EEOB 2520: Muscle Physiology and the Efferent Nervous System

Comprehensive practice flashcards covering skeletal muscle anatomy, crossbridge cycling, smooth and cardiac muscle physiology, muscle comparisons, and somatic vs. autonomic efferent pathways.

How many neurons connect the central nervous system to effector targets in the somatic nervous system, and what type of signal is transmitted?

The somatic nervous system utilizes $1$ neuron that projects directly from the CNS to innervate skeletal muscle, and it transmits only excitatory signals.

What is the definition of a motor unit in skeletal muscle?

A motor unit consists of $1$ somatic efferent motor neuron and all the myofibers (muscle fibers) that it innervates.

In the sarcomere diagram shown, what structural boundaries define the I band, A band, and H zone?

The I band is the light band consisting only of thin filaments (actin), the A band is the dark band spanning the entire length of the thick filaments (myosin), and the H zone is the central region containing only thick filaments.

What occurs to the Z lines, I band, H zone, and A band during skeletal muscle contraction?

The Z lines move closer together, and the H zone, I band, and sarcomere shorten ("HIS shortens"), while the A band remains constant in width.

Based on the illustrated thin myofilament, what are the specific functions of troponin and tropomyosin?

Troponin serves as the binding site for $Ca^{2+}$, whereas tropomyosin covers and blocks the myosin-binding sites on actin during rest and moves to unblock them when $Ca^{2+}$ binds to troponin.

What two binding sites are located on the heads of the myosin molecule shown in the diagram?

The heads of the myosin molecule contain an actin-binding site and an ATP-binding site.

What sequence of events occurs at the neuromuscular junction to trigger $Ca^{2+}$ release into the cytosol?

An action potential travels down the motor neuron to open voltage-gated $Ca^{2+}$ channels, triggering exocytosis of acetylcholine (ACh). ACh diffuses across the synapse and opens ligand-gated $Na^+$ channels, producing $Na^+$ influx and depolarization to threshold. This initiates an action potential along the myofiber and down the transverse tubules (TTs), opening voltage-gated $Ca^{2+}$ channels in the sarcoplasmic reticulum (SR) to release $Ca^{2+}$ into the cytosol.

How is skeletal muscle relaxation achieved, and what happens if ATP is completely depleted?

Relaxation requires ending motor neuron action potentials, stopping ACh release, degrading remaining ACh with ACh esterase, halting myofiber action potentials, and using an ATPase pump to return $Ca^{2+}$ from the cytosol into the sarcoplasmic reticulum. Depletion of ATP prevents crossbridge unbinding, causing rigor mortis.

What are the four sources of ATP production in skeletal muscle and their respective ATP yields?

1. Creatine phosphate: yields $1\,\text{ATP}$ (initial source via substrate-level phosphorylation; aerobic and anaerobic).
2. Glycolysis: yields $2\,\text{ATP}$ (aerobic and anaerobic; uses glucose or glycogen).
3. Krebs cycle: yields $2\,\text{ATP}$ (aerobic only; metabolizes products of glycolysis, fat, and protein).
4. Oxidative phosphorylation (ETC): yields $34\,\text{ATP}$ (aerobic only; uses NADH and $FADH_2$).

How do slow-oxidative and fast-glycolytic myofibers compare in ATPase activity, resistance to fatigue, mitochondria abundance, and glycogen stores?

Slow-oxidative fibers feature low ATPase activity (slow contraction speed), high resistance to fatigue, abundant mitochondria, and low glycogen stores. Fast-glycolytic fibers feature high ATPase activity (fast contraction speed), low resistance to fatigue, few mitochondria, and high glycogen stores.

What are the differences between isometric, eccentric, and isotonic muscle contractions regarding tension ($T$) and load ($L$)?

• Isometric: $T < L$ with an independent load; muscle develops tension with no change in length.
• Eccentric: $T < L$ with a dependent load; muscle lengthens as the load overcomes tension.
• Isotonic: $T > L$; muscle tension remains constant while the muscle shortens.

What distinguishes unfused tetanus from fused tetanus during whole muscle contraction?

Unfused tetanus occurs when high stimulation frequencies cause sustained contraction with partial relaxation between stimuli, whereas fused tetanus occurs at higher frequencies where no relaxation occurs between stimuli.

What neuronal pathways produce the knee-jerk (patellar tendon) reflex?

Tapping the patellar tendon stretches the quadriceps, activating muscle spindle receptors. Action potentials travel via sensory neurons to the spinal cord, directly stimulating somatic motor neurons to contract the quadriceps (extensor) and exciting an inhibitory interneuron to inhibit motor neurons to the hamstring (flexor), causing reciprocal inhibition.

What motor responses occur in the ipsilateral and contralateral legs during a withdrawal (crossed extensor) reflex?

In the ipsilateral (pain-stimulated) leg, flexors contract and extensors are inhibited to pull the foot away. In the contralateral leg, extensors contract and flexors are inhibited to support the shift in body weight.

How does myofilament organization in smooth muscle differ from skeletal muscle?

Smooth muscle lacks sarcomeres and Z lines; instead, thin filaments are attached to dense bodies with thick filaments interspersed, producing a radial pull. Smooth muscle possesses tropomyosin (which does not block binding sites) but lacks troponin.

What biochemical cascade initiates crossbridge cycling in smooth muscle following an elevation of cytosolic $Ca^{2+}$?

