nervous system 2

Question 1

TYPES OF MOTOR NEURONS:

Answer 1

Upper motor neurons – motor neurons that synapse either with another upper motor neuron (CNS) or with a lower motor neuron.

Lower motor neurons – motor neurons that directly stimulate effector organs in response to signals from the UMN.

Summary
Usually a command from the CNS will pass from an UMN to a LMN before stimulating the muscle or gland (effector organ).

Question 2

Electrical signals in the Neuron

Answer 2

Neurons are electrically excitable (like muscle cells) and conduct signals (action potentials) to target cells => muscles or glands.

Question 3

Neurons communicate using two types of signals…

Answer 3

1) Graded potentials (short distance only)
2) Action potentials (longer distances)
Remember muscle APs? Now we have
nerve APs

Question 4

Membrane Potential:

Answer 4

The production of nerve impulses (graded potentials and action potentials) depends on two features of the cell membrane:

Question 5

Resting membrane potential

Answer 5

– electrical difference (voltage) across the cell membrane (when the neuron is at rest).

Question 6

Ion channels

Answer 6

– the flow of ions across the membrane makes up the electrical current.

Question 7

*Graded potentials and action potentials both occur due to

Answer 7

the opening and closing of ion channels which creates a flow of ions (current).

Question 8

Membrane potential – exists due to

Answer 8

the electrical difference (separation charge, or voltage) across the cell membrane. Measured in millivolts (mV). In neurons it is -70mV (it is slightly different in other types of cells.)

The buildup of positive ions along the outside of the cell membrane, and the negative ions on the inside of the membrane creates a form of potential electrical energy.

The greater the difference in charge, the greater the membrane potential (voltage).

Question 9

Resting membrane potential is due to:

Answer 9

Unequal distribution of ions in the ECF and ICF

Question 10

ECF has a higher concentration of

Answer 10

Na+ ions,

Question 11

ICF has a higher concentration of

Answer 11

K+ ions.

Question 12

K+ has more

Answer 12

leakage channels to diffuse down its concentration gradient compared to Na+ (the membrane is more permeable to K+).
This movement of K+ makes the ECF side of the membrane more positive.

Question 13

Anions that can’t leave the cell

Answer 13

The presence of amino acids and ATP (phosphate) which are negatively charged. These molecules are unable to leave the cell and therefore make the inside of the cell membrane more negative.

Question 14

Electrogenic nature of Na+/K+ pump

Answer 14

Na+/K+-ATPase pumps 3 Na+ out of the cell and 2 K+ in each cycle. This means one more cation (1+) is pumped out than in, making the inside of the cell relatively more negative. This helps to maintain the neuron’s resting membrane potential of -70mV.

Question 15

Na+/K+ pump

Answer 15

Ubiquitous (found in all cells)
Pumps 3 Na+ OUT for every 2 K+ IN to help establish this resting membrane potential.
ACTIVE transport (pushing these ions against their concentration gradients).

Question 16

Na+ & K+ are the most

Answer 16

important electrogenic ions – I.e. their movement contributes to establishing the resting membrane potential.

Question 17

Na+/K+ pump:

Answer 17

Three Na+ OUT of the cell for every two K+

IN

Question 18

Because the ECF has a higher

Answer 18

[Na+], this ion tends to “leak” into the cell (down its concentration gradient).

.

Question 19

Because the ICF has a higher

Answer 19

[K+], this ion tends to “leak” out of the cell (down its concentration gradient)

Question 20

Sodium and potassium are considered

Answer 20

electrogenic. I.e. they contribute to establishing the resting membrane potential.

 The Na+/K+ pump actively pumps these 2 ions AGAINST their concentration gradients. This costs ATP!!!

Question 21

Ion channels

Answer 21

– ion channels are embedded in the cell membrane. When open they allow specific ions into and/or out of the cell, down their electrochemical gradient.
As ions move they create electrical current which changes the membrane potential.

Question 22

The main ions involved are:

Answer 22

Sodium (Na+) – carries a positive charge; concentrated higher OUTSIDE the cell. It wants to leak into the cell, down its concentration gradient. [This is the most abundant cation/ion in extracellular fluid]

Question 23

Potassium (K+)

Answer 23

– carries a positive charge; concentrated higher INSIDE the cell. It wants to leak out of the cell, down its concentration gradient. (There are also MORE K+ leakage channels than Na+ channels) [This is the most abundant cation in intracellular fluid.]

