MCB 32: Unit 2

Modules 9-16

neurons

basic unit of nervous sys; do most of electrical signaling

glia cells

provide support for neurons; includes Schwann cells and oligodendrocytes

myelin sheath

formed from glia cells (Schwann cells and oligodendrocytes) wrapping around neuron axons; speed up action potential conduction and prevents additional ion leak during action potentials

astrocytes

glia cells that form the BBB; function as an extra layer that molecules must go through in order to reach the brain

cell body (soma)

part of neuron that contains nucleus and most of the organelles

dendrites

receive incoming signals from neighboring neurons

axon

sends the signal to other cells

synapse

made up of presynaptic axon terminal, synaptic cleft, and postsynaptic dendrite; the region where the axon terminal communicates with its postsynaptic target cell

which part of the neuron is responsible for the output signal?

the synapse

graded potentials

small signals with variable amplitude that can summate; found in dendrites and cell body; can only be used for local signaling; can lead to depolarization OR hyperpolarization, depending on the type of channel opened

action potentials

rapid depolarization of neuron; large signals that travel down to axon terminal; causes axon terminals to release neurotransmitters, which diffuse across the synapse

membrane potential (Vm)

the charge difference across the cell membrane

true or false: neurons are positively charged at rest

false; neurons are negatively charged at rest (-70mV)

there is more K+ ____ the cell

inside

there is more Na+ ____ the cell

outside

leak channels

ion channels that are always open; good for maintaining charge when cell is at rest

sodium-potassium pump

constantly powers greater concentration of K+ inside cell and Na+ outside cell that’s NEEDED for action potentials; uses ATP to make a very significant concentration gradient; releases 3 sodium ions, brings in 2 potassium ions

two forces that drive an ion’s movement either inside or outside a cell

electrical and chemical (concentration gradients)

electrical gradient

positively charged ion is attracted to side of cell membrane that’s more negative, and negatively charged ions are attracted to side of cell membrane that’s more positive

which force is usually the driving force?

chemical, but eventually electrical becomes strong enough to overpower it; shortly after, we reach eqm, where the two forces are equal and opposite

true or false: when a cell reaches equilibrium, there is no more net movement in the cell

true; net movement ceases, but individual, random thermal motion still occurs

equilibrium potential (E)

the membrane potential at which the chemical and electrical driving forces are equal and opposite

how do you determine the direction of an ion’s movement across cell membrane?

look for the direction that brings membrane potential CLOSER to equilibrium potential (specific to a particular ion)

are there more Na or K leak channels open in cell at rest?

K leak channels; makes resting Vm negative

does the inside of a cell TEND to be negative or positive?

negative! proteins inside cell are negatively charged

depolarize

when cell becomes more positive inside (Vm gets less negative)

hyperpolarize

when cell gets more negative inside (Vm gets more negative)

the types of intracellular electrical signals neurons use are called…

graded potentials and action potentials

what causes an action potential?

a graded potential that meets the minimum threshold to cause an AP; travels out of dendrites, through cell body to the initial segment

initial segment

trigger zone for APs; large concentration of vg sodium and potassium channels

where do electrical signals go after they reach the axon?

travel to dendrites of other cells (repeats process of AP conduction) or to an effector cell

what is the membrane potential needed for an AP?

-55 mV, causes vg sodium channels to open, causing rapid depolarization

what are the two gates that control sodium voltage gated channels?

activation and inactivation gates

activation gate

opens the channel and closes it during rest period

inactivation gate

closes/blocks the channel (pore) during refractory periods, which occurs 1msec after depolarization

true or false: both sodium and potassium voltage gated channels have activation and inactivation periods

false; voltage gated potassium channels just open or close, no inactivation or activation period

where do action potentials occur?

in the axon and muscles

true or false: action potentials are regenerating (can travel long distances without losing any strength), while graded potentials aren’t

true; this is why graded potentials summate

how long does it take for voltage gated sodium channels to inactivate after depolarization?

1ms

what happens when or after voltage gated sodium channels are closing?

voltage gated potassium channels begin to open, but they’re slow to open and close, so the neuron repolarizes (gets closer to resting membrane potential), then undershoots (after hyperpolarization, neuron is more negative than resting potential); neuron returns to resting membrane potential when potassium channels close

when is the neuron able to readily undergo another action potential?

when the Na+ voltage gated channels recover and close

absolute refractory period

first 2ms after an AP when a second AP is impossible

relative refractory period

5-15 ms following the absolute refractory period; it’s possible to generate another AP, but it requires a stronger stimulus (AP threshold is higher than normal)

what is a refractory period?

time period when voltage gated sodium channels are inactive

where are Schwann cells found?

produce myelin in PNS

where are oligodendrocytes found?

produce myelin in CNS

Nodes of Ranvier

breaks in myelination; where voltage gated channels are concentrated

salatory conduction

process where action potential “jumps” from node to node; makes AP conduction very fast

chemical synapse

use neurotransmitters to communicate between cells

what does the action potential cause in the neuron?

it causes the release of a chemical messenger from the presynaptic cell which diffuses across the synaptic cleft and binds to membrane receptors on the postsynaptic cell, causing a graded potential in postsynaptic cell

what receptors are used for action potentials?

ligand-gated ion channels (ionotropic receptors) or GPCRs (metabotropic receptors)

how does action potential cause neurotransmitter release?

action potential traveling down axon causes voltage-gated calcium channels to open, and calcium enters the cell’s axon terminals, leading to exocytosis of NTs from synaptic vesicles and onto postsynaptic dendrites

what stops NT release?

