Human Phys midterm 1

Homeostasis

tendency of a system to maintain internal stability

homeostatic system

an open system that maintains itself by controlling many steady state mechanisms

what are some examples of variables under homeostatic control

osmolarity, temp, pH, movement of material inside and around the cell

another word for steady state

• dynamic equilibrium

• energy in = energy out

• water flowing into tub and draining out at the same rate

Equlibrium

• system remains constant at the lowest energy state

• no energy in and no energy out

• tub full of water

Extracellular Fluid

• third of all fluid

• plasma in blood vessels and interstitial fluid (between cells)

Intracellular fluid

• two thirds of fluid

• fluid inside cells

T or F: ECF and ICF are in disequilibrium

F: they are in steady state

Why are the ECF and ICF not in Equilibrium

• even though they are in osmotic balance (equal particle concentration) they are not in chemical or electrical balance

• Ex. there is a higher conc. of K+ in the ICF and the ICF is more negative than the ECF

T or F: the body is electrically positive

F: the body is electrically neutral

Channels/pores

• do not hydrolyze ATP

• gated (usually closed at rest)

• Ex. aquaporins

Voltage gated

opens due to membrane potential and lets ions through

Ligand Gated

can be agonistic (opened by ligand) or antagonistic (closed by ligand)

Mechanosesnitive

• channel is deformed

• ex. touch, hearing, proprioception

leak gate

usually open and ion flows down gradient

Transporters/carriers

• may or may not hydrolyze ATP

• moves ions and small molecules across membrane

3 classes of transporters

• uniporters (one molecule)

• symporters (2 molecules in the same direction, using one’s gradient)

• antiporters (2 molecules in different directions, using one’s gradient)

facilitated diffusion

• one molecule flowing down gradient using transporter/carrier

• no ATP

secondary vs primary active transporter

• secondary is coupled to primary to maintain gradient

  • utilizes a gradient

• primary uses ATP and moves against gradient

  • ex. Sodium potassium pump

    • 3 sodium out and 2 potassium in

Osmosis

• movement of water to more solute concentrated place across a semipermeable membrane

• water follows the solute until net flux is reached (steady state)

Osmolarity

• OsM

• mol. of particles/L of solvent

• isoosmotic, hyperosmotic, hyposmotic

T or F: when calculating osmolarity, CaCl2 counts as 3 particles

T! salts dissociate in solution

Tonicity

• solute content of ECF compared to ICF

• only depends on concentration of Non-penetrating solutes

Why are penetrating solutes included when calculating for tonicity

they will distribute evenly across the ICF and ECF

hypotonic solution

• ECF has lower concentration of NP solutes compared to ICF

• cell swells

• ex. distilled water

hypertonic solution

• ECF has higher concentration of NP solutes compared to ICF

• cell shrinks

isotonic solution

• ICF and ECF have equal concentrations of NP solutes

T or F: a hypoosmotic solution is always hypotonic

T! Because a hyposmotic solution already starts with fewer total solute particles than the cell interior, it will always have a lower concentration of non-penetrating solutes

Fick’s law

• membrane permeability is directly proportional to rate of diffusion

• Inversely proportional to surface area

hyponatremia

decreased sodium level in the plasma

membrane potential

electrical gradient between the ECF and ICF

T or F: at resting membrane potential, the cell is at Equilibrium

F: it is in a steady state

forces acting on ions

• chemical: ions move from area of high concentration to low

• electricostatic: ECF is positive and ICF is negative

Equilibrium potential E_ion

• membrane voltage at which chemical and electrical forces on a single ion are equal and opposite

• no net flux

• nernst equation

Nernst Equation

• helps determine conc. of ion outside/inside

• if top is smaller than bottom (out is less than in) then Eion is negative

• concentration difference determines Eion

T or F: Eion can change

F: it is fixed, determined by concentrations of ECF and ICF

Concentrations of ions K, Cl, Na

• K is mostly ICF

• Na and Cl are mostly ECF

Membrane potential (Vm/Em)

