Reflexes: Mechanisms and Classifications

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Questions and Answers

What initiates the action potential in the muscle fiber membrane?

Diffusion of K+ out of the fiber

Opening of voltage-gated sodium channels

Acetylcholine binding to receptors (correct)

Release of acetylcholine

What occurs immediately after Ca2+ ions are released from the sarcoplasmic reticulum?

They bind to actin and cause muscle contraction.

They bind to troponin C, causing a conformational change. (correct)

They cause the myosin heads to detach from actin.

They are pumped back into the reticulum.

What type of muscle contraction results in increased tension without a change in muscle length?

Eccentric contractions

Concentric contractions

Isometric contractions (correct)

Isotonic contractions

What condition occurs due to the accumulation of lactate and changes in pH from repeated muscle stimulation?

Fatigue Signup and view all the answers

What is required for myosin to detach from actin after a muscle contraction?

ATP molecule Signup and view all the answers

What type of muscle is characterized as striated and voluntary?

Skeletal Muscle Signup and view all the answers

What is the primary function of cardiac muscle?

Pumping blood Signup and view all the answers

Which component of skeletal muscle serves as the cell membrane?

Sarcolemma Signup and view all the answers

Which protein functions as an ATPase during muscle contraction?

Myosin Signup and view all the answers

What type of muscle is found in hollow organs and serves to propel contents?

Smooth Muscle Signup and view all the answers

What are the components of a reflex?

Receptor, afferent pathway, integrating center, efferent pathway, effector Signup and view all the answers

What structure stores calcium ions that are essential for muscle contraction?

Sarcoplasmic Reticulum Signup and view all the answers

Which reflex classification is represented by the knee jerk reaction?

Simple, deep, monosynaptic Signup and view all the answers

Which part of the actin filament is responsible for hiding active sites in the resting state?

Tropomyosin Signup and view all the answers

What happens when the muscle spindle is stretched?

Receptor potential is generated, leading to action potentials in the afferent nerve. Signup and view all the answers

How do myosin and actin proteins interact to cause muscle contraction?

By sliding past one another Signup and view all the answers

What characterizes the withdrawal reflex?

It is an involuntary reflex involving multiple synapses. Signup and view all the answers

In the context of muscle contractions, what does 'summation' refer to?

The increase in muscle tension resulting from repeated stimulation Signup and view all the answers

How is muscle tone primarily produced?

By γ efferent neurons maintaining partial contractions Signup and view all the answers

Which of the following accurately describes a polysynaptic reflex?

Requires one or more interneurons between sensory and motor neurons. Signup and view all the answers

What is rigor mortis?

A continuous state of muscle contraction after death Signup and view all the answers

Study Notes

Reflexes

A reflex is an involuntary, rapid movement triggered by a stimulus.
Components include:
- Receptor: Detects the stimulus
- Afferent pathway: Sensory neuron transmits the signal
- Integrating center: Spinal cord or brain processes the signal
- Efferent pathway: Motor or secretory neuron carries the response
- Effector: Muscle or gland that carries out the response

Classification of Reflexes

Site of Receptor:
- Superficial: Triggered by stimuli on the body's surface, like the withdrawal reflex.
- Deep: Triggered by stimuli within the body, like the stretch reflex or Golgi tendon reflex.
- Visceral: Triggered by stimuli in internal organs, like micturition (urination) or defecation.
Number of Synapses:
- Monosynaptic: Involves one synapse, like the stretch reflex.
- Bisynaptic: Involves two synapses, like the Golgi tendon reflex.
- Polysynaptic: Involves multiple synapses, like the withdrawal reflex.
Site of Integration:
- Simple: Integrated in the spinal cord.
- Complex: Integrated in the brain stem or cortex.

Stretch Reflex

A simple, deep, monosynaptic reflex.
Muscle contraction (response) occurs after being stretched (stimulus).
Example: Knee jerk reflex - tapping the patellar tendon stretches the quadriceps muscle, causing it to contract reflexively.

Mechanism of Stretch Reflex

Muscle spindle stretches, generating a receptor potential.
This triggers an all-or-none action potential in the afferent nerve.
The number of action potentials is proportional to the intensity of the stimulus.
Excitatory postsynaptic potentials (EPSPs) are formed at the synaptic junction.
All-or-none responses are generated in the efferent nerve.
Action potentials reaching the skeletal muscle trigger muscle contraction.