Question 24

Chloride (Cl-)

Answer 24

– carries a negative charge; concentrated outside the cell. [Most prevalent anion found in extracellular fluid.]

Question 25

Calcium (Ca2+)

Answer 25

– carries a highly positive charge; concentrated outside the cell. [Most abundant mineral/cation in the body although NOT in the ECF!]

Question 26

Phosphate (PO4-)

Answer 26

– large and highly negative – concentrated inside the cell; don’t move too much because of size and lack of channels.

Question 27

Amino acids (aa)

Answer 27

– large and mostly negative; concentrated inside the cell and don’t move too much because of size and lack of channels.

Question 28

Electrical signals rely on 4 types of ion

channels to allow the flow of ions

Answer 28

Leakage channel

Voltage-gated channel

Ligand-gated channel

Mechanical-gated channel

Question 29

Leakage channels

Answer 29

– alternate between open and closed positions. Most membranes tend to have more K+ ion channels making them more permeable to this ion.

Question 30

Voltage-gated channels

Answer 30

– open & close as a response to a change in the membrane potential. These ion channels play a key role in action potentials.

Question 31

Ligand-gated channels

Answer 31

– open & close in response to chemicals/molecules (be it a hormone, neurotransmitter, etc.). Involved in graded potentials.

Question 32

Mechanically-gated channels

Answer 32

– open & close in response to a mechanical stimulus. E.g. vibrations, pressure, and stretch. Involved in graded potentials.

Question 33

Potentials: 1) Graded potentials

Answer 33

Graded Potentials (GP) – involve ligand-gated & 
mechanically gated channels.

Small deviations from the membrane potential (-70 mV) that makes the inside of the membrane either less negative (depolarizing) or more negative (hyperpolarizing).

May vary in amplitude (depending on the strength of the stimulus). Hence the term ‘graded’.

The effects of GPs are restricted to a smaller/localized area of the membrane (dendrite and cell body). They only last a short distance unlike action potentials.

Not subject to refractory period.

Question 34

Spatial graded potential

Answer 34

– summation of GP in response to stimuli that occur at different locations in the membrane of a neuron at the same time.

Question 35

Temporal graded potential

Answer 35

– summation of GP in response to stimuli that occur at the same location in the neuron’s membrane but at different times. I.e. Same location firing several times in quick succession.

Question 36

Action Potentials

Answer 36

– an all or none electrical
impulse that occurs when a stimulus causes a
sufficiently large depolarization in the membrane
potential (reaches the AP threshold).

Question 37

AP -Arises from

Answer 37

the trigger zone at the axon hillock
Conducts AP down the axon to axon terminals
Involves voltage gated channels.
Works on the all or none principle (no summation) – like dominoes falling
2 key phases=
1) depolarization
2) repolarization

Question 38

1. Depolarization

Answer 38

– when a depolarizing GP causes the membrane of the axon to reach threshold, Na+ channels open and Na+ rushes into the cell. This makes the inside of the cell more positive. (-70 mV  - 55mV  0 mV  +30 mV)

Question 39

2. Repolarization

Answer 39

– when the membrane potential returns to resting state (i.e. becomes negative/polar again).
K+ channels open and K+ rushes out of the cell which causes the membrane potential to return to its normal resting electrical potential (-70mV)

Question 40

Related principles (action potentials)

Answer 40

All or None Law – when depolarizing stimulus reaches a threshold, the action potential is generated and is always the same size & amplitude (i.e. dominoes – pushing of the first chip faster or harder will not alter the speed or the size of the toppling effect).

Question 41

Threshold

Answer 41

– the minimum amount of voltage needed to generate an action potential. Approximately -55 mV in a neuron. (I.e. at -60, no AP will be generated.)

Question 42

Refractory period

Answer 42

– the period following an AP in which no other AP can be generated.

Question 43

Absolute

Answer 43

– this coincides with the period of Na+ channel activation and inactivation (you can think of this as the Na+ channels being open). NO amount of stimulation can cause a 2nd AP during this time.

Question 44

Relative

Answer 44

– coincides with the time that the Na+ channels have closed (returned to their resting state) but the K+ channels are open. A stronger than normal stimulus is required to generate a 2nd AP during this time.

