cell signaling and nervous system

three types of cell signalling

paracrine signaling

synaptic signaling

endocrine signaling

paracrine signaling

cells within an organ

cell targets a nearby cell

how can cells adjacent to each other communicate

through gap junctions

synaptic signaling

only between NEURONS and a target cell

neuron releases neurotransmitters

endocrine singaling

cells from endocrine glands release hormones into the blood

second messengers in cell signaling

RELAYS the signal from the extracellular environment to cytoplasm

how are second messengers in cell signaling detected

by receptor proteins at cell surface, or within the cell (if non polar)

how are second messengers produced

after first messenger triggers the system, they’re indirectly produced to spread the message

how is signal relayed from receptor to enzyme (after second messenger has triggered the process)

via G proteins

what are the G proteins coupled to

GPCRs (G protein-coupled receptors)

three subunits of G proteins

alpha, beta, gamma

beta and gamma are always together

GPCR when it’s inactive

G-proteins are coupled with the GPCRs, with alpha beta gamma together.

GDP is attached to alpha.

GPCR when it’s triggered by a ligand

alpha subunit releases GDP to the cell and binds with GTP instead.

then, alpha with GTP splits from beta and gamma

the two teams cause the effectors to work

how does GPCR cycle end

alpha units will hydrolysis GTP → GDP

GPCR goes inactive again

ligand leaves

depolarization

positive charges entering neuron cell

hyperpolarization

negative charges entering neuron cell

repolarization

neuron cell returning to the resting membrane potential

depolarization: excitatory vs inhibitory

excitatory (depolarization = going up)

hyperpolarization: excitatory vs inhibitory

inhibitory (hyperpolarization = going down)

voltage gated channels

channel only opens after a change of the resting membrane potential occurs

how are sodium channels gated in general?

by voltage

how are potassium channels gated?

by voltage or simple diffusion (leakage channels)

example of sodium channel blocker

tetrodotoxin (pufferfish)

what happens if depolarization doesn’t reach a certain threshold

goes back to the resting membrane potential, the stable equilibrium. either threshold is reached or not reached.

happens through leakage of charges through the axon

action potential process

depolarization reaches a threshold

sudden rapid change in membrane potential spikes up

• voltage-gated sodium channels open, and sodium rushes INTO cell

membrane potential hits peak

• Na+ channels close

• voltage-gated K+ channels open

• K+ rushes OUT of cell

  • resting membrane potential in neurons = -70mV. At this point, we’re at + mV.

as K+ goes out, membrane potential spikes down under -70mV.

• hyperpolarization

goes back to equilibrium at -70 mV

what kinds of loops does the Na+/K+ channels opening cause?

Na+ positive feedback loop

K+ negative feedback loop

does stronger stimulus = greater action potential?

no. all action potentials of a specific neuron has the same amplitude, shape, and duration.

does stronger stimulus = more action potentials?

yes, frequency depends on strength of stimuli

what happens if a neuron is stimulated during its action potential?

its incapable to respond to further stimulation

its REFRACTORY to further stimulation.

refractory period

when neuron cannot generate another action potential even while its stimulated

absolute refractory period

completely incapable of responding to further stimulation

due to inactivation of voltage-gated Na+ cannels.

(inhibitory domain blocks the sodium channel)

relative refractory period

during repolarization of membrane.

VERY STRONG depolarization will trigger a new action potential.

action potential conduction

stimulus starts near the head of the neuron

as it is depolarized, the nerve impulse travels down the axon.

why is the action potential unidirectional (one direction)

because as soon as one part of the axon is fired up, it inactivates the Na+ channel and cannot reopen. (refractory period)

what is myelin

insulating layer of phospholipids and proteins wrapped around axons

nodes of ranvier

parts that don’t have myelin on the axon. The only spots where action potential can occur, since sodium can’t escape through myelin

myelin function

prevents movements of ions, speeds up the process of neurotransmission

saltatory conduction

the rapid “jumping” of nerve impulses thanks to myelin. speeds up action potential.

