Pt5 Synaptic Transmission & AP

Question 1

Describe dendrites.

Answer 1

-integrate incoming info & determine if AP will be produced
-some dendrites are covered with dendritic spines
-each spine can synapse with diff axons
-1 dendrite can communicate with hundred axons
-dendritic branching pattern can change (increase/decrease)
-enriched or stimulatory environment = associated with growth of dendrites

Question 2

Describe axons.

Answer 2

-few micrometers or 10+ m long depending on species/location
-length due to cell cytoplasm & several organelles
>neurofibrils, neurotubules, sm vesicles, lysosomes, mitochondria, enzymes
-axonal proteins synthesized in soma & transported to axon
-cytoskeleton & diff proteins transport diff cargoes in both directions
-ex. Organelles; vesicles containing neurotransmitters

Question 3

Describe axoplasmic transport.

Answer 3

-anterograde transport = related with synaptic components (ex. Flow of synaptic vesicles & mitochondria) cell body to axon terminals
-retrograde transport = related with cargo for degradation (ex. Recycled membrane vesicles) axon terminals to cell body

Question 4

What are the two types of functional synapses.

Answer 4

Electrical synapses & chemical synapses

Question 5

Describe chemical synapses.

Answer 5

-most common
-unidirectional
-presynaptic neuron secretes neurotransmitter that acts on receptor protein in postsynaptic neuron
-excitatory or inhibitory

Question 6

What are the two types of neurotransmitters?

Answer 6

Conventional & unconventional

Question 7

Describe conventional neurotransmitters.

Answer 7

-stored in vesicles, released when Ca enters axon terminal in response to AP
-act by binding to receptors on membrane of postsynaptic cell
1. Sm molecule neurotransmitters (organic)
-amino acids (GABA, glutamate, etc)
-biogenic amines: dopamine, epi, serotonin, histamine
-purigenic = ATP & adenosine
-ACh
2. Lg molecule neurotransmitters (3 or more amino acids)
-endorphins, encephalins = inhibit pain
-substance P = carries pain signal
-neuropeptide Y = stimulates eating & act to prevent seizures

Question 8

Describe unconventional neurotransmitters.

Answer 8

-not stored in vesicles
-carry signals backwards (post to presynaptic)
-don’t depend on receptors - can cross membrane & act on molecule
1. Endocannabinoids
-lipid based retrograde neurotransmitter that binds to cannabinoid receptors & cannabinoid receptor proteins
-synthesized by body or found in plants like hemp
-physiological processes: appetite, mood, memory
2. Gastrotransmitters
-sm molecule of gas
-NO, CO
-blood vessels signaling to smooth muscle; NO produced is required for eye movement

Question 9

Describe the neurotransmitters.

Answer 9

Adrenaline = fight/flight
GABA = calming (I)
Noradrenaline = concentration (B)
Dopamine = pleasure
Serotonin = mood (B)
Epi = (B)
ACh = learning (B)
Glutamate = memory (E)
Endorphins = euphoria

Question 10

What are the two types of receptors in the postsynaptic membrane?

Answer 10

Ionotropic & metabotropic receptors

Question 11

Describe ionotropic receptors.

Answer 11

-neurotransmitter receptor that gate ion channels
1.cation channel = open by excitatory neurotransmitters & induce depol (ex. Na channel)
2.anion channel = open by inhibitory neurotransmitters & induce hyperpolarization (ex. Cl- channel)

Question 12

Describe metabotropic receptors.

Answer 12

-neurotransmitter receptor that acts through second messenger systems
-G protein coupled receptors
>open ion channels
>activate cAMP
>activate 1 or more intracellular enzymes
>activate gen transcription

Question 13

Describe electrical synapses.

Answer 13

-bidirectional
-interact w chemical synapses
-promotes synchronous firing of groups of interconnected neurons via gap junctions

Question 14

What are the types of synapses interfaces.

Answer 14

-synapses with another neuron
(Axosomatic, axodendritic, axoaxonic)
-neuromuscular junctions
-neuroglandular synapses
-axosecretory synapses = secrete into blood
-en passant synapses = synapse used as electrical signal passes by to the axon terminal

Question 15

How are AP’s initiated?

Answer 15

-neuron receives hundreds of inputs from other neurons
>in response to neurotransmitters from presynaptic neurons
>local changes in postsynaptic membrane
>local membrane potentials “graded potentials”
-amplitude is directly proportional to intensity of stimulus applied at synaptic sites
>synaptic site generate graded potentials
>1000 graded potentials occur at cell bodies & dendrites & travel to reach axon hillock/trigger zone

Question 16

What are the steps to initiate an AP?

Answer 16

1. 3 excitatory neurons fire. Their graded potentials sep are all below threshold
2. Graded potentials arrive at trigger zone & sum to create a Supra-threshold signal
3. AP generated

Question 17

Describe the trigger zone.

Answer 17

-where graded potentials integrate to generate AP
-sensitive to depol of local currents
-in order to start an AP, graded potentials must reach threshold potential or threshold voltage
>min voltage to change/trigger AP
>-55 mV
-once the sum of graded potentials exceeds threshold, AP is triggered
>AP propagates along axon
>depol dont reach threshold = no AP
>graded potentials decay

Question 18

Describe graded potentials (excitatory).

Answer 18

GP modulate postsynaptic neuron by shifting resting membrane potential
-shift toward threshold potential
-depol
-open cation channels (ex. Glutamate = Ca, K, Na)
-excitatory postsynaptic potentials (EPSP)
>drives MP toward threshold
>”excitatory synapses”

Question 19

Describe graded potentials (inhibitory).

Answer 19

-shift away from threshold potential
-hyperpolarization
-open anion channels (ex. GABA = ligand gated chloride channels)
-inhibitory postsynaptic potentials (IPSP)
>”inhibitory synapses”

Question 20

Describe the axon hillock.

Answer 20

-processes graded potentials by adding or subtracting potential charges
-continues to process as long as:
>sum of GP stays under threshold
>presynaptic changes occur faster than decay rate of GP in post synaptic neuron

Question 21

What are the 2 modes of summation in graded potentials?

Answer 21

Spatial & temporal

Question 22

Describe spatial summation.

Answer 22

-GP induced by diff synapses summation in postsynaptic neuron
-IPSP & EPSP = simultaneous summation of GPS

Question 23

Describe temporal summation.

Answer 23

-successive discharges from single presynaptic terminal summate in the post synaptic neuron
-occurs when 2 GP from one presynaptic neuron occur close together in time

Question 24

Describe the conduction of action potentials.

Answer 24

-AP generated at trigger zone = identical & propagate without losing strength
-dominos effect
-travel long distance
-speed of conduction:
>axon diameter
>degree of myelinization

Question 25

Describe saltatory conduction.

Answer 25

-AP occurs only at nodes of ranvier in myelinated fibers
-rich in ion channels
-AP jumps from nodes
-Na influx = depol
-electrical current flows through axoplasm inside axon
>passive and decremental Na current
-impulse transmitted from nodes
>AP is regenerated at nodes of ranvier

Question 26

Describe why signals move faster in myelinated cells.

Answer 26

1. Charge spreads down an axon & myelination (via Schwann cells) prevents ions from leaking out across PM.
2. Charge spreads unimpeded until it reaches unmyelinated section called node of ranvier = packed with Na channels
3. Electrical signals jump down axon faster than unmyelinated