Biolistrik Ada dua aspek kelistrikan dan magnetik dalam bidang kedokteran : Listrik dan magnet yang timbul dalam tubuh manusiaPenggunaan listrik dan magnet pada permukaan tubuh manusia

Apa Itu Listrik?Apakah didalam tubuh manusia ada aliran listrik?Jika ada, apa yang menyebabkan timbulnya aliran listrik?Aliran listrik hanya terjadi jika ada aliran elektron.Arus listrik terjadi jika ada beda potensial Ingat Energi listrik W = q.VMuatan listrik Q = I. t dimana Q = N.eDimana e = - 1,6 x 10-19 columb

Jenis-jenis gelombang arus listrikArus bolak balik/sinusoidal Arus setengah gelombangArus searah penuh (riple/desirArus searah murniArus Faradik Arus surged faradik/sentakan faradikArus sentakan sinusArus galvanik yang terputusArus gigi gergaji

Kelistrikan dan kemagnetan yang timbul dalam tubuhSistem syaraf dan neuron Sistem syaraf dibagi dalam 2 bagian yaitu : Sistem saraf pusat dan sistem saraf otonom. Sistem syaraf pusat Terdiri dari otak, medulla spinalis, dan syaraf periferSyaraf perifer ini adalah serat syaraf yang mengirim informasi sensoris ke otak atau kemedula spinalis disebut syaraf afferen, sedangkan serat syaraf yang menghantarkan informasi dari otak atau medula spinalis ke otot serta kelenjar disebut syaraf efferen.

SISTEM SYARAF OTONOMSerat syaraf ini mengatur organ dalam tubuh misalnya jantung, usus dan kelenjar lainnya.Suatu sel syaraf mempunyai fungsi menerima rangsangan, interpretasi dan menghantarkan arus listrik Pada dasarnya neuron terdiri dari badan sel yang menerima pesan listrik dari neuron lain melalui kontak yang dinamakan synapsis dan terletak pada dendrit atau badan sel

neuronDendrit adalah bagian-bagian neuron yang khusus untuk menerima informasi dari rangsangan atau sel lainJika rangsangan cukup kuat, neuron mentransmisi sinyial listrik sepanjang serat yang dinamakan Axon. Axon membawa sinyial listrik ke otot, kelenjar atau neuron lain.

neuronPada permukaan atau membran setiap neuron terdapat selisih potensial akibat adanya kelebihan ion negatif dibagian dalam membran, dalam keadaan ini neuron dalam keadaan terpolarisasi.Bagian dalam sel mempunyai potensial 60 sampai 90 mv lebih negatif dibandingkan dengan bagian luar selPotensial ini dinamakan potensial istirahat atau potensial Nernst dari neuron.

Konsentrasi ion didalam dan diluar sel Dari hasil penelitian diperoleh konsentrasi ion didalam dan diluar membran suatu akson berbeda.

Melalui suatu percobaan dapat ditunjukkan suatu model membran permeabel terhadap larutan KclWaktu t = 0 dimana ion k akan melakukan diffussi dari konsentrasi tinggi ke konsentrasi rendah sehingga pada saat tertentu akan terjadi membrane dipole/dua kutub

Dimana larutan dengan konsentrasi yang tadinya rendah akan kelebihan ion posistip, kebalikan dengan larutan yang tadinya konsentrasinya tinggi akan berubah menjadi kekurangan ion sehingga menjadi lebih negatif.Membran misalnya permeabel terhadap ion k, Na dan cl, namun tidak permeabel terhadap protein A-

Melalui selaput pemisah ion-ion akan menembus selaput dan proses akan berlangsung terus sampai akhirnya disisi selaput akan terkumpul ion-ion negatif sedangkan disisi kanan akan terkumpul ion-ion positip yang jumlahnya sama sehingga menimbulkan tegangan listrik antara sisi kiri dan kanan selaput.Potensial ini mencegah proses diffusi lebih lanjut dan sekaligus juga memberi keseimbangan tegangan listrik antara kedua sisi selaput.

Jika selaput tersebut dianalogikakan sebagai dinding sel, sedangkan larutan disisi kiri selaput dipandang sebagai larutan extra seluler, maka potensial keseimbangan yang timbul analog dengan potensial istirahat sel. Jadi potensial istirahat Nernst adalah fungsi dari konsentrasi ion didalam dan diluar dinding sel (membran) sel saraf.Selain itu juga potensial istirahat juga dipengaruhi oleh temperatur

Potensial istirahat dapat dirumuskan seperti dirumuskan oleh Nernst, potensial yang timbul karena perbedaan konsentrasi ion tertentu didalam dan diluar sel dapat dirumuskan sbb

Pada temperatur tubuh normal T = 310 K dan dengan memasukkan konstanta K = 1,38x10-23 J/K dan e = 1,6 x10-19 c, maka diperoleh

