Muscle Physiology Part 2: EMG and Smooth Muscle
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Questions and Answers

What happens to myosin ATPase activity when myosin phosphatase removes phosphate from myosin?

  • It becomes inactive
  • It remains the same
  • It increases
  • It decreases (correct)

What role do varicosities play in smooth muscle innervation?

  • They anchor the muscle to the bone
  • They connect muscle cells via gap junctions
  • They store and release neurotransmitters (correct)
  • They facilitate muscle fiber contraction

Which type of smooth muscle contracts as a unit due to gaps junctions between fibers?

  • Multi-unit smooth muscle
  • Cardiac muscle
  • Single-unit (visceral) smooth muscle (correct)
  • Skeletal muscle

What is the effect of decreased myosin ATPase on muscle tension?

<p>It decreases muscle tension (A)</p> Signup and view all the answers

What determines the target cell response in smooth muscle innervation?

<p>The receptor type on the target cell (B)</p> Signup and view all the answers

What is a gap junction in the context of smooth muscle cells?

<p>An intercellular connection allowing ion flow (D)</p> Signup and view all the answers

Which nervous system component can have both excitatory and inhibitory effects on smooth muscle?

<p>Autonomic nervous system (D)</p> Signup and view all the answers

What is a primary characteristic of multi-unit smooth muscle?

<p>Requires nerve stimulation for contraction (C)</p> Signup and view all the answers

What is the primary mechanism by which free Ca2+ concentration in the cytosol decreases during smooth muscle relaxation?

<p>Ca2+ is pumped out of the cell or into the sarcoplasmic reticulum. (D)</p> Signup and view all the answers

What occurs after Ca2+ unbinds from calmodulin (CaM) during smooth muscle relaxation?

<p>Myosin phosphatase removes phosphate from myosin. (C)</p> Signup and view all the answers

What is the impact of myosin phosphatase activity during the relaxation of smooth muscle?

<p>It removes phosphate from myosin, thereby decreasing ATPase activity. (A)</p> Signup and view all the answers

Which of the following processes contributes to smooth muscle relaxation?

<p>Pumping of Ca2+ into the sarcoplasmic reticulum. (B)</p> Signup and view all the answers

What role does ATP play in the process of smooth muscle relaxation?

<p>It facilitates the pumping of Ca2+ out of the cytosol. (D)</p> Signup and view all the answers

What happens to the myosin ATPase activity during smooth muscle relaxation?

<p>It decreases as the phosphate is removed from myosin. (D)</p> Signup and view all the answers

How does the binding of Ca2+ to calmodulin affect smooth muscle contraction?

<p>It activates myosin light chain kinase, promoting contraction. (A)</p> Signup and view all the answers

Which ion plays a crucial role in the cytosolic process of smooth muscle relaxation?

<p>Ca2+ (A)</p> Signup and view all the answers

What is the primary function of electromyography (EMG)?

<p>To evaluate and record the electrical activity produced by skeletal muscles (A)</p> Signup and view all the answers

Which type of electrode is primarily used in intramuscular EMG?

<p>Needle electrode or fine-wire electrode (D)</p> Signup and view all the answers

What distinguishes bipolar recording from unipolar recording in surface EMG?

<p>Both electrodes are active in bipolar recording (C)</p> Signup and view all the answers

What type of information can surface EMG provide?

<p>A general picture of muscle activation (C)</p> Signup and view all the answers

In which scenario would intramuscular EMG be particularly useful?

<p>To diagnose neuromuscular diseases (C)</p> Signup and view all the answers

Which characteristic is NOT associated with smooth muscle contraction?

<p>Higher energy consumption for a given force (A)</p> Signup and view all the answers

What does the electromyogram (EMG) record?

<p>The electrical activity of muscle cells during stimulation (D)</p> Signup and view all the answers

What feature distinguishes smooth muscle fibers from skeletal muscle fibers?

<p>Lack of striations due to no sarcomeres (C)</p> Signup and view all the answers

Which statement about surface electrodes in EMG is true?

<p>They can monitor biochemical activities of skeletal muscle (C)</p> Signup and view all the answers

How does the actin to myosin ratio in smooth muscle compare to that in skeletal muscle?

<p>It is higher than in skeletal muscle (B)</p> Signup and view all the answers

How is the electric field formed around a muscle cell during stimulation?

