SCENARIO 1
The Transmission of Impulses across a nerve fiber that lead to contraction of muscle
The Anatomy
Axons of efferent (motor neuron) that comes out of the brain pass along the spinal cord via the corticospinal tracts. The axons then pass to the muscle fibers; they then divide into several unmyelinated branches. The terminal fibers pass into myocyte to end at the neuromuscular junction. The neuromuscular junction is the junction between an axon of a motor neuron and the plasma membrane of a muscle cell. The NM occupies a hollow area in the sarcolemma(muscle plasma membrane).The muscle fiber is supplied only by one motor neuron even though one motor neuron can supply from 1 up to 25000 muscle fibers(Kandel etal,2000) .Synaptic vesicles contain neurotransmitter called acetylcholine and vesicles are found in the presynaptic zones in the terminal bouton of motor nerve. The neuromuscular junction has 2 sides, the nerve side and the muscle side. On the muscle side, prejunctional folds are formed by folding of the muscle fibers into grooves. The folds are separated by an enzyme called acetycholinesterase that breaks down Acetylcholine(Ach) after the contraction is over.Prejunctional folds are typical at skeletal muscle with voluntary activities such as those involved in stepping up a step .
Physiology
At the axon terminal, action potential arrives causing the opening up of a voltage dependent Calcium channels, stimulating the release of calcium ions from the Extracellular fluid into the cytosol of the motor neuron. This Calcium ion influx further generates a cascade of biochemical reaction leading to the fusion of vesicles to cell membrane of motor neuron. This causes acetylcholine to be released from the synaptic cleft. This is called exocytosis. There is further diffusion of Ach along the cleft and the Ach subsequently binds the nicotinic acetylcholine receptors at the motor endplate. The AchR are ligand gated ion channels. Binding to Ach causes them to open up, leading to inflow of sodium ions and outflow of potassium ions from the cytosol of the muscle.
Sodium inflow is however more than potassium outflow as a result of the variations and differences in the electrical gradient along the plasma membrane and this causes a local depolarization of the motor endplate .This is regarded as the endplate potential. The depolarization is transmitted along the muscle fiber surface into the transverse T-tubules causing calcium ion release from the sarcoplasmic reticulum and finally, the process of muscle contraction is initiated.
The contraction proper
Taking a step involves the stance and the swing phases. The swing phase starts with a push off in which the foot takes off the ground level and ends as at when the heel hits on the ground. The stance phase of the step starts as at when the foot is on the floor and bears the weight of the body. These movements involve skeletal muscles which are under voluntary control. The swing phase usually begins by a push off by the plantaflexor of the foot namely the gatrocnemius, plantaris and soleus muscles that all flex the foot by flexing calcaneous bone via their tendon attachment.
This plantaflexion can also be done by other muscles like the hamstrings and gluteus maximus. Furthermore, in the swing phase of the step, there is flexion at the hip as well as knee joint and this is followed by dorsiflexion of the ankle joint .The hip at this stage also rotates and this is done by the pectineus,sartorius and the Tensor of fascia lata muscles(anterior thigh).With
progression of the swing phase of the step, there is further dorsiflexion of the foot by means of the leg anterior compartment muscles namely the tibialis anterior muscle which flexes the cuneiform and the 1st metatarsal bone; Extensor Digitorium Longus that flexes the middle and distal phalanges bones of the lateral four digits of the foot; the Extensor halluxis Longus that flexes the distal phalanx of the great toe and the Peroneous Tertius that flexes the 5th metatarsal. The quadriceps muscles then contract to ensure that the leg is extended and that the step is taken (Moore & Dalley, 2005).At the very latter part of the stance phase of the step, the pelvis deviates towards the leg that takes the step and therefore the gluteus medius and minimus contract and act on the pelvic bone from a fixed femur bone in order to reduce the tilt towards the stance side. The invertors(mainly tibialis anterior) and evertors (Peroneus longus and brevis) of the foot then stabilize the foot in the late stance phase of the step(Moore&Dalley,2005).The Tibialis anterior acts on the 1st metatarsal and the cuneiform bones while the peroneus longus and brevis act on 1st and 5th metatarsals respectively.
