biol 1191 chapters 12 + 13

sensory function of the nervous system

the ability to detect changes in the internal and external environment

• sensory receptors detect the change and relay the information to the CNS

integrative function

the ability to analyze sensory information, store some information, and make decisions regarding appropriate behaviours

• interneurons carry out this function

motor function

when motor neurons respond to stimuli by initiating action which activates effectors such as muscles and glands

• also has the ability to regulate smooth + cardiac + skeletal muscle

contents of the PNS

all nervous tissue outside of the CNS including nerves and sensory receptors

what is a nerve

a nerve is a bundle of hundreds or thousands of axons (plus CT + blood vessels) outside of the CNS

fibre vs fascicle vs nerve

• fibre = a single axons within an endoneurium

• fascicle= a bundle of fibres within a perineurium

• nerve= a bundle of fascicles within an epineurium perineurium

note that there are numerous blood vessels within these coverings

what are the contents of a neuron cell body

• nissl bodies

• nucleus

• neurofibrils (intermediate filaments)= provide shape + support

• microtubules= move materials inside the cell/ along the axon

cell processes common name + axon/ dendrite function

aka: nerve fibres/ neurites

• axon= conducts nerve impulses from the neuron toward other cells of the body such as muscles or glands

• dendrites= recieving portion of the neuron

dendrites characteristics (conduction, #, contents)

• conduct signals toward the cell body

• most neruons have multiple dendrites

• typically short, branched and unmyelinated

• contain nissl bodies

axon characteristics (conduction, #, contents)

• conducts nerve impulses away from the cell body starting at the trigger zone

• a nervous typically only has one, long axon

• ends in fine processes called axon terminals

• contains swollen tips called synaptic end bulbs which contain vesicles filled with neurotransmitters

in your own words descrivbe the rolesof the dendrites, cell body, and axons play in signal communication

what is it called when neuroglia divide uncontrolably?

can cause gliomas

neuroglia in the CNS vs the PNS

CNS- astrocytes, ependymal cells, microglia, oligodendrocytes

PNS- satallite cells and schwann cells

astrocytes function

• star shaped cells

• processes cover blood capillaires and form the blood brain barrier

• help maintain proper ion and neurotransmitter levels in the interstital fluid

• infleuence synapse formation

• regulate neuron growth in embryos

oligodendrocytes

form and maintain the myelin sheath over around axons in the CNS

what is the function of myelin?

myelin insulates + stabilizes the axon and increases the speed of nerve impulse conduction

microglial cells function

are phagocytic cells that remove dead cells and microbes

ependymal cells

form the epithelial membrane that lines the ventricles of the brain and the central canal

forms the blood - CSF barrier

produces and circulates CSF

satallite cells function

surround the cell bodies of neurons in the PNS

regulate the exchnage of material between the cell body of neurons and the interstitial fluid

structurally support neurons

what CNS neuroglial cell is similar to sdatallite cells

astrocytes

schwann cells function

encircle axons in the PNS, whole cells produce the myelin sheath around the axons in the PNS

which CNS neuroglial cell is similar to schwann cells

oligodendrocytes

what is the difference between CNS & PNS myelination? which one can undergo axon regeneration and why?

what are clusters of cell bodies called inthe CNS vs the PNS

CNS= nucleus

PNS= ganglion

what are bundles of axons called in the CNS vs the PNS

CNS= tracts

PNS= nerves

white matter compsition vs grey matter composition

white matter= composed of myelinated axons

grey matter= composed of unmyelinated axons, cell bodies of neurons, dendrites, and neuroglia

where will you find white matter vs grey matter in a spinal cord section? in a brain section?

2 ways electrically excitable cells communicate with eachother?

• action potentials

• graded potentials

what does the productin of an AP or GP depend upon

depends upon the existence of a resting membrane potential and certain ion channels

2 reasons as to why nerve cells are excitable

• #1: becuase of their resting membrane potential (polarized), meaning that the concentration of ions inside vs outside of the cell is different

• #2: due to the presence of specific ion channels, there are more negative ions inside the cell and more postive ions outside the cell

which ions are high in the extracellular fluid and low in the cytosol

Na+ (sodium), Cl-

which ions are high in the cytosol and low in the extracellular fluid

K+ (potassium), PO4-

white active trasnportes are responsible for the concentration gradient of Na+ and K+ ions

Na+/ K+ ATPase pump

wha must happen for electrical signals to take place?

ions must flow in and out of the cell, and this ion flow occurs through ion channels located in the cell membrane

how do ions move?

when the ion channels are open, they allow for the flow of ions. ions move from an area of high concentration to an area of low concentration (diffusion)

