Animal Physiology Neurons

What is so significant about a squid in terms of neurons?

they were the first model for cellular neuroscience since they have giant axons (biggest known cell anywhere that are visible to the naked eye)

-having a giant axon is advantageous because they are really fast and can process sensory information really quick

Neuron

basal function cell of the neural system; cell that is specially adapted to generate an electrical signal most often an action potential

Endocrine

always goes through the blood

Long distance signaling

both neural and endocrine can do long distance but it can also be local

-longest neuron: spinal cord to foot

-longest endocrine: blood because it goes all throughout the body

Paracrine vs. Autocrine

-paracrine: when chemical agent is released, it relies purely on diffusion to get to its target (diffusion is slow so the only cells that are going to experience this are local/short distance)

-autocrine: cell that releases the agent will sense it (done for negative/positive feedback)

Steps for neural

1. Input in

2. Integration/processing of information

3. Response

Afferent vs. Efferent and Interneurons

Afferent: sensory information; bring sensory signals to the central nervous system

Efferent: motor information; relay control signals from the central nervous system to target cells (away from the central nervous system)

Interneurons: process what comes in, determines appropriate output, and contact motor neurons to carry out a response (lie solely with the central nervous system)

Parts of neuron

Dendrites: branching processes that receive information; where synaptic input occurs

Cell Body (Soma): where integration of incoming signals occurs; combines excitatory and inhibitory inputs and if excitatory is more than inhibitory, the neuron may respond and produce action potential(s)

Axon: conduction component; propagates action potentials along its length; signal conduction and output

Glial Cells

support cells for neurons

-oligodendrocytes: CNS, form myelin

-astrocytes: look like a star, metabolic intermediaries between capillaries and neurons

-microglia: immune cells of the CNS, causes inflammation, engulfs foreign invaders, cleans up tissues (does NOT produce antibodies)

-Schwann cells: PNS

-Ependymal cells: in part responsible for producing cerebrospinal fluid

Blood Brain Barrier

endothelial cells line capillaries and form the inner surface (have tight junctions)

-fluids that are nutrient-rich leak out into cerebrospinal fluid

-fluids that are rich in waste leak back into plasma

astrocytes form second layer of protection to determine what things should/shouldn’t pass into the blood brain barrier

What causes the blood brain barrier to break down, permitting things to pass more freely?

inflammation (immune system)

infection near or in the brain will cause inflammation that induces small leaks

Bioelectricity

carried out by ions (not electrons like in chemistry and physics)

Resting Membrane Potential

-potential difference across the membrane

-often more polarized in electrically active cells

-established and maintained by permeability to ions (more ions on one side than the other to create ion gradient), this allows ions to cross membrane to do work

-about 70 mV

Chemical gradient vs Electrical gradient

chemical: concentration gradient force (moving from high to low)

electrical: follow opposites attract rule

composes what is known as electrochemical gradient which is the driving force for ions

Nernst equation:

equilibrium potential for a single ion across a membrane; tells you which direction ions will flow across a membrane if allowed

an ion will flow in the direction that will bring the cell closer to the equilibrium potential for that ion (+61 mV for sodium so it wants to make the cell closer to that instead of its normal -70 mV)

Examples of what happens if you change concentrations inside your body, changing the driving forces

low potassium leads to muscle cramps

elevated sodium levels leads to initial hyperactivity and then a crash out

Goldman equation

calculates the membrane potential of the cell

considers equilibrium on the ions and permeability

Na+/K+ ATPase pump

most metabolically active protein in the body that uses energy to drive 3 sodium ions out and 2 potassium ions in to set up ion gradients

-important in every cell body (neuron, muscle, etc.)

depolarized vs hyperpolarized

depolarized: causes cell to be more excitable; cell becomes less negative

hyper polarized: cell is less excitable/less likely to respond; cell becomes a little too negative

Graded Potential

-can be depolarizing or hyperpolarizing

-caused by input into the cell

-passive: comes into cell and then just goes away (initial stimulus leaks out as it moves from the initial site of the stimulus)

-summation of input

-bigger input=bigger potential

Sodium flowing into the cell during depolarization is an example of what kind of feedback?

Positive feedback

-voltage-gated sodium channels keep opening as the membrane potential becomes less negative

-external stop is the inactivation gate that plugs up the sodium channels

What type of channels are involved specifically in action potentials?

voltage-gated channels

Potassium flowing out of the cell during repolarization is an example of what kind of feedback?

Negative feedback

-self-regulated and stops when hyper polarization is detected

Why does hyperpolarization occur?

Potassium channels are left open a little longer because the inactivation plug on the sodium channels does not fall out and reset until hyperpolarization occurs

How does stimulus strength affect action potential amplitude?

It doesn’t

-more frequency=more response

-stimulus strength affects graded potentials not action potentials

Absolute Refractory Period

Time when another action potential cannot be fired

-happens during the rising and falling phases of the action potential

-the sodium channels are already opening as fast as they can during the rising phase and then once they are done opening during the falling phase, they cannot reset until hyperpolarization causes the inactivation gate plug to fall out

Relative Refractory Period

Time where you can fire another action potential but the stimulus has to be much stronger than usual

-happens during hyperpolarization

-membrane potential is more negative than usual so it takes a bigger stimulus to reach threshold than it normally does

-very unlikely to fire another action potential but it is possible and can happen

How do refractory periods reinforce unidirectional flow?

current cannot flow back up the axon to the previous voltage-gated channel and generate another action potential since the channels are already open/inactive

-can only go down the axon to new channels to generate new action potentials

As axon diameter increases…

conduction velocity increases

Giant axons

allow for super fast things like escape behaviors because action potentials are able to propagate so fast

-vertebrates make up for not having a giant axon with myelin

Where are sodium and potassium channels found?

Nodes of Ranvier

Saltatory Propagation

action potentials jumps from node to node

What happens in those that have myelin disorders where they do not have enough myelin?

You lose more current than you should and so the current doesn’t make it to the next node for the next action potential to occur

-thus the message isn’t delivered since the current has more chance to leak out due to less myelin and it doesn’t make it all the way down the axon

Another word for action potential

spike

-spiking neurons are those that fire action potentials

Non-spiking neurons

when the distance is really short, the neuron can rely solely on current spread

-no voltage-gated channels or action potentials occur in these

-rely only on graded potentials

Endogenous Rhythm

pacemaker potentials that keep firing on a rhythm

-they drift up toward threshold and eventually results in burst of action potentials

-continuous cycle that happens spontaneously