Homeostasis and the Basis of Excitable Cells Flashcards
Define Homeostasis
The ability of a cell or organism to regulate and maintain its internal environment regardless of external influences
Define Set Point
The range or point at which a variable physiological state (e.g. body temperature) tends to stabilize
Define the three components of the homeostatic control systems
- the sensor detects an environmental variable
- the integrator compares the variable to its set point
-the effector initiates changes to restore the variable back to set point
Describe the homeostatic process when body temperature falls below its set point
- sensor: temperature-monitoring nerve cells
-integrator: the thermoregulation centre in the brain
-effector: stimulates the skeletal muscles to contract producing heard
-effector: smooth muscles in blood vessels constrict to minimize heat loss
What are the key differences between the two types of homeostatic regulation
- intrinsically controlled: the sensor, integrator, and effector are within the tissue (therefore the tissue can regulate its internal environment)
-extrinsically controlled: the regulatory mechanisms are outside the tissue/organ
Provide an example of an intrinsically controlled homeostatic process
- exercising skeletal muscle needs a lot of oxygen to produce ATP, when O2 conc. drops blood vessels dilate to increase the volume of oxygen they can deliver
Describe the homeostatic control system resulting in a negative loop after eating
1) the concentration of glucose is regulated in the blood to maximize the energy potential
2) after eating, blood glucose levels rise, sensed by pancreatic cells
3) the pancreas releases insulin in response to this change
4) insulin lowers blood glucose by increasing the ability of cells to uptake the glucose from the blood
5) insulin also upregulates the liver’s ability to convert glucose and store it as glycogen
Describe the homeostatic control system that occurs during labour (positive feedback loop)
1) the brain stimulates the pituitary gland to secret oxytocin
2) oxytocin is carried in the bloodstream to the uterus
3) oxytocin stimulates uterine contractions which push the baby toward the cervix
4) the head of the baby pushes against the cervix
5) the nerve impulses from the cervix are transmitted to the brain
What are the four key roles of the plasma membrane in regulating the cell environment
- regulates internal fluid composition (controls what enters/exits the cell)
- allows nutrients to enter and waste products to leave the cell
- allows cell to cell communication by absorbing chemical signals released from other cells in the body
- joins cells together to form tissues and organs
What are the three primary functions of the plasma membrane
- ensure cell survival
- maintain homeostasis
- function cooperatively (coordinated) with surrounding cells
What are the 5 key structures of the plasma membrane
- lipid bilayer composed of a hydrophilic head and a hydrophobic tail
- cholesterol is tucked between phospholipids to prevent the fatty acid chains from packing tightly forming a rigid structure
- membrane proteins are inserted into the phospholipid membrane and maintain cell structure, allow transport, and facilitate signalling
- ion channels are specialized membrane proteins that permit the entry or exit of ions
- short carbohydrate chains attach to proteins or the bilayer forming glycoproteins and glycolipids which stabilize membrane structure and act as cell surface receptors
What causes cystic fibrosis (plasma membrane disorder)
a defect in a particular chloride ion channel that controls the amount of fluid and mucous present in the lungs causing a buildup that makes breathing difficult
What causes Alzheimer’s disease (plasma membrane disorder)
the generation of reactive oxygens leads to the degradation of phospholipids in neurons which compromises the membrane integrity impacting cognitive function
What are the three ways cells physically adhere to each other to form tissues
- the extracellular matrix (ECM)
- cell adhesion molecules
- cell junctions
what are the three major protein fibres of the extracellular matrix
- collagen forms cable fibres giving tensile strength
- elastin is a rubber-like protein allowing tissues to stretch and recoil
- fibronectin promotes cell adhesion
what are fibroblasts
cells that synthesize the extracellular matrix and collagen
what are cell adhesion molecules (CAMs) and list the four families
CAMs are transmembrane proteins that interact with the cytoskeleton (intracellular side) and the ECM CAMs of other cells
- cadherins
- NCAMs
- selectins
- integrins
what is the extracellular matrix (ECM)
network of fibrous proteins embedded in a gel of complex carbohydrates that surrounds cells in tissues to hold them in place, also known as the interstitial fluid
what are the three types of cell junctions and what do they do
- desmosomes (adherens junctions) anchor adjacent cells, composed of plaques connected by glycoprotein filaments containing cadherins
- tight junctions (impermeable junction): junctional proteins creates a tight seal between cells preventing the movement of molecules from cell to cell (kiss site) found primarily in epithelial tissues