<p>By the potential differences that arise between the muscle cell and its environment (D)</p> Signup and view all the answers

What is a key structural feature of smooth muscle myosin?

<p>Has hinged heads distributed along its length (D)</p> Signup and view all the answers

Which of the following best describes the role of dense bodies in smooth muscle?

<p>Transmitting contractions from cell to cell (D)</p> Signup and view all the answers

Which component is absent in smooth muscle that is found in skeletal muscle?

<p>Myofibrils (C)</p> Signup and view all the answers

What initiates spontaneous stimulation in smooth muscle?

<p>Self-excitatory cells (C)</p> Signup and view all the answers

What structural feature helps smooth muscle to sequester Ca2+?

<p>Pouchlike infoldings known as caveolae (D)</p> Signup and view all the answers

What is the primary difference between unipolar and bipolar recording in electromyography?

<p>Unipolar records potential differences between a ground and an active electrode. (D)</p> Signup and view all the answers

During contraction, what is the effect of the longitudinal layer of smooth muscle on an organ?

<p>It leads to dilation and shortening of the organ. (C)</p> Signup and view all the answers

What are the parameters derived from the EMG signal?

<p>Amplitude, frequency, and temporal changes of the signal. (A)</p> Signup and view all the answers

Which statement accurately describes the circular layer of smooth muscle during contraction?

<p>Fibers encircle the organ, constricting the lumen and elongating it. (B)</p> Signup and view all the answers

In the context of surface electromyography, what role do surface electrodes play?

<p>They capture muscle impulses for analysis of contraction characteristics. (B)</p> Signup and view all the answers

What anatomical feature distinguishes smooth muscle from other types of muscle?

<p>It is located in hollow organs and tubes, typically in two layers. (B)</p> Signup and view all the answers

What does EMG stand for and what does it primarily measure?

<p>Electromyography; electrical activity of muscles. (A)</p> Signup and view all the answers

What occurs to the potential difference in a bipolar recording?

<p>It captures differences between two points of muscle contact. (B)</p> Signup and view all the answers

What initiates the contraction of smooth muscle cells in the sliding filament mechanism?

<p>Activation of calmodulin by calcium ions (D)</p> Signup and view all the answers

Which statement best describes the role of calcium ions during the contraction process of smooth muscle?

<p>Calcium ions facilitate the transfer of phosphate to myosin. (D)</p> Signup and view all the answers

What is essential for the relaxation of smooth muscle following contraction?

<p>Active transport of calcium ions back to the SR and ECF (B)</p> Signup and view all the answers

Which mechanism allows calcium ions to enter the cytosol during smooth muscle contraction?

<p>Voltage-dependent or independent calcium channels (B)</p> Signup and view all the answers

During the contraction process, what directly activates myosin light chain kinase?

<p>Calmodulin bound to calcium ions (C)</p> Signup and view all the answers

What is the result of phosphorylating the myosin molecule in smooth muscle contraction?

<p>Phosphorylation activates myosin for cross-bridge formation with actin. (A)</p> Signup and view all the answers

What is the primary source of calcium ions for smooth muscle contraction?

<p>Both sarcoplasmic reticulum and extracellular space (B)</p> Signup and view all the answers

What is the consequence of calcium detachment from calmodulin during smooth muscle relaxation?

<p>Reduction of myosin ATPase activity (A)</p> Signup and view all the answers

Which statement accurately describes the role of myosin phosphatase in smooth muscle relaxation?

<p>It removes phosphate groups from myosin, decreasing ATPase activity. (D)</p> Signup and view all the answers

What initiates the decrease in free Ca2+ concentration in the cytosol during smooth muscle relaxation?

<p>The active transport of Ca2+ out of the cell or into the sarcoplasmic reticulum. (A)</p> Signup and view all the answers

What happens to the calmodulin (CaM) when Ca2+ concentration in the cytosol decreases?

<p>Ca2+ unbinds from CaM, leading to relaxation. (B)</p> Signup and view all the answers

Which of the following is NOT a consequence of decreased Ca2+ binding to calmodulin (CaM)?

<p>Smooth muscle fibers initiate contraction. (B)</p> Signup and view all the answers

During which step of smooth muscle relaxation does myosin phosphatase directly engage?

