Nervous System Flashcards

Question

What are ion channels?

Answer 1

Ion channels are proteins which make pores or gates that allow ions to move across the lipid membrane Ion channels can be: - Selective - for particular ions (e.g. Na+, K+) - - Switched to be open or closed

Answer 2

Two-pore domain potassium channels... are open at rest when the cell is not excited and help set the resting membrane potential - important role - allows for the flow of K+ ions to the extracellular space (driven by conc. and electrical gradient) They are regulated by pH, oxygen tension and stretch

Answer 3

Allows us to calculate the membrane potential by looking at the ion concentrations on either side of a membrane

Answer 4

Excitable cells act like batteries, with a charge (‘resting potential’) of about -65 mV This is brought about by the activity of the sodium-potassium pump and open K+ channels (leak) in the membrane (more important)

Answer 5

Excitable cells use bioelectricity to generate brief electrical pulses called action potentials - refers to a rapid change in membrane potential Action potentials – basic functional unit of the nervous system - packets/units of information used by the nervous system These units/packets can come in multiples – encode another level of information – firing rate Hence, their timing and frequency is used to encode NS information

Answer 6

Long range communication (C.N.S. & P.N.S.) - fast efficient signalling e.g. across long axons - frequency and pattern of action potentials encode information Used for Bidirectional communication - Motor commands to muscles - Feedback sensory information (e.g. proprioception, muscle tone, pain) When action potential communication becomes disrupted: - Multiple sclerosis - Charcot-Marie-Tooth disease - Alzheimer’s disease - Locked-in syndrome - ALS

Answer 7

All-or-nothing event 1. Resting potential no stimulus - maintained by Na+/K+ ATPase pump and K+ leak channel (1) 2. Membrane depolarisation – triggered by multiple factors - causes a switch in membrane potential 3. If threshold is reached – tigger an action potential - all or nothing 4. If succesful, we get full depolarisation - opening of local voltage gated sodium channels (Rising phase 2) 5. Falling phase (3) - Repolarisation (return to resting potential) – following depolarisation as Na channels close and K+ channels open leading to K+ efflux 6. Undershoot (4) - lower potential than resting potential (neuron is desensitized) --> after which we get a return to resting potential

Answer 8

Allow for measurements of ionic currents through channels

Answer 9

Na voltage gated channels - Channels are closed at rest (-70 mV) - Depolarisation to -40 mV induces a conformational change in the protein opening a pore - Pore acts as selectivity filter to allow passage of Na+ ions - driving depolarisation - 12x more permeable to Na+ than K+ ions - Channel opening is very fast, producing a rapid depolarisation (rising phase) - Channels stay open for ~1 ms before inactivating (physical pore block) - inactivation gate blocks pore, which is different from the closed state, known as the inactivated state - Re-opening can occur when the membrane potential returns to rest (-65 mV)

Answer 10

K+ voltage gated channels - Similar to Na+ channels, K+ channels require depolarisation to open - Unlike Na+ channels, K+ channels are slow to open (~1 ms after depolarisation) - Pore acts as a selectivity filter to allow passage of K+ ions - Delay in opening and reset membrane potential = delayed rectifier - Channels close when the membrane potential returns to rest (-65 mV)

Answer 11

1. Two pore potassium channels at rest – sets MP 2. Activation of Na channels – depolarization 3. Activation of K+ channels – repolarization 4. Resting potential re-set - two pore K+ channels

Answer 12

Refractory period - the time in which an excitable cell is unable to generate a subsequent action potential Refractory period – two types 1. Absolute refractory period - when all the Na+ channels are open – you can’t introduce another stimulus as they have all been recruited – one action potential at a given time 2. Relative refractory period - repolarization phase – hyperpolarization makes the neuron less sensitive stimuli – harder to produce a action potential – greater change in membrane potential required to reach threshold

Answer 13

Axon hillock – point between soma and axon – high concentration of Na+ channels – region where action potentials are generated

Answer 14

As Na+ channels open, Na+ diffuses along the neuron in turn depolarizing regions that lay ahead (opening Na+ voltage gated channels) resulting in a propagation of the action potential – known as a wave of depolarization Upstream Na+ channels that were activated become inactivated after ~1ms & K+ channels also open, in turn blocks the bidirectional propagation of action potentials

Answer 15

Axon conduction velocity in unmeylinated axons - 1-10 m/s Factors influencing velocity 1. Resistance of the plasma membrane 2. Diameter of membrane – big axon = faster conduction 3. Sodium channel density

Answer 16

Oligodendrocytes/Schwann cells – produce myelin sheath Myelin sheath is organised following a discontinuous nature Gaps are known as nodes of Ranvier High density of Na+ channels found at the nodes - this allows for rapid saltatory conduction between nodes of Ranvier - node-to-node propagation of action potentials (described as a domino effect)

Answer 17

Myelinated axons conduct action potentials much faster than unmeylinated axons Unmyelinated axons - 1 - 10 m/s Myelinated axons - 60 - 120 m/s

Answer 18

**Mechanoreceptors** - stretch, bend or pressure sensitive unmyelinated fibres **Mechanosensitive ion channels**- gating depends on stretch of surrounding membrane - intiating membrane potential If stimulus triggers an action potential, it will propagate via the neuron(s) via the dorsal root to the spinal cord

Answer 19

Dermatome - an area of skin that contains primary afferents that transduce information from the periphery to the spinal cord Dermatomes - tile the surface area of the body, linking specific areas of skin with specific spinal nerves - think of it as a neuronal map of the skin

Answer 20

8 cervical, 12 thoracic, 5 lumbar & 5 sacral nerves….relaying sensory information from the skin to the brain, via the spinal cord.

