Membrane Potential and Ion Channels

Membrane Potential

• The voltage difference across a cell membrane is called membrane potential.
• The resting membrane potential of a neuron cell is approximately -70 mV
• The intracellular fluid has a slight excess of anions and the extracellular fluid has a slight excess of cations.
• The Na+-K+ pump maintains the resting membrane potential at the expense of energy.
• The Na+-K+ pump also functions as a leaky channel, letting potassium ions pass through the membrane.

Change of Membrane Potential

• Nerve and muscle tissues are excitable tissues, because their membrane potential can be altered.
• The change of membrane potential is categorized as polarization, depolarization, repolarization, and hyperpolarization.
• Ion movement can only be mediated by channels.

Ion Channels

• Pores that open and close provide a passageway for ions to pass through a membrane.
• There are leaky channels and gated channels.
• Gated channels can be voltage-gated, chemically gated, mechanically gated, or thermally gated.

Generation of Electrical Signals in Neuron

• Neurons generate an electrical signal via changes in membrane potential.
• Such changes in membrane potential are brought about by changes in ion movement.
• Two basic types of electrical signals are graded potential and action potential.

Graded Potential

• It is the depolarization of membrane in a small and specialized region of the total membrane.
• It is initiated by Na+ entry into the cell and the ions flow from the active site to the inactive site, causing a current along the membrane.
• It is bi-directional over a short distance.
• It functions as a short-distance signal.
• Graded potential is a localized change in the membrane potential that is graded depending on the intensity of the stimulus.

Action Potential

• It is a brief, rapid, large change in membrane potential.
• It is initiated when the membrane potentials reach threshold potential.
• It is generated at the axon hillock and propagated throughout the entire membrane, nondecrementally.
• It serves as a long-distance signal.
• It has a fixed threshold and magnitude and is all-or-none.

Conformations of voltage-gated Na+ and K+ channels

• Channels can open or close when they are in different conformations.
• The channels are responsible for the formation of action potential and are voltage-gated, meaning they are opened and closed by changes in membrane potential.

Formation of Action Potential

• Rapid fluxes of Na+ and K+ cause the action potential.
• Opening and closing of voltage-gated Na+ channels and voltage-gated K+ channels lead to the action potential.
• Graded potential activates the opening of Na+ channels (fast).
• Slow closure of Na+ channel and the opening of K+ channels (slow).
• The positive feedback of Na+ channel allows for rapid depolarization.

Refractory Period

• After the action potential, there is a time period when new action potential cannot be initiated.
• Divided into the absolute refractory period and the relative refractory period.

Myelination

• Axons covered with myelin along the length are called myelinated fibers.
• Myelin is a thick layer produced by oligodendrocytes in the CNS and Schwann cells in the PNS.
• Myelin serves as an insulator and prevents leakage of current, allowing for faster transmission of action potential.
• Nodes of Ranvier are unmyelinated gaps between myelinated segments.

Saltatory Conduction

• Myelinated axons conduct action potential faster than unmyelinated axons due to saltatory conduction.
• Action potential is only produced at the nodes of Ranvier, where Na+ channels are concentrated.
• The current moves under the myelin sheath but diminishes in amplitude.
• Na+ entry at the node reinforces the depolarization and keeps the magnitude high enough for the next action potential.

Propagation of nerve impulse

• Once an action potential is initiated, no further triggering event is needed to activate the remaining segment of the fiber.
• Conducted through contiguous conduction or saltatory conduction.
• Contiguous conduction occurs in non-myelinated axons and the action potential spreads along every patch of membrane down the axon.
• Once the original action potential is initiated it starts a self-perpetuating cycle where new action potentials are formed and propagated along the fiber.

Graded potential vs action potential

• Graded potential is a localized change in membrane potential that is graded depending on the intensity of the stimulus, whereas action potential is an all-or-none event.
• Action potentials are triggered when the membrane potential reaches the threshold.
• Graded potential only occurs in a small region of the membrane, whereas action potential is propagated along the entire membrane.

Neurotoxins

• Tetrodotoxin (TTX) is found in the pufferfish and blocks voltage-gated Na+ channels, thus preventing action potential generation.
• TTX is extremely deadly and can cause paralysis, loss of consciousness, respiratory failure, and death.

Multiple sclerosis

• An autoimmune disease, multiple sclerosis, attacks the nervous system, destroying myelin sheath.
• This slows the transmission of nerve impulse and may block the propagation of action potentials.
• Symptoms may depend on the affected axons and there is currently no cure.

Integration of information transfer between neurons

• Information is transferred between neurons via synapses, either electrical or chemical.
• It is a unidirectional process.

Chemical Synapse

• A specialized junction where information is transferred from one neuron to another.
• Synaptic vesicles containing neurotransmitters are released at the axon terminal.
• The neurotransmitters diffuse across the synaptic cleft and bind to receptors on the postsynaptic neuron.
• The neurotransmitter binding initiates a new action potential on the postsynaptic neuron.

Determination of the Post-synaptic Potential

• The combination of excitatory postsynaptic potential (EPSP) and inhibitory postsynaptic potential (IPSP) determines the postsynaptic potential.
• Temporal summation is when several EPSP occur close together in time.
• Spatial summation is when EPSP originate simultaneously from several presynaptic inputs.
• Temporal and spatial summation can lead to action potential on the postsynaptic neuron.

Memory

• Memory is the storage of acquired knowledge for later recall.
• Short-term memory lasts seconds to hours.
• Long-term memory lasts days to years and involves the process of consolidation, transferring short-term memory into long-term memory.
• There is not a single memory center in the brain, but many brain regions are involved, such as the cerebellum, prefrontal cortex, and hippocampus.
• The brain involves both structural and functional changes during the consolidation process.

Long-term potentiation

• LTP is a form of synaptic plasticity that occurs when repetitive stimulation of a particular synapse leads to an increase in the strength of synaptic connection.
• This happens when synaptic transmission is strengthened over time due to repetitive use.
• This is attributed to a strengthening of synapses in the brain due to a long-term increase in the efficiency of signal transmission across synapses.
• It is important to note that this process is not fully understood and research is ongoing to understand it better.

What determines the strength of a stimulus?

• The frequency of action potentials, the amount of neurotransmitter released, and the duration of release determine the strength of a stimulus.
• The frequency of action potentials can determine the amount of neurotransmitter release.
• Neurotransmitter release can decrease at high frequencies, leading to desensitization, a reduction in response to a stimulus.