SCENARIO 2
For an individual to raise the hand above the head .The major movement is the abduction of the shoulder joint or the glenohumeral joint between the glenoid cavity of the scapula bone and the head of the humerus bone. The first 15 degrees of the abduction of the shoulder joint is brought about by the contraction of the supraspinatus muscle. After the initiation of the first 15 degree, the deltoid muscle takes over, especially its central fibers (Moore and Dalley, 2005).This movement that eventually lead to reaching up above the head is essentially an abduction movement. The supraspinatus is essentially important at the initiation stage and it assists the deltoid in abduction of the humerus bone along the glenohumeral joint, being attached distally to the superior facet of its greater tubercule.After the first 15 degrees of abduction, the further movement is majorly by the deltoid muscle up to 120 degrees.
To further elevation of the hand after 120 degrees in order to reach above the head to the shelf, will require the action of upward rotation of the scapula bone (Moore &Dalley, 2005).Although the movement of the scapula bone occurs throughout the whole process of abduction of the humerus that lead to raising the hand above the head, it is more effective after the first 120 degrees. The deltoid has its proximal attachment on the deltoid tuberosity of the humerus and it pulls on this bone in abduction. Therefore, in summary, the movement involves abductors of the glenohumeral joints namely the deltoid and supraspinatus and the bones are the humerus and the scapular. The contraction of this group of muscles originates from the sarcomere of the muscle fibers through a series of processes inside the muscle fibers and the steps are based on the Sliding filament theory.
A cross bridge between thick (myosin) and thin (acting) filament after attaching to each other. The myosin will now further pull actin filaments to each other.This leads to a situation in which the Z disks move to each other, shortening the sarcomere units. The filaments of both the actin and myosin maintain their constant length. The arrangement of the sarcomeres is end-end pattern in a myofibril. There is shortening of sarcomeres by the sliding filaments leading to shortening of the muscle in a process called contraction. The shortening of the sarcomere results from inward pulling of actin filaments towards the centre of the sarcomere.
There are binding sites on the actin filaments for myosin fibers to attach and these attachment points are regarded as crossbridges.When the muscle is relaxed and yet to contract, the binding sites are coated with Troponin/tropomyosin protein complex. The electrical impulses that travel from the brain and the acetylcholine released from the neuron causes calcium ions to be released into the sarcoplasmic reticulum. The released calcium now triggers Troponin-Tropomyosin complex in order uncover the binding actin myosin binding regions.Furthermore,this causes heads of myosin to attach to actin.Following the attachment and binding of the myosin heads to actin,the heads of myosin rotate and draws actin filaments near the M or middle line of the sarcomere molecule. The center sarcomere is then shortened and this is by reducing the H-zone, thereby drawing together the Z-disks.Moreover, in a bid to disentangle the myosin head from actin, ATP (adenosine triphosphate) now binds to myosin head. The continuity of these steps depends on whether Calcium ion and ATP are available or not. If they are not available, the Troponin-tropomyosin complex will now draw back in order to cover the binding site.
For the muscle to relax following contraction, the following then take place:
· Following the removal of electrical impulses, the brain now instructs the muscle to relax and the release of acetylcholine to the synaptic cleft is stopped
· Similarly, calcium ion is eliminated from the sarcoplasm
· The muscle cell no longer utilizes ATP
· The Troponin-Tropomyosin molecule now covers the binding regions leading to the relaxation of the muscles
References
Moore, K.L&Dalley, A.F (2005).Clinically oriented Anatomy (5th Ed). Philadelphia: Lippincott Williams and Wilkins
Kandel etal (2000).Principles of Neural science (4th Ed).New York: McGraw-Hill
