The human body is made up of ten major organ systems that work together as a cohesive unit to ensure that the body functions properly. Although each physiological system plays a unique role in keeping the body working, the nervous system is the most important. The nervous system, which consists of the brain, spinal cord, and nerves, serves as the body’s “control center.” The brain employs the information it receives (from stimuli), interprets it, and transmits impulses to organs in order to coordinate all of the body’s actions (and reactions). Each of the body’s tissues and organs require instruction from the nervous system. The nervous system interacts with each physiological system of the body. Three examples of systems that the nervous system is linked to are the musculoskeletal, cardiovascular, and endocrine systems.
The somatic nervous system is a part of the peripheral nervous system associated with the voluntary contraction and relaxation of skeletal muscles (through somatic/motor neurons). Contraction of skeletal muscles (of the trunk and limbs) produce movement at the joints. This contraction is generated and controlled by excitation-contraction coupling, which is the physiological process of converting an electrical stimulus from the nervous system into a mechanical response. This process links an action potential generated in the sarcolemma to the muscle in order to start a contraction. First, an action potential is fired to a somatic neuron, which releases acetylcholine (a neurotransmitter) into the neuromuscular junction. Acetylcholine then binds to receptors located in the sarcolemma, which triggers the depolarization of the sarcolemma. Next, the action potential propagates along the sarcolemma and down the t-tubules. The signal travels into the sarcoplasmic reticulum, which triggers the release of stored calcium ions. The calcium binds to troponin (which changes its shape) and removes the blocking action of tropomyosin. This exposure allows myosin to bind to actin, which forms a cross-bridge. Finally, muscle contraction occurs.
The autonomic nervous system regulates processes of the cardiovascular system that are performed automatically (without conscious effort), such as blood pressure, heart rate, and breathing rate. When the autonomic nervous system receives stimuli, it responds by activating body processes (through the sympathetic nervous system) or inhibiting them (through the parasympathetic nervous system). Stress or emergency situations activate the sympathetic division, which triggers the body’s “fight or flight” response. For example, if an individual experiences a motor vehicle accident, the sympathetic nervous system makes his/her heart contract harder and faster (forcing out a larger volume of blood to hard-working body tissues more quickly). These factors combine to increase pressure in the blood vessels and widen (dilate) the airways to make breathing easier (in order for the body to take in more oxygen, which is needed for the cells to produce energy). The parasympathetic division is responsible for controlling body process under “normal” situations. The parasympathetic division conserves and restores (“rest and digest” response). After the individual survives the hypothetical car accident, his/her heart rate and blood pressure decrease as the body relaxes.
In addition, the nervous system controls vagal tone (the activity of the vagus nerve), which controls the rate at which the heart pumps. The vagus nerve belongs to the parasympathetic nervous system and acts on the sinoatrial (SA) node. Without the vagus nerve, the body’s resting heart rate would be controlled solely by the SA node (the heart’s “pace maker”), which also determines the intrinsic rate of the heart (approximately 100 beats per minute). This would result in increased blood pressure. However, this high blood pressure is detected by baroreceptors (pressure receptors), which send information to the medulla of the brain, and the brain makes adjustments to decrease heart rate and blood pressure. The vagus nerve helps the heart maintain a resting rate of 60 to 80 beats per minute.
The hypothalamus is a part of the brain that links the nervous system to the endocrine system via the pituitary gland. The hypothalamus synthesizes and secretes certain hormones (e.g. Thyrotropin (TRH), Corticotrophin Releasing Hormone (CRH), Gonadotropin Releasing Hormone (GnRH), etc.) which control and regulate the secretion of eight major pituitary hormones (e.g. Thyroid Stimulating Hormone (TSH), Follicle-Stimulating Hormone (FSH), Luteinizing Hormone (LH), Prolactin (PRL), Growth Hormone (GH), Alpha Melanocyte-Stimulating Hormone (?-MSH), Vasopressin, and Oxytocin). The HPT and HPA axes are two examples of pathways in which the hypothalamus communicates with the pituitary in order to direct neuroendocrine function. The hypothalamic–pituitary–thyroid axis (HPT axis) is part of the neuroendocrine system responsible for the regulation of metabolism. This feedback loop involves the hypothalamus, pituitary gland, and thyroid gland. First, the hypothalamus detects low levels of thyroid hormone (Triiodothyronine (T3) and Thyroxine (T4)) and responds by releasing TRH, which travels through the portal system to anterior pituitary, which stimulates the production of TSH. The TSH travels to the thyroid gland, which stimulates the release of thyroid hormone (TH) until its levels in the blood return to normal. TH has negative feedback control over the release of both TRH from hypothalamus and TSH from anterior pituitary gland. The Hypothalamus–Pituitary–Adrenal axis (HPA axis) follows a similar series of steps in order to control reactions to stress. The hypothalamus secretes CRH, which travels to the anterior pituitary and stimulates the release of adrenocorticotropic hormone (ACTH). The ACTH acts on the adrenal cortex, which secretes cortisol (hormone responsible for controlling blood sugar levels, regulating metabolism, etc.).
The nervous system is the most important physiological system in the human body. The somatic nervous system controls the movement of the trunk and limbs of the musculoskeletal system by excitation-reaction coupling. The sympathetic and parasympathetic branches of the autonomic nervous system are responsible for regulating cardiovascular processes, including heart rate and blood pressure. The vagus nerve, which belongs to the parasympathetic nervous system, also affects heart rate by utilizing baroreceptors to assist in the adjustment of the pulse to its homeostatic level (60 to 80 beats per minute). The hypothalamus of the brain sends signals to the pituitary in order to regulate neuroendocrine processes, such as in the HPT and HPA axes.