nervous system B (neurons)

Question 1

membrane potential changes

Answer 1

Membrane potential changes when:
Concentrations of ions across membrane change
Membrane permeability to ions changes
Voltage changes across the membrane can be classified into 2 basic types of electrical signals:
Action Potentials – Long-distance signals of axons that travel very rapidly
Graded Potentials – Incoming signals traveling over short distances

Question 2

graded pontetials

Answer 2

travel from dendrites to axon hillock

Question 3

action potentials

Answer 3

Action potentials are electrical signals that travel over long distances (from a neuron’s axon hillock to the end of its axon (axon terminals).

The signals are of uniform strength as they travel (do not lose strength). The action potential at the end of the axon is identical to the action potential that started at the axon hillock.

The high speed movement of an action potential along the axon is called conduction of the action potential.

In action potentials, voltage-gated ion channels in the axon membrane open sequentially as electrical current passes down the axon. They represent movement of Na+ and K+ across the membrane.

action potentials are all or none phenomenon.

voltage gated channels pass action potential down axon.

Question 4

fg

Answer 4

first key player in action potentail is voltage gated NA channels. an activation gate and an inactivation gate. when activation gates open na goes inside. inaction gates can plug hole and keep Na from going down .

voltage gated K channels only have 1 gate. when open K goes outside, closed stays inside.

Question 5

as

Answer 5

If axon hillock membrane depolarizes to Threshold Stimulus (-55 mV):
The activation gate of voltage-gated Na+ channels open
Na+ rushes into cell
3. Na+ influx causes more depolarization which opens more adjacent Na+ channels → ICF less negative

Question 6

xd

Answer 6

When Membrane potential reaches +30 mV (Action Potential peak):

1. Na+ voltage-gated channel inactivation gates close(activation gate still open)
2. Membrane permeability to Na+ declines to resting state
3. Voltage-gated K+ channels open
4. K+ exits the cell and internal negativity is restored (Repolarization)

Question 7

ex

Answer 7

Some K+ voltage-gated channels remain open, allowing excessive K+ efflux
Inside of membrane more negative than resting state. This causes Hyperpolarization of the membrane (slight dip below resting voltage)
3. Voltage-gated Na+ channels begin to reset

Question 8

propagation and conduction of action potentials

Answer 8

triggerzone where the action potentail starts.
positve A graded potential above threshold enters the trigger zone.
Its depolarization opens voltage-gated Na+ channels, Na+ enters the axon, and the initial segment of axon depolarizes.
Positive charge from the depolarized trigger zone spreads by local current flow to adjacent sections of the membrane, repelled by the Na+ that entered the cytoplasm and attracted by the negative charge of the resting membrane potential.

4. The flow of local current toward the axon begins conduction of the action potential.
   
   • When the trigger zone depolarizes, its Na+ channels open, allowing Na+ into the cell.
   • This starts the positive feedback loop causing depolarization of adjacent membrane.
5. The section of axon that has just completed its action potential is in its absolute refractory period, with its Na+ channels inactivated. For this reason, the action potential cannot move backward.

Question 9

ex

Answer 9

Some K+ voltage-gated channels remain open, allowing excessive K+ efflux
Inside of membrane more negative than resting state. This causes Hyperpolarization of the membrane (slight dip below resting voltage)
3. Voltage-gated Na+ channels begin to reset

Question 10

propagation and conduction of action potentials

Answer 10

triggerzone where the action potentail starts.
positve A graded potential above threshold enters the trigger zone.
Its depolarization opens voltage-gated Na+ channels, Na+ enters the axon, and the initial segment of axon depolarizes.
Positive charge from the depolarized trigger zone spreads by local current flow to adjacent sections of the membrane, repelled by the Na+ that entered the cytoplasm and attracted by the negative charge of the resting membrane potential.

4. The flow of local current toward the axon begins conduction of the action potential.
   
   • When the trigger zone depolarizes, its Na+ channels open, allowing Na+ into the cell.
   • This starts the positive feedback loop causing depolarization of adjacent membrane.
5. The section of axon that has just completed its action potential is in its absolute refractory period, with its Na+ channels inactivated. For this reason, the action potential cannot move backward.

