excitable cells

• Excitable cells are cells that are capable of changing their membrane potential on stimulation in an explosive and reversible manner and generate action potentials.
• An action potential is a transient alteration of the transmembrane voltage across an excitable membrane in an excitable cell generated by the activity of voltage-gated ion channels embedded in the membrane.
• The best known action potentials are pulse-like waves of voltage that travel along the axons of neurons.
• The membrane potential depends on unequal distributions of ions across the membrane.
• The body is electrically neutral, solutes include both anions (-) and cations (+).
• Sodium (Na+) is the major extracellular cation, potassium (K+) is the major intracellular cation, chloride (Cl-) is the major extracellular anion, and calcium (Ca2+) is the major intracellular anion.
• The resting membrane potential is an energy store.
• Ion channels are responsible for the transport of ions across the cell membrane.
• Electrochemical gradients across the membrane are generated by ion channels.
• The Na+-K+ ATPase pump is responsible for maintaining the electrochemical gradient across the cell membrane.
• Action potentials involve the role of voltage-activated ion channels.
• Graded potentials are the integration of neuronal inputs.
• An impermeable membrane is a type of selective permeable membrane.
• The resting membrane potential difference results from the different ions involved and the membrane permeability to each of them.
• The Na/K pump plays a key role in setting and maintaining a resting membrane potential in neurons by keeping Sodium and Potassium concentration gradients across the cell membrane.
• Cells are not permeable to only one ion, but they show different permeabilities to any of them and they can change over time.
• The Nernst Equation is used to calculate the equilibrium potential for any given concentration gradient of a single ion.
• Dissipation of Na/K ionic gradients is prevented by the Na/K pump.
• Ionic gradients across the cell membrane of a neuron are needed for the generation of Action Potentials.
• The membrane is an insulator.
• An electrical gradient is created to avoid all K+ ions to leak out of the cell.
• Opening of ion channels (Na+, K+, Cl-) occurs in response to electric stimuli, which propagate across the cytoplasm like a wave, reducing their amplitude with distance.
• Graded potentials result from receptors activation in the synapse and represent inputs to the neuron that are integrated to eventually trigger and Action Potential.
• If the potential reaches the threshold, an Action Potential will be fired.
• Triggering stimulus could be the interaction between the neurotransmitter released by pre-synaptic neuron and its receptors on the post-synaptic neurons (Ligand-gated or GPCR).
• At each node of Ranvier, the action potential is regenerated by a chain of positively charged ions pushed along by the previous segment.
• The all-or-none law states that the amplitude and velocity of an action potential are independent of the intensity of the stimulus that initiated it.
• Studies of mammalian axons show that there is much variation in the types of protein channels and therefore in the characteristics of the action potentials.
• Propagation of the action potential: the transmission of the action potential down the axon.
• Local Neurons have short axons, exchange information with only close neighbors, and do not produce action potentials.
• Refractory period: a one-way direction Action potentials cannot sum.
• Relative Refractory Period: strong graded potentials can generate an AP of smaller amplitude.
• The axon hillock and initial segment have a much higher density of voltage-gated ion channels than is found in the rest of the cell body.
• Absolute Refractory period: no generation of AP.
• Local Neurons depolarize or hyperpolarize in proportion to the stimulation.
• In a motor neuron/interneurons, the action potential begins at the axon hillock.
• Local Neurons, when stimulated, produce graded potentials: membrane potentials that vary in magnitude and do not follow the all-or-none law.
• Action potentials vary from one neuron to another in terms of amplitude, velocity, and shape.
• Graded Potentials are depolarizations (+) or hyperpolarization (-) occurring in dendrites or cell body.
• The myelin sheath of axons are interrupted by short unmyelinated sections called nodes of Ranvier.