2 - Neurons

how is the nervous system divided?

central nervous system and peripheral nervous system

describe the CNS.

brain and spinal cord

• part of NS encased in bone

describe the PNS.

neural tissue outside of the CNS

• NOT in bone → axons + connective tissues

define “nerves.”

a bundle of axons that travel together

define a “neuron.”

cell specialized for intercellular communication

what percent of the brain is comprised of neurons?

50%

axon length depends on…

cell placement

• can be meters long or very short

• bc it connects to nerves

what is the function of dendrites?

receive signals, funnel signals into soma

• receptive surfaces

what is the function of the axon hillock?

junction

• determines if signal will be passed on

what is the function of the axon?

take signal to target

what is the function of the soma?

control center of the neuron

what is the function of the synaptic terminal?

convert nerve signal into chemical signal → transmit to target cell

• where axon ends

describe the synaptic cleft.

the gap between the synaptic terminal and postsynaptic cell

what forms a synapse?

synaptic terminal and postsynaptic cell

what are the structural classifications of neurons?

• anaxonic

• bipolar

• unipolar

• multipolar

describe the structure of an anaxonic neuron?

more than two processes, but axon cannot be distinguished from dendrites

describe the structure of a bipolar neuron.

two processes separated by cell body

describe the structure of a unipolar neuron.

single elongate process with soma off to side

describe the structure of a multipolar neuron.

more than two processes, single axon and multiple dendrites

where are bipolar neurons and unipolar neurons found in the body, respectively.

• bipolar: vision cells

• unipolar: sensory

what does “polar” mean?

number of outgrowths from soma

what are the functional classifications of neurons?

afferent, efferent, interneurons

describe afferent neurons.

carry info towards the spinal cord or brain

• sensory neurons

describe the somatic and visceral funcs. of afferent neurons.

• somatic: info abt external world + body position

• visceral: info abt internal systems

describe the somatic and visceral funcs. of efferent neurons.

carry info away from the spinal cord or brain

• motor neurons

describe the somatic and visceral funcs. of efferent neurons.

• somatic: innervate skeletal muscles (vol. mvmt)

• visceral: innervate smooth and cardiac muscle, glands (invol.)

describe interneurons.

communicate between neurons

• neither sensory nor motor, but still involved in processing

describe neuroglia.

supporting cells

• 50% of all neural tissue

• diverse

what are the six main types of neuroglia?

• ependymal cells

• astrocytes

• oligodendrocytes

• microglia

• satellite cells

• schwann cells

describe the function of ependymal cells.

generate cerebrospinal fluid (CSF)

• coat inner surface of open cavities

describe the function of astrocytes.

• maintain blood brain barrier (BBB)

• regulate nutrients

• structural support (in CNS)

describe the function of oligodendrocytes.

• myelination (form myelin)

• structural support

describe the function of microglia.

• phagocytes (immune cells)

• remove dead/waste cells

describe the function of satellite cells

regulate nutrients (in PNS)

describe the function of Schwann cells.

• myelination

• repair

• ONLY IN PNS

what type of neuroglia is pictured?

astrocyte

what type of neuroglia is pictured?

ependymal

what type of neuroglia is pictured?

microglia

what type of neuroglia is pictured?

oligodendrocyte

what type of neuroglia is pictured?

satellite

what type of neuroglia is pictured?

schwann cell

what are the supporting structures of the NS?

CSF, BBB, myelin

what is the composition of CSF?

clear fluid found in brain and spinal cord

what is the function of CSF?

• protection/support

• circulation of nutrients

• removal of waste

• immune protection

T/F: the CSF is in constant circulation

true

what is the structure of the BBB?

highly selective permeable membrane

how does the BBB work?

specialized capillaries and neuroglia tightly regulate what moves into CSF from the plasma

information travels through the ____ of myelin.

gaps

what is myelin?

insulating series of membranes surrounding SOME axons

• outgrowth of specialized neuroglia

what is the function of myelin?

increase the speed an action potential moves down the axon

why do we feel different types of pain?

bc of different axons

• myelinated: sharp pain

• unmyelinated: dull pain

what is the significance of nodes of Ranvier in myelianted cells?

