Abstract
The purpose of this experiment is to investigate neural responses to voluntary functions and reflexes. Right-handed participants were chosen to complete the tasks concerning patellar reflexes in relaxed and in Jendrassik’s maneuver position in order to understand its significance on the stretch reflex. The participants were also tested on their corresponding reaction times with simple visual, auditory and tactile cues using a reaction ruler for measure. In previous studies concerning the patellar reflex, it was found that the Jendrassik’s Maneuver enhances the intensity and decreases the reaction time of the knee-jerk reflex in its subjects, which was consistent with the results of the experiment. Moreover, average response time to a visual cue was found to be the shortest in favor of the handedness of the subject. This was followed by the average reaction time in response to the auditory cue and then the average reaction time in response to the tactile cue with respect to the handedness of the subject. The reaction time of the opposite hand in response to the visual cue was found to have the longest average reaction time among all the exercises.
Introduction
Purpose
The purpose of this experiment is to investigate neural responses to voluntary functions and reflexes in consideration to the different factors that can affect response times through the observation of simple factors that can cause response time to vary.
Background
Basic to the events of signal transmission are action potential and synapse (1). Action potential is a transient change in electrical potential difference across the nerve cell membrane that propagates along the axons without deteriorating whenever a threshold is reached. Synapse, on the other hand, is the site where occurrence of a signal transmission from once cell to another takes place. In this phenomenon, a local change in electrical potential difference occurs, which adds up to generate a new action potential.
The Central Nervous System (CNS) is composed of the brain and the spinal cord. The neurons in the CNS are interconnected and transmit electrochemical signals to one another. It integrates all incoming information, interprets it, and selects the appropriate response. The Peripheral Nervous System (PNS) is composed of all the other neurons outside of the CNS. It has two divisions: the sensory division and the motor division. The sensory division is composed of sensory neurons, ganglia, and nerves connecting them to each other and to the CNS. The sensory neurons become activated when there is input from outside or inside the body. Signals are sent to the CNS concerning the ongoing events. The motor division of the PNS is composed of efferent fibers that transmit impulses processed by the CNS to effector organs in the body. The motor neurons in the motor division of the PNS connect neurons to muscles or other effector organs.
Myelination, or the formation of myelin sheath usually around the axon of a neuron, is essential for the proper functioning of the nervous system. According to Anand (2), lack of myelination or incomplete myelination implies a slower conduction of nerve impulses. Synaptic connections, on the other hand, are important since they are specialized to pass signals between individual target cells in order to conduct impulses. According to Dercksen (3), the understanding of the structural organization of the synapses when it is mapped out is crucial to understanding how impulses impinged on the system travel. In mapping out the organization, a better understanding of the nervous system and how it performs is gained.
Sensory processing varies in correspondence to visual, auditory, and tactile cues. In sensory processing of visual cues, once the cue arrives at the retina, it travels to the optic chiasm wherein in the right and left visual worlds are separated. It then travels to the optic tract, which wraps around the cerebral peduncles of the midbrain in order to get to the lateral geniculate nucleus (LGN) that is wrapped around the thalamus. The LGN is where most visual impulses synapse. The neurons in the LGN then send their axons directly to the primary visual cortex found in the occipital lobe for processing (13, 14). In auditory processing, auditory cues first go through the cochlea, then further travels through the ear as vibrations. The vibrations are then picked up by the auditory nerve to the brainstem. The cue or the impulse then synapses in the cochlear nucleus wherein the information is split into two. The first stream of information goes up to the ventral cochlear nucleus, and then is projected to the superior olive and then further up to the inferior colliculus through a fiber tract called the lateral lemniscus. The second stream of information is brought up to the dorsal cochlear nucleus instead, and then projects it directly to the inferior colliculus where both streams of stimuli meet. From the inferior colliculus, the stimuli then travels to the auditory nucleus of the sensory thalamus called the medial geniculate, which projects the information to the primary auditory cortex on both temporal lobes where the auditory data is then processed. The tactile sensory processing involves the cooperation of nerves under the skin’s surface. Light touch, pain, temperature, and pressure are the stimuli that the tactile sensory system recognizes. The tactile system can be divided into two: the discriminative tactile system and the anterolateral system. The discriminative touch system is activated when mechanoreceptors on the skin are activated. The nerve fibers travel via the dorsal root ganglion to the dorsal column nuclei in the medulla. The fibers then sysnapse, then travel to the thalamus via the medial lemniscus then onto the somatosensory cortex of the parietal lobes. The anterolateral system on the other hand, otherwise called the protective tactile system, is responsible for the processing of sensations related to pain and temperature. Receptors that respond to pain are called nociceptors while receptors that respond to temperature are called thermoreceptors. Both these receptors elicit a response called a withdrawal response in response to pain and heat.
In a study done by Ng and Chan (4) on finger response time to visual, auditory, and tactile stimuli, it was found in the study that the tactile response had the fastest response time followed by auditory then visual response.