<p>When free Ca2+ unbinds from calmodulin. (D)</p> Signup and view all the answers

What is the relationship between myosin ATPase activity and smooth muscle relaxation?

<p>Decreased ATPase activity contributes to muscle relaxation. (D)</p> Signup and view all the answers

What ion is crucial in modulating the activity of calmodulin and myosin during smooth muscle relaxation?

<p>Ca2+ (B)</p> Signup and view all the answers

During smooth muscle relaxation, what primarily leads to the inactivation of myosin?

<p>The removal of phosphate from myosin by myosin phosphatase. (D)</p> Signup and view all the answers

What characterizes the signaling mechanism of single-unit (visceral) smooth muscle compared to multi-unit smooth muscle?

<p>Single-unit smooth muscle fibers are connected by gap junctions, allowing synchronous contraction. (D)</p> Signup and view all the answers

How does the activation of myosin light chain kinase influence smooth muscle contraction?

<p>It phosphorylates the myosin molecule, enhancing its ATPase activity. (B)</p> Signup and view all the answers

Which aspect of smooth muscle innervation allows for varied responses depending on receptor type?

<p>The distinct neurotransmitters released at diffuse junctions. (B)</p> Signup and view all the answers

What primarily leads to the decrease of muscle tension in smooth muscle relaxation?

<p>The unbinding of calcium from calmodulin leads to decreased ATPase activity. (B)</p> Signup and view all the answers

What role do varicosities play in the innervation of smooth muscle?

<p>They store and release neurotransmitters at diffuse junctions. (C)</p> Signup and view all the answers

Which statement about the gap junctions in single-unit smooth muscle is accurate?

<p>They facilitate intercellular communication and synchronized contraction. (C)</p> Signup and view all the answers

What is the significance of autonomy in smooth muscle contraction?

<p>It allows for spontaneous contractions without neuronal input. (C)</p> Signup and view all the answers

In the context of smooth muscle, how does reduced myosin ATPase activity affect muscle functioning?

<p>It decreases the overall contractility and force production. (D)</p> Signup and view all the answers

What is the primary structural difference between the longitudinal and circular layers of smooth muscle?

<p>The fibers of the longitudinal layer run parallel to the long axis of the organ, while the circular layer runs around the circumference. (A)</p> Signup and view all the answers

Which of the following aspects of muscle activity can be determined from an electromyography (EMG) signal?

<p>The time course of muscle contraction, contraction force, and coordination of muscles. (D)</p> Signup and view all the answers

What type of electrodes are used for bipolar recording in electromyography?

<p>Two active electrodes measuring potential differences between muscle points. (A)</p> Signup and view all the answers

During contraction of the circular layer of smooth muscle, which of the following describes the resultant effect on the organ?

<p>The organ shortens and its lumen constricts. (D)</p> Signup and view all the answers

In what way do surface electrodes in surface electromyography provide useful data?

<p>By registering the potential differences between an active electrode and a ground electrode. (D)</p> Signup and view all the answers

What occurs during the contraction of smooth muscle when the longitudinal layer is activated?

<p>The organ dilates and shortens. (D)</p> Signup and view all the answers

Which of the following factors is NOT derived from the analysis of an EMG signal?

<p>Duration of muscle fatigue. (A)</p> Signup and view all the answers

When comparing unipolar recording to bipolar recording in electromyography, which statement is accurate?

<p>In unipolar recording, only the potential difference to a ground electrode is measured. (D)</p> Signup and view all the answers

What is the main consequence of calcium ions entering the cytosol during smooth muscle contraction?

<p>Activation of calmodulin (B)</p> Signup and view all the answers

Which of the following steps directly follows the activation of calmodulin in smooth muscle contraction?

<p>Activated calmodulin activates myosin light chain kinase (D)</p> Signup and view all the answers

What role does myosin light chain kinase (MLCK) play in smooth muscle contraction?

<p>Transfers phosphate to myosin (D)</p> Signup and view all the answers

What is required for the relaxation of smooth muscle after contraction?

<p>Dephosphorylation of myosin (B)</p> Signup and view all the answers

In which way is calcium obtained during smooth muscle contraction?

<p>From both extracellular fluid and sarcoplasmic reticulum (A)</p> Signup and view all the answers

What is the primary driving factor for the slow, synchronized contractions of smooth muscle?

