PART ONE
INTRODUCTION
Most knowledge surrounding the sensory ‘neural system’ has occurred through observing and treating patients with neurological damage. With the unfortunate neurological loss humans have experienced, it appears that scientists have gained greater insights into how our perceptions and sensors work within the human body, intercepting with the environment. There have been studies where authors continue to be under the implication that sensory motor judgement continues to be subjected to much controversy, Gandevia et al (2006)
When put under the microscope the hand on its own is an extremely complex motor structure with a vast amount of nerve endings designed to cope with the sensory signals which enable us to accomplish every day tasks. In fact, it is the hand in comparison to any other body part takes a dominant effect on the sensory motor cortex. We require this complicated ‘neural machinery’ to be able to initiate the appropriate intricate dextrous responses.
Efficient motor function within this area can heavily rely on vision, coordination and proprioception which will be discussed. It is clear that during these actions we use afferent and efferent senses to guide us in the right direction.
The document presented here will provide evidence of our sensory input to motor control, proprioception and how we can utilise these senses to enable us to perform the task as outlined above. It is our sensory system which creates a model of the body in the outside world as we move through perception-action coupling.
BACKGROUND
Our reactions and responses to stimuli are learnt over time, and will adapt according to our experiences. Fitts and Posner (1967) proposed that when learning a skills we have cognitive stage (first) associative stage (refining) autonomous stage (final). The appropriate response to picking the glass up in the most efficient form will be learnt through experience over human time.
It is fairly safe to say that we have ample experience of picking the glass up we have come to the autonomous stage of the movement and therefore at this task we are fairly skilled.
Sensory signals available for us in every activity are known as afferent or inflow signals and efferent or outflow signals. The main sensory signals which are available when reaching for, grasping a picking up a glass of water are vision and hearing which are produced outside of the body, cutaneous receptors, muscle spindles, Golgi tendon organs, joint receptors, efference copy, nociception and the vestibular system which are organised within the body and communicated via large afferent neuronal fibres. It is these senses which help us to control the posture and movement within the body.
The somatosensory system can be broken down into three main areas, discriminative touch (cutaneous, pressure, and vibration perception), pain and temperature, and lastly proprioception. The reason why these systems must be broken down in to three areas is that they work via different pathways in the spinal cord and have different targets in the brain. All the sensory systems communicate 4 different types of information (modality, location, intensity and timing). Interestingly these systems are also organised very similarly.
Once a sensory neuron is triggered by a stimulus the system will aid our response to the glass picking up scenario react via mechanoreceptors, thermorecpetors and chemoreceptor at the neuro-receptor surface. Each type of receptor described here will be mapped towards a different area on the brain in order to for relevant response to occur. In general touch pressure will head towards the thalamus and proprioceptive stimulus will be headed towards the cerebellum.
When there is sensory stimulus an excite receptor potential of the cell membrane will be initiated, we are at this point, on the cusp of an exciting journey of which there are many different avenues or processes allowing us to successfully conduct the task of picking up the glass. The information received via the receptors must undergo a three stage process via two neurons, known as primary secondary and tertiary process. We know that if we hold the glass for a longer duration, the receptor potential will also match the length of the activity.
Sensation enters the periphery via sensory axon. The sensory neuron signal passes directly from the distal axon process to the proximal process. The proximal end of the axon enters the dorsal half of the spinal cord, and immediately turns up the cord towards the brain. These axons are called the primary afferents, which are signals moving towards the brain, because they are the same axons that brought the signal into the cord. The axons ascend in the dorsal white matter of the spinal cord, once they reach the medulla the primary afferents synapse to form a secondary afferent which is joined to the post central gyrus in the cerebral cortex, thalamus and then in the third neuron will go to the cerebral cortex. Proprioception system (part of the somatosensory system) communicates via an ipsilateral system which is contained within the cerebellum. The receptors allow us to sense joint position, stretch reflexes, muscle stretch etc and it is evident that in order to carry out non restricted and precise movement, that our proprioception is the key, especially for postural control. In general our sensory system is most sensitive when it experiences rapid changes
VISION
We know that during the perception of events which is that of looking at the glass reaching for it and bringing toward the body that our vision will be our dominant sense. Although it is not vital in the control of movement, however it will help us to identify the glass in the first place. The sensory neurons enabling vision are known as ‘photoreceptors’
We will depend upon the sensory motor complex when we grasp the glass. In fact vision will be mostly dominant during the task of grasping. The parameterization of the motor behaviour is based on current visual information as well as visual and haptic information obtained during previous lifts (Säfström & Edin 2004). Säfström & Edin (2004) found within their study of maximum grip aperture (MGA), which is a function of the objects size as assed by vision haptic information in subjects was weighted more heavily when it was functionally more important for a successful execution of the grasp.
It is clear from former research that grasp and the reaching components are influenced by the size of the visual object (Tubaldi et al, 2008). It could be suggested that the retina is mostly exposed to light.
Vision is generated by photoreceptors in the retina, a layer of cells at the back of the eye. The information leaves the eye by way of the optic nerve, and there is a partial crossing of axons at the optic chiasm. After the chiasm, the axons are called the optic tract. The optic tract wraps around the midbrain to get to the lateral geniculate nucleus (LGN), where all the axons must synapse. From there, the LGN axons fan out through the deep white matter of the brain as the optic radiations, which will ultimately travel to primary visual cortex, at the back of the brain. The sensitivity of the retina is because there are a high number of receptors and receptor fields are small.
