Communication and Homeostasis

Communication and Homeostasis

1. Key Terms

   • Negative Feedback - Process that brings about a reversal of any change in conditions, ensures that optimum steady state can be maintained
   • Positive Feedback - Process that increases any change detected by the receptors, tends to be harmful and doesn't lead to homeostasis
   • Homeostasis - Maintenance of the internal environment in a constant state, despite external changes
   • Endocrine Gland - Gland that secretes hormones directly into the blood, has no ducts
   • Exocrine Gland - Gland that secretes hormones into a duct that carries the molecules to where they are used
   • Hormones - Molecules released by glands directly into the blood, act as messengers carrying a signal from endocrine gland to specific target tissue
   • Target Tissue - Possess a specific receptor on their plasma membrane, shape of receptor is complimentary to shape of hormone molecule

2. Nervous Communication

   • Sensory Receptors
     • Sensory Neurone
     • Motor Neurone
     • Features
       • Transmit action potential over long distance
       • Maintain a potential difference across plasma membrane
       • Have Na+/K+ pumps that use ATP to actively transport Na+ out and K+ in
       • Surrounded by myelin sheath
       • Many dendrites connected to other neurones
   • Resting potential
     • Na+/K+ pump actively transports 3 Na+ out of cell and 2 K+ into cell
     • Membrane more permeable to K+ so many diffuse back out
     • Cytoplasm contains large -ve ions
     • PD across membrane is -60mV
   • Action Potential
     • 1. Membrane starts in resting state
       • 2. Na+ channels open & some Na+ diffuse into cell
         • 3. Membrane depolarises & reaches threshold of -50mV
           • 4. Voltage-gated Na+ channels open & Na+ floods in
             • 5. PD across membrane reaches +40mV (inside is +ve compared to outside)
               • 6. Na+ channels close & K+ channels open
                 • 7. K+ ions diffuse out of cell (repolarisation)
                   • 8. PD overshoots slightly (hyperpolarised)
                     • 9. Back to resting state
     • All or nothing
   • Transmission of action Potentials
     • Opening of Na+ channels upsets balance of Na+ and K+ (creates local current in cytoplasm)
     • Local currents cause Na+ channels further along membrane to open
     • Na+ diffuses sideways, away from region of high conc
     • Na+ gate opens b/c of movement of Na+
     • Na+ diffuses into neurone
   • Nerve Junctions
     • Many mitochondria - lots of ATP used
     • Large amount of SER
     • Vesicles of neurotransmitter acetylcholine
     • Voltage-gated calcium ion channels
     • Acetylcholinesterase hydrolyses acetylcholine into ethnic acid and choline, stops action potential
   • Role of synapses
     • Unidirectional flow of signal
     • Filter out low-level signals
     • Acclimatise - prevents overstimulation
     • Low-level signals amplified by summation
     • Many presynaptic neurones diverge to one postsynaptic neurone (or vice versa)

3. Hormonal Communication

   • Endocrine System
     • Function of adrenal glands
       • Adrenal medulla
         • Release adrenaline
       • Adrenal cortex
         • Uses cholesterol to produce certain steroid hormones
           • Mineralocorticoids control conc of Na and K in blood
           • Glucocorticoids control metabolism of carbohydrates and proteins in liver
     • Action of adrenaline
       • Unable to enter target cell
       • Adrenaline receptor on outside cell complimentary
       • Adenyl cyclase on inner surface of receptor
       • Adrenaline is 'first messenger'
       • Adenyl cyclase activated when adrenaline binds, converts ATP to cyclic AMP
       • cAMP is 'second messenger' and causes effect inside cell
   • Blood Glucose
     • Insulin secreted by beta-cells (which detect rise in glucose) causes blood glucose to go down (converts to glycogen)
     • Glucagon secreted by alpha-cells increase blood glucose
     • Target cells are hepatocytes (liver cells)
     • Insulin binds to receptor, activates adenyl cyclase which converts ATP to cAMP
       • cAMP activates series of enzyme-controlled reactions
         • More glucose channels placed onto cell surface membrane
         • More glucose enters cell
         • Glucose in cell converted to glycogen for storage
         • More glucose converted to fats and more used in respiration
   • Insulin
     • Diabetes type I
       • Autoimmune response attacks beta-cells
       • Cannot manufacture enough insulin
     • Diabetes type II
       • Obesity, age, family history, being of Afro-Caribbean or Asian origin
       • Unable to respond to insulin
     • Regulation

4. Homeostasis

   • Stimulus ---> Receptor ---> Communication Pathway ---> Effector ---> Response
   • Maintaining Body Temperature
     • Ectotherms
       • Relies on external sources of heat
       • Advantages
         • Use less of food in respiration
         • Need to find less food and may be able to survive long periods of time without eating
         • Greater proportion of energy from food can be used for growth
       • Disadvantages
         • Less active in cooler temperatures - at risk of predation
         • May not be capable of activity in winter so must have sufficient stores of energy
       • Changes behaviour or physiology
         • Expose body to sun
         • Hide in burrow
         • Alter body shape
         • Increase breathing movements
     • Endotherms
       • Can use internal sources of heat to maintain body temp
       • Advantages
         • Fairly constant body temp whatever the outside temp
         • Activity possible when external temp is cool
         • Able to inhabit colder parts of planet
       • Disadvantages
         • Significant part of energy used to maintain body temp in cold
         • More food required
         • Less of energy from food used for growth

5. Control of Heart Rate

   • Myogenic - Can initiate its own contractions
   • SAN is pacemaker, can initiate action potential
   • Supplied by nerves from medulla oblongata
     • Affect frequency of contractions
   • Heart muscle responds to adrenaline
     • Increases heart rate, prepares body for activity
   • Stretch receptors
     • Muscles
       • Movement detected
     • Arteries
       • Blood pressure
       • Walls of carotid sinus (swelling in carotid artery)
   • Blood pH
     • CO2 from muscles when exercising reduces pH of plasma
       • Chemoreceptors in carotid arteries, aorta, brain
   • Vagus Nerve
     • Decrease heart rate
   • Accelerator
     • Increase heart rate

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