Nerves, hormones and homeostasis

1. Nerves, hormones and homeostasis Topic 6.5

2. Assessment Statements • 6.5.1 State that the nervous system consists of the central nervous system (CNS) and peripheral nerves, and is composed of cells called neurons that can carry rapid electrical impulses. • 6.5.2 Draw and label a diagram of the structure of a motor neuron. • 6.5.3 State that nerve impulses are conducted from receptors to the CNS by sensory neurons, within the CNS by relay neurons, and from the CNS to effectors by motor neurons. • 6.5.4 Define resting potential and action potential (depolarization and repolarization). • 6.5.5 Explain how a nerve impulse passes along a non-myelinated neuron. • 6.5.6 Explain the principles of synaptic transmission. • 6.5.7 State that the endocrine system consists of glands that release hormones that are transported in the blood.

3. 6.5.8 State that homeostasis involves maintaining the internal environment between limits, including blood pH, carbon dioxide concentration, blood glucose concentration, body temperature and water balance. • 6.5.9 Explain that homeostasis involves monitoring levels of variables and correcting changes in levels by negative feedback mechanisms. • 6.5.10 Explain the control of body temperature, including the transfer of heat in blood, and the roles of the hypothalamus, sweat glands, skin arterioles and shivering.

4. 6.5.11 Explain the control of blood glucose concentration, including the roles of glucagon, insulin and α and β cells in the pancreatic islets. • 6.5.12 Distinguish between type I and type II diabetes.

5. Organization of the human nervous system • Central nervous system (CNS) consists of the brain and spinal cord • CNS receives sensory information from various receptors and then interpret and process that sensory information • Motor response – response initiated by the CNS • Neurons – cells which carry electrical impulses from one point in the body to another • Sensory neurons bring info. into the CNS • Motor neurons carry response info. to muscles

6. Peripheral nervous system (PNS) is made up of the sensory and motor neurons • When many individual neurons are grouped together into a single structure, the structure is called a nerve • Peripheral nerves: • Spinal – 31 pairs emerge directly from the spinal cord; mixed sensory and motor nerves • Cranial – 12 pairs emerge from an area of the brain called the brainstem

7. Motor neuron dendrites Cell body Action potential nucleus axon Synaptic terminals of axon Myelin sheaths made of Schwann cells Nodes of Ranvier

9. Nerve impulses are conducted… • From receptors to the CNS by sensory neurons • Within the CNS by relay neurons • From the CNS to effectors by motor neurons

10. Example • You touch the arm of the person sitting next to you; Touch was accidental and you immediately remove your hand • Your touch began caused a pressure (sensory) receptor to begin an action potential • One of the spinal nerves carried the signal to spinal cord • Spinal cord made use of relay neurons to route the potential in the CNS to the appropriate area for interpretation • Your brain used relay neurons to pass action potential to spinal cord, then spinal nerves, then by way of motor neurons to cause your hand to move

11. When the action potential reaches the muscle (effector), the motor neuron sends a chemical signal to the muscle which results in a contraction

12. What is a nerve impulse? • Series of action potentials carried by axons • Axons are surrounded by a membranous structure called the myelin sheath which increase rate at which an action potential passes

13. Resting potential • The state of being where an area of neuron is ready to send an action potential; area is said to be polarized • Polarization is characterized by the active transport of sodium ions (Na+) and potassium ions (K+) • Vast majority of Na+ are transported out of the axon; majority of K+ are transported into the axon; additionally there are negatively charged ions permanently located in the cytoplasm of the axon • Net result is + outside the axon and - inside

15. Action potential of non-myelinated sheaths • Self-propagating wave of ion movements in and out of the neuron membrane • Movement is not along length of axon, but consists ions diffusing outside and inside of the axon • Requires active transport (protein channels and ATP) to set up a concentration gradient of both K+ and Na+ • Na+ actively pumped out; diffuse in when a channel opens • Channel opens for K+ to diffuse out

16. This diffusion is the “impulse” or action potential • Nearly instantaneous event which occurs in one area of an axon and is called depolarization • This area then initiates the next area of the axon to open up the channels for sodium, then potassium and thus the action potential continues down the axon • http://www.blackwellpublishing.com/matthews/channel.html • http://bcs.whfreeman.com/thelifewire/content/chp44/4402002.html

17. Myelinated fibers • Myelin insulates fiber from the extracellular fluid • Myelin sheath interrupted by nodes of Ranvier • Na+ that enters at the previous node diffused down the fiber under the axolemma • Resistance occurs and the signal becomes weaker the further it goes • Nodes are close together and the signal is just strong enough to open gates and create a new action potential

18. Return to the resting potential • Neurons may send dozens of action potentials in a short period of time • Must wait until the sodium and potassium ions have been restored to original resting potential • Active transport causes this repolarization • The time that it takes for any one neuron to send an action potential and then repolarize so it can send another is called the refractory period of that neuron

19. Synaptic transmission • A sensory pathway is unidirectional b/c the sensory neurons of the pathway are lined up so that the terminal end of the axon of the first neuron adjoins the dendrites of the next neuron • 1st neuron – presynaptic neuron • 2nd neuron – postsynaptic neuron • Synapse occurs between neurons

