CNS 4

Question 1

Motor commands from the

higher centers of the brain are transmitted through the

Answer 1

Brianstem to the spinal cord

Question 2

Sensory signals form sensory afferent neurons mediate

Answer 2

Spinal reflexes and ascend to the brianstem, cerebellum and cerebral cortex, where they provide feedback of the evolving movement

Question 3

Feedback Control

Answer 3

• The brain, cerebellum and brainstem issue a motor command (Desired limb position) to neuronal networks in the spinal cord

Question 4

Sensory receptors in muscles, joints and skin signal the actual position back to the spinal cord, which compares this to the desired position and generates an output to the

Answer 4

muscles so that the difference

between actual and desired states is minimized

Question 5

Muscle spindles are located in parallel with the force-producing muscle fibers, so their

Answer 5

sensory endings, which give rise to group Ia and II sensory afferents, respond to changes in muscle length (Ia and II refer to axonal conduction velocities: Ia: ~100 m/s, II: ~50 m/s).

Question 6

Tendon organs are in the

Answer 6

tendinous fascicles at the ends of the muscle fibers. Their sensory endings (Group Ib) respond to muscle force (Ib conduction velocity is similar to Ia conduction velocity).

Question 7

Muscle spindle group Ia afferents also respond to

Answer 7

muscle or tendon vibration.

Question 8

Prolonged vibration is used in physiotherapy to alleviate

Answer 8

spasticity (Overactive stretch reflexes). This may be because over time, Ia afferents activate interneurons that inhibit transmission in the stretch reflex pathway.

Question 9

Intrafusal

Answer 9

Inside muscle spindle

Question 10

Extrafusal

Answer 10

-outside muscle spindle

Question 11

Alpha motoneurons (MNs)

Answer 11

activate the main (Extrafusal) muscle fibres to contract

• muscle spindle afferents (Ia and II) —> signal muscle length changes

Question 12

Gamma MNs → activate tiny

Answer 12

intrafusal muscle fibers at each end of
the muscle spindle, stretching the non-contractile middle part of the spindle where the sensory endings are located. The sensory afferents then fire more rapidly, particularly if the muscle is stretched

(ie. Gamma MN activity increases the sensitivity of muscle spindles to length changes.)

Question 13

Golgi tendon organ afferents (Ib)

→

Answer 13

signal muscle force

Question 14

Alpha-gamma Coactivation

• a, b:

Answer 14

No gamma activity: spindle afferent

firing increases with muscle stretch and decreases with shortening.

Question 15

Alpha-gamma Coactivation

C:

Answer 15

Descending signals from the brain co-
Action potentials activate alpha and gamma MNs. The alpha MNs activate extrafusal muscle fibers, causing muscle shortening. The gamma MNs activate intrafusal muscle fibers.

Question 16

Compared to when a muscle is contracting, passive stretch of the
relaxed muscle produces

Answer 16

less stretch of the tendon and fewer action potentials form GTO

Question 17

Stretch reflex Stimulus: muscle stretch

Response:

Answer 17

1) Spindle-afferent-mediated monosynaptic excitation of agonist MN’s & disynaptic inhibition (via interneurons) of antagonist MN’s

– Opposes change in muscle length

Question 18

Stretch reflex
Stimulus: muscle stretch
AND, simultaneously:

Answer 18

2) GTO-mediated disynaptic inhibition of agonist MN’s and disynaptic excitation of antagonist MN’s. Opposes change in muscle force.

Question 19

During muscle stretch, spindle and GTO reflexes “compete” (Spindle feedback resists increase in length by reflexly
activating MNs, GTO feedback resists
increase in force by reflexly inhibiting
MNs). This results in muscles

Answer 19

resisting stretch in a spring-like manner (The stronger the spindle reflex, the stiffer the spring, the stronger the GTO reflex, the more compliant the spring)

Question 20

Flexor Withdrawal Reflex

Answer 20

Noxious stimulus evokes flex ion and I ipsilateral leg and extension of contralateral leg

Question 21

The primary motor cortex is also called the

Answer 21

“sensorimotor cortex,”

Question 22

Somatotopic Map

Answer 22

• First established by electrical stimulation with a probe during brain operations
• Somatotopic (topographic) map
• A somatotopic representation similar to that of the somatosensory cortex exists in the primary motor cortex
• As in the sensory cortex, hand and facial representations are largest

Question 23

Neurons in the primary motor cortex can be activated by

Answer 23

transcranial magnetic stimulation (TMS)

Question 24

By moving the coil in TMS over the different somatotopic representation
areas,

Answer 24

movements of different parts of the body are elicited

Question 25

Corticospinal Tract (CST)

Answer 25

• Axons from neurons in

sensorimotor cortex form the CST

Question 26

CST neurons make monosynaptic

connections with

Answer 26

spinal alpha motoneurons, whose axons in turn

Question 27

CST neurons are only one CNS

synapse away from

Answer 27

muscle, the “final common path” for commands from brain to muscle

Question 28

CST lesions (stroke, cerebro-vascular accident (CVA), brain attack)
result in

Answer 28

spastic hemiplegia

Question 29

spastic hemiplegia

Answer 29

1) Weakness (paresis) or paralysis of extremities
2) Exaggerated stretch reflexes (hypertonus, spasticity)
3) Spasms
4) Speech deficits (dysarthria)
5) Attentional deficits (aphasia, apraxia, hemi-neglect)

Question 30

CST lesions are also called

Answer 30

upper motoneuron lesions

Question 31

upper motoneuron lesions

Answer 31

This derives from the idea that CST neurons are the final output pathway from the brain, and synapse directly onto spinal alpha motoneurons, which are called the “lower” motoneurons

Question 32

Broca’s area:

Answer 32

1. Motor aspects of speech

2. Lesions result in motor aphasia (Slurring speech)

Question 33

Wernicke’s area:

Answer 33

1. Comprehension of language
2. Association of visual, auditory and tactile input with words
3. Lesions result in sensory aphasia & dyslexia

Question 34

Sensory aphasia

Answer 34

– eg. A person is handed a pen. When asked what it is, she says it’s a spoon. When asked to use it, she writes her name. This indicates that the sensory to motor transformation is separate from sensory to cognitive transformation.