Answer 21

Receptive fields - looks at which areas the neurons receive information from How do we map them? - Introduce a stimulus and observe whether this triggers activity in a specific neuron

Answer 22

Stimulus strength - is encoded by the action potential firing frequency Stimulus onset / offset (timing) - is encoded by the timing of the 1st / last AP

Answer 23

Excitable cells communicate via chemical neurotransmission Specialised structures called synapses are where neurons contact each other

Answer 24

1. There are precursor molecules and/or synthesis enzymes located in the presynaptic terminal 2. The chemical is present in the presynaptic terminal 3. It is available in sufficient quantity in the presynaptic neuron to affect the postsynaptic neuron 4. There are postsynaptic receptors that bind the transmitter 5. A biochemical mechanisms for inactivation is present

Answer 25

Classes of neurotransmitters: 1. Amino acids - glutamate, aspartate, serine, Ɣ aminobutyric acid (GABA), glycine 2. Monoamines - dopamine (DA), noradernaline (NA), adrenaline, histamine, serotonin (5-HT) 3. Others - acetylcholine (ACh), adenosine, nitric oxide 4. Peptides - over 50 neuroactive peptides, sometimes released with other neurotransmitters

Answer 26

Do the volume of activity - there has to be a quick replenishment of neurotransmitters --> hence, we often see recycling of molecules - cyclical

Answer 27

Sequence of events leading to neurotransmitter release: 1. Action potential invades the presynaptic terminal 2. Membrane depolarisation occurs 3. Voltage-gated calcium channels open 4. Increase in calcium promotes vesicle fusion -- caclium binds to SNARE proteins allows for vesicular fusion 5. Vesicles release neurotransmitters into the synaptic cleft - exocytosis

Answer 28

1. Docking - vesicle binds to membrane, before AP invades terminal - primed 2. Ca2+ sensing - Ca2+ entry triggers fusion of the vesicle - SNARE complex driving fusion 3. Endocytosis - new vesicle membrane “pinched” off - formation of new vesicles 4. Loading - new vesicle is filled with neurotransmitter Cycle completes and can start again

Answer 29

Ionotropic receptors = fast response (µs - ms), ion channels open when neurotransmitter binds - tend to be ion channels that allow for direct membrane depolarisation Metabotropic receptors = slow response (ms - s), activation of a second messenger - GPCRs

Answer 30

1. Phospholipase C - cleaves PIP3 into IP3 and DAG --> secondary messengers 2. Adenylate Cyclase - converts ATP into cAMP

Answer 31

Synapses are either excitatory or inhibitory - this is determined by neurotransmitter they release

Answer 32

Post-synaptic potentials (PSPs) are called excitatory (or EPSPs) if they increase the likelihood of a postsynaptic action potential occurring, and inhibitory (or IPSPs) if they decrease this likelihood. EPSP - increase movement of Na+ into the post-synaptic neuron - increasing the liklihood that it fires - depolarisation IPSP - increase movement of Cl- into the post-synaptic neuron - increasing the liklihood that it will not fire - hyperpolarisation

Answer 33

Neurons don’t just receive input from one other neuron They integrate inputs from tens to thousands of presynaptic neurons --> Example: Cerebellar Purkinje cell receives ~100,000 excitatory synaptic inputs Different types of summation 1. Spatial summation - summation of EPSPs generated at different synapses - integrated effect 2. Temporal summation - summation of EPSPs generated at the same synapse - before EPSP decays we have the arrival of another EPSP – leads to summation – domino effect Likewise, we have the arrival of IPSPs --> suppresses the excitation, reducing the membrane depolarisation and preventing the generation of an action potential

Answer 34

Agonist - mimics the effect of the endogenous neurotransmitter Antagonist - blocks the effect of the endogenous neurotransmitter Examples shown in image - Ach - endogenous neurotransmitter --> activates nicotinic and musarinic receptors - Nicotine - activates nicotinic receptors - Muscarine - activates muscarinic receptors - Curare - inhibits nicotinic receptors - poison for arrow tips, blocks muscle contraction, paralysis - Atropine - inhibits muscarinic receptors -opthalmic use, dilates the pupils - cleopatra used it for vanity

Answer 35

Act on the GABAergic system Ethanol - affects the level of inhibition, affects CNS (cognition) and PNS (motor function) Barbiturates - mild sedation to full anaesthesia (commonly misused drug) Neurosteroids - endogenous modulators, levels changed during menstrual cycle