20. Patterns of synaptic transmission • Presynaptic neuron → postsynaptic neuron • Several presynaptic neurons → postsynaptic neuron • Presynaptic neuron → several postsynaptic neurons

21. Mechanism of synaptic transmission • Far end of axons are swollen membranous areas called terminal buttons • Within these terminal buttons are many small vesicles filled with a chemical called a neurotransmitter (ex: acetylcholine) • When an action potential reaches the area of the terminal buttons, it initiates the following sequence of events

22. Calcium ions diffuse into the terminal buttons • Vesicles containing neurotransmitter fuse with the plasma membrane and release neurotransmitter • Neurotransmitter diffuses across the synaptic gap from the presynaptic neuron to the postsynaptic neuron • Neurotransmitter binds with a receptor protein on the postsynaptic neuron membrane

23. This binding results in an ion channel opening and sodium ions diffusion in through this channel • This initiates the action potential to begin moving down the postsynaptic neuron because it is depolarized • Neurotransmitter is degraded by specific enzymes and is released from the receptor protein • The ion channel closes to sodium ions

24. Neurotransmitter fragments diffuse back across the synaptic gap to be reassembled in the terminal buttons of the presynaptic neuron

25. Homeostasis • Maintaining normal limits for physiological variables • Variables include: • Blood pH • Carbon dioxide concentration • Blood glucose concentration • Body temperature • Water balance within tissues • The physiological changes that bring a value back closer to a set point are called negative feedback mechanisms

26. Endocrine system • Works cooperatively with the nervous system in order to ensure homeostasis • Consists of numerous glands which produce a wide variety of hormones • Each hormone is transported by the bloodstream from the gland where it is produced to the specific cell types in the body that are influenced by that particular hormone

27. Control of body temperature • Biological thermostat for temperature control is in the hypothalamus • You exercise, body temp. rises • Hypothalamus receives info. from thermoreceptors in your skin and begins cooling mechanisms • Increased activity of sweat glands, evaporative cooling • Arterioles in skin dilate filling skin capillaries with blood; heat leaves the skin capillaries by radiation

29. You are in cold air environment • Hypothalamus receives info. from thermoreceptors in skin and begins warming mechanisms • Constriction of skin arterioles, blood is diverted to deeper organs; less heat is lost as radiation • Skeletal muscle stimulated to shiver; results in production of heat

30. Blood glucose level • Concentration of glucose dissolved in blood plasma • Cells rely on glucose for the process of cell respiration which they are constantly carrying out • Glucose is absorbed into the bloodstream in the capillary beds of the villi of the small intestine and thus increases blood glucose level • Level “see-saws” 24 hrs a day • Maintained by negative feedback mechanism

31. Route of glucose • Intestinal villi • Capillaries, small venules, veins, hepatic portal vein • Liver • Hepatocytes • All other blood vessels • Hepatocytes are directed to action by two hormones, insulin and glucagon, which are produced in the pancreas

32. When blood glucose level goes above the set point… • First effect: • Within the pancreas β cells produce the hormone insulin and secretes the insulin which is later absorbed by the bloodstream • Insulin opens protein channels in cells’ plasma membranes allowing glucose to diffuse into the cell by facilitated diffusion • Second effect: • When blood relatively high in glucose enters the liver by the hepatic portal vein, insulin stimulates the hepatocytes to take in the glucose and covert it to glycogen • The glycogen is then stored as granules in the cytoplasm of the hepatocytes. The same effect occurs in muscles.

33. When blood glucose level goes below the set point… • Begins when someone has not eaten for many hours or exercises vigorously for a long time • α cells of the pancreas produce and secrete the hormone glucagon • Glucagon circulates in the bloodstream and stimulates hydrolysis of the granules of glycogen stored in hepatocytes and muscle cells producing glucose • Glucose enters the bloodstream, increasing blood glucose

35. Diabetes • Disease characterized by hyperglycemia (high blood sugar) • People who have untreated diabetes have plenty of glucose in their blood, but not in their body cells where it is needed • Uncontrolled diabetes can lead to many serious effects including: • Damage to the retina leading to blindness • Kidney failure • Nerve damage • Increased risk of cardiovascular disease • Poor wound healing (and possibly gangrene)

36. Type I diabetes • Caused when the β cells of the pancreas do not produce enough insulin • Can be controlled by the injection of insulin at appropriate times • An autoimmune disease where the body’s own immune system attacks and destroys the β cells of the pancreas so little to no insulin is produced • < 10% of diabetics are this type • Most often develops in children or young adults, but can develop in people of any age

37. Type II diabetes • Caused by body cell receptors do not respond properly to insulin • Controlled by diet • Known as insulin resistance • Initially, the pancreas continues to produce a normal amount of insulin, but this level may decrease after a period of time. • Most common form (90% of diabetics) • Associated with genetic history, obesity, lack of exercise, advanced age, and certain ethnic groups

38. Diary of a diabetic