2: Homeostasis

Question 1

what is a controlled variable? name an example

Answer 1

a physiological variable that is manipulated in order to maintain the regulated variable within tolerable limits
e.g. heart rate

Question 2

what is a regulated variable? name an example

Answer 2

physiological variable for which sensors are present in the homeostatic circuit and is maintained at a stable level by a negative feedback system
e.g. blood pressure

Question 3

what is a sensor? name examples

Answer 3

measures magnitude of a physiological variable by generating an output signal proportional to the stimulus
e.g. thermoreceptors, Ca+-sensing receptor

Question 4

what is a set point?

Answer 4

the range of values of the regulated variable that the homeostatic circuit attempts to maintain

Question 5

what is an error detector?

Answer 5

determines the difference betw the set point value and actual value of the regulated variable and sends out an error signal

Question 6

what is a controller?

Answer 6

receives error signal from error detector and sends output signals (instructions/commands) to increase/decrease activity of effectors

Question 7

what is an effector?

Answer 7

an entity that is activated by the controller to change the value of the regulated variable

Question 8

when should the term effector be used instead of response?

Answer 8

an effector applies to a physical entity like a cell, tissue or organ
a response is an action like a secretion or vasodilation

Question 9

describe the negative feedback loop

Answer 9

and external or internal disturbance is detected and measured by a sensor and measured by the error detector. and error signal quantifying this is sent to the controller, which sends out control signals to the effector. this responds by altering the controlled variable to return the regulated variable to its set point.

Question 10

name and describe an example of a positive feedback loop

Answer 10

self-amplifying cycle in which a physiological change leads to even greater change in the same direction
e.g. pressure on cervix -> release of oxytocin -> uterus contracts
continuing cycle until baby is expelled

Question 11

name 9 common cell shapes

Answer 11

squamous, spheroid, columnar, polygonal, discoid, fibrous, cuboidal, fusiform, stellate

Question 12

name an example of where squamous cells are found

Answer 12

oesophagus

Question 13

name an example of where spheroid cells

Answer 13

oocytes

Question 14

name an example of where columnar cells are found

Answer 14

intestine

Question 15

name an example of where polygonal cells

Answer 15

cumulus cells

Question 16

name an example of where discoid cells

Answer 16

red blood cells

Question 17

name an example of where fibrous cells

Answer 17

skeletal muscle cells

Question 18

name an example of where cuboidal cells

Answer 18

liver cells

Question 19

name an example of where fusiform cells

Answer 19

smooth muscle cells

Question 20

name an example of where stellate cells

Answer 20

nerve cells

Question 21

what is simple diffusion?

Answer 21

passive movement of particles from a place of low concentration to a place of higher concentration. say “down the concentration gradient”

Question 22

what is osmosis?

Answer 22

passive transport that refers to the movement of water across cell membrane through channel proteins

Question 23

what is filtration?

Answer 23

passive process in which water and solutes are driven through channel protein, but only certain particles can pass, depending on size of membrane pores.

Question 24

what is hydrostatic pressure?

Answer 24

force driving solutes from a high pressure area to a low pressure area

Question 25

what is facilitated diffusion?

Answer 25

carrier mediated transport in which a solute binds to a transmembrane protein to move substances through the membrane

Question 26

what is active transport?

Answer 26

carrier mediated transport of a solute through a membrane up the concentration gradient using energy provided by ATP

Question 27

describe how the Na+/K+ pump works

Answer 27

- Na+ binds to cytoplasmic site of protein -> induces ATP-mediated phosphorylation of protein -> triggers conformational change in protein (binding site and opening) - conformational change induces expulsion of Na+ and allows extracellular K+ to bind -> triggers release of phosphate - loss of phosphate reverses conformational changes - K+ is released into cytoplasm, binding site is again receptive to Na+ -> cycle can repeat

Question 28

what is vesicular transport? what is the difference between endocytosis and exocytosis?