<p>Low energy consumption due to slow ATPase activity (A)</p> Signup and view all the answers

What occurs after myosin is phosphorylated in the contraction process of smooth muscle?

<p>Myosin forms cross-bridges with actin (C)</p> Signup and view all the answers

What mechanism is involved when calcium ions are transported back into the sarcoplasmic reticulum during smooth muscle relaxation?

<p>Active transport using ATP (C)</p> Signup and view all the answers

What characterizes single-unit (visceral) smooth muscle compared to multi-unit smooth muscle?

<p>Single-unit smooth muscle has numerous gap junctions allowing coordinated contraction. (C)</p> Signup and view all the answers

Which of the following locations is typical for multi-unit smooth muscle?

<p>Ciliary muscle of the eye (A)</p> Signup and view all the answers

What is the main neurotransmitter associated with the sympathetic innervation of smooth muscle?

<p>Noradrenaline (A)</p> Signup and view all the answers

In multi-unit smooth muscle, how does contraction occur?

<p>As a result of individual fiber innervation with neural stimulation. (D)</p> Signup and view all the answers

Which type of receptor is predominantly involved in parasympathetic control of gastric muscle?

<p>Muscarinic receptors (A)</p> Signup and view all the answers

What primarily differentiates the contraction duration between single-unit and multi-unit smooth muscle?

<p>Single-unit muscle contractions are prolonged and take time to fade. (D)</p> Signup and view all the answers

What effect do alpha-1 adrenergic receptors have on vascular smooth muscle?

<p>Contraction, leading to vasoconstriction (A)</p> Signup and view all the answers

Which option correctly describes the autonomic innervation characteristics of multi-unit smooth muscle?

<p>Each muscle fiber requires its own nerve supply and has minimal gap junctions. (D)</p> Signup and view all the answers

What distinguishes intramuscular EMG from surface EMG in terms of recording technique?

<p>Intramuscular EMG requires needle electrodes inserted into muscle tissue. (B)</p> Signup and view all the answers

Which statement about the recording process of electromyography is incorrect?

<p>Electrodes need to be placed in the intracellular environment for accurate readings. (B)</p> Signup and view all the answers

In which scenario is the use of surface EMG indicated over intramuscular EMG?

<p>For a general picture of muscle activation in a larger area. (C)</p> Signup and view all the answers

What is a primary advantage of using needle electrodes for intramuscular EMG?

<p>They allow for the measurement of action potentials from individual muscle fibers. (B)</p> Signup and view all the answers

Which of the following electrodes configuration is utilized in unipolar recording?

<p>One electrode is placed on the muscle with another far away as a ground. (B)</p> Signup and view all the answers

What is the primary difference between bipolar and unipolar surface EMG configurations?

<p>Bipolar configuration measures the difference in potential between two electrodes on the skin. (A)</p> Signup and view all the answers

What can be determined from the electromyogram (EMG)?

<p>The overall muscle activation trends during specific tasks. (B)</p> Signup and view all the answers

During an examination, what is the main limitation of employing surface EMG without intramuscular EMG?

<p>Surface EMG cannot record signals from deep muscle structures. (A)</p> Signup and view all the answers

Flashcards

Electromyography (EMG)

A technique to measure and record electrical activity from skeletal muscles.

Electromyogram (EMG)

The record produced by an electromyograph, showing muscle electrical activity.

Intramuscular EMG

EMG using a needle electrode inserted into muscle tissue to record individual muscle fiber activity.

Surface EMG

EMG using electrodes placed on the skin to record overall muscle activity.

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Unipolar Recording

Surface EMG method where one electrode records activity and a separate reference electrode is used.

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Bipolar Recording

A surface EMG method using two active electrodes placed near each other for recording.

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Needle electrode

A type of electrode used in intramuscular EMG, usually a fine wire or needle, that's inserted into the muscle tissue.

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Electrode placement

The strategic positioning of electrodes on or in the muscle tissue during EMG.

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Unipolar EMG recording

Measures potential difference between a single active electrode and a reference electrode (ground).

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Bipolar EMG recording

Measures potential difference between two active electrodes placed on the muscle.

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EMG signal parameters

Amplitude, frequency, and change over time of the EMG signal used to derive muscle contraction force, time course, and coordination.

