short axon where the cell body and the dendrite are found outside the spinal cord while the cell body is found in the dorsal root ganglion. They take somatic nervous system signals from the skeletal muscles, joints, the skin, and sensory organs (Beloozerova, Sirota & Swadlow, 2003, p.1087). Also, they carry autonomous nervous system signals in the lungs, heart, and the vessels.
Types of neurons and their functions
Motor/efferent neurons; are multipolar in shape and conduct action potentials from the CNS. It has short dendrites plus long axons (Burnstock, 2008, p.14). The cell bodies, as well as the dendrites, are found in the CNS where their axons are inside the nerves to the other organs.
Interneurons; also known as a relay or intermediate neurons carry information from the sensory through motor neurons and are found in the CNS or within the spinal cord. The neuron has short dendrites plus short/long axons.
How a neuron generates and propagates nerve impulses
Neurons are said to be in a resting state when they do not conduct impulses. Here, the axon membrane is permeable to potassium ions (K+) plus impermeable to sodium ions (Na+ ) as well as the negatively charged proteins in the axoplasm (Burnstock, 2008 p.17). The axon plasma is rich in proteins and (K+) and low in Na+. Also, the fluid surrounding the axon is rich in Na+ and low in (K+). Due to such, a concentration gradient is formed. Also, active transport of ions occurs across the membrane through the sodium-potassium pump. Here three sodium ions are drawn outside and two potassium ions inside the cell. This makes the exterior of the cell to be positively charged while the interior negatively charged, a state of polarization.
When a stimulus is applied to the region of the polarized membrane, the membrane at the region becomes permeable to sodium ions, hence leading to sodium ions move inside the cell. This makes the interior of the cell to be positively charged while the exterior negatively charged, a depolarization state. The region becomes a stimulus for the neighboring section of the membrane that becomes depolarized. Also, the formed membrane gets repolarized because of movement of sodium ions outside the cell.
How synapse receives and relays nerve impulses
The action potential gets into the pre-synaptic neuron that changes its voltage leading to a voltage-gated calcium (Ca) channel on a presynaptic neuron to open. Here, the Ca2+ move into the pre-synaptic neuron where a high Calcium concentration leads to synaptic vesicle (Katz & Miledi, 2001 p.410). The vesicle diffuses into the membrane where a neurotransmitter is taken into the area between the two neurons. Next, the acetylcholine neurotransmitter fuses across the synaptic cleft to the membrane of the postsynaptic neuron. An increase in neurotransmitter concentration causes chemically gated channels in the postsynaptic neuron depolarize the membranes of the neuron hence leading to excitatory postsynaptic potentials, EPSPS (Kacsoh, 2000 p. 566). When the EPSPS attain a certain limit, the action potential is started in the postsynaptic neuron where an impulse is positively transferred from one neuron to the other.
Propagation of nerve impulses in a neuron
Conclusion:
Although the nervous and the endocrine systems are all regulatory, it should be noted that the latter involves the use of chemical signals while the nervous system uses neural impulses. The transmission of the signals in the systems vary due to interconnection of the neurons and time taken for the hormones to pass via the blood stream.
Hormones found in the endocrine system
Testosterone; it is a male sex hormone produced by the testes as well as in small quantities in the female ovaries. Testosterone helps the growth of male gonads as well as male secondary sex traits.
Aldosterone; it is an adrenal gland hormone that helps in blood pressure regulations through raising blood pressure as well as volume.
Growth hormone; stimulates the growth of bones plus tissues and also the emotional wellness of a person.
Thyroid stimulating hormone (TSH); helps the thyroid gland to give thyroid hormones
Mode of action of a steroid hormone
Steroid hormones leads to changes in a cell by passing via the plasma membrane of the target cell. They achieve this since they are soluble in fats. The plasma membrane comprises of a phospholipid bilayer that averts fat insoluble compounds from spreading into a cell (Kloosterboer, 2001 p.235). When inside the cell, the hormones bind with a receptor in the cytoplasm of a target cell and then travels into the nucleus. Here it binds to a different receptor on the chromatin. Once in the chromatin, the steroid hormone receptor complex calls for the synthesis of RNA molecules by transcription (Nieuwenhuys, Voogd, &Van, 2007). The molecules are changed as well as moved to the cytoplasm. The mRNA molecules then code for the synthesis of proteins via translation.
Mode of action of a water soluble hormone.
The hormone molecule attaches to the receptor molecules extending from the edge of the target cell. Attachment of the hormone to the receptor causes a chemical reaction inside the cell without a hormone molecule entering the cell (Marieb, & Hoehn, 2004 p.610). Next, the chemical reaction activates the enzymes inside the cell and then change the biochemical action of the cells such that the rate of activities executed by the cell is decreased or increased (Westenbroek, Hoskins, & Catterall, 2008). That is to say, some processes occur very quick or slow because of the instruction a cell receives by the binding of the hormone to a receptor.