Answer 28

movement of large particles/fluid droplets/molecules through cell membrane contained in bubbles of membrane endocytosis: brings material into cell exocytosis: brings material out of cell

Question 29

what is phagocytosis?

Answer 29

type of endocytosis; engulfing particles like bacteria/dust/debris

Question 30

what is pinocytosis?

Answer 30

type of endocytosis; taking droplets of ECF containing molecules of use to cell

Question 31

what is receptor mediated endocytosis?

Answer 31

phago- or pinocytosis in which specific molecules bind to specific receptors on the cell membrane and are taken into cell in clathrin-coated vesicles w a minimal amount of ECF

Question 32

what are intercellular channels between cells used for? what are they made up of?

Answer 32

intercellular channels function as a means of cell-to-cell signalling. they have two halves in each cell called connexons, made up of 6 connexins each

Question 33

what is autocrine signalling?

Answer 33

a chemical messenger diffuses a short distance through the ECF and binds to a receptor on the same cell

Question 34

what is paracrine signalling?

Answer 34

chemical messenger diffuses a short distance through the ECF and binds to a receptor on a nearby cell

Question 35

what is endocrine signalling?

Answer 35

a hormone is released into the bloodstream and binds to a specific target cell receptor

Question 36

what is nervous signalling?

Answer 36

involves rapid transmission of action potentials, often over long distances, and the release of a neurotransmitter at a synapse

Question 37

what is neuroendocrine signalling?

Answer 37

involves the release of a hormone from a nerve cell and the transport of the hormone by the blood to a distant target cell

Question 38

when do chemically-gated channels open?

Answer 38

they open in response to specific chemical messengers (signalling molecules) that bind to the extracellular region of channel aka ligand-gated channels

Question 39

when do voltage-gated channels open?

Answer 39

in response to changes in the membrane potential (electrical changes)

Question 40

when do mechanically gates channels open?

Answer 40

in response to physical distortion of membrane surface

Question 41

when do leak channels open?

Answer 41

open and close at random, no actual event that induces opening. have an intrinsic rate of switching betw open and closed states

Question 42

what is the membrane potential?

Answer 42

a value, measured in millivolts mV, representing the charge in the intracellular side of the membrane based on the outside being zero (relative)

Question 43

what is the resting membrane potential? how is it achieved?

Answer 43

-70mV - achieved when Na+ concentration (c) outside cell is > c inside cell - K+ c inside cell > K+ c outside cell - anions and negatively charged proteins in cytosol

Question 44

how is the resting membrane potential maintained?

Answer 44

by ion leakage through leakage channels - plasma membrane has more leak channels for K+ than for Na+ -> K+ leaks out of cell -> cell becomes negatively charged -> stabilises at -90 mV - at same time Na+ leaks into cell -> offsets negative charge -> stabilises potential -70 mV

Question 45

what is an action potential?

Answer 45

a rapid rise and subsequent fall in a membrane potential across a cellular membrane

Question 46

when does an action potential occur?

Answer 46

when the balance of Na+ and K+ is disrupted within a cell membrane

Question 47

how does depolarisation happen?

Answer 47

voltage-gated Na+ channels open and positively charged Na+ ions flood into cell following the concentration gradient -> cytosol becomes less negative - such a strong concentration gradient that Na+ continues to enter cell after 0, reaches peak at +3- mV

Question 48

what happens during repolarisation?

Answer 48

- build-up of positive charge in cell causes the Na+ channels to close, induces opening of K+ channels - K+ leaves cell, takes positive charge with it -> membrane potential moves back towards resting voltage -70 mV

Question 49

why does hyperpolarisation occur?

Answer 49

K+ ions reach equilibrium when membrane voltage is below -70 mV. Voltage-gated K+ channels are delayed in closing, causing a short overshoot

Question 50

what is the threshold for depolarisation? what happens if this is not reached?

Answer 50

threshold is -55 mV, membrane depolarisation not reaching this will not result in an action potential