Purves C16 - Reflexes

Question 1

Location of first subsystem of neural centres responsible for movement control

Answer 1

Within the gray matter of the spinal cord and the tegmentum of the brainstem.

Question 2

Relevant cells in the first subsystem of neural centres responsible for control of movement

Answer 2

Lower motor neurons and local circuit neurons.

Question 3

Lower motor neurons

Answer 3

See their axons out of the brainstem and the spinal cord to innervate the skeletal muscles fo the head and body respectively.
Innervated by local circuit neurons.

Question 4

Why are lower motor neurons called the final common path by Sherrington

Answer 4

Commands for movement, whether reflexive or voluntary, are ultimately conveyed to the muscles by the activity of the lower motor neurons.

Question 5

Local circuit neurons

Answer 5

innervate the lower motor neurons; receive sensory inputs as well as descending projections from higher centres. The circuits they form provide much of the coordination between different muscle groups that is essential for organized movement.

Question 6

What happens after you disconnect the spinal cord from the brain? And why?

Answer 6

Because of the circuits formed by local circuit neurons. Even after the spinal cord is disconnected from the brain in an experi- mental animal, appropriate stimulation of local circuits in the isolated spinal cord can elicit involuntary but highly coordinated limb movements that resemble walking.

Question 7

The second subsystem of neural; structures involved in the control of movement

Answer 7

Descending control centres in the cerebral cortex and brainstem.

Question 8

What does the second subsystem involved in control of movement consist of?

Answer 8

Upper motor neurons

Question 9

Upper motor neurons

Answer 9

Their cell bodies lie in the brainstem or cerebral cortex, and their axons descend to synapse with the local circuit neurons or (more rarely) with the
lower motor neurons directly.

Question 10

The upper motor neuron pathways that arise in the cortex are essential for what?

Answer 10

For the initiation of voluntary movements and for complex spatio-temporal sequences of skilled movements. In particular, descending upper motor neurone pathways from cortical areas in the frontal lobe, including the primary motor cortex and parts of the pre-motor cortex, are essential for planning, initiating, and directing sequencings of voluntary movements involving the head, trunk, and limbs.

Question 11

The third and fourth subsystems - general/what they control

Answer 11

massive, complex neural circuits with output pathways that have no direct access to either the local circuit neurons or the lower mo- tor neurons. Instead, they control movement indirectly by regulating the activity of the upper motor neurons in the cerebral cortex and brainstem.

Question 12

Third subsystem

Answer 12

the cerebellum; functions via its efferent pathways to the upper motor neurons as a servomechanism, detecting and attenuating the difference, or “motor error,” between an intended movement and the movement actually performed. The cerebellum mediates both real-time and long-term reductions in these inevitable motor errors (the latter being a form of motor learning).

Question 13

Damage to cerebellum effect

Answer 13

exhibit incoordination with persistent errors in controlling the direction and amplitude of ongoing movements.

Question 14

Fourth subsystem

Answer 14

the basal ganglia. The basal ganglia prevent upper motor neurons from initiating unwanted movements and prepare the mo- tor circuits for the initiation of movements. the importance of this subsystem in the regulation of transitions from one pattern of voluntary movements to another.

Question 15

Damage to basal ganglia - effect

Answer 15

Parkinson’s disease and Huntington’s disease; shows the importance of this subsystem in the regulation of transitions from one pattern of voluntary movements to another.

Question 16

Motor neuron pool

Answer 16

all the motor neurons innervating a single muscle; they are grouped together into a rod-shaped cluster that runs parallel to the long axis of the spinal cord for one or more spinal cord segments.

Question 17

How is the motor neuron - muscle relationship organised?

Answer 17

This spatial organization of motor neuron pools in the ventral horn.

Into maps. Seen both along the length of the spinal cord and across the me- dial-to-lateral dimension of the cord. Provides spatial map of the body.
All the motor neurons innervating a single muscle (called the motor neuron pool for that muscle) are grouped together into a rod-shaped cluster that runs parallel to the long axis of the spinal cord for one or more spinal cord segments.

also a map, or topography, of motor neu- ron pools in the medial-to-lateral dimension of the spinal cord. Motor neurons that innervate the axial musculature (i.e., the postural muscles of the trunk) are located most medially in the ventral horn of the spinal cord, whereas neurons that innervate the muscles of the shoulders (or pelvis in the lumbar spinal cord; see Figure 16.2) are lateral to the axial neurons.

Question 18

What do medial lower motor neuron pools govern?

Answer 18

postural control and the maintenance of balance; receive input from upper motor neurons in the brainstem vestibular nuclei and reticular formation.

Question 19

first and second motor subsystems vs third and fourth motor subsystems

Answer 19

The third and fourth have no direct access to either the local circuit neurons or the lower motor neurons. Control movement indirectly by regulating the activity of the upper motor neurons in the cerebral cortex and brainstem.