• collective voltage across the membrane at any given time of all permeant ions

• shifts

• sits between all permeant ions Eion

• uses GHK equation

GHK equation

• all ion permeabilities into account

• gives more weight to more permeant ions

• if membane is impermeable, Vm=0

Driving force

• Vm-Eion

• if negative: inward push for cations and outward push for anions

• if positive: outward push for cations and inward for anions

• 0 means no net movement

T or F: permeability of K is twice as much as Na in neurons

T: so Vm is closer to Ek

How is concentration gradient maintained

Sodium potassium pump

Ion concentrations for K, Na, Cl

• K: 5 (ECF), 150 (ICF) Eion is -90

• Na: 145 (ECF), 15 (ICF), Eion is +60

• Cl: 108 (ECF), 10 (ICF), Eion is -63

pathway from stimulus to response in neuron

stimulus, sensory receptor, afferent signal, integration in CNS, efferent motor signal, target, response

T or F: sensory neurons are multipolar

F: sensory neurons are pseudounipolar (signal doesn’t pass through cell body) and motor neurons are multipolar

How does the signal change from neuron to neuron

travels as electrical signal, changes to chemical in the cleft, and back to electrical in the cell body

Graded potential

• local change in membrane potential (Vm)

• occurs when membrane permeability changes

• signal loses speed and amplitude as it propagates

T or F: in a graded potential, the amplitude can vary

T: varies depending on the intensity of the stimulus

where do graded potentials occur

in cell body and dendrites

Why do graded potentials lose amplitude

• Rm (membrane resistance): depends on how holey the membrane is

  • increases if gates are closed (signal goes farther)

  • decreases if gates are open

• Rc (cytoplasmic resistance): depends on diameter of the axon

  • increases if the diameter is smaller (less room for signal)

  • decreases if the diameter is bigger (signal goes farther)

T or F: Rc is constant and Rm is variable

True!

length constant

• if bigger, current flows a longer distance

• want a big Rm and a small Rc

Action Potentials

• usually occur because of summation of graded potentials at the trigger zone

• amplitude does not vary

• signal travels the entire axon and doesn’t decrease in voltage

• occurs in axon

when membrane potential is -70mV…

• neuron is resting

• between -70 and -55: graded potentials

when membrane potential is -55mV…

• action potential threshold

• many voltage gated sodium channels open

• sodium floods into the cell

• rapid depolarization (rising phase)

when membrane potential is greater than 0 mV…

overshute phase

when membrane potential is 30mV…

• peak

• driving force of sodium is lowest here (we are the closest to E_Na that we will get)

• voltage gated sodium channels start to close as we move away from peak and enter repolarization phase

T or F: voltage gated potassium channels start to open during the repolarization phase

F: start to open at threshold, but open much slower than sodium channels (so, potassium is slowly leaving the cell)

when membrane potential is less than -70mV…

• hypolarization/ undershute phase

• more potassium out than sodium in

How does the cell return to resting potential

• sodium leaks back into the cell and the cell retains it because of electrical attraction

• Sodium potassium pump DOES NOT restore resting potential, and neither do voltage-gated channels

T or F: during an action potential, the concentration gradients of sodium and potassium flip

False!! there is never a change in concentration gradient

voltage gated sodium channels cycle

1. resting: activation gate is closed and the inactivation gate is open

2. when there is depolarization, gate opens: both gates are open and ions can flow through for a defined amount of time

3. when time ends (miliseconds): activation gate remains open and inactivation gate closes due to conformational change (unrelated to voltage)

4. as membrane repolarizes: both gates close until conformational change resets sodium channel to resting

why do sodium channels have double gates

limits amplitude of APs and keeps them constant

Absolute refractory period

• after AP threshold, AP is irreversible and has fixed amplitude

• another AP in this region is not possible

• sodium channels are still resetting

Relative Refractory period

• enough voltage gated sodium channels have reset (activation gate closed, inactivation gate open)

• AP can be generated

• membrane is farther away from AP threshold, so a stronger stimulus is needed

VG channels vs. leak channels

• VG: effect AP

• leaf: effect resting

Action Potential Propagation

1. graded potential above threshold reaches trigger zone

2. VG sodium channels open and sodium enters the axon (more positive inside the cell)

3. positive charge flows into adjacent sections of the axon by local current flow

4. local current flow causes new sections to depolarize

   1. refractory region prevents backflow of AP

Refractory regions

• potassium channels are slower to open, so they open behind the AP

• repolarizes the membrane

latency

• time between nerve stimulation and response

• if too long, may signal nerve damage

Variables that effect conduction speed

• Rc

• Rm

• delayed opening of VG channels

myelin functions

• insulates the axon

• increases Rm

• increases conductance rate

• saves energy

what cells create myelin in the CNS and PNS

• schwann cells in PNS

• oligodendrites in CNS

AP vs passive current slow

• AP is powerful but slow

• passive current is fast but weak due to signal loss

Saltatory conduction

• VG sodium channels in nodes of ranvier (regenerative flow)

