Physiology: Nerve Cells and Electrical Signaling

What is the resting membrane potential voltage for a typical nerve cell?

-65 mV to -85 mV (usually -70 mV).

Saltatory conduction

A specialized electronic conduction that action potentials are propagated by axons that are sheathed with myelin.

Supra-threshold

A stimulus greater than the threshold; also elicits action potentials

5 Types of glial cells:

Astrocytes - CNS Ependymal Cells - CNS Microglia - CNS Oligodendrocytes - CNS Shwann Cells - PNS

Glial Cells

Account for 90% of all cells in the nervous system. Main function is to provide structural and metabolic support to the nervous system.

Retrograde Transport

Backward flow. From axon to soma

Central Nervous System (CNS)

Brain and Spinal Cord

Excitatory

Depolarization - moving to the threshold (UP)

Anterograde Transport

Forward flow. From soma to axon terminal

Frequency coding

Frequency with which an action potential can be generated by a single neuron. Intensity of a stimulus is encoded by the changes of frequency in action potentials.

Differentiate between a nerve cell and a glial cell.

Glial cells lack axon and dendrites and they cannot generate action potentials. They support nerves. They surround neurons, hold them in place, provide nutrition, help maintain homeostasis, provide electrical insulation, destroy pathogens, regulate neuronal repair, and removal of dead neurons, and participate in signal transmissions on the nervous system.

Inhibitory

Hyperpolarization - going away from the threshold DOWN

Efferent

Information OUTPUT from CNS to PNS Include motor neurons extending to skeletal muscle and neurons of the autonomic nervous system.

Threshold

Level of depolarization necessary to elicit action potential. If the graded potentials do not reach the threshold it will not generate the action potential.

Resting potential of a neuron

Membrane potential is more permeable to K+. More K+ leaves the cell than Na+ enters. Inside the cell becomes more negative. Electrical forces develop: Na+ into cell K+ into cell Due to electrical forces: K+ outflow is slower and Na+ inflow is faster. Inflow of Na+ is balanced by the outflow of K+. Na+/K+ Pump maintains the resting potential. Resting membrane potential = -70mV

Multipolar neurons

Multipolar neurons The most common of the neurons. Have multiple projections from the cell body. One projection is the axon, all the others are dendrites.

What are the resting potential charges inside and outside of the cell membrane?

Outside the cell + inside the cell -

Axon

Sends information. Transmits electrical impulses called action potentials.

The two divisions of the autonomic nervous system that work antagonistic to each other.

Sympathetic - fight or flight Parasympathetic - rest or digest

Electrotonic conduction

The spread of voltage or current along the membrane by movement of charge.

Dendrites

Tree-like projections. Receive information.

Effect of local anesthetics

lidocaine, procaine -blocks voltage gated sodium channels

Two factors that determine resting membrane potential:

1. Concentration gradients of ions 2. Presence of ion channels in the plasma membrane. Membrane permeability.

Describe the 4 types of gated ion channels

1. Voltage gated-membrane potential. 2. Chemically gated-neurotransmitter. 3. Mechanically gated- physical deformation of receptors. 4. Thermally gated-local temperature.

How many ions are pumped in and out of the Na+/K+ - ATPase pump?

3 Na+ are pumped out and 2 K+ ions are pumped in Net: +1 out

A Na+/K+ Pump

A Na+/K+ Pump Directly due to 20% of the resting membrane potential. Indirectly due to 80% of resting membrane potential

What is Hyperpolarization?

A change to a more negative value The membrane becomes more polarized. K+ Channels are still open causing the membrane potential to become even more negative. The Na+/K+ pump returns Na+ and K+ to maintain concentration gradient; Returning to resting membrane potential.

Subthreshold

A depolarization that is below the threshold. May open some Na+ channels, but not enough to produce an action potential.

What is used to force the ions in opposite directions?

A sodium/potassium ATPase pump, which uses ATP

Interneurons

Accounts for 99% of all neurons in the body. Located entirely in the CNS. Perform all the functions of the CNS, including processing sensory information from afferent neurons, and carrying out complex functions of the brain such as memory and emotions.

Consequences of the refactory period

All or none principal Frequency coding Unidirectional propagation of action potential - anterograde (forward flow)

Equilibrium Potentials

An ion is at equilibrium when there is no net force for it to move across the membrane Chemical force=negative electrical force or Electrochemical force=0

Sodium Equilibrium Potential

As Na+ diffuses into the cell, the cell becomes positive. Electrical driving force acts to push Na+ out of the cell. The cell eventually reaches equilibrium and the chemical and electrical driving force go in opposite directions and equal in magnitude. When membrane potential = +60mV, sodium is at equilibrium.

Action potentials are generated through ________ and propagated through ________.

Axon hillock Axon that are sheathed with myelin

Need videos for reference and help? Check these links out!