Harga-harga konsentrasi beberapa ion seluler dapat dilihat pada tabel

Ion Konsentrasi extraseluler (mol/l)Konsentrasi inraseluler (mol/l)

K+0,0050,141Na+0,1420,010Na+0,1470,151Cl-0,1030,004A-0,0440,147A-0,1470,151

Telah diketahui bahwa sel mempunyai lapisan yang disebut membran sel, didalam sel ini terdapat ion Na, K, Cl dan protein (A-).Sel mempunyai kemampuan memindahkan ion dari satu sisi ke sisi yang lain dan kemampuan seperti ini disebut aktifitas kelistrikan sel.Dalam keadaan biasa (istirahat) konsentrasi ion Na+ lebih besar diluar sel daripada didalam sel.

Suatu syaraf atau membran otot pada keadaan istirahat (tidak ada proses konduksi impuls listrik) konsentrasi ion Na+ lebih banyak diluar sel daripada didalam sel, didalam sel akan lebih negatif dibandingkan dengan diluar sel.Apabila perbedaan potensial diukur dengan galvanometer akan mencapai -90 mv, dan membran sel ini disebut dalam keadaan polarisasi. Apabila suatu rangsangan terhadap membran dengan mempergunakan listrik, mekanik atau zat kimia, maka butir-butir membran akan berubah dan beberapa ion Na+ akan masuk dari luar sel ke dalam sel.

Didalam sel akan kurang negatif daripada diluar sel dan potensial membran akan meningkat dan keadaan membran ini dikatakan menjadi depolarisasi.Suatu rangsangan yang cukup kuat mencapai titik tertentu sehingga dapat menibulkan depolarisasi membran titik tertentu ini disebut nilai ambang (treshold) dan proses depolarisasi akan berkelanjutan serta irreversibleIon-ion Na+ akan mengalir kedalam sel secara cepat dan dalam jumlah yang banyak.+ + + + + + + + + + + + + + +- - - - - - - - - - - - - -

Pada keadaan ini potensial membran akan naik dengan cepat mencapai overshoot 40 mv.Terjadinya depolarisasi sel membran secara tiba-tiba disebut potensial aksi yang berlangsung kurang dari 1 milli sekon.Segera setelah potensial aksi mencapai puncak mekanisme pengangkutan didalam sel membran dengan cepat mengembalikan ion Na+ keluar sel sehingga mencapai potensial membran istirahat (-90mv) keadaan ini disebut keadaan repolarisasi ( kembali ke keadaan polarisasi).

Sel-sel penyusun JantungSel-sel pacemakerSebagai sumber bioelektrik jantungBerada di Nodus Sino Atrial (SA Nodes) yang terletak diatrium kanan dekat muara vena cava superiorSel KonduksiMerupakan jaringan neuromuskuler yang membentuk traktus internodal atrium, berkas his atau serat purkinje.Sebagai kawat penghantar arus bioelektrikSel-sel otot jantung (miokardium)Berfungsi untuk kontraksi.

The Heart's Electrical Sequence (rentetan) The synchronized electrical sequence of the heart is initiated (diprakarsai) by the SA node, the heart's natural pacemaker. The firing of the SA node sends out an electrical impulse via its neurons to the right atrium, left atrium, and AV node simultaneously.

SA nodeSince the right atrium is closer to the SA node, it depolarizes first, resulting in pumping action by the right atrium before the left atrium. At the AV node, the impulse is delayed to allow for the ventricles to fill up with blood. After the delay, the AV node sends the impulse to the Bundle of His and the Purkinje fibers. This triggers the contraction of the ventricles to send blood either to the lungs (paru-paru) or out to the body.

The Sinoatrial Node: The Body's Natural Pacemaker In the upper part of the right atrium of the heart is a specialized bundle of neurons (serabut neuron) known as the sinoatrial node (SA node). Acting as the heart's natural pacemaker, the SA node "fires" at regular intervals to cause the heart of beat with a rhythmn (irama) of about 60 to 70 beats (denyutan) per minute for a healthy, resting heart.

SA nodeThe electrical impulse from the SA node triggers a sequence of electrical events in the heart to control the orderly sequence of muscle contractions that pump the blood out of the heart. The depolarization and repolarization of the SA node and the other elements of the heart's electical system produces a strong pattern of voltage change which can be measured with electrodes on the skin. Voltage measurements on the skin of the chest are called an electrocardiogram or ECG.

While it is the norm for nerve cells that they require a stimulus to fire, the SA node can be considered to be "self-firing (menghidupkan sendiri). It repetitively goes through a depolarizing discharge and then repolarizes to fire again. This action is analogous to a relaxation oscillator in electronics. Such an oscillator is routinely used to produce a periodic flash from a light.

