Neurophysiology - Aidan Helfant's Digital Garden

# Neuron Signaling ## Intra-Neuron Signaling ==Neurophysiology== is the study of electrical and chemical processes in neurons.  All cells have a ==negative== electrical energy inside the call compared to outside.  Neurons at rest exhibit a characteristic resting membrane potential of around 50-80 ==millivolts (mV)== Cell membranes permit some substances to pass through them but not others by ==selective permeability==  The cell ion pumping system is at equilibrium when every K+ ion pushed one direction by ==diffusion== would be matched by the force of it getting pulled in the other direction by ==electrostatic charge==.  The ==Sodium Potassium Pump== pumps three ==sodium== ions out for every two ==potassium== ions it pumps in which requires significant energy. This causes a net build up of positively charged particles outside the neuron contributing to the net negative charge inside the neuron.  ==Hyperpolarization== brings the membrane potential farther from zero.  ==Depolarization== brings the membrane potential closer to zero  After depolarizing to around -40 mV for most neuron cells a ==treshold== is reached and an action potential is sparked.  What are action potentials :: Action potentials are very brief but large changes in neuronal polarization that arise in the initial segment of the axon and are propagated at high speed along the axon’s length.  After the onset of an action potential neurons enter a are said to be ==refractory== because they are more unresponsive to subsequent electrical signals.  Refractoriness has what two stages :: absolute refractory phase in which no amount of stimulation can produce an action potential and the relative refractory phase during which only a very strong stimulus can produce another action potential. It's important to understand the action potential travels in a ==regenerative depolarization== of subsequent axon segments.  Unlike the axons terminals and axon segments, the ==cell body and dendrites== have very few ion channels and thus can't effectively spread action potentials.  The myelin sheath that encases some axons is interrupted by ==nodes of Ranvier==, small gaps spaced about every millimeter along the axon ![[Pasted image 20220826084401.png]]  ## Inter-Neuron Signaling ==Neurotransmitters== are chemicals released from presynaptic terminals that serve to communicate with postsynaptic terminals.  What is the difference between excitatory and inhibitory neurons? :: excitatory neurons make it more likely a action potential will continue in a postsynaptic neuron where as inhibitory neurons make it less likely.  It is important to remember that ==excitatory== and ==inhibitory== neurons get their names from their actions on postsynaptic neurons, not from their effects on behavior.  Neurotransmitters released into synapses briefly alter the membrane potential of the postsynaptic cell. We call these brief changes ==postsynaptic potentials.== Depolarizing postsynaptic potentials are called ==Excitatory postsynaptic potentials (EPSP)== where as Hyperpolarizing postsynaptic potentials are called ==Inhibitory postsynaptic potentials (IPSP)==  At ==excitatory== synapses the ions ==Na+ and K+== are allowed to enter the cell leading to depolarization where as at ==inhibitory== synapses the ion ==Cl-== is allowed to enter leading to hyperpolarization.  A molecule of the correct shape called a ==ligand== can fit into a receptor protein and open it.  What are the four types of synaptic connections? :: ![[Pasted image 20220829090441.png|]] A group of neurons and their synaptic connections form ==neural circuits==  The simplest form of neural circuit is a straightforward linking of a series of neurons called a ==neural chain==.  This summation of potentials from different physical locations across the cell body is called ==spatial summation== and the summation of action potentials in time is called ==temporal summation.==  Common to many neural circuits is ==convergence== in which many neuron signals converge on a single cell and ==divergence== in which many signals diverge from a single cell.  The transfer of information across a synapse is called ==synaptic transmission==. What are the seven steps involved in the process? ==1.The action potential traveling down the axon arrives at the axon terminal.== ==2.This depolarization opens voltage-gated calcium channels in the membrane of the axon terminal, allowing calcium ions (Ca2+) to enter the terminal.== ==3.The Ca2+ activates synaptotagmin which fuses synaptic vesicles filled with neurotransmitter to the presynaptic membrane alongside the help of V-SNARE and T-SNARE proteins. The vesicles rupture releasing the neurotransmitter molecules into the synaptic cleft. ![[Pasted image 20220831102708.png]]== ==4. Neurotransmitter molecules cross the cleft to bind to special receptor molecules in the postsynaptic membrane, leading to the opening of ion channels in the postsynaptic membrane.== ==5.This ion flow creates a local EPSP or IPSP in the postsynaptic neuron.== ==6.Synaptic transmitter is either (a) inactivated (degraded) by enzymes or (b) removed from the synaptic cleft by transporters in a process called reuptake for reuse, so the transmission is brief and accurately reflects the activity of the presynaptic cell.== ==7.Synaptic transmitter may also activate presynaptic autoreceptors, regulating future transmitter release.== ^56ad3d  ## Brain Imaging A recording of spontaneous brain potentials, or brain waves, is called an ==electroencephalogram (EEG)==  ==Seizures== feature widespread synchronization of electrical activity.  Gross potential changes evoked by discrete sensory stimuli, such as light flashes or clicks, are called ==event-related potentials (ERPs)==. Typically, many ERPs are averaged to obtain a reliable estimate of stimulus-elicited brain activity.  ==Proteins== tend to be large and negative so they don't flow from inside to outside the neuron cells. They contribute to the inside having more of a negative charge compared to the outside. ![[Pasted image 20220829102412.png]] In neuron cells ==Na+== ions can't come back in after it flows out where as ==K+== ions can flow out and in using passive K+ ion channels. What is the only way Na+ ions can come back into a neuron on the outside of the cell? :: If a voltage gated ion channel opens after the threshold is reached. Why does reaching the the threshold in a neuron cause an action potential? :: Voltage gated ion channels open allowing for Na+ to come in down the concentration gradient and depolarizing the neuron cell. After Na+ flow stops through voltage gated ion channels during the spread of an action potential, ==voltage gated K+== ion channels open allowing K+ to flow out and reestablish a resting potential. Generally the action potential on a neuron begins at the ==axon hillock.== ![[Pasted image 20220829104757.png]] One of the reasons we can support such large brains is because of the invention of ==fire==. It allowed us to eat vastly more things allowing us to have bigger brains. What are the three types of ion channels? 1. Passive ion channels: ==Ion channels that are always open allowing for passive transport== 2. Voltage gated ion channels: ==Ion channels that open when voltage reaches a threshold.== 3. Ligand gated channels: ==Ion channels that open when a specific molecule binds.== Puffer fish have a toxin in their ovaries which block sodium channels. Some cultures see puffer fish as a delicacy making it crucial that chefs remove the ovaries correctly during preparation. Otherwise, people won't be able to breathe.