The Conduction Velocity of Human Motor Nerve - Essay Example

EXPT The Conduction Velo Of Human Motor Nerve The anterior horn cell, its axon and all the single muscle fibers supplied by that nerve are referred to as a motor unit (Braddom, 2000). The resting membrane potential of a neuron is -70 mV (Ganong, 2005), maintained relatively at that level primarily by the Na++ - K+ pump and the permeability of ion channels. As the electrical stimuli were gradually applied at the ulnar nerve starting at 20mV in increasing increments, each produced an excitatory postsynaptic potential (EPSP). These EPSP summated and finally reached the threshold potential, which is +15 mV or at -55 mV. When the threshold potential or firing level was reached, an action potential was created causing Na++ influx. This in turn made the inside of the cell more positive causing it to be depolarized. This action potential was propagated along the entire length of the nerve by salutatory conduction. As the nerve nears its termination, it loses its myelin sheath and divides into terminal buttons. The action potential caused the voltage-gated Ca++ channels to open resulting in Ca++ influx. The Ca++ then triggered the synaptic vesicles containing the neurotransmitter (in this case, acetylcholine (AcH)) to fuse with the plasma membrane causing it to release its contents to the neuromuscular (NMJ) junction. The AcH then fused with the nicotinic receptors on the motor end plate of the muscle consequential in the opening of Na++ channels. This created an end plate potential until it reached its threshold potential and brought about the potential to be conducted away in both directions along the muscle fiber. The potential traversed down the T- tubules and reached the sarcoplasmic reticulum. The end plate potential caused a conformational change in the dihydropyridine receptor (DHPR) which initiated the ryanodine receptor (RYR) to release the Ca++ (Berne, et.al, 2006). The Ca++ then binds with the Troponin C, causing exposure of the active site in the actin. The myosin head then attached to the active site, and with ATP, performed a "power stroke" causing shortening and contraction of the muscle. This is the contraction seen when the thumb moves towards the palm (abduction). However, the contraction seen as a response to the electric stimulation is not tetanic, meaning it is not sustained; therefore a relaxation period occurs as well. In order for the muscle contraction to stop, the Ca++ attached to the Troponin C must be sequestered back to the sarcoplasmic reticulum. This is done by the Ca++ -Mg++ ATPase. Only when there is cessation of the interaction between actin and myosin can muscle relaxation occur. Consequently, in order for repolarization of the motor end plate, the AcH that has attached to the nicotinic acetylcholine receptors on the motor end plate is broken down by acetylcholinesterase, which hydrolysis Ach to acetate and choline. Re-uptake of these products then occurs by the pre-synaptic neuron for synthesis of new AcH. 2.) The normal nerve conduction velocity for myelinated nerves is 50-60 m/s, depending on the diameter of the nerve and as to its myelination. The greater the diameter size, the faster the conduction velocity. Similarly, myelinated neurons are also faster conductors than unmyelinated neurons. Our results yielded a NCV of 44 m/s which is not within the range but is near the accepted values. Latency or time that the impulse took to travel within the axon is expected to be longer from the elbow compared to the wrist due to the increased distance from the thumb. However, if we are to consider the fiber type of the ulnar nerve, which is an -motor neuron, the NCV that we got was significantly less than the known conduction velocity of 70-120 m/s. Several factors can affect the NCV such as age and temperature. The conduction velocity decreases significantly with decrease temperature and the distal latencies become correspondingly prolonged. (DeLisa, 2005). The standard temperature used in laboratories is 32oC. It was also found out that the conduction velocities decrease slightly after the ages 30-40 years old. EXPT 2: Monosynaptic Stretch Reflex 1.) A reflex arc is begins with a stimulus applied to or detected by a sensory organ. The stimulus is carried as an impulse by an afferent neuron to a central integrating station or sympathetic ganglion. After it has been integrated and a response has been generated, the impulse is carried by the effector neuron to an effector organ where the response is carried out. The simplest reflex arc is one with a single synapse between the afferent and efferent neurons and is also termed as a monosynaptic reflex (Ganong, 2003). The stretch reflex is the best example of a monosynaptic reflex. For our experiment, the stretch of the tendon due to the tapping on the tendon causes the muscle fibers to be stretched also. The sensory organ is the muscle spindles, also called the intrafusal fibers, which is sensitive to stretch. The intrafusal fibers are composed of two types: the nuclear bag, which is responsible for the dynamic response and the nuclear chain, which is responsible for the static response. The nuclear bag is the primary sensory organ for quick stretches such as a stretch reflex whereas the nuclear chain is more active in prolonged stretches. Sensory innervation for the nuclear bag is the annulospiral or type Ia endings whereas the nuclear chain is innervated by the flower-spray or type II endings, in addition to the annulospiral endings. Motor innervation for both nuclear bag and nuclear chain is the -motor neuron. The central portion of the muscle spindles is