big themes in physiology

Question 1

assuming that an analogous reflex circuit will fire in this scenario as the one we discussed controlling the knee jerk reflex, please describe one sensory neuron that is likely being stimulated, the reflex pathway initiated by the activation of that sensory neuron, and one neural pathway that allows this strong individual to over-ride the relevant reflex pathway. (with image)

Answer 1

When the tricep is stretched, it physically opens the 1A afferent muscle spindle stretch receptor- that isthe mechanoreceptor here- Na+ and K+ can flow in, which causes an action potential through a sensory neuron that synapses with an inhibitory interneuron in the spinal cord. That inhibitory interneuron synapses with a motor neuron (that leads to a neuromuscular junction in the bicep), which itself is excitatory. The inhibition of an excitatory signal leads to a net decline in excitation, which is why the bicep muscle does not contract (at least as much). The same sensory neuron also has branches that lead to the brain as well as to a motor neuron (that leads to a neuromuscular junction in the tricep), which excites the tricep and causes it to contract. This one sensory neuron – one motor neuron reflex arc is why this is a monosynapticreflex. For our sake, motor neurons are excitatory.

However, the brain is able to make the bicep and not the tricep contract.

Signals from the brain travel from a CNS neuron (not sure what kind but I do not think that is necessary knowledge for this exam) and synapse with a motor neuron that leads to the bicep, causing it to contract (excitatory signal). It overrides the signal from the inhibitory interneuron, applying more frequent EPSPs than the IPSPs of the inhibitory interneuron that was explained above. Signals from the brain also likely synapse with inhibitory interneurons that themselves synapse with motor neurons leading to the tricep, leading to less contraction there. Whenever we are lifting something heavy, the brain is overriding reflex arcs just as we have explained here.

Question 1

Does the knee jerk reflex involve the brain?

Answer 1

No

Question 2

voltage gated channels and ligand channels are examples of _____ transport

Answer 2

passive! The flow of ions through any channel does not require a net input of energy.

Question 3

How does a G protein initiate it’s signaling pathway?

Answer 3

o Receptor is associated with heterotrimeric G protein (⍺, β, γ).
o When ligand binds, GTP physically replaces the GDP bound to the alpha subunit
o The three subunits dissociates from receptor, and ⍺ subunit further separates.
o The ⍺ subunit interacts with and ac?vates a transmembrane enzyme or ion channel
o The ⍺ subunit has intrinsic GTPase ac?vity, turning itself off by catalyzing cleavage of
third P in GTP.
o Subunits reassociate with each other and receptor

Question 4

Characterize a key difference between primary active transport and secondary active transport

Answer 4

Both require ATP and involve movement from an area of low concentration to an area of high concentration. However, in primary active transport ATP is used directly and in secondary it is not.

Question 5

Negative feedback mechanisms are crucial in regulating ____________(relatively stable conditions). Negative feedback (set up to oppose change from the steady state). They will regulate from _________ __________.

Answer 5

homeostasis, either direction

Question 6

controlled variable

Answer 6

Controlled variable: variable that is measured and controlled (in negative feedback mechanism)

Question 7

Sensor

Answer 7

device that measures the current value of a controlled variable or output variable; the output of the sensor is the input to the controller (in negative feedback loop, positive feedback loop)

Question 8

Set Point

Answer 8

Value of a desired level of a controlled variable (in negative feedback loop)

Question 9

Controller

Answer 9

Receives input from the sensor and set point - has a comparator to calculate the error signal (only found in negative feedback cycle)

Question 10

Error Signal

Answer 10

set point - value of the controlled variable (in negative feedback loop)

Question 11

Actuating signal

Answer 11

output of controller, produced adaptive value in effector, serves as the input of the effector (in negative feedback loop and positive feedback loop)

Question 12

Steady-state

Answer 12

Controlled variable is within a small range for a period of time

Question 13

Closed loop system

Answer 13

All components work

Question 14

Open loop system

Answer 14

Systems do not work

Question 15

Plateau

Answer 15

Maximum value of output of an effector

Question 16

Threshold

Answer 16

Level of output variable at which point system rapidly drives towards plateau

Question 17

What are some biological examples of positive feedback loops

Answer 17

• Action potential
• Labor and childbirth

Question 18

Give an example of the steps for a negative feedback cycle in a thermostat (and sketch out the general outline!)

Answer 18

controlled variable: temperature
effector: the furnace
sensor: thermometer
set point: degrees
controller: thermostat

Question 19

Give an example of the steps for a positive feedback cycle in a woman giving birth (and sketch a general outline!)

Answer 19

output variable: pressure in cervix wall
sensor: sensory neurons
controller: hypothalamus
actuating signal: oxytocin
effector: uterine smooth muscle

Question 20

what are the three types of passive transport

Answer 20

1. simple diffusion (osmosis)
2. diffusion through channel
3. movement through uniporter carrier proteins

Question 21

what are the two types of active transport

Answer 21

• Primary active transport
• secondary active transport

Question 22

hypotonic

Answer 22

-lower solute concentration outside the cell than inside the cell

Question 23

hypertonic

Answer 23

-lower solute concentration inside the cell than outside the cell

Question 24

characterize transport across the plasma membrane

Answer 24

The smaller, and less polar or charged, the more likely a substance can cross the membrane without needing a membrane protein, i.e. via osmosis
Examples: O2, Steroid hormones, H2O (is polar but SMALL enough to make it through the membrane on it’s own). Water makes it across the membrane but not as readily as oxygen gas.
The larger, and more polar or charged, the more likely a substance cannot cross the membrane without the help of protein
Examples: H+, Glucose

Question 25

Describe mechanisms of passive transport

Answer 25

Channel proteins: Allow diffusion across the membrane
Ion channels: allow specific ions to diffuse across membranes. Vary in conditions that cause ion channels to open. E.g. voltage-gated (example of positive feedback) Na+ channels open and allow Na
**Water channels: aquaporins
Likely to result in more rapid transport than in the case of a uniporter
Uniporter carrier (or transporter) proteins: Allow passive movement across the membrane, requiring binding site(s) and conformational change in protein
E.g. glucose transporters (glut1, 2, 3, 4)
Will involve a conformational change

Question 26

Describe mechanisms of active transport

Answer 26

Primary active transport via ion pumps
Harness energy from ATP
Drive ions against their gradients a
E.g. Na+/K+ ATPase (we use around 30% of our energy fueling this pump, with 3Na+ out of the cell and 2K+ into the cell)
Secondary active transport via co-transporters or exchangers
Harness energy from one substance (e.g. Na+ moving down it’s gradient)
Drive molecules against their gradient
E.g. SGLT1 and 2 are NA+, glucose cotransporters

Question 27

Lipid-soluble messengers can diffuse into cells, bind _________ The receptor-hormone complex stimulates transcription of specific genes.

Answer 27

to receptors in the cytoplasm or nucleus.

Question 28

Water-soluble messengers bind _________________

Answer 28

transmembrane protein receptors

Question 29

Ionotropic receptors

Answer 29

are ion channels, ex: nicotinic acetylcholine receptor, allows both Na+ to flow in and K+ to flow out

Question 30

Metabotropic receptors

Answer 30

interact with intracellular enzymes to initiate signaling pathways
We will see many examples of different classes of metabotropic receptors
One example now: G-protein coupled receptors
They are our largest class of receptors: we have close to 1,000
G-protein coupled receptors