The sensorimotor system Flashcards
2 association cortex for sensorimotor
- posterior parietal cortex (plays an important role in integrating these two kinds of information, in directing behavior by providing spatial information, and in directing attention) much of the output of the posterior parietal cortex goes to areas of motor cortex which are located in the frontal cortex: to the dorsolateral prefrontal association cortex, to the various areas of secondary motor cortex, primary motor cortex and to the frontal eye field (a small area of prefrontal cortex that controls both eye movements and shifts in attention)
- dorsolateral prefrontal cortex (It receives projections from the posterior parietal cortex, and it sends projections to areas of secondary motor cortex, to primary motor cortex, and to the frontal eye field.) (The activity of some neurons depends on the characteristics of objects; the activity of others depends on the locations of objects; and the activity of still others depends on a combination of both.) (The activity of other dorsolateral prefrontal neurons is related to the response rather than to the object. These neurons typically begin to fire before the response and continue to fire until the response is complete. Neurons in many cortical motor areas begin to fire in anticipation of a motor activity, but those in the dorsolateral prefrontal association cortex tend to fire first. The response properties of dorsolateral prefrontal neurons suggest that decisions to initiate voluntary movements may be made in this area of cortex, but these deci- sions depend on critical interactions with posterior parietal cortex and other areas of frontal cortex
damage to posterior parietal cortex:
- apraxia (disorder of voluntary movement that is not attributable to a simple motor deficit (e.g., not to paralysis or weakness) or to any deficit in comprehension or motivation) Although its symptoms are bilateral, apraxia is often caused by unilateral damage to the left posterior parietal cortex or its connections
- contralateral neglect (a disturbance of a patient’s ability to respond to stimuli on the side of the body opposite (contralateral) to the side of a brain lesion in the absence of simple sensory or motor deficits. The disturbance is often associated with large lesions of the right posterior parietal cortex, though damage to other brain regions has also been implicated. ‘egocentric left’. These patients, who are said to suffer from object-based contralateral neglect, fail to respond to the left side of objects (e.g., the left hand of a statue) even when the objects are presented horizontally or upside down
Secondary motor cortex
Areas of secondary motor cortex are those that receive much of their input from association cortex. and send much of their output to primary motor cortex.
- supplementary motor area
- premotorcortex
- 8 areas of secondory motor cortex in each hemisphere each with its own subdivisions
- -To qualify as secondary motor cortex, an area must be appropriately connected with association and sec- ondary motor areas
- Electrical stimulation of an area of sec- ondary motor cortex typically elicits complex movements, often involving both sides of the body. Neurons in an area of secondary motor cortex often become more active just prior to the initiation of a voluntary movement and continue to be active throughout the movement.
-In general, areas of second- ary motor cortex are thought to be involved in the programming of specific patterns of movements after taking general instructions from dor- solateral prefrontal cortex
- ventral premotor cortex: mirror neurons, activation when you do a movement but also when you see someone else doing it, helps with social cognition Mapping the actions of others onto one’s own action repertoire might facilitate social understanding, cooperation, and imitation. Support for the idea that mirror neurons play a role in social cognition has come from demonstrations that these neurons respond to the understanding of the purpose of an action, not to some superficial characteristic of the action itself. Mirror neurons have been found in several areas of the macaque monkey frontal and parietal cortex
primary motor cortex
- located in the precentral gyrus of the frontal lobe
-Recent studies using longer bursts of electrical stimulation have shown that the primary motor cortex (M1) elicits complex, natural response sequences, such as feeding actions, rather than simple muscle contractions. This suggests a looser somatotopic organization, with neurons responsive to movement endpoints rather than specific directions. These findings indicate that M1 is highly plastic and contains an action map that guides movements to specific targets. Overall, M1 directs complex, coordinated actions and can adapt to various contexts, including potentially controlling machine movements.
damage of primary motor cortex lesions
not that much of effect.
Large lesions to the primary motor cor- tex may disrupt a patient’s ability to move one body part (e.g., one finger) independently of others, may produce astereognosia (deficits in ste- reognosis), and may reduce the speed, accuracy, and force of a patient’s movements.
Such lesions do not, however, eliminate voluntary movement, presumably because there are parallel pathways that descend directly from secondary and association motor areas to subcortical motor circuits without passing through primary motor cortex.
cerebellum
-it constitutes only 10 percent of the mass of the brain, the cerebellum contains more than half of the brain’s neurons
-receives information from primary and secondary motor cortex, information about descending motor signals from brain-stem motor nuclei, and feedback from motor responses via the somatosensory and vestibular systems. The cerebellum is thought to compare these three sources of input and correct ongoing movements that deviate from their intended course
-By per- forming this function, it is believed to play a major role in motor learning, particularly in the learning of sequences of movements in which timing is a critical factor
-The effects of diffuse cerebellar damage on motor function are devastating. The patient loses the ability to accurately control the direction, force, velocity, and amplitude of move-ments and the ability to adapt patterns of motor output to changing conditions. It is particularly difficult to maintain steady postures (e.g., standing), and attempts to do so frequently lead to tremor. There are also severe disturbances in balance, gait, speech, and the control of eye movement. Learning new motor sequences is difficult.