non-contractile, only the ands are contractile. When the muscle spindle is stretched, its sensory endings are distorted and receptor potentials are generated (Ganong, 2003). The impulses travel through the spinal cord and synapses with the -motor neuron, which is the effector neuron, innervating the extrafusal fibers of the same muscle. In this experiment, this causes the gastrocnemius to fire eliciting the ankle jerk or plantarflexion response. In addition, the Ia fibers pass through the spinal cord, synapsing with a Golgi bottle neuron, which is an inhibitory interneuron, that in turn, synapses with the -motor neuron of the antagonist muscle (in this experiment, the tibialis anterior), causing it to relax. This phenomenon is called reciprocal inhibition, which ensures the contraction of the agonist muscle. For our experiment, the reflex arc can be stated in the table below: Stimuli Sensory Organ Afferent Neuron Integrating Center Efferent Neuron Effector Organ Response Stretch of the muscle spindles Muscle spindles Ia & II endings CNS - Spinal Cord Tibial nerve Gastrocnemius muscle Ankle jerk (Plantar Flexion) The central integrating system for stretch reflexes is only at the level of the spinal cord. This ensures a faster response time since it does not reach consciousness, and can be a protective mechanism. Consequently, when the extrafusal fibers contract as a response to the stretch of the intrafusal fibers, this becomes a protective measure to prevent the muscle from becoming overstretched. 2.) Latency in this experiment involving the stretch reflex is longer than the first experiment because the pathway in the stretch reflex involves a longer pathway compared to the nerve conduction velocity experiment. The stimulus has to be propagated through the afferent pathway then goes to the spinal cord where it will synapse with the efferent pathway where upon reaching the effector organ, and only then will a response be seen. The first experiment didn't use the afferent pathway and central integration system. The electrical stimulation applied on the ulnar nerve only traversed the effector pathway, then proceeded towards the effector organ which was the abductor pollicis muscle thereby having a faster reaction time. 3.) In our experiment, the hard tap elicited a greater response compared to the soft tap. This is because the action potentials in the sensory fibers fire at a frequency proportionate to the degree of stretching. The hard tap stretches the intrafusal fibers more than the soft tap. There is a smaller response for a weaker stimulus because smaller motor units have smaller diameter size for its nerve supply, thereby having a lower threshold potential and is stimulated first compared to larger motor units. Summation is the addition of individual twitch contractions to increase the muscle contraction as a whole (Guyton, 2000). There are two types of summation: temporal and spatial. Spatial summation occurs when more than 1 neuron is stimulated at the same time and each facilitates the other neuron to reach firing level. Temporal summation occurs when a neuron is stimulated repeatedly and the EPSPs produced reaches the firing level. Summation is important in reflexes because activity in the reflex arc starts in a sensory receptor with a receptor potential whose magnitude is proportionate to the strength of the stimulus (Ganong, 2003). Therefore, this generates an all-or-none response, wherein the threshold potential should be reached or else the stimulus will not be able to elicit a response. In the CNS, the responses are graded in terms of EPSP (Ganong, 2003) and stretch reflexes are integrated in the CNS. 4.) Control of -efferent discharges are regulated by descending tracts from the brain which includes the large facilitatory area in the brainstem reticular formation and a smaller inhibitory brainstem area that is driven by impulses from the cerebral cortex and cerebellum (Ganong, 2003). Other factors can also influence -efferent discharges such as anxiety and stimulation of the skin of other body parts. The Jendrassik's maneuver done in this activity elicited a greater response with a lower tapping stimulus compared to other trials performed without the Jendrassik's maneuver. This may be due to increased -efferent discharge initiated by afferent impulses from the hands (Ganong, 2003) which causes increased -motor neuron excitability. Many of the homonymous -motor neurons are not discharged by the Ia afferent fiber input, but have EPSPs evoked in them that do not achieve the critical firing level. This increases the excitability of these neurons, putting them in an excited state called subliminal fringe. The maneuver by itself does not cause the -motor neurons to discharge, but when added to the subthreshold excitation of the subliminal fringe caused by the tap-induced muscle stretch, causes the neurons in the subliminal fringe to discharge (Mann, 2008). Bibliography Braddom, R. L. (2000). Physical medicine and rehabilitation. London: W.B. Saunders. DeLisa, J. A., & Gans, B. M. (2005). Physical medicine and rehabilitation: principles and practice. Philadelphia: Lippincott Williams & Wilkins. Ganong, W. F. (2003). Review of medical physiology. New York, N.Y., [etc.]: Lange Medical Books/McGraw-Hill. Guyton, A. C., & Hall, J. E. (2000). Textbook of medical physiology. Philadelphia: Saunders. Levy, M. N., Berne, R. M., Koeppen, B. M., & Stanton, B. A. (2006). Berne & Levy principles of physiology. Philadelphia, Pa: Elsevier Mosby. Mann, M.D. (2008). The Nervous System in Action [Internet]. Retrieved from:< http://www.unmc.edu/physiology/Mann/mann15.html> [Accessed 20 October 2008] Read More