- gap junctions: six connexins form a connexon, two of these from two adjacent cells align to form a tunnel connecting their intracellular spaces (allowing for direct communication)
what are the two main factors which determine a molecules membrane permeability
- size: small substances (ions) can enter through ion channels, whereas larger macromolecules (glucose) require a transport protein to move across a membrane
- solubility: lipophilic substances (uncharged/nonpolar molecules) can easily cross the plasma membrane, whereas lipophobic molecules cannot (without help)
substances that can freely penetrate and cross the membrane on their own are driven by two primary forces
1) their concentration gradient
2) their electrical gradient
what is the process of diffusion
molecules spread from areas of high density to areas of low density, or down the concentration gradient until it reaches dynamic equilibrium (no net movement)
what is osmosis
diffusion when a membrane separates unequal solutions
what occurs in the process of osmosis with a non-penetrating solute
the non-penetrating solute will not diffuse across the membrane, but the water will move down its concentration gradient until the concentration of the substance and water are equal on both sides
what two forces underlie the movement of water
- osmotic pressure moves water down its concentration gradient
-hydrostatic pressure is the force created by a given volume of water
at a steady state osmotic pressure (before it moves down conc. gradient) is equal to the net hydrostatic pressure (steady state)
what are the two types of carrier-mediated transport
- facilitated diffusion does not require energy, uses a carrier to transport a substance down its concentration gradient (eg glucose transport)
- active transport uses a carrier protein to move a substance against its concentration gradient thus requiring energy (ATP) (eg Na+-K+-ATPase pump)
what are the three important characteristics that determine what will be transported across the membrane
- specificity each carrier protein recognizes and transports one specific substance (>1 when there are structurally related substances)
- saturation the limited number of carrier proteins means there is a transport maximum (Tm)
- competition when several related substances share the same carrier protein no substance can achieve its Tm
what are the two types of vesicular transport
-endocytosis: when ions/large molecules have no specific mechanism to be transported into the cell so the cell wraps them in a membrane-enclosed vesicle for transport
-exocytosis: a membrane-enclosed vesicle fuses with the plasma membrane and releases its contents to the extracellular environment
what are the three classifications of endocytosis
- pinocytosis is when the cell membrane engulfs and internalizes a small amount of extracellular fluid (caused by membrane-deforming coat proteins) forming an endocytic pouch pinched off by dynamin (protein)
- receptor-mediated endocytosis is similar to pinocytosis but the trigger to create a vesicle is dependent upon the binding of a substance to a specific cell surface receptor
-phagocytosis internalizes large multimolecular particles; it only occurs in few special cell types (eg white blood cells)
what are the two purposes that materials produced by the endoplasmic reticulum and the golgi complex use exocytosis
- provides a mechanism to release large polar molecules (eg protein molecules)
- enables a cell to move proteins (eg carrier proteins)
what are the steps of exocytosis
1) secretory vesicle formation
2) budding from the golgi
3) uncoating
4) docking at plasma membrane
5) exocytosis
What is the relationship described by Ohms Law V=IR
Ohms law explains the membrane potential of excitable cells:
V is the membrane potential
I is the current of ions moving across the membrane
R is the ability of the plasma membrane to resist ion movement
What is the relationship described by Ohms Law V=IR
Ohms law explains the membrane potential of excitable cells:
V is the membrane potential
I is the current of ions moving across the membrane
R is the ability of the plasma membrane to resist ion movement
Describe the four main classes of ion channels
- voltage-gated ion channels open and close in response to changes in membrane potential
- chemically gated ion channels open when a messenger (ligand) interacts with it
- mechanically ion channels open in response to physical deformations (eg stretch)
- thermally gated ion channels respond to changes in temperature
explain how the electrochemical gradient of potassium can work together or oppose each other
- the direction of the concentration gradient will depend on the area with a higher concentration
- the direction of the electrical gradient depends on the ionic charges caused by non-permeable anions
these two gradients can push in opposing directions, in the potassiums electrochemical gradient the electrical gradient has a stronger driving force than the conc. gradient therefore potassium flows into the cell
What do the variables of the Nernst equation represent?
