Skeletal Muscles And Alpha Motor Neurons Essay

Skeletal Muscles And Alpha Motor Neurons Essay

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Skeletal muscles have the ability to contract whether it be voluntary or involuntary. Purves (2008) describes the motor unit which is comprised of an a motor neuron and striated muscle fibres. These motor units are distributed within the muscle and are responsible for providing the force that is required for generating movement and posture. Purves (2008) explains that the contraction of skeletal muscles is initiated by lower motor neurons (LMN) located in the spinal cord and brain stem. There are two types of LMN’s, gamma motor neurons and alpha motor neurons. Purves (2008) discusses the roles of these, gamma motor neurons innervate muscle fibres as well as regulate sensory input by adjusting the length of the intrafusal muscle fibres. Alpha motor neurons innervate the extrafusal muscle fibres which generate the force that is required for movement and posture. The upper motor neurons (UMN) synapse with local circuit neurons (LCN) in the spinal cord and brain stem, which then connect with LMN. The key role of these LMN’s is to send information to skeletal muscle from various locations. According to Mendel (2005), the amount of the force that is produced is determined by the number and types of motor units that are recruited. Mendel explains that the smallest motor neurons are recruited first as they are activated by weak synaptic stimulation. As stimulation is increase, more force is generated by progressively larger motor neurons being recruited as well as an increase in axon activity enough to provide enough force to sustain the increase in muscular tension. This recruitment of motor units in terms of size is called the Hennenman’s size principle.
The stretch reflex that occurs is a response to the muscle stretching as a respons...


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...motor or sensory pathways can result in upper motor neuron lesions which can give rise to a set of symptoms termed upper motor neuron syndrome, according to Purves (2008). These lesions can cause significant weakness and spasticity of the muscle as well as increased tone and exaggerated reflexes. In a clinical setting, Fuller (2005) explains that having an understanding of the normal reflexes, the location of specific structures in the brain and the understanding of the underling physiology means that it is possible to determine the location of the lesion. This can be done by assessing the extent of sensory loss, changes in reflex and weakness. The experiments in this lab focused on looking at the generation of reflex and tone as well as developing an understanding of the mechanical properties of muscle and being able to apply this information to a clinical example.

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