Lecture 11

When describing the electrical potential of a cell membrane, what unit is used?

Measured in millivolts (mV). There are 1000 mV per volt.

Enough graded potentials will lead to an action potential occurring. Where on a neuron are action potentials initiated. What type of channel is responsible for action potentials occurring?

- Action potentials are initiated on the axon hillock of the neuron, and go down the axon terminals. - Voltage gated channels are responsible for action potentials - The whole reason for the action potential is it causes the voltage-gated calcium channels to open, so that calcium can enter the axon terminal. Calcium is the stimulus to release the neurotransmitter (signaling molecule/ligand). -In skeletal muscle, calcium is stored in the endoplasmic reticulum of the cell, and when there is action potential, it releases calcium into the intracellular fluid/ cytosol so that the calcium can bind with contractile proteins that allow for the contraction to occur. Muscle action potentials initiate at the sarcolemma.

What is resting membrane potential? What is the resting membrane potential of a typical neuron? What about a skeletal muscle cell? Is resting membrane potential the same for every neuron or skeletal muscle cell?

- At rest, ions aren't crossing membrane in large numbers. - Resting potential of a typical neuron is -70 mV. - Resting potential of a typical skeletal muscle cell is -85mV - Resting potential of a typical cardiac muscle is -90mV -Resting membrane potential is not the same for every neuron or skeletal muscle, there can be variation at any given time due to the functions that are being carried out

voltage-gated ion channel

- Change in membrane potential causes opening/closing. Channels open and close in response to changes in membrane voltage. - Found in the sarcolemma (plasma membrane of skeletal muscle cell) and in axon of neurons

Where on a neuron do grade potentials occur? What type of channel is responsible for these potentials?

- Graded potentials occur in the dendrites and cell bodies of the neuron. - Ligand-gated channels are responsible for these potentials.

hyperpolarization

Make potential more negative than resting membrane potential Going from -70 mV to -80 mV

leak channel

- Randomly open and close - Potassium leak channel is the most common that we will discuss, and to some extent sodium leak channels - They will have some effect on resting membrane potential and movement of ions.

repolarization

- Returning to our RMP (resting membrane potential). - Make potential more negative, but not below RMP. - Depolarization from a RMP of -70 mV to -10 mV, then repolarization back to -70 mV

In a general sense, how does depolarization occur?

Open transmembrane proteins called ion channels that are tunnels into the cell. You could open a Na+ channel and allow more sodium ions to flood in.

When describing a cell's electrical potential, what is our reference point for comparison (i.e. are we comparing the extracellular surface of the plasma membrane to the intracellular surface/vice versa/etc.)?

The reference point for comparison would be the resting membrane potential. Inside of a cell membrane is negative relative to outside.

ligand-gated ion channel

- MOSTLY for SODIUM Na+ also K+ - Ligands are chemicals that have effects on cells (signaling molecules) - Hormones and neurotransmitters are ligands that affect gates of ligand-gated channels - You would find ligand-gated channels in the motor end plate (MEP) of skeletal muscle cells, and in the dendrites or bodies of neurons - Can alter membrane permeabilities, activate second messengers, and alter gene expression - Specificity between ligand and target cell receptor

mechanically gated channels

- Mechanoreceptors in the body where the physical distortion of them releases a ligand or neurotransmitter that bind to ligand-gated ion channels - Tactile cells are mechanoreceptors, when you press on them the gates on some of these different ion channels open, and ions will move in or out of cell, and stimulates the cell electrically and causes a neurotransmitter to be released onto a neuron

Describe what potential energy is. Give one example of ion concentrations as a source of potential energy in biology.

- Potential energy is stored energy, ready to go, ready to be converted to kinetic energy that would allow you to do some kind of work within the body. Energy that is stored up in ATP, when you break that chemical bond, it will give you the energy you need to do some chemical work. Water stored behind a dam, on the upriver side of the dam you have water stored, open the dam and allow that potential energy to become kinetic energy. A cell has a resting membrane potential, -70 mV, then allows the positive ions to come in the cell, depolarization of the cell. - When using the electron transport train, pumping of hydrogen ions across the inner membrane of mitochondrion.

What protein in cell membranes is primarily responsible for maintaining resting membrane potential? How does this protein maintain resting membrane potential, and why does it have to do this?

- Sodium-potassium pump - Uses ATP to pump ions against concentration gradients. For every 1 ATP, 3 Na+ flow out, and 2 K+ flow in. - It needs to maintain resting membrane potential so that it can be ready for whatever command comes its way, be it a muscle contraction or electrical message. If it is not at resting membrane potential, it might not be ready to "go." - Because there is a difference in charges (and inside is negative), we say the membrane is polarized.

What accounts for the negative resting membrane potential of excitable cells?

- The concentration of ions that typically exist in the intracellular vs extracellular environment. - There are more negatively charged proteins that stay in the cell. There is more phosphate PO4^(3-) in the cytosol. Wherever you have a high phosphate concentration, you want to have a lower calcium concentration (Ca^(2+)). When calcium and phosphate come together they form a precipitate, which makes up your bone matrix, like hydroxyapatite, you don't want these two coming together and forming bone crystals in the soft tissues. Intracellular K+ is much higher than in extracellular fluid, and cells have K+ leak channels, so they are constantly losing a positive charge as K+ goes down its concentration gradient. - There are more positively charged ions outside of the cell, much higher concentration of Na+ extracellularly, as well as Ca^(2+).

depolarization

- When we flip the polarity and make the intracellular environment more positive. Make membrane potential more positive. - Going from -70 mV to -60 mV, -10 mV. It doesn't necessarily have to reach +, but it becomes MORE + than it was previously. - Open transmembrane proteins called ion channels that are tunnels into the cell. You could open a Na+ channel and allow more sodium ions to flood in. - Ions move down their concentration gradients when channels open. - Many types of channels discussed on the next slide.

Define what an electrical potential is. What two things determine electrical potentials?

Differences in electrical charges across a plasma membrane. 1. Concentration of intracellular and extracellular ions such as Na+, K+, Ca^(2+), and Cl- across the plasma membrane. 2. Concentration of charges (attributed to the ions) across the membrane - Intracellular relative concentration trends: Na+ is lower, K+ is higher, Ca^(2+) is lower - Extracellular relative concentration trends: Na+ is higher, K+ is lower, Ca^(2+) is higher