• ions also move toward an area of opposite charge

why are ion channels necessary for ion diffusion across the cell membrane

becuase the plasma membrane is only semi permeable, therefore ion channels are necessary

leak channels desc/ location

description

• open/ close randomly

• there are more K+ channels than Na+ channels

location

• cell body, axons, + dendrites of all types neurons

ligand gated channels desc/ location

description

• lock + key

• requires the binding of a specific chemical to open or close (hormone or neurotransmitter)

location

mechanically gated channels desc/ location

description

• open and close in response to mechanical stimuli

• vibration, pressure, stretching

location

voltage gated channels desc/ location

description

• open/ close and are the key to generating long distance AP’s

location

• located in the axons of all types of neurons

what is a graded potential? what causes it? what happens if threshold is reached?

• graded potential = local membrane chnages

• occurs in repsonse to a stimulus

• can be tiny or large, and occur in either direction (increase or decrease) = graded

• if sufficient (if threshold is reached) it triggers an action potential, which is an all or nothing event that can travel long distances

where do graded potentials occur?

they occur in the dendrites and cell body of a neuron

what are the 2 channels associated with graded potentials?

• mechanically gated channels (respond to mechanical stimuli like vibration, pressure, stretching)

• ligand gated channels (lock + key motion that requires the binding of a specific chemical to open like a neurotransmitter or hormone)

what is hyperpolarization

this is when the membrane becomes more NEGATIVE than the resting membrane potential, which is further away from threshold so therefore an action potential is LESS LIKLEY to occur

• membrane becomes more negative by taking in anion or losing cation

what is depolarization

this is when the membrane becomes more POSITIVE than the resting membrane potential, which is closer to the threshold, and therefore MORE LIKLEY to trigger an AP

how does a larger stimulus lead to larger changes in membrane potential

what is an action potential?

an AP is a rapid, complete depolarization of the membrane followed by the restoration of the membrane potential

• an AP is an all or none principle

what channel does an AP involve

an AP involves voltage gated channels, NA+ and K+ (this is the channel that is used to propagate AP’s)

• this travels the entire length of the axon, and results in the release of neurotransmitters at the next synapse

what are the 2 main phases of an AP

• depolarization (excitatory)

• hyperpolarization (inhibitory)

what are the 3 main phases of an AP

#1 depolarizing phase

#2 repolarizing phase

#3 refactory period (includes: absolute and relative refactory period)

what happens during the depolarizing phase

• voltage gated Na+ channels open and Na+ rushes into the cell making the inside more postiive and chnages the membrane potential to +30mV (positive feedback)

what happens during the repolarizing phase

• Na+ channels inactivate (no more inflow of Na+)

• K+ channels open and K+ leaves the cell

• membrane potential returns to -70mV

what happens during the refectory period of an AP

this is the period of time in which a neuron cannot generate another AP

#1: absolute refractory period:

• when inactive Na+ channels must return to thier resting state before they can be reopened, not even a very strong stimulus can trigger another AP

#2: relative refractory period:

• when a suprathreshold stimulus will be able to generate another AP, K+ channels are still open but Na+ channels are closed, hyperpolarization

how can the Na+ & K+ gradients be restored after repolarization

Na+ / K+ ATPase pump

subthreshold vs threshold vs suprethreshold

• subthreshold = stimulus leads to a graded potential that is insufficient in reaching threshold, no AP

• threshold= stimulus leads to sufficient depolarization, 1 AP

• suprathreshold= stimulus leads to multiple rounds of depolarization, frequent AP’s

how do we differentiate a light touch from a firmer touch

light touch

• small stimulus = less frequent AP’s

firmer touch

• larger stimulus = more frequent AP’s and more activation of the sensory neurons

in the poragation of an AP, where does the AP arise and end?

action potentials arise at the trigger zone, and ends at the axon terminals

what is propagation?

propagation is the travel of an action potential all the way from where they arise (at the trigger zone) to where they end (at the axon terminals)

what are the 2 methods action potentials propagate?

• continuous propagation in non-myelinated axons

• saltatory propagation in myelinated axons

what is continuous propagation?

it is the movement of an action potential along an un-myelinated axon where each segment of the membrane depolarizes sequentially along the entire length of the axon

what is saltatory conduction

it is the movement of an action potential along a myelinated axon where the impulse “jumps” from one node of ranvier to another

what is the consequence of decreased myelination

slowed down AP signal

what are the 3 factors that affect propagation speed

#1: axon diameter

• larger diameter axons propagate AP’s faster

#2: amount of myelination

• myelin increases the speed of AP propagation

#3: temperature

• higher temperatures increase the speed of AP propagation