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Smooth muscle location

Found in walls of hollow organs and tubes (except heart), often in two layers (longitudinal and circular).

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Longitudinal smooth muscle layer

Outer layer of smooth muscle, fibers parallel to organ's long axis. Contraction shortens and dilates the organ.

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Circular smooth muscle layer

Inner layer of smooth muscle, fibers run around the circumference. Contraction constricts lumen and lengthens the organ.

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EMG information

Muscle contraction time-course, force, and coordination of multiple muscles in a movement sequence.

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Muscle Coordination

How muscles work together during movement.

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Smooth Muscle Contraction Speed

Smooth muscle contracts and relaxes more slowly than skeletal muscle.

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Smooth Muscle Energy Use

Smooth muscle requires less energy to produce the same amount of force compared to skeletal muscle.

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Smooth Muscle Force Maintenance

Smooth muscle can sustain force for extended periods, unlike skeletal muscle.

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Smooth Muscle Structure - Fibers

Smooth muscle fibers are small, spindle-shaped, and shorter than skeletal muscle fibers.

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Smooth Muscle Nucleus

Smooth muscle cells have a single nucleus located centrally within the cell.

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Smooth Muscle Striations

Smooth muscle lacks striations, unlike skeletal muscle; this is due to the absence of sarcomeres and myofibrils.

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Smooth Muscle Actin/Myosin ratio

Smooth muscle has a higher proportion of actin to myosin, and the filaments are longer compared to skeletal muscles.

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Smooth Muscle Myosin Heads

Myosin filaments in smooth muscle have heads arranged along their entire length, which are diagonally oriented.

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Smooth Muscle Structure

Smooth muscle lacks striations, has a single nucleus in each cell, and is found in the walls of hollow organs and tubes.

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Single-unit (Visceral) Smooth Muscle

The most common type of smooth muscle, where cells contract synchronously as a unit due to connections called gap junctions.

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Multi-unit Smooth Muscle

A type of smooth muscle where cells contract independently, each controlled by a nerve ending.

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Gap Junctions

Specialized connections between smooth muscle cells, allowing molecules and ions to pass through, promoting synchronous contraction.

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Smooth Muscle Contraction

Smooth muscle contracts and relaxes more slowly than skeletal muscle, requires less energy to produce force, and can maintain force for prolonged periods.

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Smooth Muscle Regulation

Smooth muscle contraction is regulated by the autonomic nervous system, with sympathetic and parasympathetic influences.

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Diffuse Junctions

The points where autonomic nerve fibers release neurotransmitters to regulate smooth muscle contraction.

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Smooth Muscle Myosin

Myosin filaments in smooth muscle have heads along their entire length, differently arranged compared to skeletal muscle.

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Autonomic Neurotransmitters

Chemicals (neurotransmitters) released by neurons in the autonomic nervous system, controlling involuntary functions like heart rate and digestion.

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Calcium's Role in Smooth Muscle

Calcium ions (Ca2+) initiate smooth muscle contraction by triggering a cascade of events, starting with binding to calmodulin.

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Calmodulin's Role in Smooth Muscle

A calcium-binding protein that activates the myosin light chain kinase (MLCK) enzyme when bound to Ca2+.

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Myosin Light Chain Kinase (MLCK)

An enzyme activated by calmodulin that phosphorylates (adds a phosphate group to) myosin, activating its ATPase activity.

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Activated Myosin in Smooth Muscle

Phosphorylated myosin binds to actin filaments in the thin filaments, enabling cross-bridge formation and muscle contraction.

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Dephosphorylation of Myosin

A phosphate group is removed from myosin, deactivating its ATPase activity and relaxing the smooth muscle.

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Calcium (Ca2+) in Smooth Muscle Relaxation

Calcium ions (Ca2+) play a key role in smooth muscle relaxation. When their concentration in the cytosol decreases, it triggers a series of events leading to relaxation.

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Sarcoplasmic Reticulum's Role

The sarcoplasmic reticulum (SR) is a network of membranes within muscle cells that stores and releases calcium ions. In relaxation, Ca2+ is pumped back into the SR.

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Calcium & Calmodulin (CaM)

When free calcium levels fall, calcium detaches from calmodulin (CaM), a binding protein that helps activate myosin light chain kinase.

