Physiology - chapter 7: nervous system (neurons and synapses)

Nervous System

includes CNS & PNS

Central Nervous System (CNS)

Consists of the brain and spinal cord

- functioning in control and integration (how do you see me? how can you smell things?)

Peripheral Nervous System (PNS)

- Includes cranial nerves and spinal nerves that branch off of or send signals into brain & spinal cord

- responsible for communication

- connects CNS to receptors, glands, etc

Neurons

conduct electrical signals to & from the CNS

Neuroglia

Majority of all nerve tissue cells that support neurons.

what are the structures of the neuron?

- dendrites

- cell body

- axon hillock

- axon (axon transport)

- axon terminal

Dendrites

Branch-like structures of a neuron that receive signals.

Cell Body

The part of a neuron that contains the nucleus and organelles.

Axonal Hilock

The region of a neuron where the axon begins.

Axon

The long, slender projection of a neuron that conducts electrical impulses away from the cell body.

Myelinated Axons

Axons that are covered with a myelin sheath, which increases the speed of electrical impulses.

what is myelin?

a fatty/lipid covering around the axons

why is myelin important?

Because it allows us to send a signal from the axon hillock region to the terminal ends much faster

what are nodes of ranvier?

regions where the electrical signals can jump/skip over to allow for faster signaling to the terminal end branches

what happens to the myelin when we get older?

EX: touching hot stove

- the myelin degrades which slows down the signaling, slower rxn times

Afferent Neurons

- Neurons that carry sensory information to the CNS

- sends signals from other parts of the body back to CNS

Efferent Neurons

- Neurons that carry signals away from the CNS to effectors

- somatic & autonomic nervous system

- motor response out to the sensory receptors of body parts

Interneurons

Neurons that connect afferent and efferent neurons within the CNS.

Schwann Cells

Neuroglial cells in the PNS that form the myelin sheath around axons.

Oligodendrocytes

are what myelinate neurons or axons in the CNS

Astrocytes

Star-shaped glial cells in the CNS that support neurons and maintain the blood-brain barrier.

what are Microglia "glial cells" and their fucntion?

Immune cells in the CNS that act as macrophages to break down pathogens

Ependymal Cells

- type of epithelial cell

- lines the cavities CNS & PNS

why do ependymal cells line the CNS & PNS?

to produce CSF which nourishes the neurons and neuroglia cells

Regeneration of a Cut Neuron

Occurs in the PNS where the severed part degenerates and a regeneration tube is formed by Schwann cells.

why do CNS not regenerate and provide an example?

- cannot regenerate severed neurons

- people who get paralyzed in spine are part of the CNS, which cant regrow

what can excitable tissues like nerve and muscles do?

- can undergo rapid changes in their resting membrane potentials

- can change their resting potentials into electrical signals

EX: can go from resting membrane potential of -70 to then less negative -50

Resting Membrane Potential

The electrical potential difference across the membrane of a neuron at rest, typically around -70 mV.

Polarization "neutral"

A membrane potential other than 0 mV.

Depolarization

- going closer to 0 or polarization

- less negative than resting membrane potential

Repolarization

- going more negative to -70mV

- membrane returns to resting potential after

Hyperpolarization

- going past -70 resting potential

- more negative than resting membrane potential

example of depolarization

if we push a bunch of NA+ into the cell at resting membrane potential, the inside of the cell will become less negative compared to the outside of the cell

what are the types of channels?

1. leaky channels (always open)

2. ligand-gated channels

3. voltage-gated channels

4. mechanical-gated channels

what are ligand-gated channels?

- any type of chemical thats going to bind to that channel and open/close it

- A type of ion channel that opens in response to the binding of a chemical messenger.

what are mechanical-gated channels?

mechanoreceptors, how do you detect whats on your arm? --> pressure changes

Voltage-gated channels

- channels that will open or close when theres a change in voltage

what are the 2 types of K+ channels?

- leaky

- voltage-gated K+ channels

Leaky K+ channels

- Channels that are not gated and are always open

- ions will still need to slowly squeeze through these channels

- relatively permeable

Voltage-gated K+ channels

Channels that open when a particular membrane potential is reached and are closed at resting potential.

Voltage-gated Na+ channels

Channels that are closed at rest and open when a particular membrane potential is reached.

EX: ~55 mV --> this threshold will open the NA+ channels

what happens when a nerve cell "fires"?

an electrical signal moves down the neuron

what are the 2 types of electrical signals?

1. graded potentials

2. action potentials

what are graded potentials?

- Electrical signals that vary in magnitude with stimulus strength

- are decremental

- produced by some specific change in environment acting on specialized region

what do we have to reach in order for an A.P to occur?

a threshold (-55 mV)

what are action potentials?

- Brief, rapid, large changes in membrane potential

- All or none (do we reach the threshold or not?)

