PSYC 427 Study Guide - Winter 2018, Comprehensive Midterm Notes - Motor Cortex, Postcentral Gyrus, Muscle
12 Oct 2018
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PSYC 427
MIDTERM EXAM
STUDY GUIDE
Fall 2018
PSYC 427 – LECTURE 1
DECEREBRATED CAT EXHIBITING 3 DIFFERENT GAIT PATTERNS ON A TREADMILL (CONTINUATION FROM LAST CLASS)
Despite absence of descending input from cerebral cortex, locomotion pattern is normal. Not only is it normal, it changes in exactly the same way it would in a normal
cat as speed changes.
• At low speed, cat alternates legs one relative to the other
• At higher speed, cat moves fore and hind limbs in synchrony
Besides the at’s iailit to support its o eight ad olitioal aspet of loootio, all eessar iruitr is preset at the level of the midbrain and spinal cord
Similar to the wiping motion seen in frogs, complex sensory motor patterns strictly on basis of the spinal cord can be seen in sophisticated animals such as cats
(though not in humans).
BASIC STRUCTURES OF THE MOTOR SYSTEM
The motor system has three basic levels of control
1) Forebrain (cerebral cortex)
2) Brain stem
3) Spinal cord
Cortical motor areas act on the spinal cord either directly or indirectly through descending systems in the brain stem
All three levels receive sensory inputs and are under the influence of two subcortical structures: the cerebellum and basal ganglia
• The basal ganglia and cerebellum act on the cerebral cortex through nuclei in the thalamus
Unlike cerebral cortex, cerebellum and basal ganglia have very limited direct output to the spinal cord but are important in producing smooth movements.
• Damage to the cerebral cortex causes significant motor problems
o Stroke patients
o Inability or great difficulty with movements
• With damage to the basal ganglia or cerebellum, movement is still possible, though far from being normal
o Parkiso’s patiets
o Produces very different symptoms, such as tremor, posture irregularity and difficulty initiating movements.
o Ataxia: discoordination and loss in accuracy of movement seen in patients with cerebellum damage
sagittal view of brain and axial view of spinal cord
The spinal cord contains circuits that mediate reflexes- some of which can be very complex- and rhythmic behaviors such as locomotion and scratching
• These reflexes function even when the spinal cord is separated from the brain
o Operates independently of the cerebral cortex
o Possible for decerebrate animal to produce patterns of rhythmic neural activity comparable to what is seen when they are chewing
• The patterns of these refle ehaiors are rearkal sesitie to sesor iputs
• Similar circuits governing chewing and swallowing are found in the brainstem
Majority of reflexes are polysynaptic, with multiple interneurons between sensory inputs and motor outputs
The same networks of interneurons involved in reflex behaviors are also involved in voluntary movements
Myotatic (stretch) reflex: simplest reflex that weakly resists muscle stretch
Single synapse that produces a motor output in response to a sensory input. The sensory input is usually displacement of the limb.
1. Sensory afferent inputs originate in muscle receptors and skin
2. Tendons enter the spinal cord through the dorsal root
3. There are terminations mostly in the intermediate zone; some on the alpha motor neurons, which have efferent axons innervating peripheral muscles
I Professor’s point of view, it is ot reall a refle. It a e a useful liial sig or a partiipate i ore oplicates reflexes but does’t have functional
autonomy comparable to, for example, locomotor patterns in a spinal cat.
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Reflexes are organized in a fashion that is sensitive to the structure of the sensory input.
Outputs from the motor cortex synapse onto the 1. Intermediate zone (pink circle) or 2. Alpha motor neurons (green circle)
• The iterediate zoe does’t hae diret atio o usles ut has itereuros that sapse oto alpha otor euros
• Alpha motor neurons are the output cells of the motor neurons that produce direct movement (contraction of peripheral muscles)
The motor neurons in spinal cord resemble neural conduction from the brain to the periphery. More specifically, when you get muscle contraction, electrical
recordings show an assemble or group of action potentials from all target motor units within the muscle.
Sensory (afferent) inputs from muscle receptors, skin and tendons enter the spinal cord through the dorsal root
• Most terminate in the intermediate zone of the spinal cord
• Some terminate monosynaptically on alpha motor neurons
o Alpha motor neurons have efferent axons that innervate peripheral muscles and result in muscle contraction
o The monosynaptic stretch reflex is mediated through this pathway
Descending commands that drive flexor/stretch muscles inhibit antagonist muscles through the same inhibitory interneuron that is activated during the stretch reflex
• Joints are controlled by two opposing sets of muscles, extensors and flexors, which must work in synchrony. Thus, when a muscle spindle is stretched and
the stretch reflex is activated, the opposing muscle group must be inhibited to prevent it from working against the resulting contraction of the
homonymous muscle. This inhibition is accomplished by an inhibitory interneuron in the spinal cord.
• Reflex circuitry involved strictly at level of the spinal cord is acted on by cortical outputs to get inhibition of the antagonist
Motoneuron: mediates the myotatic reflex
• “herrigto’s alled the otor euro the fial oo patha eause sigals fro desedig trats as ell as iput from the periphery all
integrate on the motor neuron, which conducts the appropriate signal out to the muscle.
• Everything sums at the motor neuron to produce volitional movement
Spindles are mechanically in parallel with the main muscle, like a string.
• Stretches and collapses together
• Responds to changes in length of muscle in which it is embedded
When you produce a myotatic reflex, the tendon is deformed and main muscle is stretched
Then, a signal is sent from the sensory receptors in the spindle to the spinal cord via the 1A afferent pathway
• Afferents: from periphery to the brain; some of the fastest connections
• Cell bodies are within the dorsal root ganglion, not directly in the spinal cord
There are synaptic terminations with other neurons within the spinal cord
• Green (excitatory): homonymous muscle in which the spindle is embedded and the synergist muscle in which there is the same mechanical action
o One termination involves alpha motor neurons that project back to homonymous muscle
o Produces cascade of events involving neurotransmitter acetylcholine, causing contraction
• Grey (inhibitory): inhibition of the antagonist muscle via the 1A inhibitory interneuron
o Prevents opposing force
However, this is all fiction. An ideal robot would resist displaeet ad restore positio, ut the streth refle is eak ad does’t produe orretie atio.
find more resources at oneclass.com
find more resources at oneclass.com