1. NT diffuse out of synaptic cleft

2. Transporters power reuptake of NTs by presynaptic cell

3. Enzymes in synaptic cleft inactivate NT

true or false: both graded and action potentials are powered by the opening of voltage-gated sodium channels

false; only action potentials are powered by voltage gated channels, which are found in the axon; graded potentials powered by opening of ligand-gated channels, which are found in the dendrites and cell body

how do metabotropic receptors allow neurotransmitters into postsynaptic cell?

binding either activates G-protein, allowing it to separate and activate second messenger that opens/closes the metabotropic receptor, or the G protein opens/closes the receptor itself

excitatory synapse

brings membrane potential closer to the threshold for an action potential (the graded potential is depolarizing)

what are examples of activity that leads to excitatory synapse (not the nature of a graded potential)

opening channels that only let Na+ in, opening cation channels, and closing K+ channels

inhibitory synapse

decreases the likelihood of a postsynaptic neuron reaching threshold for an action potential (the graded potential is hyperpolarizing)

what are examples of activity that leads to an inhibitory synapse (not the graded potential itself)

opening K+ channels (more K+ leaves cell) and opening Cl- channels (Cl- enters cell)

synaptic integration

the process by which a neuron (at axon hillock) combines multiple excitatory and inhibitory signals received from other neurons to decide whether to fire an action potential

which part of the neuron adds up all the arriving excitatory and inhibitory signals to decide whether to trigger an action potential?

the axon hillock (part of cell body right before the axon begins)

temporal summation

occurs when one synapse generates two postsynaptic graded potentials in rapid succession

spatial summation

occurs when two postsynaptic graded potentials from nearby synapses are generated at nearly the same time

agonists

activators; shaped similarly to a neurotransmitter, so they can bind to its receptor

antagonists

inhibitors; shaped similarly to neurotransmitter, so they can bind to its receptor

methamphetamine and amphetamine are agonists for …

norepinephrine/epinephrine at adrenergic receptors

acetylcholine

neurotransmitter used by neuromuscular junction (neuron onto muscle) and in ANS; has ionotropic receptors in PNS and metabotropic receptors in CNS and other organs

glutamate

main excitatory neurotransmitter in the brain; most common NT in the brain

GABA

most common inhibitory neurotransmitter in CNS

what happens when there’s too much excitation in the brain?

seizures and excitotoxicity (neurons die)

what happens when there’s too much inhibition?

coma

true or false: agonists can bind at excitatory AND inhibitory synapses

true; at inhibitory synapses, they would cause hyperpolarization instead of depolarization at excitatory synapses

CNS

Part of the NS that contains the brain and spinal cord

PNS

Neurons going into (afferent/sensory) or out of (efferent/motor) the CNS

Afferent neurons

Send signals (action potentials) towards CNS; sense external or internal environment (sensory neurons)

Efferent neurons

Transmit information from CNS to PNS; tell muscles and other target cells to do something

Interneurons

Located entirely in CNS and do most signal processing; send signals either to brain/spinal cord or efferent neuron

Meninges

three connective tissue membranes between outer bones and CNS; protects nervous system

Cerebrospinal fluid (CSF)

Type of ECF that fills space between meninges and surrounds entire brain; also found in ventricles, cavities in the brain

Blood brain barrier

physically separates blood from CSF and is made up of glia cells called astrocytes, which provide an extra layer for molecules to pass through before entering the brain, tight junctions, and endothelial cells

White matter

Clusters of axons

Grey matter

clusters of dendrites and cell bodies in spinal cord interior

Dorsal

Side facing back half of body

Ventral

side facing the front of body

Dorsal root

Where sensory/afferent neurons enter the body

Ventral root

Where efferent/motor neurons exit the body

Dorsal root ganglion

collection of sensory neuron cell bodies that sits just outside spinal cord; includes afferent neuron cell bodies

Ganglion

Collection of neuron cell bodies in periphery

Nucleus

Collection of neural cell bodies in CNS

True or false: ventral root ganglion exists with dorsal root ganglion just outside of spinal cord

false; there’s no such thing as a ventral root ganglion

spinal cord

column of nerves that branch off and go to periphery

tight junctions

present in capillaries in the brain; close spaces in endothelial cells that permit leakage

cerebral cortex

outermost part of the brain; most advanced part of the brain, highly convoluted/folded; higher-order processing of sensory information, integrating this information and controlling muscle movements via motor cortex

cerebral cortex lobes

frontal, parietal, occipital, and temporal

cerebellum

in back of brain; controls motor coordination and balance; ensures movements are well-controlled

corpus callosum

axons that connect the left and right sides of the cerebral cortex

diencephalon

beneath cerebral cortex, consists of thalamus and hypothalamus

thalamus

functions as relay station for sensory information before it’s sent to cerebral cortex

hypothalamus

functions in body temperature regulation, ANS management, pituitary hormone release, and in general regulates mechanisms that control basic homeostatic properties

brainstem

connects to spinal cord and regulates ANS and unconscious physiological function