E = 2.3026 (RT/zF) log (Co/Ci)
E = equilibrium for an ion
R = universal gas constant
T = absolute temperature
z = valence of the ion
F = Faraday constant
explain the three key terms for membrane potential changes: depolarization, repolarization, and hyperpolarization
-depolarization is when the membrane moves towards 0 mV
-repolarization when it begins to return to the RMP
-hyperpolarization becomes more negative than the RMP
what are graded potentials
local changes in membrane potential used for short-distance signalling
what causes depolarization to spread
the charge moves along the membrane regardless of the triggering event occurring in a localized area
what is an action potential
caused by triggering events resulting in localized depolarization that conducts or propagates through the entire membrane without losing strength
what are the structural differences between potassium and sodium ion channels
- sodium ion channels are one large transmembrane protein with a single inactivation gate
- potassium ion channels have 4 subunits that contribute to the activation/inactivation of the whole channel
directly after an action potential no amount of energy/signalling can trigger another action potential, what is the cause of this
the refractory period
what are the four functional zones of the neuron
1) input zone receives incoming signals (contains dendrites and cell body)
2) trigger zone where action potentials are initiated (axon hillock)
3) conducting zone where action potentials are conducted to a target location (axon)
4) output zone releases chemical messengers (axon terminals)
why is it essential that axonal conduction is unidirectional
the backward conduction of action potentials results in the breakdown of neuronal communication
what causes refractory periods and one-way propagation
when the action potential triggers adjacent action potentials, the original is still finishing so the Na+ channels are inactive and cannot be immediately re-opened
what causes refractory periods and one-way propagation
when the action potential triggers adjacent action potentials, the original is still finishing so the Na+ channels are inactive and cannot be immediately re-opened
what is the difference between absolute and relative refractory periods
absolute is when under no conditions can another action potential can be triggers
relative is when it is extremely difficult
what occurs when a strong signal is sent to trigger an action potential
does not cause a larger action potential, only increases the frequency
what are the two traits that influence the rate of conduction down an axon
diameter (larger propagates faster) and myelination (myelinated propagates faster)
what cells for myelin sheaths
oligodendrocytes (CNS) and Schwann cells (PNS)
what are the nodes of Ranvier
regions of exposed fibres at regular intervals within myelinated axons
what is saltatory conduction
when the wave of excitation jumps from one Node of Ranvier to the next
why is synaptic transmission chemical in nature rather than electrical
because neurotransmitters (chemicals) cross the synapse
what are the three steps in synaptic transmission
1) electrical potential reaches axon terminal in the presynaptic neuron which opens voltage-gated CA2+ channels so they can flow down the concentration gradient
2) calcium flows into axon terminal triggering exocytosis of synaptic vesicles (contain neurotransmitters)
3) the neurotransmitters diffuse across synaptic cleft interacting with specific postsynaptic receptors opening chemically gated ion channels into the cell
what are the two types of postsynaptic potentials
- inhibitory postsynaptic potential (IPSP) when the neurotransmitter interacts with its receptor to activate either Cl- channels or K+ channels causing a graded hyperpolarization (brings away from threshold)
- excitatory postsynaptic potential (EPSP) when the interaction of the neurotransmitter with its receptor opens nonselective cation channels (Na and K+) causing a graded depolarization (brings it closer to threshold)
in which direction do graded potentials diffuse in nerves
towards the axon hillock where they impact membrane potential
When will postsynaptic potentials fire
when the EPSPs outweigh IPSPs enough to reach threshold
what are the two types of postsynaptic summation
- temporal summation is the summing of several EPSPs that occur close in time due to the repetitive firing of a single presynaptic neuron.(each EPSP has an additive effect)
- spatial summation is the summing of EPSPs and IPSPs from several presynaptic inputs that have simultaneous effects of membrane potential