Muscle Tone

Resistance to muscle stretch caused by continuous partial contraction.
Present in all muscles, produced by γ efferent neurons.
Functions:
- Maintains erect posture at rest and during movement against gravity.
- Holds viscera in place through abdominal muscle tone.
- Aids venous return in lower limbs.
- Contributes to maintaining normal body temperature.

Withdrawal Reflex

A spinal, superficial, polysynaptic reflex protecting the body from damaging stimuli.
Noxious stimuli trigger action potentials in receptors, transmitted to sensory neurons.
Sensory neurons synapse with interneurons, which connect to motor neurons.
Motor neurons cause flexor contraction and extensor relaxation to withdraw the limb from the stimulus.
The opposite occurs in the contralateral limb to maintain posture.

Types of Muscle

Striated or Non-striated:
- Striated: Muscles with visible striations (bands) under microscopy.
- Non-striated: Muscles lacking striations.
Voluntary or Involuntary:
- Voluntary: Muscles under conscious control.
- Involuntary: Muscles not under conscious control.

Muscle Types

Skeletal Muscle: Striated, voluntary, attached to bones, responsible for voluntary movement.
Smooth Muscle: Non-striated, involuntary, found in hollow organs and tubes, responsible for propelling contents.
Cardiac Muscle: Striated, involuntary, found only in the heart, responsible for pumping blood.

Skeletal Muscle Anatomy

Sarcolemma: The cell membrane of a muscle fiber.
Sarcoplasm: The intracellular fluid surrounding myofibrils, containing mitochondria.
Sarcoplasmic Reticulum: The endoplasmic reticulum of muscle fibers, storing calcium ions crucial for contraction.

Sarcomeres & Filaments

Sarcomeres: Repeating units within muscle fibers, composed of actin (thin) and myosin (thick) filaments.
Myosin Filament (Thick): Composed of myosin molecules, each with two heads and a tail. Myosin heads function as ATPase, hydrolyzing ATP for energy during contraction.
Actin Filament (Thin): Composed of actin, tropomyosin, and troponin proteins.
- Actin forms the backbone of the filament as a double helix.
- Tropomyosin wraps spirally around actin, covering active sites in a resting state.
- Troponin is attached to tropomyosin and binds calcium ions.

General Mechanism of Skeletal Muscle Contraction

An action potential travels along a motor nerve to its endings on muscle fibers.
Acetylcholine (Ach) is released as a neurotransmitter.
Ach binds to receptors, opening acetylcholine-gated channels on the muscle fiber membrane.
Sodium ions (Na+) diffuse into the muscle fiber, initiating an action potential.
Depolarization of the muscle membrane triggers the release of calcium ions (Ca2+) from the sarcoplasmic reticulum.
Ca2+ binds to troponin C, causing conformational changes in the troponin-tropomyosin complex, uncovering actin active sites.
Myosin heads bind to actin, and the sliding of actin filaments along myosin filaments causes muscle contraction.
Calcium ions are pumped back into the sarcoplasmic reticulum, ending the contraction.
ATP is required to detach myosin from actin and allow for relaxation.

Types of Muscle Contraction

Isometric: Muscle tension increases but length stays the same.
Isotonic: Muscle tension stays the same, but muscle length shortens.

Muscle Twitch: A single contraction of a muscle fiber.
Summation: A second contraction occurring before the first subsides, leading to a stronger contraction.
Tetany: Sustained contractions requiring ATP.
Fatigue: Weakening of contractions under repeated stimulation, caused by lactate accumulation, pH changes, and depletion of energy resources.
Hypertrophy: Increase in muscle mass.
Rigor Mortis: Extreme muscle rigidity after death, caused by the permanent binding of actin and myosin due to lack of ATP.

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Description

Explore the fascinating world of reflexes, focusing on their components and classifications. This quiz covers the involuntary movements triggered by stimuli, including the different types and mechanisms involved in reflex actions. Test your understanding of superficial, deep, and visceral reflexes along with monosynaptic and polysynaptic reflexes.