Membrane Physiology

MEMBRANE PHYSIOLOGY Anita Ocampo-So, MD, FPSP, DPBA, MHA

Professor

Nervous System A complex array of specialized structures which serve to: • Receive • Store • Transmit information • Consist of the CNS and the PNS • Neuron- basic unit of the NS – 1 trillion neurons

Neurons • • • • •

Made up of dendrites Soma Axon- axon terminal Has different organelles but no centromere Classification according to 1. structure 2. Form 3. covering – Schwann cells

Neurons

Neurons

•

structures aside from organelles 1. astrocytes 2. microglia 3. oligodendroglia – apoptosis

Neurons • • • •

Bell-Magendie Law Afferent – sensory Efferent – motor Afferent  200,000 interneuron  10 efferent

Neurons

Classification of Nerve Fibers Class

Conduction Velocity (m/sec)

Size ( μm)

Functions

A α

70-120

12-20

Proprioception Somatic motor

β

30-70

5-12

Touch pressure

3-6

Motor to muscle spindle

12-30

2-5

Pain,touch, cold temp

B

3-5

<3

C

0.5-2

0.4-1.2

γ δ

15-30

Preganglionic, Autonomic pain, temperature, mechanoreception

Classification of Nerve Fibers

Coverings of Nerve Fibers 1. endoneurium 2. perineurium 3. epineurium

Coverings of Nerve Fibers

Properties of Nerve Fibers

2. Excitability 4. Conductivity

Myelinated Fibers • Schwann cells • Nodes of Ranvier • Saltatory Conduction – Advantages • faster conduction • saves energy • less loss of ions

Myelinated Fibers

Resting Membrane Potential • Cells under resting conditions have an electrical potential difference across their plasma membrane. This potential is the Resting Membrane Potential (RMP) • The cytoplasm is electrically (-) relative to the ECF • The RMP plays a vital role in the excitability of nerve and muscle cells and in other cellular responses

Genesis of the RMP 1.Distribution of ions • movement of ions depend on: a. Concentration b. Electrical potential difference

•

if forces are equal = no movement – Nernst equation – Nernst equilibrium EMF (mo)

= (-)61log conc 1/conc 0 = (+)61

Genesis of the RMP – –

Gibbs-Donnan equilibrium Goldman-Hodgkin-Katz

 

Selective Permeability of the Membrane Na+ - K+ pump

•

RMP – –

skeletal muscle (-)90mV nerve (-) 70 mV

Action Potential

Action Potential • All-or-Nothing Principle • Strength – duration curve – Rheobase – Utilization time – Chronaxie

Refractory Periods • Absolute Refractory Period • Relative Refractory Period – nerve fibers are not susceptible to fatigue

• Accommodation

Refractory Periods Local Excitatory States • subthreshold stimulus • inadequate duration of applications According to their location • receptor potential • synaptic potential • pacemaker potential • end-plate potential In general, moves the RMP closer to the threshold potential

Synapse and Synaptic Transmission Synapse • an anatomically specialized junction between two neurons • important in the transport of nerve signals from one neuron (presynaptic) to the next neuron (postsynaptic) – 100 quadrillon synapses in the CNS – 2 types a. Electrical b. Chemical

Presynaptic and Postsynaptic Neuron Presynaptic neuron • has lots of mitochondria • has NTs inside vesicles • usual forms are ; round, dense, flat and sphenoid • intertwining – cerebellum • basket and climbing – midbrain Postsynaptic neuron • contains the receptor proteins

Presynaptic and Postsynaptic Neuron Presynaptic

Postsynaptic

Presynaptic and Postsynaptic Transmission 1. axo-axonic 2. axo-dendritic 3. axo-somatic

Characteristics of Synapses 1.convergence 2.divergence 3.facilitation 4.post-tetanic potentiation 5.fatigue 6.after discharge

Characteristics of Synapses 1. 2. 3. 4. 5.

subliminal fringe occlusion reverberation inhibition summation – temporal – spatial

Synaptic Responses 1. excitatory postsynaptic potential – can be graded response which will bring the RMP closer to the TP 2. inhibitory postsynaptic potential – makes the membrane hyperpolarized

Classes of Neurotransmitters 1. Acetylcholine 2. Biogenic amines – catecholamines, serotonin, histamine 3. Amino acids – glutamate, GABA 4. Neuropeptides – endogenous opioids 5. Miscellaneous – nitric acid, adenosine

Factors Affecting Synaptic Effectiveness 1. Presynaptic factors 2. Postsynaptic factors 3. General factors

Modulators

Chemical messengers that elicit complex responses that cannot be described as either IPSPs or EPSPs

Factors Affecting Synaptic Transmission 1. acid-base states a. alkalosis b. acidosis 2. oxygen and CO2 concentrations • Drugs – those that excite – those that inhibit

Neuromuscular Junction and N-M Transmission

Neurotransmitter Receptors • metabotropic receptors • ionophore

Acetylcholine Synthesis and Metabolism

• Acetyl choline cholinesterase acetate + choline

Events during N-M Transmission Action potential in Presynaptic Motor axon terminal

Increase in Ca++ ion permeability and influx of Ca++ into the Axon Terminal

Release of Ach from the Synaptic Vesicles into the Synaptic Cleft

Diffusion of Ach to Postjunctional membrane

Events during N-M Transmission Combination of Ach with specific receptors on postjunctional membrane

Increase in permeability of postjunctional membrane to Na++ and K+ causes EPP

Depolarization of areas of Muscle membrane adjacent to end plate and initiation of AP

Factors affecting NM Transmission

1. 2. 3. 4.

Ions Drugs – curare, antibiotics Acid – base conditions Temperature

Clinical Applications • • • • •

Hemicholiniums Curare Myasthenia Gravis Denervation supersensitivity Eaton-Lambert Myasthenia Syndrome

Thank You and Good Day