Question 45

Propagation (conduction) of APs:

2 main types:

Answer 45

Continuous – nerve impulse travels along the membrane due to opening of each subsequent and adjacent ion-channel. This occurs in unmyelinated axons and is (relatively) very slow

Saltatory – nerve impulses “jump” along the axon at the Nodes of Ranvier (remember these are the gaps between myelin sheaths where there are concentrated amounts of voltage gated channels).This is not only a LOT faster (usually 10 fold), it’s also a lot more energy efficient!

Question 46

Speed of transmission also depends on size/diameter of the axon; there are 3 types of axon fibers found throughout the body.

Answer 46

1. A fibers – 100 % myelinated, speeds up to 130 m/sec. Found in sensory nerves of touch, temp & pressure, & motor neurons of skeletal muscles. VERY FAST!
2. B fibers – moderately myelinated, speeds up to 15 m/sec. Found in ANS & visceral organs. MODERATELY FAST!
3. C fibers – No myelination, slowest of the 3 fibers. Found in reproductive, urinary, excretory, digestive neurons, nociceptors from skin and viscera. SLOW!

Question 47

Signal Transmission between neurons:

Answer 47

Synapse: the site of communication between two neurons or a neuron and effector cell (mm/gland)
Synapses allow information to be filtered and integrated – they can be electrical or chemical.

a. pre-synaptic neuron – the neuron before the synapse
b. post-synaptic neuron – the neuron after the synapse
c. axo-dendritic – connection b/w an axon & a dendrite
d. axo-somatic – connection b/w an axon & cell body

Question 48

2 types of synapses:

Electrical

Answer 48

– uses gap junctions & found in the ANS, smooth muscle & cardiac muscles. The signal passes through tubes made of connexon proteins.
Advantageous for faster communication & synchronization of signals (e.g. heart/digestive organs).
FAST!!!

Question 49

Chemical

Answer 49

– involves the transmission of an AP across a synapse. This is slower!!!

Question 50

1) AP signal arrives at the synaptic end bulb of the pre-synaptic neuron.
2) Depolarization phase opens voltage gated Ca2+ channels that allow Ca2+ to flow in.
3) Stimulates the exocytosis of synaptic vesicles. Neurotransmitters from within the vesicles are released into the synaptic cleft.

Answer 50

4) The released neurotransmitters diffuse across the synapse and attach onto corresponding receptors on the post-synaptic membrane.
5) Binding of the neurotransmitter to the receptor (AKA ionatropic receptors-specific to NTs) opens the channel and thus triggers a graded potential (post synaptic potential).
The post synaptic potential may be excitatory (EPSP) or inhibitory (IPSP).

Question 51

If the neurotransmitter causes an opening of Na+ channels,

Answer 51

an excitatory post synaptic potential occurs (EPSP) – if that potential reaches threshold (-55mV), we get an action potential in the post synaptic neuron.

Question 52

If the neurotransmitter released causes hyperpolarization,

Answer 52

then we get an inhibitory effect (IPSP). In this case Cl- ion channels open and the cell membrane becomes more negative/hyperpolar (making an AP less likely to occur).

Question 53

Neurotransmitters cause graded potentials (post synaptic potentials) that are either a:

Answer 53

Excitatory post synaptic potentials (EPSP) – depolarizing, bringing the membrane closer to threshold.
Inhibitory post synaptic potential (IPSP) –hyperpolarizing, membrane becomes more negative, further away from threshold and therefore the generation of an AP becomes more difficult.

Question 54

Ionotropic receptors

Answer 54

– neurotransmitter receptor connected directly to an ion channel. I.e. An acetylcholine receptor at a NMJ is connected to a sodium ion channel

Question 55

Metabotropic receptors

Answer 55

– uses a messenger protein to open certain ion channels. These are usually inhibitory.
NB: ACh can be excitatory at some synapses (ionatropic) and inhibitory at others (metabotropic).

Question 56

Post synaptic neurons receive input from numerous pre-synaptic neurons.

Answer 56

Some release excitatory NTs, others inhibitory.
The total post synaptic potentials (the total excitatory effects vs the inhibitory effects) must reach threshold in order for an AP to be generated.