There are three basic kinds of reaction time used in common reaction time experiments. The most basic kind of experiment is the basic reaction time experiment. In this sort of experiment, only one stimulus and one corresponding response is recorded . Another type of experiment is called the recognition reaction time experiment wherein a certain stimuli called the memory set should be responded to while another stimuli called the distractor set should get no response at all. In choice reaction time experiments, the person must give a response in correspondence to the certain stimulus given. The presentation of the stimuli must be given in random in this kind of experiment (5).
The system that governs the operation of reflexes is called the reflex arc. It is the simplest nerve pathway in the nervous system. The reflex arc is composed of a receptor, which detects the stimulus; a sensory neuron, which transmits afferent impulses an integration center, which processes sensory impulses into motor impulse; a motor neuron, which conducts an efferent impulse from the integration center to the effector and lastly, the effector, which can be a muscle or a gland that produces the reflex action of the stimulus. In order for skeletal muscles to function normally, it is important for muscle spindles, which initiates stretch reflexes, to maintain a healthy muscle tone (11).
The Jendrassik’s Maneuver (JM) is routinely used in patellar reflex test to enhance the amplitude of tendon reflexes in the lower limb (6, 15). A firm tap, which is the stimulus, on the tendon on the knee during a patellar reflex causes the tendon to draw the patella down causing a stretch on the extensor muscle and its spindles, then a contraction of the quadriceps femoris muscle that results to an extension of the knee. When a JM is used, the subject is distracted from voluntarily suppressing the reflex and decreasing the amount of inhibition and thus, amplifying the needed effect of the procedure (7,12).
Hypothesis
If the Jendrassik’s Maneuver is utilized in a patellar reflex exercise, then response time will be faster since the positioning of this maneuver will require the focus of the subject to be on other muscles thus, decreasing the voluntary inhibitions the subject may exhibit.
Neural response time may vary for the reactions to visual, auditory, and tactile cues considering that different pathways are involved in each of these reflexes. It may be that this investigation will exhibit similar results to that of Ng and Chan (4) with tactile responses having the fastest followed by auditory then visual response.
Moreover, if the subject is right-handed, it may be presumed that a faster response time will be observed with right hand reactions compared to left hand reactions since it is often the more utilized by the subject of the experiment.
Materials and Method
As given in the Anatomy and Physiology the Unity of Form and Function, pages 36-39 (8). Please note that for part 1, only patellar reflex was focused on.
Results
The following graphs show the difference in reaction times in response to visual, auditory and tactile cues in consideration of the handedness of the person performing the exercises.
Figure 1: The different reaction times in response to different reaction cues
As seen in figure 1, the average reaction time in response to visual cues for a right-handed person using his right hand was observed to be the fastest amongst the other cues. Auditory cue placed second fastest while the tactile cue followed. The slowest average response time is seen in the results of the left hand response time to the visual cue conducted by a right-handed person.
Figure 2: Difference of reaction times with handedness
Figure 2 shows the stark difference of reaction times with respect to the handedness of the subject exercising the experiment. It is apparent that the right-handed subject had a much faster reaction time with his dominant hand as compared to his response to a visual cue with his left or opposite hand.
Discussion
Consistent with the hypothesis presented at the beginning of the experiment, the Jendrassik’s Maneuver enhances the response time and intensity of the knee-jerk reaction. Since observations were collected in a qualitative manner, it was apparent in the subjects that the JM enhanced the intensity and reaction time of the knee-jerk reaction in one-second conditions during the trial. The mechanism for such enhancement is still under debate. Two conflicting theories have been brought up to explain such. In studies measuring muscle spindles in the tibialis anterior, an enhanced sensitivity was observed as compared to when the subject is in a resting condition. The second theory explains that the JM reduces the presynaptic inhibition when the brain is focused on a different muscle contraction (9).
In the second part of the experiment, the results were not consistent with what was earlier hypothesized in order to be in coherence with the experiment by Ng and Chan (4). The reaction time in response to the visual cue was found to be the shortest followed by the response time to the auditory cue and then the tactile cue with respect to the handedness of the subject. The inconsistency may be due to probable sources of error when the experiment was done. It is important to consider the human errors present such as the time of release of the auditory cue and tactile may not have been released at the same time as the ruler.
Handedness, which is genetic, is related with hemisphere specialization. In previous studies done on visual reaction time with respect to handedness, limb dominance showed no difference. Other studies show conflicting results suggesting that the dominant hand contributes to a significance reaction time (10). In the experiment however, it was found that handedness played a significant role in the length of reaction time. The dominant hand showed a shorter reaction time in response to a release of a visual cue as compared to the response time of the other hand. The significance of reaction time in relation to handedness can be applied to everyday tasks such as driving. Right-handed individuals may benefit from driving left-hand drive cars since the right hand is being used for both the shifting of gears and the maneuvering of the steering wheel.
Conclusion
The Jendrassik’s Maneuver was able to shorten the reaction time as compared to the subject being in resting state. The experiment was not sufficient enough to determine which of the two theories is responsible for such.
The exercise found that the subject had the fastest reflex time in response to the release of a visual cue followed by auditory cue and then finally, tactile cue with respect to handedness. Handedness showed significant response time difference whereby the dominant hand resulted to a shorter response time than the opposite hand.
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