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Myosin & Myosin Phosphatase

When calcium detaches from calmodulin, myosin phosphatase becomes active. It removes phosphate from myosin, thereby deactivating it and decreasing myosin ATPase activity.

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Smooth Muscle Myosin ATPase

Myosin ATPase is an enzyme that breaks down ATP to provide energy for smooth muscle contraction. During relaxation, its activity decreases, reducing muscle contraction strength.

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Smooth Muscle Relaxation Summary

Smooth muscle relaxation is a multi-step process that involves decreasing free calcium levels in the cytosol. This triggers a cascade of events that ultimately deactivates myosin, resulting in muscle relaxation.

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Global Electromyogram

Surface EMG can be used to monitor the general activity of a large muscle group.

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Action Potentials

Electrical signals generated by muscle fibers during contraction, which are recorded by EMG.

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What does EMG measure?

Electromyography (EMG) measures the electrical activity produced by muscles during contraction, which can be used to assess a range of muscle functions.

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Calcium's Role

Calcium ions (Ca2+) act as the trigger for smooth muscle contraction. They initiate a chain of events that lead to muscle shortening.

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Calmodulin

A calcium-binding protein that activates myosin light chain kinase, enabling myosin to bind to actin and initiate contraction.

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Calcium Removal

Calcium ions are removed from the cytosol, causing a decrease in calcium levels, which ultimately stops contraction.

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Myosin Deactivation

The phosphate group is removed from myosin, deactivating it and reducing its ability to bind to actin, resulting in relaxation.

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Smooth Muscle Properties

Smooth muscles are slow, synchronized, very energy-efficient, and can maintain force for extended periods.

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Calcium's Role in Relaxation

Decreased calcium concentration in the cytosol initiates smooth muscle relaxation by triggering a cascade of events.

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Sarcoplasmic Reticulum (SR) in Relaxation

The SR pumps calcium back into its storage, reducing free calcium levels in the cytosol, which is crucial for relaxation.

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Calmodulin (CaM) and Relaxation

When free calcium levels fall, calcium detaches from calmodulin, which in turn deactivates the myosin light chain kinase.

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Myosin Phosphatase in Relaxation

After calcium detaches from calmodulin, myosin phosphatase removes phosphate from myosin, deactivating it.

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Myosin ATPase Activity during Relaxation

The removal of phosphate from myosin decreases its ATPase activity, reducing energy production for contraction.

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Why is Smooth Muscle Relaxation Important?

Smooth muscle relaxation is vital for maintaining normal organ functions. It allows organs to return to their resting state after contraction, ensuring proper function.

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Autonomic Nerve Fibers

Nerve fibers that control involuntary functions, like smooth muscle contraction, and release neurotransmitters at diffuse junctions.

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Single-unit Smooth Muscle

A type of smooth muscle where cells are connected by gap junctions, allowing them to contract synchronously as a unit.

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What is the difference between single-unit and multi-unit smooth muscle?

Single-unit smooth muscle cells are linked by gap junctions, allowing for synchronized contraction, while multi-unit smooth muscle cells contract independently, with each cell receiving its own innervation.

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What is the location of single-unit smooth muscle?

Single-unit smooth muscle is found in the walls of hollow organs and tubes, such as the intestinal tract, respiratory tract, reproductive system, and blood vessels.

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What is the location of multi-unit smooth muscle?

Multi-unit smooth muscle is found in locations where fine, controlled contractions are needed, such as the iris and ciliary muscle of the eye, and the large airways.

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What are gap junctions?

Gap junctions are specialized connections between smooth muscle cells that allow ions and small molecules to flow freely between cells, facilitating synchronized contractions.

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What is the role of the autonomic nervous system in smooth muscle function?

The autonomic nervous system controls smooth muscle contraction via neurotransmitters like acetylcholine and norepinephrine, regulating functions like digestion, blood flow, and breathing.

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What is the role of calcium in smooth muscle contraction and relaxation?

Calcium ions (Ca2+) play a crucial role in smooth muscle contraction and relaxation, triggering the activation and deactivation of myosin to regulate muscle activity.