Intracellular $Ca^{2+}$ binds to calmodulin (CaM). The $Ca^{2+}$-calmodulin complex activates myosin light chain kinase (MLCK), which phosphorylates myosin light chains on the myosin heads, activating myosin ATPase and initiating crossbridge cycling.

What enzyme mediates smooth muscle relaxation by removing phosphate from myosin heads?

Myosin phosphatase (MLCP) dephosphorylates the myosin light chains, decreasing myosin ATPase activity and halting crossbridge cycling.

What specialized axonal structures release neurotransmitters along smooth muscle cells in place of motor end-plates?

Autonomic postganglionic axons contain swelling structures called varicosities that release neurotransmitters along the surface of smooth muscle cells.

In the cardiac intercalated disc shown, what are the distinct functional roles of desmosomes and gap junctions?

Desmosomes mechanically hold adjacent cardiac muscle fibers together to withstand contractile stress, while gap junctions provide low-resistance cytoplasmic channels allowing action potentials to spread rapidly between cells for synchronized contraction.

How does "$Ca^{2+}$-induced $Ca^{2+}$ release" operate during myocardial contraction?

An action potential propagates along the plasma membrane into the T-tubules, opening voltage-gated $Ca^{2+}$ channels. Entering extracellular $Ca^{2+}$ binds to troponin to trigger crossbridge cycling and simultaneously opens chemically-gated $Ca^{2+}$ channels in the sarcoplasmic reticulum, releasing additional $Ca^{2+}$ into the cytosol.

According to comparative muscle tissue characteristics, where are transverse tubules located in skeletal versus cardiac muscle, and are they present in smooth muscle?

In skeletal muscle, transverse tubules are aligned at each A–I band junction; in cardiac muscle, they are aligned at each Z disc; in smooth muscle, transverse tubules are completely absent.

How do skeletal, cardiac, and smooth muscle tissues compare in their capacity for regeneration?

• Skeletal muscle: limited regeneration capacity via satellite cells.
• Cardiac muscle: limited regeneration capacity under certain conditions.
• Smooth muscle: considerable regeneration capacity (compared to other muscles, though limited compared to epithelial tissue) via pericytes.

What are three structural and functional distinctions between the autonomic nervous system and the somatic nervous system?

1. Neuron count: Autonomic pathways contain $2$ neurons in series (preganglionic and postganglionic), whereas somatic pathways contain only $1$ neuron.
2. Effectors: Autonomic innervates smooth muscle, cardiac muscle, glands, and adipose tissue, whereas somatic innervates skeletal muscle.
3. Action: Autonomic signals can be excitatory or inhibitory, whereas somatic signals are excitatory only.

Where are the ganglia located, and what are the relative neuron lengths in sympathetic versus parasympathetic pathways?

In sympathetic pathways, ganglia are located close to the spinal cord, resulting in short preganglionic neurons and long postganglionic neurons. In parasympathetic pathways, ganglia are located on or near target organs, resulting in long preganglionic neurons and short postganglionic neurons.

Where do the preganglionic neurons of the sympathetic and parasympathetic divisions emerge from the central nervous system?

Sympathetic preganglionic neurons emerge from the thoracic and lumbar segments of the spinal cord ($T_1 - L_2$), whereas parasympathetic preganglionic neurons emerge from the brainstem and sacral segments of the spinal cord.

What neurotransmitters and receptor types are utilized at the preganglionic and postganglionic synapses in parasympathetic pathways?

Preganglionic neurons release acetylcholine (ACh) onto nicotinic receptors in the autonomic ganglion, and postganglionic neurons release ACh onto muscarinic receptors on target tissues.

Why is the adrenal medulla categorized as a specialized sympathetic pathway?

The adrenal medulla acts as a modified postganglionic sympathetic ganglion innervated by a preganglionic neuron; upon stimulation, it secretes epinephrine (adrenaline) directly into the bloodstream, eliciting widespread and prolonged effects.

What autonomic control centers are housed within the hypothalamus, pons, and medulla?

• Hypothalamus: temperature control, water balance, and eating behavior.
• Pons: urinary bladder control and secondary respiratory center.
• Medulla: blood pressure control and primary respiratory center.

How do sympathetic and parasympathetic inputs contrast in their effects on the pupils, heart rate, airways, and digestive activity?

• Pupils: Sympathetic dilates; Parasympathetic constricts.
• Heart rate / cardiac output: Sympathetic increases; Parasympathetic decreases.
• Airways: Sympathetic relaxes (dilates); Parasympathetic constricts.
• Digestive activity: Sympathetic decreases motility and secretion; Parasympathetic increases motility and secretion.

What are the specific agonists and antagonists for cholinergic muscarinic and nicotinic receptors?

• Muscarinic: Agonist is muscarine; antagonists are atropine and scopolamine.
• Nicotinic: Agonist is nicotine; antagonists are $\alpha$-bungarotoxin (muscle only), tetraethylammonium / TEA (ganglia only), and curare.

What agonists and antagonists selectively target adrenergic alpha ($\alpha$) and beta ($\beta$) receptors?

• Alpha ($\alpha$) receptors: Agonist is phenylephrine; antagonists are "Alpha-blockers".
• Beta ($\beta$) receptors: Agonists are isoproterenol and albuterol; antagonists are "Beta-blockers" including propranolol ($\beta_1$ and $\beta_2$) and metoprolol ($\beta_1$ only).