Question 57

Neurotransmitters (NTs): Two types:

Small molecule NTs

Answer 57

Acetylcholine (Ach)
Excitatory NT at NMJ, inhibitory NT in cardiac muscle in response to PSNS.
Amino Acids
Excitatory:
Glutamate and aspartate
Inhibitory: 
GABA – Gamma-aminobutyric Acid (CNS)
Glycine (inhibitory effects at NMJ)

Question 58

Biogenic Amines

Answer 58

Epinephrine (adrenaline) – released by adrenal glands and also as a NT in a small amount of neurons in the brain.
Norepinephrine (NE) (noradrenaline)
Dopamine (DA) – E.g. mood & pleasure centers.

Serotonin (5-HTP) – E.g. sensory perception & mood.

Question 59

Parkinson’s disease

Answer 59

=> not enough DA

Question 60

Nitric oxide (NO)

Answer 60

– vasodilator, relaxes blood vessel smooth muscles but also an important excitatory NT in the brain.

Question 61

Neuropeptides: large molecules

Answer 61

Found in CNS and PNS – both excitatory and inhibitory (depending on the receptor).
Bind to metabotropic receptors and many also play a role as hormones.

Certain neurons in the brain have receptors for opiate drugs which led to the discovery of the first neuropeptides:

Endorphins and enkephalins – natural painkillers – 200x stronger than morphine!
Substance P – also a neuropeptide – enhances the perception of pain.

Question 62

Removal of the NTs from the synaptic cleft is necessary for proper communication. If they aren’t cleared away they will continue to have an effect on the neuron/mm/gland. 3 ways:

Answer 62

Diffusion – NTs diffuse out of the cleft

Enzyme degradation – i.e. acetylcholinesterase at the NMJ.

Uptake by cells (reuptake) – NTs are actively transported back to the neuron that released it.

Question 63

Modifying the effects of NT

Answer 63

Delaying NT reuptake – cocaine delays reuptake of dopamine by blocking its active transporters.
NT synthesis can be inhibited – patients with Parkinson’s produce less dopamine.
NT release can be inhibited or enhanced –amphetamines enhance the release of dopamine and norepinephrine.
NT receptors can be blocked or activated, enhancing or inhibiting their effect – usually by drugs or toxins.

Question 64

Neural Circuits:

Answer 64

Functional groups of neurons that process specific types of information.
Types of neural circuits:

Question 65

1. Simple

Answer 65

– simplest of all – 1 neuron stimulates another which in turn stimulates another (very rare).

Question 66

2. Diverging

Answer 66

– a single neuron stimulates a few neurons, which then in turn stimulate more (signal amplification).
Sensory signals, and some neurons in the brain that are responsible for certain body movements will stimulate numerous motor neurons.

Question 67

Converging

Answer 67

– multiple neurons will stimulate 1 single neuron, which will allow for summative effects (reverse of diverging circuits). E.g. a single motor neuron receiving numerous signals from the brain.

Question 68

4. Reverberating

Answer 68

– similar to simple circuits – neurons will reverberate backwards to stimulate earlier neurons. E.g. coordinated movement, short term memory, breathing.

Question 69

5. Parallel after-discharge

Answer 69

– a single pre-synaptic cell stimulates multiple neurons which all synapse back to a common post-synaptic cell. E.g. precise activities like solving mathematical equations

Question 70

Damage and Repair of Nervous Tissue:

Answer 70

Plasticity – the capability to adapt/change based on experiene.

Neurogenesis – the birth of new neurons from undifferentiated stem cells.

Epidermal growth factor (EGF) – in 1992 it was discovered that this chemical can trigger mitosis in neuronal cells.

Question 71

CNS damage:

Answer 71

Our current understanding is that damage to
axons and cell bodies within the CNS (brain
& spinal cord) will result in permanent
damage. It is suspected that this is due to
the formation of myelin sheaths from
oligodendrocytes and NOT Schwann cells.
Research is ongoing.

Question 72

PNS damage:

Answer 72

Mild to moderate damage to neurons of the PNS will still permit good recovery of nerve function. This is b/c most cells of the PNS are covered with a protective sheath called a neurolemma (from Schwann cells).

Question 73

Chromatolysis

Answer 73

– degranulation of nissl bodies within the neuron as part of the repair process.

Question 74

Wallerian degeneration

Answer 74

– degeneration of the distal portion of the axon and myelin sheath as part of repair.