Anatomy & Physiology Lab Bio 1012 SO1
A myofibril is a rod-like unit of a muscle fibre, and is made up of many sarcomeres. Each sarcomere is composed of actin and myosin filaments which are arranged in a hexangular fashion. The overlapping pattern of these filaments provides a banded striated appearance to the myofibrils. These bands are designated with letters, namely A, I, H, M, and Z. Muscolino (2016) states that the two main bands of a sarcomere are the A-band and the I-band. A-band is dark in appearance, and derives its name from the optical term, anisotropic. It refers to the effect of light on the tissue when viewed under a microscope. A-band is located at the center of a sarcomere with myosin present in it. I-band is isotropic when examined under a microscope, and is light in appearance. It is characterized by the presence of actin filaments, and shares regions partially with adjacent sarcomeres. H-zone refers to the region in the A-band containing myosin filaments, exclusively. M-line lies with the H-zone, and indicates the center of the myosin molecule. Z-disc present at the center of I-band represents the border between two adjacent sarcomeres.
A neuromuscular junction (NMJ) or a neuromuscular synapse is a synapse between the axon of a motor neuron and a skeletal muscle fibre. The structure of the NMJ can be functionally and anatomically divided into four distinct regions (Blottner & Salanova, 2014). These include one each of presynaptic, synaptic, and postsynaptic regions, and a sub-synaptic microdomain. The NMJ plays significant role in controlling muscle function through neuromuscular transmission. The impact of the transmission is dependent on the release of acetycholine (Ach) from the motor nerve terminal, and its interaction with Ach receptors on the postsynaptic muscle surface. It results in generation of local potential change which may not be large enough to cause local depolarization of the muscle fibre membrane. However, it induces a regenerating action potential through activation of voltage-gated sodium channels (VGSCs) (Karpati, Hilton-Jones, Griggs, 2001). Opening of sufficient number of VGSCs results in the depolarization which regenerates action potential in the muscle fibre.
References
Blottner, D., & Salanova, M. (2014). The NeuroMuscular System: From Earth to Space Life Science: Neuromuscular Cell Signalling in Disuse and Exercise. Springer.
Karpati, G., Hilton-Jones, D., & Griggs, R. C. (2001). Disorders of Voluntary Muscle. Cambridge University Press.
Muscolino, J. E. (2016). Kinesiology: The Skeletal System and Muscle Function (3rd Ed.). Elsevier Health …
