NA and NP (5) - motor systems

Question 1

What are the 3 levels of motor control?

Answer 1

1) Precommand (highest)
2) Projection (middle)
3) Segmental (lowest)

Question 2

Describe the lowest level of motor control

Answer 2

Segmental level ; spinal cord

• controls automatic movements
• included locomotions and other repeated patterns = central pattern generators (CPGs)
• CPGs = networks of oscillating inhibitory and excitatory neurons that set rhythms

Question 3

Describe the middle level of motor control

Answer 3

Projection ; motor cortex

• direct and indirect motor fibers (descending tracts)
• upper motor neurons initiate direct pyramidal pathways to muscles
• brain stem motor nuclei help control reflexes and CPG controlled motor actions
• conveys the instructions to spinal cord motor neurons and sends a copy of info to higher levels

Question 4

Describe the highest level of motor control

Answer 4

Precommand ; cerebellum and basal nuclei

• starting and stopping movements
• coordinates movements with posture and blocks unwanted movements
• monitors muscle tone
• highest level because it controls the output of the 2 lower levels by providing instructions as to what they should do next
• BOTH happen before the actual motor response

Question 5

What are the functions of the cerebellum and basal nuclei in the precommand level?

Answer 5

Cerebellum = has no access to spinal cord, not able to stimulate it direction, communicates with projection areas of brain stem and motor cortex via the thalamus

Basal Nuclei = recieve inputs from all cortical areas and sent output to premotor and prefrontal cortex via thalamus

Question 6

What are the different descending pathways and are they direct/indirect?

Answer 6

Direct pathways = pyramidal tracts

• Only corticospinal tracts

Indirect pathways = all others

• Rubrospinal, reticulospinal, vestibulospinal, tectospinal tracts

Question 7

Where do upper and lower motor neurons travel

Answer 7

Upper = motor cortex

Lower = spinal motor neurons / cranial motor neurons

Question 8

What is the function/role of direct pathways ?

Answer 8

AKA pyramidal tracts - allows direct access to muscles

Axons descend without synapsing from primary motor coretx through the length of the spinal cord

They regulate fine/fast (skilled) movements - such as typing or playing an instrument

Synapsing with lower motor neurons occurs in anterior horn at the level where the neuron will exit spianl cord to activate skeletal muscle

Question 9

What are 4 main spinal cord injuries and paralysis?

Answer 9

1. Paralysis = loss of motor function
2. Paraplegia = transection between T1 and L1 (lower limbs)
3. Quadriplegia = transection in cervical regions (upperlimbs and lower limbs)
4. Hemiplegia = due to brain injury rather than spinal cord ( some side of the body) - damage to the left brain results in paralysis in the right side of the body

Question 10

Difference between flaccid and spastic paralysis?

Answer 10

Flaccid paralysis = damage to the ventral roots / anterior horns

• impulses do not reach muscles i.e. no voluntary or invluntary muscle control

Spastic paralysis = damage to only upper motor neurons of primary motor cortex

• loss of voluntary muscle control but but spinal reflexes can still occur

Question 11

What is the function/role of indirect pathways?

Answer 11

Contains all motor pathways that are not a part of the pyramidal system

Rubrospinal, reticulospinal, vestibulospinal, tectospinal

More so for supporting movement; muscles for balance and posture and for controlling coarse limb movement (thighs), head, neck and eye movements

Question 12

List the steps for motor function (6)

Answer 12

1. Premotor cortex
2. Precommand areas (basal nuclei and cerebellum) to provide timing and patters

• Basal Nuclei - required to start movements
• Cerebellum - calculates the best way to perform by sending the blueprint to motor cortex

3. Primary motor cortex - sends signals down direct pathways
4. Indirect pathways (brain nuclei) - assist with balance, eye movement and muscle tone
5. Lower motor neurons - receive signals from both pathways
6. Sum total of inhibitory + excitatory = final response

Question 13

What is the function of reflex arcs?

Answer 13

To permit rapid and predictable responses

Can be inborn (instrinsic) or learned (acquired)

Question 14

What is an inborn reflex vs a learned reflex?

Answer 14

Inborn;

• Unlearned, involuntary and simply a reaction
• To help maintain posture, avoid pain and control visceral activities

Learned;

• results from rpactice or repetition and can modify an inborn reflex by learning and conscious effort
• Somatic reflexes activate skeletal muscle
• Visceral reflexes activate smooth muscle, cardiac muscle, glands

Ex. Think about driving carefully but almost automatic after being an experienced driver compared to when you first started driving

Question 15

What are the two things the NS needs to know about for a stretch and tendon reflex?

Answer 15

1) the length of a muscle (muscle spindles) - when a muscle is contracted it is shorter vs when it is relaxed it is longer

2) tension in the muscle and tendons (tendon organs)

Question 16

What is a muscle spindle composed of?

Answer 16

Each is about 3-10 modified intrafusal fibers enclosed in a CT capsule to monitor muscle lengths

The central region of intrafusal fibers lack myofilaments and is noncontractile (receptive part)

Question 17

What is the difference between intrafusal and extrafusal fibers?

Answer 17

Intrafusal = noncontractile (the receptive region)

Extrafusal = controls msucle contractions

Question 18

What are the 2 types of afferent nerve endings?

Answer 18

1) Annulospiral;

• Larger nerve axons that monitor both rate and degree of stretch
• primary sensory endings

2) Flower spray;

• Smaller nerve axons that monitor only degree of stretch
• Secondary sensory endings

Rate = rate of change of the muscle length

Question 19

What fibers are used during a stretch and tendon reflex?

Answer 19

γ (gamma) efferent motor fibers = associated with motor spindles and contribute to contractile activity at either end

α (alpha) efferent motor fibers go directly to a muscle = stimulates direct muscle contraction

Intrafusal fibers have actin and myosin = can be stimulated to contract by gamma efferent muscle fibers

Question 20

What factors can affect muscle stretch?

Answer 20

• External force/contraction of antagonistic muscles
• activating gamma motor neurons that put stretch on the middle of the spindle

Question 21

Which fibers/pathways do the descending pathways activate?

Answer 21

Activates both alpha and gamma pathways so that the brain can continue to be notified about what muscles are doing

Contracts both muscle spindles (gamma) and fibers (alpha)

Question 22

What is reciprocal activation vs inhibition?

Answer 22

Reciprocal activation = acts in the opposite direction; the contracting muscle relaxes as its antagonist is activated

Reciprocal inhibition = contraction of stretched muscle and inhibition of its antagonist

Question 23

What happens when a muscle is damped?

Answer 23

There are 2 synapses;

1) Induces muscle directly stimulating contraction
2) Synapses with interneuron going to antagonistic muscle

Question 24

What are some key differences in the somatic vs autonomic systems?

Answer 24

Somatic;

• heavily myelinated axons for inducing very quick responses
• stimulatory effect is on skeletal muscle
• NT is ACh (acetylcholine)
• voluntary movement

Autonomic;

• lightly myelinated preganglionic axons and typically nonmyelinated postganglionic axons
• subdvisions of parasympathetic and sympathetic
• can have stimulatory or inhibitory effects depending on the NT and receptors on effector organs
• sympathetic ; NT produced is NE (norepinephrine) and uses ACh
• parasympathetic ; NT produced is ACh and uses ACh