Neurotransmission (Sept 17)

Question 1

Describe conduction of nerve impulses

Answer 1

-wave of depolarization along the axon -in excitable tissue positive potential moves away from the point where the stimulus is initiated -in an axon, this wave is initiated at the axon hillock -wave can only move generally from hillock to terminal because the area behind it is in its refractory period

Question 2

Steps of nerve impulse conduction (continuous)

Answer 2

1. Stimulus triggers action potential which depolarizes the local membrane 2. Inside of axon membrane becomes briefly positive which triggers the next area of membrane to become positive because of opening of voltage sensitive Na+ channels 3. Voltage increase is terminated shortly after it begins by closure of Na+ channels and opening of K+ channels -continuous: depolarize every segment of the axon (pretty slow)

Question 3

What is the purpose of the refractory period?

Answer 3

-assures that transmission is one a one way event -assures that APs are separate events (signal would not make sense if there were no spaces between signals) -stimulus strength (eg. light touch versus hard) is coded for by frequency of discharges (APs) not the amplitude since the APs are the same (same threshold and amplitude with each AP) -allows cell to recover to the resting state (keeps neurons from running down due to loss of ions and energy shortages brought about by the use of large quantities of energy to shift ions to reestablish RMP)

Question 4

Saltatory Conduction

Answer 4

-jumps from one region of the axon to the next -action potentials are created at nodes of Ranvier (bare sections of axon where ion channels are) -myelin sheath (Schwann cell wrapped around axon) speeds up process because only short section of the axon has to be depolarized then it jumps to next node (like the ball game where you pull back then the force gets transferred to the ball on the other end; in this case sodium ions push each other like the balls) -thick axon speeds up this process too because larger membrane has less resistance to current flow -pain signals are not myelenated

Question 5

Electrical synapse

Answer 5

-electrical signal is transmitted across the synapse by inducing ionic movement in the adjacent cell -ex: smooth muscle cells (no control of these eg. respiratory system, heart) -work as one functional unit to create a wave of contraction (eg. bottom of ventricle contracting first to squeeze blood out)

Question 6

Gap junction

Answer 6

-hole between 2 adjacent cells that allows for electrical synapses -like a straw that connects the cytoplasms of both cells -Na+ ions can flow from one neuron through gap junction into adjacent neuron so signal is propogated

Question 7

Chemical synapse

Answer 7

-action potential gets to nerve terminal axon -arrival of AP causes opening of voltage gated Ca2+ channels and Ca2+ flows in (it’s more abundant outside cell) -Ca2+ binds to vesicle containing NTs which allows vesicle to bind to plasma membrane, fuse with membrane, and release NTs into synapse -NT docks onto ligand gated ion channel which causes it to open and create a graded potential -if cell gets to threshold, then AP is generated -NT doesn’t stay bound forever- it releases and gets broken down then the products end up back in the presynaptic membrane

Question 8

Postsynaptic potentials

Answer 8

-can be excitatory or inhibitory -these are just graded potentials because they are created by ligand gated ion channels -can be summed over space and time

Question 9

What does this graph of a nerve impulse show?

Answer 9

temporal summation of postsynaptic potential (two signals on one neuron one after the other)

Question 10

What does this graph of a nerve impulse show?

Answer 10

spatial summation of a postsynaptic potential (two neurons close together providing excitatory stimulus on target neuron)

Question 11

What is a neurotransmitter?

Answer 11

• chemical that transmits signals across the synapse
• released from presynaptic membrane usually
• binds to ligand gated ion channel on post synaptic membrane causing excitatory or inhibitory potential

Question 12

What is acetylcholine

Answer 12

• most abundant neurotransmitter
• located at all ganglia of ANS and neuromuscular junctions
• necessary for thought, memory, skeletal muscles (voluntary movement)

Question 13

Biogenic amines

Answer 13

• catecholamines: norepinephrine, epinephrine, dopamine (all contain functional group R-NH2) made from tyrosine–> LDOPA–>DA–>NE–>EP
• indolamines: serotonin, histamine

Question 14

Norepinephrine

Answer 14

• released from adrenals into blood and released at synapses
• necessary for cortex to be awake so you can process info

Question 15

Epinephrine

Answer 15

• blood vessel constriction, flight or fight, pump heart
• released from adrenal glands (on top of kidney) into blood and activate body

Question 16

Dopamine

Answer 16

• found largely in CNS
• implicated in Parkinson’s (lack of DA)
• necessary for nuclei in brain to perform motor functions (swing arms, turn around, etc.)

Question 17

Serotonin

Answer 17

• associated with sleep and dreaming
• CNS
• released by enteric NS (gut) gives sensation of nausea
• affective tone (emotional well being)

Question 18

Histamine

Answer 18

• CNS
• helps keep cortex awake (antihistamines block histamine from keeping cortex awake and you get drowsy)
• allergic responses (immune cells release it)
• will have different effects in different areas of body (cause airway muscle to constrict, blood vessels to change, etc.)

Question 19

Amino acids

Answer 19

• CNS
• glutamate, aspartate, glycine
• use to build other proteins in body
• some can be excitatory or inhibitory

Question 20

Peptides

Answer 20

• amino acids strung together
• substance P: pain NT, released in tissues that are damaged and stimulates free nerve endings to give pain sensation
• oxytocin: important for labour (contracts uterus), lactatino (myoepithelial cells), CNS NT (helps increase bonding behaviour between couples or mother child)

Question 21

Enkephalins

Answer 21

• endogenous opioid like substance
• brain stem produces and put on spinal cord to inhibit pain reception
• gives you high when running (pushes pain sensation away after a certain point of pain)

Question 22

What is NT inactivation?

Answer 22

• enzyme breaks down NT in synapse so it can no longer stimulate ligand gated ion channels
• catecholamines: monoamine oxidase
• ACh: acetylcholinesterase

Question 23

Describe the effects of monoamine oxidase

Answer 23

-converts epinephrine into an aldehyde (also converts NE and dopamine)

-

Question 24

Describe the effect of acetylcholinesterase. What can inhibit acetylcholinesterase and what effects are observed?

Answer 24

• acetylcholinesterase breaks down acetylcholine into acetate and choline
• nerve gases (eg. Sarin gas) inhibit acetylcholinesterase resulting in SLUDD or spastic paralysis

Question 25

What is the major method of removal for catecholamines? How can this method be blocked?

Answer 25

• reuptake (usually into the presynaptic neuron)
• reuptake is specific to the NT
• SSRIs (eg fluoxetine): inhibits reuptake of serotonin from the synapse (can be used for depression because increased serotonin is associated with increased affective tone)
• blocked catecholamine uptake: cocaine blocks epinephrine reuptake leading to manic feeling, tricyclic antidepressants block reuptake of NE and serotonin

Question 27

What could be given as an antidote to acetylcholinesterase inhibitors?

Answer 27

-cholinergic blockers

• atropine: to treat SLUDD (blocks muscarinic receptors of structures innervated by postganglionic parasympathetic neurons)
• curare: to treat spastic paralysis (blocks nicotinic receptors at NMJ)