- the cerebellum plays an important role in learn- ing from one’s errors and in the prediction of errors
basal ganglia
The basal ganglia are a complex, heterogeneous collection of interconnected nuclei, unlike the systematically organized cerebellum. They perform a modulatory function, forming neural loops with cortical areas and the cerebellum rather than contributing directly to descending motor pathways. Theories of basal ganglia function have evolved to include roles in cognitive functions, motivation, and learning, such as habit learning and classical conditioning. A key theory posits that the basal ganglia control movement vigor based on motivation and also suppress inappropriate or unwanted motor activity. Failures in this inhibitory function can lead to neurological or psychiatric symptoms.
sensory feedback
They continuously monitor the effects of their own activities, and they use this information to fine-tune their activities. The sensorimotor system does the same. The eyes, the organs of balance, and the receptors in skin, muscles, and joints all monitor the body’s responses, and they feed their information back into sensorimotor circuits. In most instances, this sensory feedback plays an important role in directing the continuation of the responses that produced it. The only responses that are not normally influenced by sensory feedback are ballistic movements—brief, all-or-none, high- speed movements, such as swatting a fly.
differences dorsolateral motor tracts (2) and ventromedial tracts (2)
- The ventromedial tracts are much more diffuse. Many of their axons innervate interneurons on both sides of the spinal gray matter and in several different segments, whereas the axons of the dorsolateral tracts terminate in the contralateral half of one spinal cord segment, sometimes directly on a motor neuron.
- The motor neurons activated by the ventromedial tracts project to proximal muscles of the trunk and limbs (e.g., shoulder muscles), whereas the motor neurons activated by the dorsolateral tracts project to distal muscles (e.g., finger muscles).
dorsolateral motor tracts
control the movements of
the limbs
ventromedial tracts
involved in the control of posture and whole-body movements (e.g., walking, climbing) and that they can exert control over the limb movements involved in such activities
Describe the components of a motor unit and distinguish between the different types of muscles
Motor units are the smallest units of motor activity. Each motor unit comprises a single motor neuron and all of the individual skeletal muscle fibers that it innervates. Motor units differ appre- ciably in the number of muscle fibers they contain; the units with the fewest fibers—those of the fingers and face— permit the highest degree of selective motor control.
Acetylcholine, which is released by motor neurons at neuromuscular junctions, activates the motor end-plate on each muscle fiber and causes the fiber to contract.
All of the motor neu- rons that innervate the fibers of a single muscle are called its motor pool.
skeletal muscle fibers two basic types: fast and slow.
-Fast muscle fibers, that contract and relax quickly. Although they are capable of generating great force, they fatigue quickly because they are poorly vascularized (have few blood vessels, which gives them a pale color). (quick movements like jumping)
-slow muscle fibers, although slower and weaker, are capable of more sustained contraction because they are more richly vascularized (and hence much redder). (gradual movements like walking)
Many skeletal muscles belong unambiguously to one of two categories: flexors or extensors. Flexors act to bend or flex a joint, and extensors act to straighten or extend it (biceps and triceps) the flexor and extensor, respectively, of the elbow joint. Any two muscles whose contraction pro- duces the same movement, be it flexion or extension, are said to be synergistic muscles; those that act in opposition, like the biceps and the triceps, are said to be antagonistic muscles.
Activation of a muscle can increase the tension that it exerts on two bones without shortening and pulling them together; this is termed isometric contraction. Or it can shorten and pull them together; this is termed dynamic contraction.
The activity of skeletal muscles is monitored by two kinds of receptors:
- golgi tendon organs –> in tendons, connect each skeletal muscle to bone. respond to increases in muscle tension but insensitive to changes in muscle length. the function of Golgi tendon organs is to provide the central nervous system with information about muscle tension, but they also serve a protective function. When the contraction of a muscle is so extreme that there is a risk of damage, the Golgi tendon organs excite inhibitory interneurons in the spinal cord that cause the muscle to relax.
- muscle spindles –> in the muscle the muscle tissue itself. respond to changes in muscle length, but they do not respond to changes in muscle tension.
-> each muscle spindle has its own intrafusal muscle, which is innervated by its own intrafusal motor neuron. shortening the intrafusal muscle each time the extrafusal muscle becomes shorter, thus keeping enough tension on the middle, stretch-sensitive portion of the mus- cle spindle to keep it responsive to slight changes in the length of the extrafusal muscle.
Describe the stretch reflex and explain its mechanism.
he resulting leg extension is called the patellar tendon reflex (patella means “knee”). This reflex is a stretch reflex—a reflex elicited by a sudden external stretching force on a muscle.
The stretch reflex, such as the patellar tendon reflex, is an automatic response to the sudden stretching of a muscle. When the knee tendon is struck, it stretches the thigh muscle, activating muscle-spindle receptors. These receptors send action potentials to the spinal cord via the dorsal root. In the spinal cord, these signals excite motor neurons, which send impulses back to the muscle, causing it to contract. This reflex helps maintain body posture and stability by providing immediate compensatory muscle contractions in response to unexpected external forces.
Describe the withdrawal reflex and explain its mechanism.
The withdrawal reflex is an automatic response to painful stimuli, such as touching a hot object, resulting in the rapid retraction of the affected limb. When pain receptors are activated, the signal travels through sensory neurons to the spinal cord. Unlike the monosynaptic stretch reflex, this reflex involves at least one interneuron. The interneuron relays the signal to motor neurons, which then activate the muscles to pull the limb away. This response occurs rapidly, protecting the body from injury.