- not decremental (constant A.P happening)

what happens if we dont reach the threshold?

we end up getting a graded potential (depolarization)

Threshold potential

The membrane potential that must be reached to trigger an action potential.

what happens at the peaks of the A.P graph?

the Na+ channels close, while stimulates the K+ channels to open allowing the K+ to flow out of cell

*SEQUENCE of Action potentials*

• depolarization

- (can push more Ca+/Na+/K+ into cell to get it to be less negative

• Peak

- no more Na+ can get into cell, channels close

- K+ channels open to allow flowing out of cell

• Repolarization

- K+ leaving the neuron

• After hyperpolarization

- K+ still leaving neuron slowly

• Return to resting membrane potential

- (-70 mV)

*Voltage gated Na+ channel Process*

when we reach the -55 threshold

↓

it will stimulate the voltage gated Na+ channel to open

↓

when open, Na+ can now run into the cell

↓

when we reach the peak of the A.P, the charge difference is what will close the gate

How do we return back to resting membrane potential?

with the use of Na+/K+ ATP pumps

when do the K+ voltage gated channel close?

during the depolarization phase & when we reach hyperpolarization

what does "all or nothing" refer to with action potentials?

amount of depolarization (amplitude) is always the same as long as depolarized to threshold

how do you know if somethings hot? How do you know if something is crawling on your arm vs. pinching your arm?

because of the frequency of these A.P. which can get bigger & more quicker indicated from differences in stimulus

Refractory Period

A period following an action potential during which a neuron is less responsive to stimuli.

what are absolute refractory periods?

- The period during which no new action potential can be initiated

- due to inactivated Na+ channels

what are relative refractory periods?

can still send an A.P., but is hyperpolarized so will need another greater stimulus to generate another A.P

where do A.P get produced and how often?

A.P. get produced continuously along the plasma membrane of unmyelinated axons

Saltatory conduction

- the propagation of an A.P. down the axon hillock of a myelinated axon

- the A.P. "jump" from one node to the next (50x faster)

white mater & grey matter

• White matter is myelinated, allowing for faster signaling

• Grey matter is unmyelinated

Synapse

The communication junction between a neuron and another neuron, muscle, or gland cell. (gap in-between)

what are the types of synapses?

electrical synapse & chemical synapse

what do synapses stimulate?

stimulates physiological change (usually in membrane potential) in the recipient cell

Excitatory postsynaptic potential (EPSP)

- A postsynaptic potential that makes the postsynaptic cell membrane depolarize.

- will depolarize the cell & want to reach a threshold

- will send a signal onward

EX: receptor/neurotransmitter stimulation

if you eat something but dont taste anything

↓

this is because the taste receptors on your tongue didn't release enough neurotransmitters and couldn't get stimulated well

↓

now, your brain doesn't perceive the taste receptor because the signal couldn't get up to the brain

*SYNAPSE PROCESS*

the presynaptic neuron sends an A.P. to its terminal end branches which will release a neurotransmitter

↓

then, the neurotransmitter is released into the synaptic cleft in this vesicle

↓

then, binds to the postsynaptic neurons specifically (Na+ ligand-gated ion channels)

↓

then, Na+ rushes into the cell, which then releases a threshold

↓

this triggers/stimulates a voltage gated Na+ channel to open the axon hillock

↓

this propagates an A.P. down to its axon

Inhibitory postsynaptic potential (IPSP)

- A postsynaptic potential that makes the postsynaptic cell membrane hyperpolarize.

- make more negative/hyperpolarization

what happens if an Excitatory postsynaptic potential (EPSP) is strong enough depolarization to reach a threshold?

an A.P. will form in postsynaptic cell

how do we excite the cell by depolarizing?

need to push positively charged ions into the cell OR we can remove negative charged ions

excitatory neurotransmitter

- often times effects Na+ channels

- opens sodium channels

EX: acetylcholine

inhibitory neurotransmitter

- opens Cl- channels

- EX: GABA

Neuronal integration

The process by which neurons communicate with many other neurons to determine whether to fire an action potential.

Chemical synapse

A type of synapse that uses neurotransmitters to communicate between neurons.

G-Protein Coupled Channels

the neurotransmitter binding to a integral protein/receptor is separate from the protein that serves as the ion channel

*PROCESS of G-proteins*

AcH binds to receptors

↓

G-protein subunit dissociates (alpha, beta, gamma)

↓

the beta and gamma proteins will move over

↓

G-protein binds to K+ channel, causing it to open

catecholamines

use cAMP as second messenger

Acetylcholinesterase (AChE)

- functions to break down acetylcholine into its components for reuse.

- found in synaptic cleft

*PROCESS of Acetylcholinesterase (AChE)*

the re-uptake receptor found in presynaptic neuron will take the AcH & bring it back to presynaptic neuron

↓

then we can resynthesize the neurotransmitter

what are the 2 types of communication pathways for neuronal integration?

- convergence

- divergence

what does convergence refer to?

when many presynaptic neurons converge to 1 postsynaptic neuron

what does divergence refer to?

where 1 presynaptic neuron diverges to several postsynaptic neurons

when a postsynaptic neuron fires an A.P. what does it depend on?

1. the # of active synapses

2. whether the synapses are excitatory of inhibitory

what mist the presynaptic neuron do in order to release more neurotransmitter?

needs to send more A.P.

Summation

- The process of adding together multiple inputs to achieve an effect

- the sum of your excitatory & inhibitory inputs

what are the 2 types of summation?

- temporal summation

- spatial summation

what is temporal summation?

the sum reaching threshold because 1 presynaptic neuron is stimulating the postsynaptic neuron threshold

what is spatial summation?

- signals from multiple synapses are combined to produce an effect.

- means many presynaptic inputs (this neuron, then this neuron, then this one, etc)

Overshoot

going past the neutral/polarization stage, getting positive

Electrical Synapse

A type of synapse where electrical signals pass directly between neurons.

Neurotransmitter Release

The process by which neurotransmitters are released from the axon terminal into the synaptic cleft.