Week 5 - Lecture 1 - Alterations in neuronal impulse conduction

Question 1

What are the two principle type of cells of nervous tissue

Answer 1

neuroglia and neurones

Question 2

What are neuroglia cells

Answer 2

small cells that surround and wrap delicate neurons

in the central nervous system there’s astrocytes, microglial, ependymal and oliodendrocytes

in the PNS there is satellite and Schwann cells

Question 3

what are neurons

Answer 3

excitable cells that transmit electrical signals

Question 4

5 neuron characteristics

Answer 4

1. extreme longevity
   - 100 years or more
2. amitotic ( do not divide)
3. high metabolic rate
   - require continuous supply of oxygen and glucose
4. large, highly specialised cells - conduct impulses
5. All have cell body and processes
   - processes are dendrites and an axon

Question 5

Functional classification of neurones

Answer 5

grouped by direction in which nerve impulse travels relative to CNS

1. sensory - afferent
2. motor - efferent
3. interneurones (association)
   - lie between motor and sensory neurones
   - 99% of body’s neurones

Question 6

dendrites

Answer 6

dendrite are the receptive (input) region of a neurone, they receive incoming signals toward cell body as graded potentials (changes in membrane potential)

Question 7

Axons

Answer 7

Axons are the conducting region of a neurone

Question 8

What is the myelin sheath

Answer 8

The myelin sheath is a whitish, protein lipoid substance
- it is segmented sheath around most long, or large diameter axons

myelinated fibres

Question 9

What are the functions of myelin

Answer 9

protects and electrically insulates axon
increase speed of nerve impulse transmission
nonmyelinated fibres conduct impulses slowly

Question 10

Plasma membranes of myelinating cells have less protein, why

Answer 10

because there are no channels or carriers, good electrical insulators and interlocking proteins bind adjacent myelin membranes

Question 11

what are and why are there myelin sheath gaps

Answer 11

gaps between adjacent Schwann cells

called nodes of ranvier

regions rich in sodium channels to promote nerve impulse over long distances

Question 12

Membrane potentials

Answer 12

neurons communicate with other neurons in the body through action potential (nerve impulse/electrical signal)

• nerve impulse is always in the form of an action potential regardless of stimulus (eg. pain, light, heat)
• Action potential is a result off charged ions

Question 13

What are the two main types of ion channels

Answer 13

leakage (non gated) and gated

Question 14

Leakage non gated channel

Answer 14

always open

Question 15

Gated channels

Answer 15

part of protein changes shape to open/close channel

3 channels

1. chemically gated (ligand - gated) channel
2. voltage gated channel
3. mechanically gated channel

Question 16

chemically gated ( ligand - gated) channel

Answer 16

open with binding of a specific neurotransmitter

Question 17

voltage gated channels

Answer 17

open and close in response to changes in membrane potential

Question 18

mechanically gated channels

Answer 18

open and close in response to physical deformation of receptors, as in sensory receptors

Question 19

The resting membrane potential

Answer 19

the bulk solution inside and outside of the cell are electrically neutral

potential difference exists in a very defined area only across the membrane of resting cells

• approx - 70mV In neurons
  (cytoplasmic side of membrane negatively changed relative to outside)

membrane termed polarised

This is generated by the differences in ionic makeup of ICF and ECF
differential permeability of the plasma membrane
- ECF has higher concentration of sodium NA+ than ICF, balanced chiefly by chloride ions (Cl-)
-ICF has higher concentration of K+ than ECF, balanced by negatively charged proteins

K+ plays most important role in membrane potential

Question 20

Differences in plasma membrane permeability

Answer 20

slightly permeable to Na+ (through leakage channels)
- sodiums diffuses into cell down concentration gradient

25 times more permeable to K+ than sodium (more leakage channels)
- potassium diffuses out of cell down concentration gradient

more potassium diffuses out of the cell than sodium diffuses into the cell

• cell more negative inside
• establishes resting membrane potential

sodium potassium pump stabilises resting membrane potential

Question 21

How do membrane potentials change

Answer 21

membrane potential changes when

• concentrations of ions across membrane change
• membrane permeability to ions changes

Question 22

Changes in membrane potential

Answer 22

terms describing membrane potential relative to resting membrane potential

Depolarisation

• decreased in membrane potential (towards zero and above)
• inside of membrane becomes less negative than resting membrane potential
• increase probability of producing a nerve impulse

Hyperpolarisation

• an increase in membrane potential (away from zero)
• inside of cell more negative than resting membrane potential)
• reduces probability of producing a nerve impulse

Question 23

threshold

Answer 23

not all depolarisation events produce an action potential

for an axon to ‘fire’ depolarisation must reach threshold
- that voltage at which the action potential is triggered

at threshold

• membrane has been depolarised by 15 to 20 mV
• Na+ permeability increases
• Na+ influx exceeds K+ efflux

an action potential either happens completely, or it does not happen at all

Question 24

Propagation of an action potential

Answer 24

propagation allow an action potential to serve as a signalling device

Na+ influx causes local currents

• local currents depolarisation of adjacent membrane areas in direction away from action potential origin (toward axon’s terminal)
• local currents trigger an action potential there
• this causes the action potential to propagate away from the origin

since Na+ channels closer to an action potential origin are inactivated no new action potential is generated there

once initiated an AP is self propagating

Question 25

The synapse

Answer 25

nervous system works because information flows from neurone to neurone neurons functionally connected by synapses - junctions that mediate information transfer 1. from one neurone to another neurone 2. or from one neurone to an effector cell synapses are electrical or chemical in nature

