Motor Control and Supplementary Motor Area

"

Motor control requires the integration of sensory information which are relayed as action potential’s down the neural pathways by receptors such as mechanoreceptors, photoreceptors, thermoreceptors. The sensory systems are used for vision, touch, hearing, proprioception, smell and taste, while receptors are used to detect the stimulus internally and externally. Each types of receptors are sensitive to specific stimuli. For example, photoreceptors can transduce light into electrical signals or chemoreceptors which respond to changes in the blood like oxygen/carbon dioxide concentration. Following the detection of the stimulus the sensory neuron (also known as afferent neuron) transduce the signal into an action potential or a graded potential.

The sensory information travels along the afferent nerve fibres following the ascending sensory tracts path towards the brain via the spinal cord. The sensory information is carried towards a specific area in the brain like the visual cortex (located in occipital lobe), auditory cortex (located in temporal lobe) and somatosensory cortex (located in parietal lobe). The somatosensory system leads to the primary somatosensory cortex, located in the post central gyrus, where touch, pressure, pain, sensation of movement and proprioception are experienced. Electrical impulses which travel via the spinal cord travel along the ascending sensory tracts that are called dorsal column tract and spinothalamic tract. Spinothalamic tract is a sensory pathway from the skin consisting of two pathways: lateral spinothalamic and anterior spinothalamic. Lateral spinothalamic tract carries sensory information for pain (nociception) and temperature while the anterior spinothalamic tract information carries sensory information for non-discriminative and crude touch to the brain.

The dorsal column tract on the other hand is responsible for proprioception, fine touch, two-point discrimination and vibrations from the skin and mechanoreceptors. The ascending sensory tracts in the spinal cord use three different neurons to convey information from the receptor to the brain via the spinal cord: primary afferent neurons, second order neurones and thalamus neurons. The primary afferent neurons detect the stimulus, for example mechanoreceptors in the skin are sensitive to changes in pressure and instigate an action potential. Primary afferent neurons have their cell bodies in the dorsal root ganglion and an axon which projects towards the spinal cord. The dorsal root ganglion contains lots of sensory cell bodies for different sensory modalities. The action potential reaches the cell body in the dorsal root ganglion and then the second order neurons. Second order neurons starting from the nuclei relay the information towards the thalamus and their axons cross over to the medulla – this is known as decussation. The thalamic neurons travel upwards to the somatosensory cortex where the axons form a synapse with the primary somatosensory cortex and sensory information is sent to the cortex via the thalamus. Ascending tracts of the spinal cord have different pathways. The dorsal column pathway has tracts that travel through the medial lemniscus. While, the spinothalamic tract has its own ascending pathway. Both ascending sensory tracts relay their information to the cerebral cortex via the axons from the thalamus.

The somatosensory cortex is part of the cerebral cortex that receives all the sensory input from the peripheral nervous system throughout the body, and the sensory information received is processed by the cerebral cortex. Located in the postcentral gyrus of the cerebral cortex is the primary somatosensory cortex that consists of three areas defined as Brodmann areas 1,2 and 3. The primary somatosensory cortex receives a large amount of thalamocortical projection which run between the thalamus and cerebral cortex, it also contains an area where parts of the body are mapped contralaterally. The Brodmann areas 1,2 and 3 are designated as Area 3, Area 1 and then Area 2 from anterior to posterior. Brodmann area 3 is further divided into 3a and 3b, the 3a division is focused with proprioceptive information while 3b is receiving sensory information to do with other stimuluses.

Area 3b projects to area 1 regarding the texture information and projects information about the shape and size to area 2. These areas contain neurons that are used to project to the secondary somatosensory cortex. The secondary cortex is also in the cerebral cortex and, it solely responds to pain related stimuluses that are received from the peripheral nervous system (Colman, 2008). The secondary cortex also aids with the ability to identify and recognise three dimensional objects just from the sensation of touch. The primary somatosensory cortex relays its processed information to the motor cortex which is located anterior to the primary somatosensory cortex. The motor cortex, located in the pre-central gyrus, uses the sensory information to plan movement. After planning the motor cortex executes and controls the voluntary actions.

"