Question 11

absolute refractory period

Answer 11

When voltage-gated Na+ channels open, a neuron cannot respond to another stimulus
Absolute refractory period
Time from opening of voltage-gated Na+ channels until resetting of the channels
Ensures that each AP is an all-or-none event
Enforces one-way transmission of nerve impulses

Question 12

relative refractory period

Answer 12

Follows absolute refractory period
Most Na+ voltage-gated channels have returned to their resting state
Some K+ voltage-gated channels still open
Repolarization is occurring
Threshold for AP generation is elevated
Inside of membrane more negative than resting state
Only exceptionally strong stimulus could stimulate an AP

Question 13

graded potential

Answer 13

Graded potentials are variable-strength signals that travel over short distances and lose strength as they travel through the cell.

They are used for short-distance communication.

Graded potentials in neurons are depolarizations or hyperpolarizations that occur in the dendrites and cell body or, less frequently, near the axon terminals.

If a depolarizing graded potential is strong enough when it reaches the trigger zone of the axon hillock, the graded potential initiates an action potential.

These changes in membrane potential are called “graded” because their size, or amplitude, is directly proportional to the strength of the triggering event. A large stimulus causes a strong graded potential, and a small stimulus results in a weak graded potential.

In neurons, graded potentials occur when chemical signals from other neurons open chemically-gated ion channels, allowing ions to enter or leave the neuron.

Question 14

why do graded potentials lose their strength

Answer 14

Why do graded potentials lose strength as they move through the cytoplasm?
Current leak – the membrane of the neuron cell body has open leak channels that allow positive charge to leak out into the ECF. Some positive ions leak out of the cell across the membrane as the depolarization wave moves through the cytoplasm, diminishing the strength of the signal inside the cell.
Cytoplasmic resistance – the cytoplasm provides resistance to the flow of electricity
Graded potentials that are strong enough eventually reach the region of the neuron known as the trigger zone.
In efferent neurons and interneurons, the trigger zone is the axon hillock and the very first part of the axon, a region known as the initial segment.
In sensory neurons, the trigger zone is immediately adjacent to the receptor.

Question 15

another name for axon

Answer 15

nerve fiber

Question 16

speed of action potential conduction

Answer 16

Two key physical parameters influence the speed of action potential conduction in a mammalian neuron:
1. The diameter of the axon
- the larger the diameter of the axon, the faster an action potential will move
- large axons offer less total resistance
2. The resistance of the axon membrane to ion leakage out of the cell
- Nonmyelinated axons have low resistance to current leak because the entire axon membrane is in contact with the ECF and has ion channels through which current can leak.
- Myelinated axons limit the amount of membrane in contact with the ECF. In these axons, small sections of bare membrane (the nodes of Ranvier) alternate with longer segments with myelin sheaths. The myelin sheath creates a high resistance wall that prevents ion flow out of the cytoplasm.
Myelinated axons allow for Saltatory Conduction which is faster and is an effective alternative to large-diameter axons.

Question 17

myelintated sheaths

Answer 17

lots of NA voltaged gated channels in high density in nodes of ranvier. when action potential reaches nodes they open cuz membrane potential becomes more positive. myelin blocks leak channels and keeps them insulated (as in keeps ions from leaking)

Question 18

Cell-to-Cell Communication in the Nervous System

Answer 18

Neurons communicate at Synapses
Each synapse has two parts:
1. The axon terminal of the presynaptic cell
2. The membrane of the postsynaptic cell
In a neural reflex, information moves from a presynaptic cell to postsynaptic cell

The postsynaptic cells may be neurons or non-neuronal cells.

In most neuron-to-neuron synapses, the presynaptic terminals are next to dendrites or the cell body of the postsynaptic neuron.

Question 19

chemical synapse

Answer 19

Majority of Synapses
Mainly use neurotransmitters to carry information from cell to cell.
Presynaptic axon terminals have many small synaptic vesicles filled with neuro-transmitters that are released on demand.
Neurotransmitters stored in vesicles in the axon terminal are released into the synaptic cleft by exocytosis.
Axon terminals also contain mitochondria to produce ATP for metabolism and transport.

Question 20

neurotransmitter termination

Answer 20

pumped back through terminal, pumped away by glial cell

diffused out into the blood.

enzymes inactivate them

Question 21

neurotransmitters

Answer 21

Language of the nervous system
50 or more neurotransmitters have been identified
Most neurons make two or more neurotransmitters
Neurons can exert several influences
Usually released at different stimulation frequencies
Classified by chemical structure and by function

Question 22

acetylcholine

Answer 22

Neurotransmitters can be informally grouped into 7 classes.
1. Acetylcholine (ACh)
- First identified; best understood
Synthesized in axon terminal and made from acetyl-CoA and choline.
Neurons that secrete ACh and receptors that bind Ach are described as cholinergic.
Released at neuromuscular junctions and by some CNS and ANS neurons.