Mode of action of steroid and water-soluble hormones
3.Examples of negative feedback and positive feedback
Positive feedback; is when production of a reaction leads to an increase in that reaction. For a system, in homeostasis, the positive feedback shifts the system far from the equilibrium target (Cannon &Rosenblueth 2007). It amplifies the effects of the product plus occurs when an event has to happen suddenly. Examples of positive feedback include fruit ripening, childbirth, and blood clotting.
Negative feedback; the sensory cells in the body of the animal with vertebral column helps monitor the reference area of the internal environment. Changes in the internal environment can trigger, the receptor cells that relay information to the control center (Cannon &Rosenblueth 2007). The latter determines the deviation as well as activates the effectors which respond to the stimulus plus correct the reference area by increasing/decreasing its activation. Immediately the system is corrected, the effectors plus the control center are switched off. In negative feedback mechanism, changes occurring in a system automatically triggers the corrective mechanism, which reverses the changes and brings the system to normal (Harris & Levine, 2005). Examples of negative feedback mechanism include regulation of thyroxine hormone, blood pressure, blood sugar and PH.
Mechanisms of positive and negative feedback
Childbirth (positive feedback); when labor starts a baby’s head is moved downwards thus resulting much pressure on the cervix hence triggering receptor cells to relay chemical signals to the brain (Harris & Levine, 2005). This releases oxytocin which diffuses to the cervix via the blood where it’s stimulated more contractions. Such contractions trigger the release of oxytocin until a baby is born.
Regulation of thyroxine hormone (negative feedback); low thyroxine hormone concentration in blood changes the cellular activities that are a decline in basic metabolic rates or temperature (Johansson et al., 2009 p.30). Such decline stimulates neurosecretory cells of the hypothalamus to synthesize thyrotropin-releasing hormone (TRH) that leads to the secretion of thyroid stimulating hormone (TSH) by the pituitary gland. The TSH then triggers thyroid gland to release thyroxine. The latter leads to an increase in the metabolic activity, producing ATP energy, heat and ultimately restores homeostasis (Karsch et al., 2013). The raised body temperature plus the higher thyroxine levels in the body feedback to lower the release of TSH and TRH.
The nervous system helps collect, process, as well as respond to information via electric signals. It comprises of the central nervous system and the peripheral nervous system.
On the other hand, the endocrine system helps in regulation. It releases hormones that travels via the blood stream and also controls the action of organs and cells. The rationale of this report is to explain how the systems coordinate functions of different systems in the human body and how such relates to homeostasis.
Reference:
Beloozerova, I.N., Sirota, M.G. and Swadlow, H.A., 2003. Activity of different classes of neurons of the motor cortex during locomotion. Journal of Neuroscience, 23(3), pp.1087-1097.
Burnstock, G., 2008. Non-synaptic transmission at autonomic neuroeffector junctions. Neurochemistry international, 52(1-2), pp.14-25.
Cannon, W.B. and Rosenblueth, A., 2007. Autonomic neuro-effector systems. The Macmillan Company.
Harris, S.L. and Levine, A.J., 2005. The p53 pathway: positive and negative feedback loops. Oncogene, 24(17), p.2899.
Johansson, C.B., Momma, S., Clarke, D.L., Risling, M., Lendahl, U. and Frisén, J., 2009. Identification of a neural stem cell in the adult mammalian central nervous system. Cell, 96(1), pp.25-34.
Karsch, F.J., Dierschke, D.J., Weick, R.F., Yamaji, T., Hotchkiss, J. and Knobil, E., 2013. Positive and negative feedback control by estrogen of luteinizing hormone secretion in the rhesus monkey. Endocrinology, 92(3), pp.799-804.
Katz, B. and Miledi, R., 2001. A study of synaptic transmission in the absence of nerve impulses. The Journal of physiology, 192(2), pp.407-436.
Kacsoh, B., 2000. Endocrine physiology (pp. 566-567). New York: McGraw-Hill.
Kloosterboer, H.J., 2001. Tibolone: a steroid with a tissue-specific mode of action. The Journal of steroid biochemistry and molecular biology, 76(1-5), pp.231-238.
Nieuwenhuys, R., Voogd, J. and Van Huijzen, C., 2007. The human central nervous system: a synopsis and atlas. Springer Science & Business Media.
Marieb, E.N. and Hoehn, K., 2004. The endocrine system. Human Anatomy & Physiology, pp.603-643.
Westenbroek, R.E., Hoskins, L. and Catterall, W.A., 2008. Localization of Ca2+ channel subtypes on rat spinal motor neurons, interneurons, and nerve terminals. Journal of Neuroscience, 18(16), pp.6319-6330.
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