Graded Potentials & Action Potentials

Synapses between neurons

Axodendritic - axon is connected to dendrite (most common) Axosomatic - connected directly to cell body Axosomatic - connected to axon hillock

Summation

If several Presynaptic end bulbs release their neurotransmitter at about the same time, the combined effect may generate a nerve impulse due to summation - summation may be spatial or temporal

Signal transmission at a chemical synapse

1) An action potential arrives and depolarizers the synaptic terminal 2) extracellular Ca enters the synaptic terminal, triggering the exocytosis of ACh (released for muscle contraction) 3) ACh binds to receptors and depolarizes the postsynaptic membrane - graded potential 4) ACh is removed by AChE (half shaped moon) When neurotransmitters leave, they are recycled or returned to the cell Floating around is ACh esterase - will pick up ACh and recycle it

Action potentials

A sequence of rapidly occurring events that decrease (make more positive) and eventually reverse the membrane potential (depolarization) and eventually restore it to the resting state (repolarizing) From -70 to +30 then go back to resting state Voltage gated channels only open up at a certain value Present at the axon hillock and all the way down the axon As one opens up, it causes a domino effect all the way down the axon/ membrane At -55mV, voltage gated Na channels open and Na goes into the cell until the cell is +30mV Open up all the way down the axon til it hits the terminals At +30mV, K voltage gated channels open and they flood out of the cell as the Na channels close The cell then repolarizes back to resting, a little more negative than -70mV

Graded potentials

A small deviation from the RMP that makes the membrane either more polarized (more negative inside) or less polarized (less negative inside) Graded potentials can either be Hyperpolarizing or Depolarizing events Depolarizing brings it closer to zero or higher Hyperpolarizing makes it more negative Occurs in response to the opening of a mechanically-gated or ligand-gated ion channel (found on cell body and dendrites of a neuron) If the membrane is more negative, it's harder to stimulate and depolarize it

Refractory periods

Absolute: the membrane cannot respond to further stimulus Relative: the membrane can respond only to a larger-than-normal stimulus

AP: stimulus strength

Action potentials can only occur if the membrane potential reaches threshold

Resting membrane potential (rmp)

An electrical potential difference (voltage) that exists across a plasma membrane of an excitable cell when the cell is in an undisturbed state (resting)

Continuous conduction

An unmyelinated neuron Voltage gated sodium channels open up and it causes the channels in the next space to keep opening up Domino effect down the neuron All or none principle

Factors that affect propagation speed

Axon diameter The larger the axon is, the faster it will occur because of increased surface area Amount of myelination The thicker the sheets are, the faster we go Temperature Action potentials increase

Information processing

Axon hillock integrating EPSPs and IPSPs from the cell body and dendrites Axon hillock decides what the response from the neuron will be - whether it will depolarize or hyperpolarize Added info is called summation

Synaptic delay

Delay btwn release of neurotransmitters and the reuptake of it -not available during delay

Signal transmission at synapses

Electrical - gap junctions connect cells and allow the transfer of info to synchronize the activity of a group of cells Chemical - one-way transfer of info from a Presynaptic neuron to a postsynaptic neuron

Neurotransmitters

Excitatory - depolarization - excitatory post synaptic potential Anything that brings the cell to be more positive Inhibitory - repolarization/ Hyperpolarizing - inhibitory post synaptic potential Anything that brings the cell to be more negative EPSP or IPSP dependent upon receptors in post synaptic membrane Dependent on channels present - what can come in and leave

Postsynaptic potentials

Excitatory postsynaptic potentials - a Depolarizing postsynaptic potential Inhibitory postsynaptic potentials - a Hyperpolarizing postsynaptic potential A postsynaptic neuron can receive many signals at once

Electrical synapse

Gap junctions -very rare in cns and pns More abundant in cardiac tissue of muscle fibers In GI tract

Summation

Graded potentials can be add together to become larger in amplitude Individually, weren't enough for action potentials to occur Summaries together to form an action potential

Spatial summation

If both fire at the same time, an action potential is caused in the next neuron

AP: All or none principle

If threshold is reached, an action potential is generated

Depolarizing

Making resting potential more positive; decrease the range or strength the action potential would have to occur

Saltatory conduction

Myelinated neuron Myelin sheaths Schwann cells Nodes of ranvier - action potentials skip over the myelin sheaths from one node to the other Action potentials occur faster Can occur in cns and pns All or none principle

Chemical synapse

Neurotransmitters Ca gated channels present at synaptic bulbs Vesicles are filled with neurotransmitters Fuse with synaptic bulb and is released Bond to synaptic membrane Neurotransmitters bind and sodium enter those channels in the postsynaptic membrane Presynaptic membrane - cell before the synapse Synaptic cleft - empty space Postsynaptic membrane - cell that receives neurotransmitter Action potential travels down the axon and arrive at the synaptic end bulb What action potential will do, voltage gated Ca (important for intercellular communication) channels open up due to voltage changes and Ca comes in Presence of Ca will cause synaptic vesicles to fuse with the plasma membrane and release neurotransmitter Neurotransmitters will bind to ligand gated channels to open up and will cause Na to rush into the next cell -graded potential

Acetylcholine

One of the most abundant neurotransmitters

Temporal summation

Over a period of time If pre fires neurotransmitters fast enough, its graded potential can add up to cause action potential to be caused in the post

Graded potentials

Small deviations in resting membrane potential Ligand and mechanical channels - cause some changes but not action potentials Hyperpolarizing - making more polar; making it more negative - need more negatively charged stuff to come in or positively charged stuff to leave the cell

GP: Stimulus strength

The amplitude of a graded potential depends on the stimulus strength Some stimulating of cell body and dendrites

Synapses

Transference of information from cell to another Where cells connect/ communicate Electrical - action potential can jump from one cell to another Chemical - more common