biomedical physics (10) + (11)

Biomedical physicsLecture (10)

Electricity in Human Body• The origin of almost every electrical potential

which arises within the body is a semi-permeable membrane .

• A single nerve consists of a cylindrical semi-permeable membrane surrounding the axon of a neuron.

Cont…

• The membrane is called semi-permeable because it is partially permeable to ions such as potassium (K +) and sodium (Na+), which can pass more freely in one direction through the membrane than the other.

Cont…

• The result of these electrical properties of the membrane is that a potential difference is generated across the membrane.

• Changes in potentials of this type are the origin of signals such as the EEG, EMG and ECG.

Resting Potential

• Resting Potential :Is the potential difference between the inside and

outside of the membrane of a nerve cell when the cell is not conducting an impulse (at rest ).

• The resting membrane potential (RMP) for nerve cells is from -40 mV to -90 mV.

• The most common value is -70 mV, and the membrane is said to be polarized.

Cont…• The properties of the membrane normally( at

rest ) give rise to a high potassium ion concentration and low sodium ion concentration inside the nerve fibre.

Cont…

• The negative charge inside the cell membrane and the positive charge outside the cell membrane are maintained because the Na+-K+

pump pumps out 3 sodium ions for each 2 potassium ions pumped inside.

• This unequal ratio results in the outside being positively charged compared to the negative interior.

• The membrane potential in the polarized state is maintained until some kind of disturbance upsets the equilibrium.

Action Potential

• When a section of the nerve membrane is excited, either by an externally supplied stimulus or by the flow of ionic current , the membrane characteristics change and begin to allow sodium ions to enter and potassium ions to leave the nerve axon.

• This causes the trans-membrane potential to change which, in turn, causes further changes in the properties of the membrane.

Depolarization• This process is called “depolarization” and it

results in the inside of the nerve becoming positive with respect to the outside; the process of depolarization is the beginning of a nerve action potential

Ionic Currents

• Because a nerve fiber is immersed in a conducting fluid (extracellular fluid ), ionic currents will flow around it from the polarized to the depolarized parts.

• These external currents are very important because they are the only external evidence that an action potential is present; it is these external currents which give rise to the bioelectric signals which can be recorded.

• For example, the heart gives rise to external currents of approximately 100µA when it is active and it is these currents which give rise to the ECG.

Cont…

• External current flow around a nerve fibre is also responsible for the transmission of an action potential along the nerve.

• The external current flow at the point of depolarization disturbs the trans-membrane potential further along the fibre and this causes depolarization to spread.

Muscle Action Potentials

• All the muscles in the body produce electrical signals which also control contractions.

• Muscles are subdivided into involuntary and voluntary types .

• The muscle from which our intestines are made and the muscle in the walls of blood vessels is smooth(involuntary ) muscle, but the muscles which move our limbs are of the voluntary type.

Muscle Contraction Stages

The following list gives the steps which lead to a muscle twitch:

(1) A nerve impulse (NAP) is initiated by the brain and travels down an axon within the spinal cord.

(2) The nerve impulse will cross a synapse within the spinal cord and initiate an action potential, which then travels down a motor nerve at a speed up to about 100 m/s.

Cont…(3) The motor nerve may branch a few times along

its course but, a few millimetres before it reaches the muscle, it branches into many terminal nerve fibers. Each of these fibres supplies one muscle fiber.

(4) This arrival of action potential (in nerve fibers ) triggers the voltage-regulated calcium gates to open, allowing the passage of calcium ions into the axon terminal.

Cont…

(5) The calcium then triggers the release of acetylcholine (ACh) into the space between the nerve terminal and the muscle fiber, known as “ motor end plate “.

Acetylcholine diffuses across the gap to the muscle fibre.

(6) The ACh then binds to protein receptors found on the muscle fiber. When two molecules of AChhave bound to each receptor, the gate opens, allowing sodium to enter the cell and potassium to exit the cell (depolarization).

Cont…

(7) Depolarization of the muscle fibre gives rise to a conducted action potential (MAP) which travels in both directions from the end plate. This change in trans-membrane potential is about 100 mV in amplitude and it travels along the muscle fiber at about 1–5 m/s.

Cont…

(8) Following the action potential, a single contraction of the fibre takes place over a period of about 100ms.

Electrical Conduction system of the

heart

• The heart is composed primarily of muscle tissue.

• A network of nerve fibers coordinates the contraction and relaxation of the cardiac muscle tissue to obtain an efficient, pumping action of the heart.

Electrical conduction system of the

heart is composed of :

The Sinoatrial Node• The Sinoatrial Node (SA) is a group of cells

positioned on the wall of the right atrium, near the entrance of the superior vena cava.

• It sends the electrical impulse that triggers each heartbeat.

• During the normal heartbeat, the electrical impulse from the SA node spreads through the myocardial cells of the right atrium and is conducted rapidly to the left atrium along a specialized bundle of fibres, so that the contraction of the two atria takes place together.

Cont…

• The impulse that originates from the SA node strikes the Atrioventricular node (AV node) which is situated in the lower portion of the right atrium.

• The atrioventricular node in

turn sends an impulse through

the nerve network to the ventricles,

initiating the same contraction

of the ventricles.

Cont…

• The electrical network serving the ventricles leaves the atrioventricular node through the Right and Left Bundle Branches (Bundle of His ).

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• The AV node delays the excitation by about 100 ms, to give time for the atria to contract completely, and the impulse then spreads rapidly down the specialized conducting fibres of the bundle branches and through the myocardial cells of the ventricles.

• This ensures that the whole of the muscle of each ventricle contracts almost simultaneously.

Absolute refractory periode

• Skeletal muscle has a short absolute refractory period of 1–2 ms following contraction, during which the membrane is completely insensitive to a stimulus.

• Cardiac muscle has an absolute refractory period of about 250 ms, starting after depolarization of the membrane.

Cont…

• This is almost as long as the contraction and is an important safeguard as the muscle will always relax before contracting again ,thus ensuring that the heart will continue to act as an effective pump, even if stimuli are arriving at many times the normal rate.

The electrocardiogram(ECG)• The electrocardiogram recorded from the right arm and

the left leg has a characteristic shape shown in figure below .

• The start of the P wave is the beginning of depolarization at the SA node. The wave of depolarization takes about 30 ms to arrive at the AV node.

• There is now a delay in conduction of about 90 ms to allow the ventricles to fill. The repolarization of the atria, which causes them to relax, results in a signal of opposite sign to the P wave.

Cont…

• The conduction through the His –Purkinje system takes about 40ms, and the depolarization and contraction of the ventricles then begins, giving rise to the QRS complex.

Cont…

• Finally, repolarization of the ventricles takes place. This is both slower than the depolarization , so that the resulting T -wave is of lower amplitude and longer duration than the QRS wave, but has the same polarity