BIO 122 LECTURE 3: NEUROMUSCULAR PHYSIOLOGY J4R4FFE

I. SIGNALLING IN NERVES NERVE IMPULSE

- transmission of action potential from the axon hillock to the axon terminals and into the adjacent neuron

- action potential = all or nothing; does not diminish based on distance from origin

- action potentials jump from node to node - unmyelinated vs. myelinated nerve fibers (voltage-gated

Na+ channels are present only at the nodes of Ranvier) Why action potential jump down axon

1. As charge spreads down an axon, myelination (Schwann cells) prevents ion from leaking out the PM.

2. Charge spreads unimpeded until it reaches an unmyelinated section of the axon (node of Ranvier, which is packed with Na+ channels)

3. Electrical signals continue to jump down the axon much faster than down an unmyelinated cell.

Normal conduction of myelinated fibers high density voltage-gated Na+ channel at node; saltatory conduction of signal Demyelination of nerve fibers in MS increased Na+ channels along demyelinated axons (multiple sclerosis) SYNAPSE

- Charles Sherrington, 1897 - specialized intercellular spaces between a neuron and an

effector cell or another neuron - synaptic transmission vs. axonal transmission - presynaptic and postsynaptic terminals

1. Electrical Synapse

- occurs in gap junction (nexus) present - transmission occurs without measurable delay - little or no fluctuation in AP (continuous) - gap junction are formed exclusively from hexameric

pores (connexons) = connect cells with each other for robust electrical coupling

- functions: metabolic (diffusional exchange); local inhibitory network in CNS

2. Chemical Synapse - most common synaptic transmission - synaptic cleft: 20 to 50 nm - time lag occurs - AP may fluctuate - mediated by NEUROTRANSMITTERS (from

terminal bulb of presynaptic axon) synthesized by neuron (1 neuron : 1 transmitter

type) present in presynaptic terminals bind to specific receptor on postsynaptic

membrane associated with specific mechanisms of

deactivation e.g. Ach and Ne

- no. of vesicles is reduced with: - decreased Ca2+ and Na+ in ECF - previous depolarization making the AP weaker

Synaptic Transmitter at Neuromuscular Junction (NMJ): Acetylcholine Synthetic and Storage

- Quantum: amount of neurotransmitters in one vesicle that determines the minimum size of postsynaptic potential; e.g. Ach = quantal units of 3000 molecules

- *cholinergic vesicle = ~103 molecules of Ach - Quantal release = transmitter is released in quantum (there

is a certain amount that is released) - miniature EPSPs (mEPSPs) change in the membrane

potential of a muscle cell produced by a single quantum - mEPSPs EPSP threshold AP to postsynaptic

terminal - normal neurotransmission requires the release of many

vesicle simultaneously - regulated fusion of synaptic vesicles with the nerve

terminals and release of neurotransmitter to synaptic cleft: docking priming fusion

SYNAPTIC POTENTIAL

- generated during the transmission of a nerve impulse across a synapse

- graded, with longer duration but lower amplitude (unlike AP which is all-or-none)

- magnitude related to amount of neurotransmitter released

1. Presynaptic Potential AP arriving at the terminal end of an axon

2. Postsynaptic Potential a. EPSP

- depolarization leads to an AP resulting from opening of ligand-gated ion channels

- permeability changes generating EPSP are VOLTAGE-INDEPENDENT, instead are TRANSMITTER-DEPENDENT

- e.g. Na+ ions flow inward generates EPSP - increases postsynaptic potential

depolarization b. IPSP

- tends to hyperpolarize so that AP is not generated

- e.g. Cl- ions flow inward and/or K+ ions flow outward (membrane is simultaneously permeable to Cl- and K+ ions) generates IPSP

- decreases postsynaptic potential hyperpolarization

Multiple excitatory and inhibitory inputs onto dendrites and the soma SUMMATE EPSP + IPSP.

Spatial Summation Temporal Summation several neurons single neuron stimulating at the same time stimulating at different times occurs at different sites of membrane

occurs at the same site of membrane

DENDRITES - unable to transmit AP due to:

few voltage-gated channels too high threshold for excitation

- transmit ELECTRONIC CURRENT down to the soma - dendrites are long with thin membranes at least partially

permeable to K+ and Cl- leaky to electric current decremental conduction (not saltatory)

- current spreads bc fluid (without generation of AP) - dendrites summate the excitatory and inhibitory potentials

Termination of Synaptic Transmission

- resorption (active reuptake) of neurotransmitter or its breakdown products

- enzymatic degradation of neurotransmitter (e.g. Acetylcholinesterase)

Pre and Postsynaptic Inhibitions

NMJ vs. Neuron-neuron Synapse

- NMJ = more powerful synapses; AP in motor neuron produces AP at target muscle fiber

