Essay on Human Anatomy and Physiology: Tracing a Nerve Impulse

(Part one) Trace the impulse, listing the steps in as much detail as possible, from which your brain sends the message to the appropriate muscles to step up one step. Specifically include the steps involved in transmitting the impulase from the neuron to the muscle fiber at the neuromuscular junction. Specify which muscles are pulling on which bones and what type of joints are involved.

The starting point of a voluntary movement impulse is the Central Nervous System (CNS), which includes the brain and the spinal cord. For example, a typical movement may be initiated in the motor cortex of the forebrain, later transmitted downwards through the brain stem into the spinal cord and from the latter to the Peripheral Nervous System (PNS), which connects between the CNS and the organs. Both systems function as sensory as well as motor control mechanisms; this review, however, focuses merely on voluntary motor control.

Motor neurons are typically multipolar, meaning that they their soma (bodies) receives transmissions from many other motor and/or interneurons. Axons leave the soma of cortical motor neurons and eventually connect to a dendrite of a lower brain motor system, which has the ability to alternate the impulse through the receptor configuration in the postsynaptic cell. From there, the impulse enters the spinal cord, namely into somatic (in our case) motor cells in the anterior horn of the spinal cord.

Once leaving the spinal cord at some point along its length (according to the target muscle), the nerve fibre (i.e. an axon) flows through the PNS and enters the neuromuscular junction (motor endplate). In this junction, the motor axon expands to cover a certain length of the muscle fibre (it can also cover several fibres, but a single fibre has only one neuromuscular junction). In the junction, the axons release molecules of neurotransmitters into the synaptic cleft, which bind with neuroreceptors (proteins) in the postsynaptic membrane. The result is a depolarization of muscle sercolemma (the mechanism that contracts the muscle fibre), which creates a movement.

Most of the skeleton muscles on the extremities are spindle forms, pinnate muscle (single or double) and multi-headed (up to four heads). Muscles between the trunk and the extremities can be flat and wide. Joints can be classified according to the axes or to the degree of freedom. The shapes of joints are the hinge joint (e.g. elbow), saddle joint (e.g. in the thumb) and the ball-and socket joints (e.g. in the shoulder).

(Part two) Trace the impulse, listing the steps in as much detail as possible, from which your brain sends the message to the appropriate muscles to reach up above your head to the shelf. Include in this the steps involved in the actual muscle fiber contraction – sliding filament mechanism. Again, specify which muscles are pulling on which bones and what type of joints are involved.

The impulse comes out of the brain stem and into the spinal cord. The nerves (=axons) that are destined to the hands leave the spinal cord on the lower area of the neck vertebrae. They contract the elevators of the upper arm in the shoulder joint (i.e. those elevators whose insert are in the shoulder belt), which are mainly the deltaideus, the brachilaris and the biceps. The main joint is the shoulder joint, which is a ball-and socket joint between the shoulder belt bones (scapula and clavicle) and the upper arm bone (humerus).

Each skeleton muscle contains a vast number of muscle fibres (cells), each is 10-100 µm thick and up to 15 cm long. The sercolemma, or the contraction mechanism, of a single muscle fibre, is defined as the zone between two delicate intermediate striations (Z), each is connected to isotropic (I) bands, which move (in reaction to depolarization in a neuromuscular junction) one toward the other between wider anisotropic fibres (A) and reach the middle light zone (H). In simple words, the contraction of a singe muscle fibre occurs when I bands move towards the central H zone, while isolated by the A fibres. When they move, each I band pulls the Z “plate” to which it is connected, leading to a contraction in the muscle.