  • signal regenerates here

• passive flow in myelinated regions

  • signal speeds through here

• signal never leaves the axon

How is myelin an energy saver

saves energy by using sodium potassium pump less because the gradiance is less diminished

how does anesthesia work

• blocks VG sodium channels

• pain is recieved, but the signal doesn’t propagate

Electrical synapses

• direct current flow via gap junctions

  • signal never enters ECF

• bidirectional

• sign conserving

• fast

• uncommon in mammals

Chemical synapse

• indirect signaling

• unidirectional

• can be sign inverting

• in mammals

Calcium regulation of NT release

1. AP depolarizes axon terminal

2. depolarization leads to VG calcium channels open and Calcium flowing into terminal

3. calcium triggers exocytosis of vesicles

4. NT diffuses across synaptic cleft and binds receptors on postsynaptic cell

5. NT binding initiates response in postsynaptic cell

ionotropic channels

• ion channel

• binding causes channel to open and ion flows down its gradient

• fast, but every receptor must be bound

What type of channels do glutamate, GABA, and nicotinic ACh bind

• all bind ionotropic channels

• glutamate is excitatory in CNS

• GABA is inhibiting in CNS

• ACh is excitatory in PNS

G protein coupled receptors

• binding causes dissociation of g protein

• g protein activates ion channel

• amplification: one receptor can activate many proteins

• ex. muscarinic ACh receptors

3 ways to stop NT activity

1. NT is returned to axon terminals for reuse or transported to glial cells

2. enzymes inactivate NT

3. NT can diffuse out of cleft

Signal summation

• either excitatory or inhibitory

  • depends on postsynaptic receptors

  • excitatory depolarizes and inhibitory hyperpolarizes

• summed input from presynaptic neuron determines charge of AP on the postsynaptic neuron

Trigger zone

• site of stimulus integration

• high density of VG sodium channels

excitatory/inhibitory postsynaptic potential

• graded potential

• excitatory: net positive charge flows in (depolarization)

  • makes AP more likely

• inhibitory: net negative charge flows in (hyperpolarization)

  • makes AP less likely

Temporal summation

• presynaptic

• if graded potentials are close enough in time from a single presynaptic neuron causes an AP

Spatial Summation

• postsynaptic

• if multiple graded potentials on one postsynaptic neuron fire close enough in time and space, AP is generated

• post synaptic inhibition: inhibitory signal can stop AP if close enough

Receptor Potential

• conversion of stimulus into electrical signal by opening or closing ion channels

• graded potential

Touch receptors

• ruffini: stretch

• pacinian: vibration

• nociceptors (free nerve endings): pain, itch

• meissner’s: light touch

• merkel’s: course touch

white and grey matter in the spinal cord

• white surrounds grey

• white matter: myelinated axons and few cell bodies

• grey matter: unmyelinated cell bodies, dendrites, and axons

Dorsal Root

• carries sensory (afferent) info to CNS from outside

• ganglion receives sensory info

  • made of primary sensory neuron cell bodies

  • terminates in spine or brain

  • aka nucleus in CNS

Ventral root

carries motor (efferent) into the muscles and glands from CNS

tract

• bundle of axons running through white matter that connects CNS

• aka nerve in PNS

T or F: thalamus integrates all sensory information for the brain

F: not smell (olfaction)

T or F: info can switch tracts

F: information will flow down or up one tract to or from the brain

overall pathway to somatosensory cortex

1. PNS to CNS (primary and secondary synapse at spine or medulla)

2. CNS to brain (secondary and tertiary synapse in thalamus)

3. thalamus to cortex (tertiary terminates in specific location in the cortex)

T or F: secondary and tertiary neurons are interneurons

True! they are completely in the CNS

Dorsal column medial lemniscus (DCML) pathway

• carries info for more nuanced senses

  • fine touch, vibration, propioception

• neurons cross midline at medulla and synapse in medulla then again in thalamus

spinothalamic tract

• recieves survival info

  • pain, temp, coarse touch

• neurons cross midline at spinal cord, then thalamus to cortex

• reflexes