- N-N = require simultaneous inputs from presynaptic neurons to generate AP to postsynaptic neuron

Synaptic Plasticity

- changes in synaptic efficacy over time - amplitudes of synaptic potentials are not constant over time

a. Facilitation - increase in amplitude of PSPs in response to

successive presynaptic impulses - basis of occurrence of sensitization (increased

intensity of an effector response)

b. Depression - decrease in amplitude of PSPs with successive

presynaptic impulses - basis for occurrence of habituation (decreased

intensity of an effector response)

II. SENSORY RECEPTION Receptor Cell

- specialized cell responsive to internal and external stimuli - has the ability to convert energy into neural signal

stimulus receptor afferent nerve CNS

RECEPTOR POTENTIAL - graded depolarizartion of a receptor in response to

stimulation - ionic basis: increased permeability of receptor membrane to

all small ions, esp to Na+ Characteristics:

1. An adequate stimulus elicits a graded RP, the amplitude of which is a function of stimulus intensity

Adequate stimulus form of stimulus energy to which the receptor is most sensitive or to where it normally responds

2. The frequency of resultant AP in a receptor is a coded representation of the intensity of the adequate stimulus

Receptor potential is graded and non-propagated. Action potential is non-graded and propagated.

Sensory Transduction: absorption of stimulus energy (SE) transduction of SE to electrical signal amplification of energy integration and conduction Sensory Reception and Processing: stimulus sense organ (accessory structure) transducer (sensory cells) action potential (nerve transmission) decoder (CNS) TRANSDUCTION: Excitation of Receptors to Generate RP Altered permeability of membrane to ions:

1. mechanical deformation of receptors 2. chemical application 3. temperature change 4. effects of electromagnetic radiation

Receptor Adaptation

- decrease in the response of a receptor to a steadily maintained stimulus over time

- decrease in firing of AP despite maintained depolarization Types:

1. Tonic Receptors slowly adapting; respond for the duration of the stimulus

2. Phasic Receptors radily adapting; adapts to a constant stimulus and turn off

Adaptation Curves (Examples)

1. Muscle spindle receptors - sensory neurons that detect change in muscle length - intrafusal muscle fibers distributed among extrafusal - ion channels connected by spectrin = responds to

membrane deformation/stretch 2. Inner hair cell transducers

- auditory and vestibular apparatus - stereocilia and kinocilium (true for vestibular,

degenerate for auditory) - inner hair cells innervated by sensory + motor nerves - extracellular fluid (i.e. endolymph) around hair cells

= potassium-rich - tip link = connects stereocilia at one end to an ion

channel, one that admits potassium and calcium - depolarization = movement of cilia towards

kinocilium 3. Olfactory receptors

- neurons that have ciliated terminal ends projected into the mucus of olfactory epithelium

- odorant receptors located in the cilia - each olfactory receptor cell expresses only one type

of odorant receptor = binding protein 4. Gustatory receptors

- tongue papillae taste buds taste cell innervated by sensory nerve

- can be produced in different ways: a. through cationic channels (Na+, Na+/H+

cotransport) b. blocking of K+ channels c. through secondary messengers that work close

to K+ channels (bitter and sweet) d. through secondary messengers that open Cl- or

non-specific ion channel 5. Visual receptors

- rods and cones of the retina - rhodopsin and cone pigments = light sensitive

chemicals found in the outer segment - light rhodopsin decomposition + hyperpolarization

of rod receptor potential (not depolarization)

Dark State Light Stimulation cGMP-gated Na+ channels, which are open in the dark

rhodopsin decomposition

membrane less negative cGMP-gated Na+ channels close active transport of Na+ membrane more negative membrane more negative hyperpolarization RMP = -40 mV normal in dark conditions

III. MOVEMENT AND LOCOMOTION

A. SKELETAL MUSCLES Skeletal Muscle Structure

- muscle cells/muscle fiber - multinucleated; diameter = 10-80 m - several myofibrils (1-2 m) comprise each muscle fiber - sarcomere = functional unit of a myofibril

Skeletal Muscle Innervation

- motor unit: composed of motor neuron + all muscle cells innervated

- there are many motor units in a muscle - a single motor neuron may innervate several fibers

fewer muscle fibers per neuron the finer the movement (e.g. fingers)

many muscle fibers per motor unit coarse movement (e.g. trunk muscles)

Myofibrils

- contain contractile elements of muscles - A band (dark band) thick filaments - M line center of the A band - I band (light band) thin filaments - Z line/disk center of I band

Sarcomere

- Z-M-Z - Z line -actinin/titin binds actin of adjacent sarcomeres - M line Mittel of sacromere

