Graded & Action Potentials

Generation of Action Potential at the trigger zone

*trigger zone usually located at the axon hillock region for the efferent neurons and inter-neurons - for the sensory neurons, it is the area immediately adjacent to the sensory receptor where the dendrites join the axon -the trigger zone is the integrating center of a neuron and contains high density of voltage-gated Na+ channels - if the graded potential can depolarize the membrane at the trigger zone to or above the threshold voltage, voltage-gated Na+ channels open and an action potential is generated. -if the GP causes the depolarization to the membrance at the trigger zone, this graded potential is considered to be excitatory. -if the GP causes the hyper-polarization to the membrane at the trigger zone, this GP is considered to be inhibatory -the threshold for a typical mammalian neuron is about -55mV; if GP is above threshold AP is generated - the excitability of cell is refer to its ability to respond to a stimulus and fire an AP

Propagation of AP along an axon

- the AP (spikes) are of uniform strength that travel to the end of the axon without any degredation (decay) to their amplitude (not for GP) throughout their propagation path -ion channels in the axon membrane open sequentially at the Node of Ranvier as the action potential move down the axon - the high speed propagation of an AP along the axon is called conduction of the AP -AP are "all or none" phenomena ex. an AP will fire once the threshold is reached. **Amplitude of the AP is the same regardless the strength of the GP as long as the GP is = or greater than the threshold. - propagation of AP down the axon is a wave of electrical energy move along the axon in a constant amplitude.

The strength (amplitude) of GP will be degraded along the cytoplasmic space of dendrites and soma because

1) current leak: all +ve ions to leak out and weaken the current flow inside the cytoplasm 2)cytoplasmic resistance: resistance to electrical flow inside the cell *combo of cytoplasmic resistance & current leak is the reason why GP decreases its strength (amplitude) over distance.

Graded potentials in neurons

graded potentials in neurons are the events of depolarization (excitatory) OR hyper-polarization (inhibitory) that occur in the dendrites and soma - less frequently occurs near the axon terminals -stimuli can be in the form of: chemical (ex. Neurotransmitters - ligand-gated channels) or mechanical (ex. pressure receptor - mechanically-gated channels -strength of the stimulus is positively correlated to the amplitude of the graded potential generated (stronger the stimulus = larger the amplitude) -Amplitude of the graded potential is a reflection on the number of gated ion channels are activated (opened), allowing ions to enter (influx) or to leave (efflux) the cell of the stimulated neuron. -mechanical stimuli or chemical stimuli open gated ion channels in the sensory neurons - the ionic flow controlled by the gated channels created the local current flow which is the genesis of the graded potentials - graded potentials may also occur when an open ion channel becomes close -graded potential begins when the stimulus causes the opening of the Na+ gated channels and allows the influx of +ve Na ions to spread through the cytoplasm (AKA wave of polarization) - the strength of the stimulus determines how many Na+ gated channels will be activated(open) - more Na+ channels open = higher amplitude =farther the GP can spread along neuron

Speed of propagation for an AP

speed of AP in neuron is influenced by: 1) Diameter of axon - larger axons are faster 2)Myelinated axons are faster - saltatory conduction between nodes of ranvier