CNS: https://www.youtube.com/watch?v=q8NtmDrb_qo PNS: https://www.youtube.com/watch?v=QY9NTVh-Awo Nervous System: https://www.youtube.com/watch?v=qPix_X-9t7E Action Potential: https://www.youtube.com/watch?v=OZG8M_ldA1M Synapses: https://www.youtube.com/watch?v=VitFvNvRIIY

Structural organization of neurons in the CNS

Cell bodies are grouped into nuclei and axons travel together in bundles called pathways, tracts or commissions.

Structural organization of neurons in the PNS

Cell bodies of neurons are clustered together in ganglia and the axons travel together in bundles called nerves.

Two forces acting on sodium:

Chemical driving force: To move IN Electrical driving force: To move OUT

Ganglion

Cluster of neural cell bodies located outside the CNS

What are the differences between contiguous and salutatory conduction?

Contiguous-continuous; lack of myelination; gray matter. Salutatory-myelination; white-fatty matter; tissue is white.

Phases of action potential in order:

Depolarization --> Repolarization -->Hyperpolization

Viseral receptors

Detect information pertaining to conditions in the interior of the body

Sensory receptors

Detect information pertaining to the outside environment.

The strength of the net force of resting membrane potential and potassium

Difference between -70 and -94 = 24 Electrical force is INTO the cell (lower) Chemical force is OUT of the cell (higher) Net force is weak: K+ flows out of the cell, but the membrane is still HIGH permeability to K+ Small K+ leak at rest (low force, high permeability) Na+/K+ pump return Na+ and K+ to maintain balance

The strength of the net force of resting membrane potential and sodium.

Difference between -70 and 60 = 130 Electrical force is INTO the cell Chemical force is also INTO the cell Net force is strong: Na+ flows into the cell, but the membrane has LOW permeability to Na+ Small Na+ leak at rest (high force, low permeability) Na+/K+ pump return Na+ and K+ to maintain balance

Peripheral Nervous System (PNS)

Everything else

What is a neuron?

Excitable cells. Communicate by transmitting electrical impulses. Produce and conduct electrochmical signals. Respond to external stimuli by altering their membrane potential. *Smallest functional unit of the nervous system. A special cell that sends and recieves messages in the nervous system. 10% of he body

Action Potentials

Excitable neurons have the ability to generate action potentials. A distinct charge in membrane potential that occurs in response to stimulation. Action potentials travel along the axons from the cell body (anterograde) to the axon terminal. Or if an afferent neuron (information IN), from the receptor to the terminal.

Which solution is Na+ in the greater concentration?

Extracellular Fluid

Maintaining neural stability

Graded potentials tend to dissipate Na+ and K+ concentration gradients. Only a small percentage of ions actually move. The Na+/K+ pump prevents dissipation to perform normal levels of activity.

What are the biggest differences between a graded potential and an action potential?

Graded potentials-travel short distances and AP travels farther. GP voltage intensity decreases with distances, whereas AP does not. GP is sub-threshold, whereas AP passes the threshold. GP is directly proportional to the triggering event, whereas AP impulses due to crossing the threshold.

What is the concentration of Potassium ions (K+) inside and outside the cell?

High concentration inside the cell. Low concentration outside the cell. The chemical driving foce pushes Potassium ions OUTSIDE the cell. Balanced electrically by organic anions (A-)

Afferent

Information INPUT from PNS to CNS Transmit sensory information from sensory receptors. Visceral information form visceral receptors. Somatic senses - skin, muscles and joints Special senses - vision, hearing, equilibrium, smell, and taste. Viseral senses - Fullness of the stomach, blood pressure, and blood pH Mostly pseudo-unipolar neurons with the cell body located outside the CNS in the ganglion.

Myelin

Insulating wrap around axons of neurons to transmit action potentials more efficiently and rapidly.

Axon hillock

Junction between soma (cell body) and axon. Where Action Potentials are initiated.

Which three ions are the major contributors to the resting membrane potential?

K+ Potassium Na+ Sodium Cl- Chloride ions

Potassium Equilibrium Potential

K+ chemical driving force out of the cell. K+ diffuses out of the cell As K+ diffuses out of the cell, the inside of the cell becomes more negative. Electrical driving force act to "pull" K+ back into cell. The cell eventually reaches equilibrium and the chemical and electrical driving force go in opposite directions and equal in magnitude. When membrane potential = -94mV postassium is at equilibrium.

Propagation of action potentials

Mechanisms depend on the presence or absence of myelin

If those videos didn't make it clear also check out these!

Membrane Potential (part 1) https://www.youtube.com/watch?v=PtKAeihnbv0 Membrane Potential (part 2) https://www.youtube.com/watch?v=eROhIFBGKuU Sodium Potassium Pump https://www.youtube.com/watch?v=C_H-ONQFjpQ Electronic/Action Potentials https://www.youtube.com/watch?v=gkQtRec2464

Leak Channels

Non-Gated channels. Found in the plasma membrane throught a neuron. Always open. Responsible for resting membrane potential

Relative refactory period

Occurs immediately after an absolute refactory period. Spans the last part of the repolarization phase and hyperpolarization. Second action potential CAN be generated - with a stronger stimulus to reach the threshold under resting conditions. Some Na+ gates are closed; some are inactivated.