Answer 36

Skeletal - somatic motor cells

Answer 37

Chain of sympathetic ganglion (body of neuronal bodies) Preganglionic neurons release acetylcholine Postganglionic neurons release noradrenaline Sympathetic neurons located next to targets Participates in ‘fight-or-flight’ response By binding to different postsynaptic adrenoreceptors: 1. Relaxes airways in the lungs 2. Inhibits digestion 3. Accelerates heart rate 4. Constrict blood vessels Adrenaline - used to treat patients in cardiac arrest (increases peripheral resistance & accelerates heart rate)

Answer 38

Releases acetylcholine Compliments sympathetic nervous system, acts as a brake to the sympathetic system Controls actions that do not require an immediate reaction: 1. digestion 2. metabolic functions (liver, GI tract) 3. regulates kidneys, liver etc 4. Vasodilation of blood vessels By binding to different postsynaptic acetylcholine receptors, results in: 1. constriction of airways 2. stimulates digestion 3. slows heart rate ACh acts on two types of receptors, the muscarinic and nicotinic cholinergic receptors. - Preganglionic neuron releases ACh at the ganglion, which acts on nicotinic receptors - Postganglionic neuron then releases ACh to stimulate the muscarinic receptors of the target organ Parasympathetic neurons travel large distances

Answer 39

Somatic motor system: under voluntary control and generates behaviour Divided into upper and lower motor neurons

Answer 40

Lower motor neurons - located in spinal cord and conntect to muscle fibres - Directly command muscle contraction - Skeletal muscle movements, initiate by ‘lower’ motor neurons - Activated by local spinal cord circuits - Each motor neuron innervates a single muscle

Answer 41

Lower motor neurons are distributed within the ventral horn in a predictable way --> Spatial organization/map of the body in the spinal cord - muscles located more distally are located laterally in the spinal cord (visa-versa) Spatial map of body musculature Neurons innervating flexors are dorsal to those innervating extensor muscles

Answer 42

The alpha motor neuron and all the muscle fibres it innervates is classed as the motor unit

Answer 43

Muscle contraction - results from the individual and combined action of motor units Motor neuron pool - collection of alpha motor neurons that innervate a single muscle (e.g. biceps) - drive contraction of a single muscle This arrangement maintains normal muscle activity when damage to a single motor neuron occurs - redundancy in the system

Answer 44

Spread contractile load - ensures that the entire muscles produces force in an even manner

Answer 45

Muscle contraction - initiated by Ach release and binding to postsynaptic ACh receptors 1 presynaptic action potential is sufficient to trigger 1 postsynaptic action potential in a muscle fibre Ach activity 1. Ach – binds to receptor 2. AChE breaks down Ach into choline and acetic acid 3. Choline recycled and pumped back into neuron – symport with Na+ 4. Choline + Acetyl CoA used to resynthesize Ach

Answer 46

Muscle contraction - thin (actin) filaments slide along the thick (mysoin filaments) Contraction coupling - 1. Ca2+ binds to troponin, exposing myosin binding sites on actin 2. Myosin binds actin, myosin head pivots, sliding actin down 4. Myosin disengages at the expense of ATP hydrolysis 5. Cycle repeats

Answer 47

1. AP in the alpha motor neuron 2. Exocytosis of ACh 3. Postsynaptic depolarisation 4. Ca2+ release from sarcoplasmic reticulum 5. Sliding actin/myosin filaments 6. Muscle contraction Relaxation occurs when Ca2+ or ATP levels reduce

Answer 48

Number of motor neuron recruitment will change the level/force of contraction – higher Hz more motor unit activity 80 Hz action potentials = full contraction

Answer 49

An involuntary, nearly instantaneous movement in response to a stimulus (does not require the brain) Important role in responding to noxious stimuli E.g. Platellar or Knee-jerk reflex

Answer 50

Myotatic or stretch reflex: reciprocal innervation of flexors and extensors One contracts the other is inhibited

Answer 51

Basically interneuron is present which sends an inhibitory signal to the flexor (hamstring), whereas the extensor (quad) receives an excitatory stimulus.

Answer 52

Reflex pathway 1. Painful stimulus 2. Activation of sensory (afferent) axons 3. Activates excitatory interneurons 4. Motor neurons (efferent) induce contraction But on its own this reflex would make you unstable --> Muscles don’t work separately - coordinated activity to ensure stability Crossed extensor reflex – accounts for loss of balance induced by initial reflex in response to noxious stimulus – compensation to ensure body remains stable/balanced Consists of a system of interneurons

Answer 53

The vestibulo-ocular reflex (VOR) is a reflex acting to stabilize gaze during head movement, with eye movement due to activation of the vestibular system. How does it fundamentally work? Contraction of opposite muscle relative to direction of head rotation – allows to fix gaze on a specific target Heads rotates to the right - the left extraocular muscles contract whereas the right one's are inhibited