Myosin

- composed of two coiled polypeptide chains - tails are oriented towards the center of the sarcomere (M

line) Actin

- composed of two coiled actin molecules + regulatory proteins TROPOMYOSIN = covers actin binding sites TROPONIN = theww binding sites (for

tropomyosin, actin and Ca2+ ions) EXCITATION

NMJ: Chemical Synapse Ach

Drugs that cause muscle spasm - through Ach-like action - metacholine, carbachol, nicotine - destroyed very slowly by cholinesterase or not at all - through Ach-ase inactivation - neostigmine, physostigmine bind with Ach-ase for

several hours but reversible - diisopropyl fluorophosphates a nerve gas that binds with

Ach-ase for weeks (lethal) Drugs that block transmission at NMJ

- prevent impulse transmission - curariform drugs

Myasthenia gravis

- autoimmune disease where antibodies attack, block or alter the Ach receptors at NMJ prevents muscle contraction

- mostly affect voluntary muscles - muscle weakness

How is Ca2+ released from sarcoplasmic reticulum?

1. Plunger Model - ryanodine receptors block Ca2+ channels - AP: calcium lifts the ryanodine

2. Enzyme- or messenger-mediated mechanism

CONTRACTION Sliding Filament Mechanism

- decreases in width: sarcomere, I band, H zone - no change: A band, myosin and actin filaments

Cross-Bridge Cycle

Energy Sources for Contraction Main source = ATP

- limited - must be regenerated through:

Direct Phosphorylation creatine phosphate/phosphocreatine (CP) high energy molecule found in muscle fibers creatine phosphokinase transfers PO4 from CP

to ADP

RELAXATION Contraction-Relaxation Steps Requiring ATP

- splitting of ATP by myosin ATPase provides energy for power stroke of cross bridge

- binding of fresh molecule of ATP to myosin leads to cross bridge detachment from actin filament at end of power stroke so cycle can be repeated

- active transport of Ca2+ back to sarcoplasmic reticulum during relaxation

- calsequestrin (in sarcoplasmic reticulum)

Isotonic Contractions Isometric Contractions muscle shortens with constant tension

muscle remains same length during contraction; tension is variable

load < tension load > tension

Slow Muscle Fibers Fast Muscle Fibers slow but prolonged response rapid contraction but short

response slow contraction, longer duration extensive SR for rapid release of

Ca2+ associated with smaller fibers innervated by smaller nerves

associated with large fibers which elicit great strength of contraction

extensive blood supply and mitochondria

less extensive blood supply and mitochondria

high myoglobin red/dark muscles

low myoglobin white muscles

low levels of myosin ATPase and glycolytic enzymes

high levels of myosin ATPase and glycolytic enzymes

Type I/Slow Oxidative = depends on aerobic processes

Type IIA/Fast Oxidative Glycolytic (FOG) = intermediate Type IIB/Fast Glycolytic = anaerobic processes

B. SMOOTH MUSCLES Smooth Muscle Structure

- no striations - no sarcomere - no troponin - no T-tubules - less developed SR

Smooth Muscle Innervation: Autonomic

- no NMJ - neurotransmitters are released from varicosites

Types: 1. Multi-unit - one nerve per muscle cell neurogenic - e.g. muscles found in iris of eyes, trachea, arteries 2. Single-unit - one nerve + gap junctions myogenic - e.g. peristaltic wave in GI tract

Smooth Muscle Contraction

1. With excitation-contraction coupling - through neural input (autonomic nervous system)

Parasympathetic: Ach as NT binding with muscarinic receptor

Sympathetic: NE as NT 2. Without E-C coupling - through hormones, paracrine agents (effects/signals

neighboring cells), etc. involving secondary messengers

C. CARDIAC MUSCLES Cardiac Muscle Structure

- with striation - with troponin - with developed SR - with T-tubules

- INTERCALATED DISCS + gap junctions (unique

feature) - regions of low electrical resistance for action potential

transmission - marks adjacent muscle cells - innervated by autonomic nervous system - myogenic - ECF + SR = calcium sources - Ach is inhibitory to contraction (vs. skeletal = induces

contraction) - NE is excitatory (responsible for fast heart rate)

Action Potential in Cardiac Myocyte

Dyhydropyrinidine receptors are unable to affect function of ryanodine receptors,

instead, the release of large amounts of Ca2+ opens RyRs.