Voltage-gated Channels

Open and close in response to changes in membrane potential. Sodium and Potassium channels are located throughout the neuron but more in the axon (especially axon hillock) Action potentials are initiated. When these channels are open, calcium enters the cytosol of the axon terminals and triggers the release of the neurotransmitter.

Ligand Gated Channels

Open and close in response to ligand binding of a chemical messenger to a specific receptor in the plasma membrane. Located in the dendrites and cell body. Synaptic potentials

What is the refactory period?

Period of reduced excitability. Happens during and immediately after an action potential, the membrane is less excitable that it is at rest.

Phases of Action Potentials

Phase 1 - Depolarization Phase 2 - Repolarization Phase 3 - After-Hyperpolarization

What + charged ions are inside the cell?

Potassium (K+)

Voltage =

Potential

Oligodendrocytes

Primary function is to form an insulating wrap of myelin around the axons of neurons. Located in the CNS. ONE oligodendrocyte forms SEVERAL myelin sheaths. Myelinates SEVERAL sections for SEVERAL axons.

Schwann Cells

Primary function is to form an insulating wrap of myelin around the axons of neurons. Located in the PNS ONE Shwann cell forms ONE myelin sheath. Myelinates only ONE section of an axon

Factors affecting propagation

Refactory period - unidirectional Axon diameter - Larger, less resistance, FASTER Smaller, more resistance, SLOWER Myelination - Saltatory conduction - Jumping action potentials. Faster propagation

Axon Terminal

Releases the neurotransmitter on arrival of an action potential.

Resting Membrane Potential

Resting membrane potential of all neurons is approximately -70 mV. A cell and rest is not receiving or transmitting any signals. All cells have a resting membrane potential of -5mV to -100mV Exists because there are more negative charges inside the cell and more positive charges outside of the cell.

Graded potentials

Small changes in membrane potential that occur when ion channels open or close in response to a stimulus acting on the cell. Communicate over a short distance. Affects movement of ions across plasma membrane. Electrical signals (nerve cell language) continues down the axon. Magnitude varies - The stronger the stimulus the large graded potential. If strong at stimulus, once it reaches the axon hillock it will generate an action potential. Decremental - Decays with distance

An action potential is originated through ________ travels along _______ to reach the _________.

Soma Axons Axon terminal (anterograde)

Three main components of a neuron:

Soma Dendrites Axon

Soma

Soma Cell body. Contains the nucleus and most organelles.

Absolute refactory period

Spans all of the depolarization phase and most of the repolarization phase. Second action potential CANNOT be generated. Na+ gates are inactivated

Pseudo-unipolar

Subclass of bipoloar neurons. The dendrite is modified to function like an axon, and is the functional continuation of the axon.

Spatial summation

The addition of graded potentials generated at different locations that occurs when they are stimulated more or less than simultaneously. Different stimuli Overlap in time

Synapse

The gap between one neuron and another where neurotransmitters are released and bound. Chemical conduction from one neuron to the next neuron or muscle cell. Stimulus - Calcium ions Synapitc vessicles move to end and fuse with membrane. (exocytosis) Neurotransmitter is expelled and diffuses across synaptic cleft. Neruotransmitter binds to receptor.

In which solution is K+ in the greater concentration?

The intracellular fluid.

Where are the majority of sodium ions (Na+) ?

The majority of sodium ions are outside of the cell membrane. Sodium ions are more highly concentrated outside the cell. The chemical driving force to push sodium ions into the cell. Balanced electrically by the presence of Chloride ions (Cl-) Permeable only to sodium.

What is Repolarization?

The membrane becomes less polarized. Occurs after depolarization, when the membrane potential returns to the resting membrane potential. K+ Channels open. Na+ Channels close. K+ ions diffuse out Na+/K+ Pump restores normal concentrations.

What is Depolarization?

The membrane becomes less polarized. A change to a less negative or to a positive potential. Na+ Channels are open Na+ diffuses from outside of the cell to inside the cell. Inside the cell becomes more polarized Na+ Channels close K+ Channels open.

Bipolar neurons

The sensory neurons with two projections, an axon and dendrite coming off the soma. Functions: Senses and vision.

True/false. A neuron is capable of transmitting messages to an effector organ or receiving information from a sensory organ is said to innervate that organ.

True. Neurons in the efferent division transmit information from the central nervous system to organs in the periphery called effector organs, that perform functions in response to commands from the neurons. Effector organs are usually muscles and glands.

True/False. The membrane is 25 times more permeable to potassium inside the cell than sodium?

True. The number of open K+ channels far exceed the number of open Na+ channels. Resting membrane potential is closer to the K+ equilibrium because of the permeability to K+

Types of gated channels:

Voltage gated - Changes in electrical signals open channels Ligand gated - Chemical messengers open gates

All or none principal

Whether a membrane is depolarized to threshold or above, the amplitude of the resulting action potential is the same; if the membrane is not depolarized to the threshold, no action potential occurs. Threshold depolarization - Action potential Subthreshold depolarization - NO action potential Supra-threshold depolarization - Action potential Action potentials from threshold and supra-threshold stimuli are the same magnitude