Vestibulo-ocular reflexes (VOR) control the eyes when the head moves to look at the glass; essentially it is the vestibular system which controls the movement of the head in space (cited in http://thalamus.wustl.edu/course, 2009). We rely on this system to coordinate the motions of the eyes head and hands
Vision greatly influences postural control.
Reaching out to grasp a glass is a fine motor skill, which requires a certain amount of visual stimulus to be able to carry out the initial impulse phase and current control phase processes (Woodworth 1899). Vision can be easily seen for its importance if we were to close our eyes and trying to conduct a fine motor task the skills, the skill with your eyes closed can be greatly hindered. From Magill 2004, p. 104; reprinted from Smyth and Silvers 1987):
Neuropsychological studies suggest that there are separate visual systems sub serving cognition and action (FLANAGAN et al, 1996). It appears that our vision will help us to identify the fact that there is a glass filled with water in front of us.
Very many tasks that involve aiming, including reaching and grasping actions, rely on visual feedback in order to maximise endpoint accuracy. In order to acquire visual information, the eyes must fixate on the object for a significant length of time prior to manual contact. When an object appears or moves rapidly, or changes its location unexpectedly, the localisation of the object must be fixated as rapidly as possible in order to provide sufficient time to process the visual information. In these situations, the VOR provides crucial support for coordination between eyes, head and hand.
Our perception of the object will allow us to make the necessary judgment based on perception of weight and size of the glass on how we should pick this glass up. The shape we should mould our hand in order to successfully pick the glass up largely depends on our perception of the glass. Two functional streams of visual information processing within primate neo cortex steams support different visual functions occurring via two different areas of the motor cortex. The first is a ventral stream and the second is within the dorsal cortex
Dorsal Stream -involved with visual processing for the control of skilled motor action (MOTOR STREAM), localises objects in visual space (Ungerleider and Mishkin ,1982). mediate the control of goal-directed actions.(Goodale et al 1997)
Ventral Stream involved with processing that supports conscious visual perception and cognitive judgements plays a special role in the visual identification of objects, enduring characteristics of objects and their spatial relations with each other (Goodale et al 1997)
COMPLEX MUSCLE TRANSDUCERS
The muscle spindles will help us to detect changes in muscle length and strength and are located within the muscles intra-fusal fibres. They are used by the CNS to sense relative positions of the body segments. In the example given we will experience changes in muscle spindles in the forearm hand, upper arm extremity, and lower body and trunk regions.
Muscle spindles are arranged in parallel with muscle fibres and respond to the passive stretch of the muscle, but cease to discharge if the muscle contracts isotonically, thus signaling muscle length. The muscle spindles are the receptors responsible for the stretch or myotactic reflex (reflex, stretch). (http://www.mondofacto.com/facts/dictionary, 12 Dec 1998)
In the example of grasping the glass this is an isometric contraction so we know that the receptor potential will decays to a lower value proportional when the isometric contraction occurs during griping, however when we initially reach for the glass we know that , the discharge rate of the sensory afferents increases as the sensory endings are stretched. Steady-state, neuron discharge rate will regulate once the hand is placed on the glass, the opposite will occur once we let go of the glass of water.
GOLGI TENDON ORGANS & JOINT RECEPTORS
Are sensory receptors located at the junction between muscle fibres and tendon, they sense and send responses of the tension on the joints and muscles When GTO is stretched, the afferent axon is compressed by the collagen fibres and its rate of firing increases. GTO’s excite antagonist α-motoneurons, inhibit homonymous and synergist α-motoneurons, this means that they will control the muscles flexibility and will not allow them to. They are the ‘speed limit’ road signs in the body. GTO’s Useful when making fine movements but not ballistic ones
Mechanoreceptors in the joints (joint receptors) play a secondary role in the sense of movement, however we must not irradiate them from the equation. They will usually kick in at the limits of angular excursion joint afferents do have a strong synaptic coupling to higher-order sensory neurons in the central nervous system and that, should a joint receptor be exposed to an adequate tensile strain within the joint capsule or extra capsular ligament, it could provide useful information kinaesthetic acuity is substantially impaired if we do not have joint receptors. Together these studies indicate that the central nervous system has access to information about joint movement which is derived from joint receptors. Under normal circumstances, for movements in the mid-range of joint motion, the discharge of joint receptors may 'duplicate' the kinaesthetic input from muscle spindle endings.
EFFERENCE COPY
Efference copy encompasses the notion of control and predicative response of mechanisms of muscle control. Efference copy is an extra retinal signal affecting vision but not originating at the retina. It is also called ‘outflow’ or a feed forward mechanism because a signal flows out from the oculomotor centres viable explanation of static position perception and sensorimotor interaction (Bridgman 2007). It can be thought of as a copy of the efferent signals and is derived from the afferent motor signals. When a motor command is sent through the nervous system this copy is used to predict the expected sensation that will occur.
An illusion of increased force or heaviness occurs when signals related to an increased central command or effort are required to generate the same force (for review see Gandevia, 1987).Within our glass picking up scenario our somatosensory system will send afferent information to our periphery will send an afferent signal to the CNS. An efferent signal is produced, a copy of which is sent to the motor system. Based on past experience efference copy will aid us to predict the sensory consequence. If the expected feedback (efference copy) matches the actual feedback they will cancel each other out.
Because the notion of efference copy can be very complicated and confusing, models were formed to enable the explanation of efference copy in their simplest form, forward models take the input of a motor command to the “plant” and output a predicted position of the body
Thanks for reading I hope you are looking forward to the part two of this article....
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