The circuit involves a capacitor which is charged by the energy of a battery(roles played by the membranes of the SA node and the ion transport processes which repolarize it) and a resistor which controls the rate of flashing of the light. In the case of the SA node, there is input from the physiology of the body related to oxygen demand and other factors which control the rate of firing of the SA node and therefore the heartrate.

Electrocardiograms

The electocardiogram or ECG is a major diagnostic tool for the assessment of the health of the heart. It is a measurement taken at the surface of the skin which reflects the electrical phenomena in the heart when the SA node triggers the electrical sequence that controls heart action.

Heart Electrical Phenomena

The rhythmic contractions of the heart which pump the life-giving blood occur in response to periodic electrical control pulse sequences. The natural pacemaker is a specialized bundle (susunan syaraf) of nerve fibers called the sinoatrial node (SA node). Nerve cells are capable of producing electrical impulses called action potentials. The bundle of active cells in the SA node trigger a sequence of electrical events in the heart which controls the orderly (secara teratur) pattern of muscle contractions that pumps the blood out of the heart.

The electrical potentials (voltages) that are generated in the body have their origin in membrane potentials where differences in the concentrations of positive and negative ions give a localized separation of charges. This charge separation is called polarization. Changes in voltage occur when some event triggers a depolarization of a membrane, and also upon the repolarization of the membrane

The depolarization and repolarization of the SA node and the other elements of the heart's electical system produce a strong pattern of voltage change which can be measured with electrodes on the skin. Voltage measurements on the skin of the chest are called an electrocardiogram or ECG. The heart's electrical control system must properly (sebagaimana mestinya) synchronize the pumping functions illustrated above.

Cell Membrane PotentialsCell membranes in general, and membranes of nerve cells in particular, maintain a small voltage or "potential" across the membrane in its normal or resting state. In the rest state, the inside of the nerve cell membrane is negative with respect to the outside (typically about -70 millivolts). The voltage arises from differences in concentration of the electrolyte ions K+ and Na+. If the cell membranes were simply permeable to these ions, they would approach an equilibrium with equal concentrations on each side of the membrane, and hence no voltage difference. This makes it clear that the processes which produce the membrane potential are not simply diffusion and osmosis.

Cell Membrane PotentialsIn the selectively permeable cell membranes are ion channels which allow K+ ions to pass to the interior of the cell, but block Na+ ions. Negatively charged proteins on the interior of the cell are also denied passage across the membrane. In addition, there are active transport mechanisms at work.

Cell Membrane PotentialsThere is a process which utilizes ATP to pump out three Na+ ions and pump in two K+ ions. The collective action of these mechanisms leaves the interior of the membrane about -70 mV with respect to the outside. For the nerve cell, this equilibrium is disturbed by the arrival of a suitable stimulus. The dynamic changes in the membrane potential in response to the stimulus is called an action potential. After the action potential the mechanisms described above bring the cell membrane back to its resting state.

Action Potentials

Action potential from a giant squid axon. In response to the appropriate stimulus, the cell membrane of a nerve cell goes through a sequence of depolarization from its rest state followed by repolarization to that rest state. In the sequence, it actually reverses its normal polarity for a brief period before reestablishing the rest potential.

Action Potentials The above example of the squid action potential was patterned after a measured action potential shown in West's Medical Physics. The approximate time intervals shown were scaled from time markers on the experimental trace. The times seem very short to me. I thought the recovery time to rest potential was more like 100 msec.

The action potential sequence is essential for neural communication. The simplest action in response to thought requires many such action potentials for its communication and performance. For modeling the action potential for a human nerve cell, a nominal rest potential of -70 mV will be used. The process involves several steps:A stimulus is received by the dendrites of a nerve cell. This causes the Na+ channels to open. If the opening is sufficient to drive the interior potential from -70 mV up to -55 mV, the process continues.

Having reached the action threshold, more Na+ channels (sometimes called voltage-gated channels) open. The Na+ influx drives the interior of the cell membrane up to about +30 mV. The process to this point is called depolarization.

The Na+ channels close and the K+ channels open. Since the K+ channels are much slower to open, the depolarization has time to be completed. Having both Na+ and K+ channels open at the same time would drive the system toward neutrality and prevent the creation of the action potential. With the K+ channels open, the membrance begins to repolarize back toward its rest potential.

The repolarization typically overshoots the rest potential to about -90 mV. This is called hyperpolarization and would seem to be counterproductive, but it is actually important in the transmission of information. Hyperpolarization prevents the neuron from receiving another stimulus during this time, or at least raises the threshold for any new stimulus.

Part of the importance of hyperpolarization is in preventing any stimulus already sent up an axon from triggering another action potential in the opposite direction. In other words, hyperpolarization assures that the signal is proceeding in one direction. After hyperpolarization, the Na+/K+ pumps eventually bring the membrane back to its resting state of -70 mV .