Smooth Cardiac excitation: requires both extra and

intracellular sources of Ca2+ contraction: Ca2+ binds with calmodulin (vs. skeletal = troponin) contracts more slowly and exhibit more prolonged contraction with less ATP

contraction: mechanism is similar to skeletal but more calcium is released for AP generation more actin-myosin interaction = to avoid tetany of heart

relaxation: requires myosin phosphatase enzyme to dephosphorylate myosin

relaxation: 1. active transport of Ca2+

back into SR during relaxation

2. 3 Na : 1 Ca antiport in SR and sarcolemmal pump

sources of Ca2+ are both extracellular and intracellular (SR)

extracellular Ca2+ allows prolonged contraction intracellular increase in Ca2+ = due to nerve stimulation or hormonal/local factors

Thick Filament Regulation (Phosphorylation of Myosin): calmodulin + Ca2+ activates myosin light-chain kinase (MLK) adds phosphate to myosin phosphorylated myosin binds to actin contraction [vs. skeletal = Thin Filament Regulation]

- depends on the uncovering of the thin filament (actin)

D. NON-MUSCLE CELLS Cytoskeleton

- network of protein filaments in eukaryotic cell cytoplasm that provides shape, support and movement

- cytomuslculature Three types of protein filaments:

1. Actin Filaments (Microfilaments) - maintain cell shape by resisting tension (pull) - move cells via muscle contraction or cell crawling - divide animal cell into two - move organelles and cytoplasm in plants, fungi and

animals 2. Intermediate Filaments

- maintain cell shape by resisting tension (pull) - anchor nucleus and some other organelles

3. Microtubules - maintain cell shape by resisting compression (push) - move cells via flagella or cilia - move chromosomes during cell division - move organelles - provide tracks for intracellular transport

Actin Filaments in Crawling Cells (Amoeboid)

1. Trailing edge - where most actin is heavily distributed

2. Leading edge - pulls the cell forward

3. Stress fibers - lie on ventral surfaces of cells - made up of actin-myosin - form in response to tension generation within cell;

adhesion and deadhesion of cell to substratum

Contractile bundle

- stress fibers Cell cortex

- gel - beneath the PM - actin-myosin - support + stiffens the fluid-like (gel) membrane - randomly arranged

Filopodium - thinner projections of cells - actins are tightly arranged in parallel bundle

Growth cone - developing axon (terminal portion) not yet synaptically

connected - guides the axon in looking for synaptic target - lamellipodium + filopodium + microfilaments

Addition of G-actin to F-actin

- filopodium Cells with amoeboid movement

- of amoebas - embryonic cells during development - invasion of tissues by leukocytes (macrophages) - migration of cells during wound-healing - metastasis of cancer cells

Actin-binding Proteins: -actin, Filamin, Fimbrin

- determine the form + function of actin filaments -actin = contractile bundles filamin = gel fimbrin = parallel bundles

Steps in Cell Crawling

1. Protrusion - of the leading edge - polymerization of actin subunits to form actin fils

2. Adhesion - to certain substrates - mostly stress fibers contribute to adhesion - cortex, hindi humahaba pero nagmo-move - *Deadhesion trailing end

3. Traction - interaction of actin-myosin

Cytoplasmic Gel-Sol Conversion: Role in Locomotion Gel-Sol

- actin cytoskeleton transitioning between solid-like elastic material (gel/gelation) and a solution-like viscous material (sol/solation)

- from gel to sol; gel at rest, sol for contraction - induced by presence of calcium

CILIA

- occur in groups; shorter - power stroke: counters a force; cilium bends at the base - recovery stroke: bend propagates up the cilium (bottom to

top high energy) - motion with single bend

FLAGELLA

- single; longer - wave-like motion = reverse bend-forward bend - recovery stroke: bend propagates up the flagellum - motion with several bends

Where cilia is found/used:

- respi tract to remove mucus - uterus for propulsion of egg cell - attachment is called basal body - beat metachronically - smoke (cigarette) loosens cilia lining in lungs

Axoneme in Cilia and Flagella - similarly organized - 9+2 arrangement of microtubules - -tubules = 13 profilaments - -tubules = 11 profilaments - central = 13 profilaments - basal body = no central fils 9+0 - radial spokes = linkages of outer doublets to central - nexins = links adjacent doublets - dyein arms

inner and outer protein motor molecules that walk along adjacent

microtubules ATPase activity:

hydrolysis of ATP associated with reattachment of the dyein arms to the adjacent -tubule but at a different location a sliding motion of adjacent outer tubule structures

binding of ATP release of the dyein arms from the adjacent -tubule

sliding microtubule

In a cilium/flagellum, two adjacent doublets cannot slide far because they are physically restrained by proteins so they bend.

(A) Trypsin-treated axoneme of sperm tail nexin linkers and radial spokes cleaved ATP addition sliding of microtubules axoneme is 7-8 times longer

(B) ATP-dependent movement of outer doublets restricted by cross-linkage proteins in order for sliding to be converted into bending of axoneme