Part 1: Image-Mapped Tutorial
Part 2: Matching Self-Test:
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33b |
33c
Part 3: Multiple-Choice Self-Test
1 - Fibers from Cerebral Cortex
The primary motor cortex is located in the precentral gyrus of the frontal lobe and receives information from a number of surrounding areas before controlling impulses are sent via the upper motor neurons to the lower motor neurons of the spinal cord. The cortical areas that project to the primary motor cortex are the somatosensory cortex, prefrontal cortex, premotor cortex, and the supplementary motor cortex. The somatosensory cortex is located in the post-central gyrus of the parietal lobe and provides ongoing information from the senses of touch and proprioception. This proprioceptive information is necessary for the proper guidance of movement. The remaining cortical areas that project to the primary motor cortex are located in the frontal lobe. Each of these regions is involved in the preparation of movement. The supplemental motor cortex and the premotor cortex are most active during the planning of movement, even when the movement isn't executed, and are reciprocally connected to the primary motor cortex and each other. Input to the premotor cortex is primarily visual, whereas input to the supplemental motor cortex is primarily somatosensory. The supplemental motor cortex seems particularly involved in the planning and learning of new movements containing coordinated sequences. Neuronal activity in the supplemental motor cortex is triggered in particular by internal representations rather than by external events, whereas the premotor cortex is triggered in particular by external events. Each of these cortical regions is located just anterior to the primary motor cortex, with the supplemental motor cortex wrapping around into the longitudinal fissure separating the two hemispheres (see Tutorial 21).
Neuronal activity within the prefrontal cortex is associated in particular with sensory signals just preceding a movement. The prefrontal cortex of the dorsolateral frontal lobe receives projections from the posterior parietal cortex (association sensory cortex) and projects to the secondary motor cortex and the frontal eye fields as well as to the primary motor cortex. The prefrontal cortex may provide a mental representation of a stimulus to be the target of a movement and in addition may make the choice to initiate a voluntary response to that stimulus.
Based on the diversity of information reaching the primary motor cortex, this region controls the complex and coordinated movement of several muscles. The primary motor cortex projects downward through the medulla to the ventral horn of the spinal cord where the activation of appropriate alpha motor neurons occurs.
Particular regions of the primary motor cortex control a particular region of the body, although each region is activated as well with movement of adjacent regions of the cortex and corresponding body regions. The representation of body space in this somatotopic organization of the primary motor cortex is called the motor homunculus, and was first mapped out by electrical stimulation of patients during the work of neurosurgeon, Wilder Penfield (1937, Montreal Neurological Institute). Within a given region of the primary motor cortex, the neurons appear to be specialized for movement in a particular direction. Some of the neurons in a region stimulate movements of flexion, other neurons stimulate movements of extension, and yet others stimulate movements necessary to create various angles between these two extremes. This characteristic is known as the movement vector of the neuron.
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Lesions due to stroke or traumatic injury that affect the primary motor cortex may result in upper motor neuron syndrome (Parent, 1996). Demyelinating disease may also affect the pyramidal motor system. Flaccid paralysis typically occurs immediately following damage to the primary motor cortex. Movement of the affected muscles is impossible and reflexes are weak. Symptoms change after a period of recovery, with paresis or muscle weakness replacing the complete paralysis. There is also an increase in tone (hypertonia) and reflexive response. This syndrome usually affects large groups of muscles, with muscles of the limbs and face affected most severely. Independent movement of the fingers and toes may be permanently lost following damage to the primary motor cortex. Tone may be normal upon rest, but active resistance may occur with high-speed movements. Lesions of either the upper or lower motor neurons will often result in spasticity. In this condition, resistance during slow movements may disappear suddenly (a response called the clasp knife phenomenon). Unlike the lower motor neuron syndrome, the upper motor neuron syndrome is not characterized by muscle fasciculations (small and local involuntary contractions of muscles that are visible under the skin) and atrophy.
Damage to the premotor cortex and supplementary motor cortex causes complex disruptions in the ability to develop appropriate strategies for movements. These disorders are called apraxias. In apraxia, muscle strength and sensory function are intact, but the ability to perform complex, sequenced movements such as brushing one's teeth or combing one's hair is disrupted. The bimanual orientation of the hands and discrete fingers needed to perform complex tasks is severely disrupted by damage to the supplemental motor cortex.
| Reference |
Parent, A. (1996). Carpenter's human neuroanatomy (9th ed.). London: Williams & Wilkins.