Question 23

synthesis and recycling of acetylcholine

Answer 23

acetylcholine is made from choline and acetyl coa

in synaptic cleft ACh is broken down by acetylcholinase enzyme

choline transported back into axon terminal by cotransport of NA

recycled choline used to make ach.

mitochondira good source of choline

Question 24

biogenic amines

Answer 24

2. Biogenic Amines
   Derived from single amino acids.
   Catecholamines: Dopamine, norepinephrine (NE), and epinephrine
   Synthesized from amino acid tyrosine
   Neurons that secrete NE and receptors that bind norepinephrine are described as adrenergic.
   Indolamines: Melatonin, Serotonin and histamine
   Serotonin synthesized from amino acid tryptophan; histamine synthesized from amino acid histidine
   Broadly distributed in brain and play roles in emotional behaviors and biological clock
   Some ANS motor neurons (especially NE)
   Imbalances associated with mental illness

Question 25

amino, peptides, purines, gases, lipids

Answer 25

Amino Acids
Glutamate and Aspartate are excitatory neurotransmitters
Glycine and gamma-aminobutyric acid (GABA) are inhibitory neurotransmitters.
4. Peptides
- Substance P (mediator of pain signals); Endorphins (reduce pain perception)
5. Purines
- Adenosine, ATP can act as neurotransmitters and bind to purinergic receptors in the CNS and other excitable tissues such as the heart.
6. Gases
Nitric oxide (NO), Carbon monoxide (CO), and Hydrogen sulfide (H2S)
Bind with G protein–coupled receptors in the brain
NO involved in learning and formation of new memories; brain damage in stroke patients, smooth muscle relaxation in intestine
7. Lipids – include several eicosanoids.

Question 26

diverging and converging circuit

Answer 26

Diverging circuit
• One input, many outputs
• An amplifying circuit
• Example: A single neuron in the brain can activate 100 or more motor neurons in the spinal cord and thousands of skeletal muscle fibers

 Converging circuit • Many inputs, one output • A concentrating circuit • Example: Different sensory stimuli can all elicit the same memory

Question 27

postsynaptic responses

Answer 27

A neurotransmitter combining with its receptor set in motion a series of responses in the postsynaptic cell.
The initial response of the postsynaptic cells are graded potentials called Postsynaptic Potentials

If the synaptic potential is depolarizing, it is called an excitatory postsynaptic potential (EPSP) because it makes the cell more likely to fire an action potential.
If the synaptic potential is hyperpolarizing, it is called an inhibitory postsynaptic potential (IPSP) because hyperpolarization moves the membrane potential farther from the threshold and makes theq cell less likely to fire an action potential.

Question 28

excitatory synapses adn EPSPs

Answer 28

Neurotransmitter binding opens chemically gated channels
Some target cells have channels that allow simultaneous flow of Na+ and K+ in opposite directions
Na+ influx greater than K+ efflux  net depolarization called EPSP (not AP)
EPSP help trigger AP if EPSP is of threshold strength
Can spread to axon hillock, trigger opening of voltage-gated channels, and cause AP to be generated

Question 29

inhibitaroy syanpses and IPSPs

Answer 29

Reduces postsynaptic neuron’s ability to produce an action potential
Makes membrane more permeable to K+ or Cl–
If K+ channels open, it moves out of cell
If Cl- channels open, it moves into cell
Therefore neurotransmitter hyperpolarizes cell
Inner surface of membrane becomes more negative
AP less likely to be generated

Question 30

synaptic integration:summation

Answer 30

A single EPSP may not induce an AP
EPSPs can summate to influence postsynaptic neuron
IPSPs can also summate
Most neurons receive both excitatory and inhibitory inputs from thousands of other neurons
Only if EPSP’s predominate and bring to threshold  AP

Question 31

two types of summation

Answer 31

Spatial summation
Postsynaptic neuron stimulated simultaneously by large number of terminals at same time
Temporal summation
One presynaptic neuron transmits impulses in rapid-fire order