topic 10

Topic 10: Effects of disuse on NM system
→ e.g., being on crutches.

Effects of Changes in Activity on Neuromuscular System:
Decreased use. In humans, muscle unloading causes rapid loss of strength that is related to duration of unloading.

14 d of unloading results in 25% ↓ in strength, 7d of unloading results in 20% ↓ in strength

• *rapid loss not associated with significant difference in muscle mass or myofiber size, but highly correlated to ↓ EMG activity + neural activation of muscle ↓ by 30–50%

• more severe in soleus than EDL EMG activities.
  → Posture, extensor fibers (condensed) vs locomotors (ankle flexor)

• Loss of muscle mass dependent upon fiber type composition & function of muscle.

• Fastest shock within first week (loss of muscular strength) unilteral lower limb sus[ension, electromyography, superimposed electrical stimulation

Hindlimb suspension model used to mimic unloading

• By 4 weeks of unloading in humans, see atrophy at whole muscle & myofiber level (most pronounced in postural muscles)

• At 4 weeks of unloading, whole muscle CSA ↓ by 9%, but in 10 d CSA (cross-sectional area) smaller by 5%
  → Type II fibers show greatest atrophy (36% in type IIx, 23% in Type IIa, 11% in Type I)
  → Unloading results in ↓ in specific tension (mainly due to decreased myofiber density)
  → Unloading results in shift from Type I to Type II fibers (increases power, rate of contraction)

Effects of aging on unloading-related myofiber atrophy:
→ In soleus of aged rats, 4 wks of unloading = Type I: 45% atrophy, Type IIa: 28% atrophy, Type IIx: 32% atrophy.
→ With fiber types combined, aged soleus show 48% atrophy vs 29% in young fibers as a result of unloading.
→ In young rats, 4 wks of unloading causes NO fiber atrophy in EDL muscles.
→ In aged rats, 4 wks of unloading causes significant atrophy only in Type IIb/X fibers (28%) of EDL.
→ In aged men, 7d of ULLS results in greater decline in strength than young men, but only at faster rates of isokinetic contraction.
→ Muscle endurance unaffected by unloading both in young & aged men.
→ 4 wks of unloading causes no morphological change in NMJ’s of young rats, but in aged rats see signs of degeneration (expansion of pre- & post-synaptic areas, increases in presynaptic branching, increased NCAM expression).

Gender effects of unloading - induced adaptations:
→ In humans, ULLS causes greater decline in strength in women than men (29% vs. 16%), (23% vs. 13%) *Different Studies
→ Associated with greater EMG decreases in women than men
→ neither 7 nor 14 days of ULLS results in significant changes in myofiber size or thigh muscle mass
→ Gender differences also not due to differences in neuromuscular transmission efficiency

Effects of exercise training on NMJ structure:
→ Both endurance training & resistance training result in ↑ size at post-synaptic endplate (increased # of ACh receptors, no change in density) *adaptations of ET greater than those to RT
→ *Training increases nerve terminal branching (more branches, same length)
→ ↑ branching associated with greater # of ACh vesicles
→ No change in pre- or post-synaptic coupling

Neurophysiological Effects of Endurance Training:
→ Endurance training ↑ quantal content (~30%)
→ No change in quantal size → single vesicle
→ Endurance training decreased rate of EPP “ruundown” (during train of stimuli (increased resistance to fatigue)
→ Training ↓ incidence of spontaneous release of ACh (MEPP) mini endplate potential → have much control nerve terminals have over release -70mV
→ Training results in hyperpolarization of resting nerve terminals & sarcolemma (more Na⁺/K⁺ pump)

Effects of Resistance Training on Neuromuscular Physiology (also weight lifting):
→ Resistance training ↑ motor drive to trained muscle (primary movers)
→ Resistance training ↓ motor drive to antagonist
→ Resistance training ↑ synchronization in motor unit firing (affects power, not strength)
→ Resistance training ↑ conduction velocity in trained muscles
→ Resistance training causes greater myofiber hypertrophy in young vs. aged men (aged still show 30–40% myofiber hypertrophy)