Question 26

axodendritic

Answer 26

between axon terminals of one neurone and dendrites of others

Question 27

axiomatic

Answer 27

between axon terminals of one neurone and soma of others

Question 28

Electrical synapses

Answer 28

less common than chemical synapses neurones electrically coupled (joined by gap junctions that connect cytoplasm of adjacent neurones and can allow the passage of charged ions) - communication very rapid - may be unidirectional or bidirectional More abundant in embryonic nervous tissue

Question 29

chemical synapses

Answer 29

specialised for release and reception of chemical neurotransmitters typically composed of two parts 1. axon terminal of presynaptic neurone - contains synaptic vesicles filled two parts separated by synaptic cleft : fluid filled space electrical impulse changed to chemical across synapse, then back into electrical transmission across synaptic cleft - chemical event (slower than an electrical one) - depends on - release of neurotransmitter - diffusion of neurotransmitter - receptor binding of neurotransmitter - ensure unidirectional communication between neurones

Question 30

information transfer across chemical synapses 9 stages

Answer 30

1. action potential arrives at axon terminal of presynaptic neurone 2. causes voltage-gated Ca2+ channels to open, Ca2+ floods into the cell 3. intracellular proteins binds Ca2+ and promote fusion of synaptic vesicles with axon membrane 4. exocytosis of neurotransmitter into synaptic cleft occurs 5. neurotransmitter diffuses across synapse 6. binds to receptors on post synaptic neurone 7. ion channels are opened 8. causes an excitatory or inhibitory event 9. neurotransmitter effects terminated

Question 31

termination of neurotransmitter effects

Answer 31

within a few milliseconds neurotransmitter effect terminated in of three ways 1. reuptake - by astrocytes or axon terminals 2. degradation - by enzymes 3. diffusion - away from synaptic cleft

Question 32

post-synaptic potentials

Answer 32

neurotransmitter receptors cause changes in membrane potential that vary in strength with - amount of neurotransmitter released and - time neurotransmitter stays in the area types of post synaptic potentials - EPSP - excitatory postsynaptic potentials - IPSP - inhibitory postsynaptic potentials

Question 33

Neurotransmitters

Answer 33

50 or more neurotransmitters have been identified most neurones make two or more neurotransmitters - neurons can evert several influences classified by chemical structure and by function

Question 34

Acetylcholine

Answer 34

first identified : best understood released at - neuromuscular junctions - some ANS neurons - some CNS neurones - synthesised from acetic acid and choline by enzyme choline acetyltransferase - Degraded by enzyme acetylcholinesterase (AChE)

Question 35

Biogenic amines

Answer 35

1. catecholines - dopamine, norepinephrine (NE) and epinephrine - excitatory/inhibitory 2. indolamines - serotonin (inhibitory) and histamine (excitatory/inhibitory) Broadly distributed in brain - play role in emotional behaviour and biological clock - imbalanes associated with mental illness

Question 36

Amino acids

Answer 36

glutamine : excitatory - stroke neurotransmitter - excess release : cell stimulated to death aspartate : excitatory glycine : inhibitory - spinal cord main inhibitory GABA - gamma - aminobutyric acid : inhibitory = main inhibitory neurotransmitter of the brain

Question 37

classification of neurotransmitters : function

Answer 37

great diversity of functions can classify by effects: excitatory versus inhibitory actions: direct versus indirect function of the neurotransmitter is determined by the receptor to which it bunds effects - excitatory versus inhibitory - neurotransmitter effects can be excitatory (depolarising) and or inhibitory (hyper polarising) - GABA and glycine usually inhibitory - glutamine usually excitatory - acetylcholine and NE bind to at least two receptor types with opposite effects ACh excitatory at neuromuscular junction in skeletal muscles ACh inhibitory in cardiac muscles

Question 38

Direct vs indirect actions

Answer 38

direct action - neurotransmitter bind to and opens ion channels - promotes rapid responses by altering membrane potential - examples : Each and amino acids indirect action - neurotransmitter acts through intracellular second messengers, usually G protein pathways - broader, longer lasting effects similar to hormones - example : biogenic amines, neuropeptides and dissolved gases

Question 39

neurotransmitter receptors

Answer 39

1. channel linked receptors : mediate fast synaptic transmission - ligand gated ion channels - action is immediate and brief 2. G protein linked receptor : mediate slow synaptic responses - responses are indirect, complex, slow, and often prolonged

Question 40

neurotransmitters vs neuromodulators

Answer 40

nitric oxide, adenosine often referred to as neuromodulators

Question 41

neuromodulators

Answer 41

chemical messengers released by the neurone does not can EPSP or IPSP affects the strength of synaptic transmission (may inhibit, potentiate, or prolong the effects of neurotransmitters)

Question 42

Processes of neuronal injury

Answer 42

mature neurones do not divide - new neurones do not replace damaged neurone that lost function loss of neurones leads to impaired neurologic transmission impaired neurologic transmission leads to neurologic disorders - cell damage - ageing manifestations reflect - site of injury - functions that are controlled by the dysfunctional neuronal signals

Question 43

processes of neuronal injury

Answer 43

result of injury at the cellular level chromatolysis - for of apoptosis - swelling of a neurone and dissolution of chromophil substance in neurones - in certain pathological conditions or following an injury to the cell or axon atrophy - decrease in cell size neuronophagia - phagocytosis and inflammatory responses causes by a dead neurone damaging neighbouring cells intraneuronal inclusions - distinctive structures formed in the nucleus or cytoplasm