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Study Notes

Muscle Physiology Part 2: Electromyography (EMG) and Smooth Muscle

  • Electromyography (EMG): A technique for evaluating and recording the electrical activity of skeletal muscles.
  • EMG measures potential differences between the interior of a muscle cell and the extracellular environment, which create an electrical field around the muscle cell.
  • Special electrodes positioned within or on the surface of the muscle are used to record these potential differences.
  • EMG is performed using an electromyograph to produce an electromyogram (a record of the electrical activity).
  • Intramuscular EMG: A needle electrode or needle containing fine-wire electrodes is inserted into the muscle tissue.
  • This type of EMG records the action potentials of individual muscle fibers, a part, or the entire motor unit.
  • Intramuscular EMG is primarily used to diagnose neuromuscular disease and motor control disorders.
  • Surface EMG: Recording electrodes are placed on the skin surface above the muscle to pick up the electrical activity.
  • Surface EMG can show the overall muscle activation (global electromyogram) or be used for biochemical studies of skeletal muscle.
  • Unipolar Recording: The active recording electrode is placed above the tested muscle, and a separate, reference zero electrode is placed some distance away. This records potential difference between the zero and active electrode.
  • Bipolar Recording: Two active electrodes are placed on the muscle, and the potential difference between the two points is recorded.
  • What can be learned from an EMG? EMG analysis reveals the time course of muscle contractions, contraction force, and coordination of several muscles in a movement sequence.
  • EMG Parameters: These parameters are derived from the signal's amplitude, frequency, and changes over time.

Smooth Muscle

  • Location: Walls of most hollow organs and tubes (excluding heart). Usually in two layers (longitudinal and circular).

  • Structure: Small, slender, and spindle-shaped fibers. Single, centrally-located nucleus. No T-tubules, myofibrils, or sarcomeres (no striations). No tendons.

  • Function: Longitudinal layer → organ dilates and shortens. Circular layer → constricts the lumen and elongates the organ.

  • Characteristics: Slower contraction and relaxation, uses less energy to generate force. Maintains force for long periods with low oxygen consumption rates.

  • Smooth Muscle Contraction:

    • Sliding filament mechanism: Actin and myosin filaments slide past each other.
    • Ca2+: Obtained from the sarcoplasmic reticulum and extracellular space, triggers contraction by binding to calmodulin.
    • Calmodulin: The activated calmodulin activates the myosin light chain kinase (MLCK) enzyme.
    • MLCK: MLCK catalyzes transfer of phosphate to myosin, activating the myosin ATPases.
    • Cross-bridge formation: Activated myosin forms cross-bridges with actin, and filament shortening begins.

  • Smooth Muscle Relaxation:

    • Active transport of Ca2+ back into the sarcoplasmic reticulum and extracellular space.
    • Ca2+ detaches from calmodulin.
    • Dephosphorylation of myosin reduces myosin ATPase activity.
    • Myosin phosphatase removes phosphate from myosin.
    • Reduced muscle tension.

Neural Regulation of Smooth Muscle

  • Innervation: Innervated by the autonomic nervous system (sympathetic and parasympathetic).
  • Varicosities: Bulbous swellings of nerve fibers store and release neurotransmitters.
  • Diffuse junctions: Autonomic nerve fibers innervate smooth muscle at diffuse junctions.
  • Target cell response: The smooth muscle response depends on the receptor type.

Types of Smooth Muscle

  • Single-unit (visceral) smooth muscle:

    • Most common type.
    • Muscle fibers contract synchronously as a unit (connected by gap junctions).
    • Often exhibit spontaneous action potentials (pacemaker cells).
    • Innervation to few cells
    • Stimulation and contraction last long period and slowly disappear.
    • Location: intestinal tract, respiratory, reproductive system, etc.

  • Multi-unit smooth muscle:

    • Few, if any, gap junctions (each fiber acts individually).
    • Doesn't have automaticity (pacemaker cells).
    • Graded contractions only occur in response to neural stimuli.
    • Arranged in motor units (each muscle fiber receives its own innervation).
    • Very dense innervation.
    • Location: large airways and arteries, eye (ciliary muscle and iris).
    • Stimulation of muscle contraction is short and soon disappear.

Innervation

  • Sympathetic (Norepinephrine): Effects depend on receptor type.
  • Parasympathetic (Acetylcholine): Effects depend on receptor type.
  • Gastric Muscle: Types of receptors and action.
  • Aortic Muscle: Types of receptors and action.

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