what is action potential used for

to relay signals through the body

central nervous system

composed of brain + spinal cord

peripheral nervous system

made of nerves outside the CNS (basically everything else)

three principal regions of neurons

cell body (has nucleus of neuron)

dendrites

axon

cluster of cell bodies in CNS

nucleus

cluster of cell bodies in PNS

ganglion

contains nucleus of the neuron

cell body

cytoplasmic extensions that provide a receptive area for electrochemical stimulation

dendrites

long cytoplasmic projection that conduct action potential

axon

where axon originates from

axon hillock

branched out part of the neuron

collateral axon

motor neurons

carry out the impulse from CNS to effector organs

messenger

motor neurons aka

efferent neurons

somatic motor neuron

stretches from CNS to skeletal muscles

autonomic motor neuron

connects the end of a CNS neuron signal to the smooth and cardiac muscle or glands

sensory neurons

conduct (pass) impulses from sensory receptors into the CNS

stretches from PNS to CNS

lets you feel things (woah sensory)

sensory neurons aka

afferent neurons

interneurons

relays the signals between two other neurons, only located in CNS

multipolar neurons

several dendrites and one axon

most common

pseudounipolar neurons

one branch receive stimuli

other branch relay signal to CNS

branched off cell body in the middle

(most sensory neurons are pseudounipolar)

bipolar neurons

neurons with two extensions, one either side of cell body

cell body in the middle, part of axon

neuron type?

multipolar

neuron type?

pseudounipolar

neuron type?

bipolar neuron

multiple axons form together in PNS

nerve

multipole axons form together in CNS

tract

mixed nerve

has both sensory and motor neuron

neuroglia

nervous system cells, five times more abundant than neurons

brings protection and support to the nervous system

can’t produce impulse

neuroglia is composed of

schwann cells

neuroglia function

produces myelin sheaths around axons and surround all PNS axons to form a neurilemmal sheath

neurilemmal sheath

outermost layer of schwann cell. Made up of Schwann cells cytoplasm and nucleus

schwann cell function

wraps around the axon to form protective layer (myelin sheath)

do schwann cells exist next to unmyelinated axons

yes, they’re just not wrapped around the axons

neuroglia (PNS) is made of

satellite cells

satellite cells

cover the surface of neuron cell bodies

provide support

might control microenvironment

4 types of neuroglia in CNS

astrocytes (structural support)

oligodendrocytes (myelin formation)

microglia (immune defense)

ependymal cells (cerebrospinal fluid circulation).

oligodendrocytes

produce myelin sheaths around axons

white matter

area of CNS where myelin sheaths are prominent

because myelin sheaths give a white color

grey matter

cell bodies and dendrites of neurons in the CNS

neuroregeneration in PNS

when axon is cut, the severed portion degenerates and is phagocytosed by schwann cells

schwann cells then form a regeneration tube, acts as guidance track for regenerating axon

neurotrophins

secreted by schwann cells to promote axon regeneration

neuroregeneration in CNS

neurons die upon injury or via apoptosis

regeneration prevented by inhibitory proteins (e.g. Nogo)

forms a glial scar

• further prevents axon repair

microglia

main form of immune defense in CNS

able to detect sites of infection or damage

also involved in synaptic pruning

synaptic pruning

eliminating unused or weaker neural connections (synapses) to create more efficient, specialized neural circuits

microglia origin + classification

distinct embryonic origin from other types of neuroglia

considered myeloid cells

microglial activation

cells become ameboid and become phagocytic cells

ameboid cells

able to change shape due to low consistency of cytoplasm

what can cause microglial activation

altered state of the extracellular environment

how do microglia find damaged cells

detect ATP released from damaged cells

how do microglia end inflammatory responses

releasing anti-inflammatory chemicals, contributes to neuroprotection

astrocytes

help regulate external environment of neurons.

what do astrocytes look like

numerous cytoplasmic projections rating outwards

(thus astro- aster: star in greek)

most abundant neuroglia in CNS

astrocytes

how do astrocytes work

they encircle the endothelial cells of blood capillaries with projections named end feet

in other words, the end feet attach themselves to different blood vessels and axons

• blood brain barrier is formed

what things can astrocytes do

K+ uptake (kidnap K+ that’s exiting the neuron during action potential)

glucose uptake from the blood, release lactate for neurons

how do astrocytes help synapse

maintenance, formation, and maturation

what could astrocytic dysfunction be a sign of

neurodevelopmental disorders

alexander’s disease

caused by a gain of function point mutation in an astrocyte-specific protein

gliotransmitters

messengers released by astrocytes to regulate neuron function

fenestration

new opening is formed

endothelial cell

surrounds the blood cell

sometimes leaves gaps (through fenestration) to allow some filtering to go through