6.5 Nerves, Hormones and Homeostasis

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.

2. Draw and label the structure of a motor neuron.

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.

4. Define resting potential and action potential (depolarization and repolarization).

resting potential = an electrical potential across a cell membrane when not propagating an impulse

action potential - the localized reversal (depolarization) and then restoration (repolarization) of electrical potential between the inside and outside of a neuron as the impulse passes along it

5. Explain how a nerve impulse passes along a non-myelinated neuron.

role of Na+ ions:

• resting potential: Na+ ions concentrated outside membrane
• action potential: Na+ ions rush to inside of membrane

role of K+ ions:

• resting potential: K+ ions concentrated inside membrane
• action potential: K+ ions rush to outside of membrane

role of ion channels:

• resting potential: Na+ & K+ ion channels closed
• action potential: Na+ channels open 1st, then close, just as K+ ion channels open

role of active transport:

• Na+/K+ pump moves Na+ to outside & K+ to inside of membrane
• pumps ions against concentration gradients; ATP expended
• changes in membrane polarization:

     1. resting potential: - 70 mV potential across membrane
     2. action potential: 
          a. Na+ channels open, reversing membrane potential to + 30 mV
          b. K+ channels open, restoring membrane potential to - 70 mV 

6. Explain the principles of synaptic transmission.

Ca+2 influx and release:

• action potential open Ca+ channels; Ca+ flows in
• Ca+ cause exocytosis of neurotransmitter at synaptic cleft

diffusion and binding of neurotransmitter:

• neurotransmitter diffuses across synaptic cleft
• neurotransmitter binds to post-synaptic receptor

polarization of the post-synaptic membrane:

• EPSP: Na+ channels open; post-synaptic membrane depolarization
• IPSP: K+ channels open; post-synaptic membrane hyperpolarization

subsequent removal of neurotransmitter:

• enzymes hydrolyze neurotransmitter; pre-synaptic recycling

communication between neurons & glands or muscles takes place in synapses

7. State that the endocrine system consists of glands that release hormones that are transported in the blood.

8. State that homeostasis involves maintaining the internal environment between limits, including blood pH, carbon dioxide concentration, blood glucose concentration, body termperature and water balance.

9. Explain that homeostasis involves monitoring levels of variables and correcting changes in levels by negative feedback mechanisms.

• set-point: a constant value to which a variable is constrained, such that any time the variable fluctuates outside a given set-point range, negative feedback takes actions to return the variable to its set-point
• sensors: sensors respond to stimuli, gathering information about a variable in question, signaling when its value fluctuates from the set-point
• control center: receives information from sensors, comparing the value to a set-point, and if necessary, directing actions to return the variable to its set-point
• effectors: a mechanism for taking action to return a variable to its set-point, switching on or off under the direction of the control center
• responses: the resulting action produced by an effector, returning a variable to its set-point value

10. Describe the control of body temperature including the transfer of heat in blood, the roles of the hypothalamus, sweat glands, skin arterioles and shivering.

body temperature:

• set-point: core body temperature = 37°C
• sensors: stimulus = body and blood temperatures above and below 37°C

          a. hypothalamus thermostat sensitivity to blood temperature
          b. skin warmth receptors
          c. skin cold receptors

• control center:

          a. hypothalamus thermostat
          b. cerebral cortex

• effectors:

          a. if T > 37°C
                    1) involuntary responses by sympathetic nervous system
                              a) vasodilation => increases heat loss
                              b) decreased basal metabolic rate => decreases heat production
                              c) sweating => increases heat loss
                              d) lethargy => decreases heat production
                    2) voluntary responses directed by cerebral cortex
                              a) rest => decreases heat production
                              b) behavioral responses (fanning, change to cooler clothing, cool drink)

          b. if T < 37°C
                    1) involuntary responses by sympathetic nervous system
                              a) vasoconstriction => decreases heat loss
                              b) increased basal metabolic rate => increases heat production
                              c) shivering => increases heat production
                              d) piloerection (goose bumps) => decreases heat loss
                    2) voluntary responses directed by cerebral cortex
                              a) rest => decreases heat loss
                              b) behavioral responses (muscular activity, change to warmer clothing, warm drink, curling up, eating)

• response:

          a. if T < 37°C, effectors:
                    1) increase heat production, 
                    2) decrease heat loss
                    3) until T = 37°C
          b. if T > 37°C,  effectors:
                    1) decrease heat production, 
                    2) increase heat loss
                    3) until T = 37°C

11. Explain the control of blood glucose concentration, including the roles of glucagon, insulin and å and ß cells in the pancreatic islets.

levels of blood glucose

• set-point: blood glucose = 90 mg/100 ml
• sensors: stimulus = blood glucose levels above and below 90 mg/100 ml

          a. glucose detectors in pancreas islet beta cells detect high glucose levels
          b. glucose detectors in pancreas islet alpha cells detect low glucose levels

• control center:

          a. pancreas islet beta cells 
          b. pancreas islet alpha cells

• effectors:

          a. if blood glucose > 90 mg/100 ml, then pancreas beta cells produce and release insulin
          b. if blood glucose < 90 mg/100 ml, then pancreas alpha cells produce and release glucagon

• response:

          a. if blood glucose > 90 mg/100 ml
                    1) insulin binds to receptors in muscle and liver cell membranes
                    2) moving glucose from the blood into liver and muscle cells
                    3) where glucose is either metabolized or stored as glycogen or fatty acids
                    4) in fat cells, insulin promotes glucose entry where it is converted to triglycerides
                    5) until blood glucose = 90 mg/100 ml
         b. if blood glucose < 90 mg/100 ml
                    1) glucagon binds to receptors in liver cell membranes
                    2) which activates a cascade of enzymes which degrade glycogen into glucose
                    3) glucose moves from the liver into the blood
                    4) until blood glucose = 90 mg/100 ml

12. Distinguish between type I and type II diabetes.

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