exchange can only happen there

where does exchange happen on unmyelinated cells?

along the length of the axon

describe this step in the formation of a myelin sheath by a Schwann cell.

step 1

• Schwann cell first surrounds a portion of axon within the groove of its cytoplasm

describe this step in the formation of a myelin sheath by a Schwann cell.

step two

• SC begins to rotate aroiund axon

describe this step in the formation of a myelin sheath by a Schwann cell.

step three

• as SC rotates, myelin is wound around axon in mult. layers

  • forms tightly packed membrane

define potential.

difference in charge

define the action potential.

the signal that travels down the length of axon

define the synaptic activity.

release of chemical signal, travels across synapse, targets cell

from start to end, state the zones a signal travels across in neurons.

resting potential (stimulus produces) → graded potential (may produce) → action potential (triggers) → synaptic activity → information processing

define voltage.

the difference in electrical potential between two points

• measures potential

• two pts = inside vs outside of cell

what is voltage measured in?

mV (millivolts)

which is more negative: intracellular vs extracellular

intracellular

define current.

the mvmt of charge to eliminate potential

• any direction

• to equilibrium

define resistance.

anything which impedes the mvmt of charge

what is the significance of proteins?

• allow ions to flow

• dictate resistance

what is Ohm’s law?

I = V / R

• I = current

• V = voltage

• R = resistance

where are anaxonic neurons found?

in the brain

most neurons are ____.

multipolar

describe the absolute refractory period.

period where cell cannot be stimulated to fire another action potential

• bc we started depolarization + we need it to finish

what is the mechanism behind ARP?

• volt. gated Na+ channels are open (depolarization) OR

• inactviation gates of the volt. gated Na+ channels are closed (repolarization)

describe the relative refractory period.

period where cell can only be stimulated to fire another action potential if depolarization event is greater than usual

what is the mechanism behind RRP?

cell is hyperpolarized (further away from threshold)

define resting membrane potential.

the electrical potential difference across the cell membrane during resting conditions

how is RMP measured (2 factors)?

inside relative to outside; mV

what is the RMP of a typical neuron?

-70 mV

why is the inside of a neuron more negative than the outside?

• outside: + charged ions (calcium, sodium)

• inside: - charged proteins

what factors contribute to RMP?

• passive chemical gradients (conc. gradients)

• passive electrical gradients (+ vs - charge)

• active transport (Na K pump)

define active transport.

the mvmt of an ion against a gradient using ATP

what are the main ions regarding membrane potential?

K+ and Na+

which is more permeable to the membrane: K+ or Na+

K+

at resting RMP, there is more mvmt of ___ than ___.

K+ than Na+

if the plasma membrane were freely permeable to K+, …

outflow of K+ would continue until the equilibrium potential (-90 mV) was reached

at equilibrium of RMP…

Na+ and K+ would no longer move

what regulates mvmt of ions at RMP?

leak channels

• protein that allows an ion to “leak” through

the membrane contains many more ____ leak channels than ____ leak channels.

K+ than Na+

what force gives Na+ a smaller current?

greater resistance

define the electrochemical gradient.

force of chemical gradient plus electrical gradient

if the plasma membrane were freely permeable to Na+, …

the influx of Na+ would continue until the equilibrium potential (+66 mV) was reached

define polarized.

change in charge btwn inside and outside

• inside of cell is negative relative to outside

define depolarization.

process of making inside of cell less negative/polarized (move toward zero)

define repolarization.

process of returning membrane potential BACK to RMP

define hyperpolarization.

process of making inside cell MORE negative than resting membrane potential

• if charge is below RMP, even if steadily increasing back to eq., still counts as hyperpolarization

define graded potential.

localized change in membrane potential (dendrites and soma)

define action potential.

propagated change in membrane potential (down the length of the axon)

what propagates the action potential?

voltage gated Na+ and K+ channels along length of axon

• opens at -55 — -60

define threshold.

the voltage level a cell membrane must reach to trigger an action potential

how long is the absolute refractory period?

1 msec

how long is the relative refractory period?

~0.8 msec

how many gates do Na+ channels and K+ channels have respectively?

• Na+ 2 gates

• K+ 1 gate