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1

- neural pathways that control the sequence and pattern of muscle contractions
- distributed throughout brain and spinal cord

Motor System

2

- consists of a single motor neuron and the muscle fibers that it innervates
- for fine control, a single motor neuron innervates only a few muscle fibers
- EXAMPLE: eye muscle
- for larger movements, a single motor neuron may innervate thousands of muscle fibers
- EXAMPLE: postural muscles

Motor Unit

3

- is the set of motoneurons innervating fibers within the same muscle

motoneuron pool

4

- force of muscle contraction is graded by recruitment of additional motor units (graded response)
- as additional motor units are recruited, more motor neurons are involved and more tension is generated

Size Principle of Muscles

5

Types of Motor Neurons

1. Alpha Motor Neurons
2. Gamma Motoneurons

6

- innervate extrafusal skeletal muscle fibers
- action potentials in α motor neurons lead to action potentials in the extrafusal muscle fibers they innervate, which results in contraction

Alpha Motor Neurons

7

- innervate specialized intrafusal muscle fibers
- adjust the sensitivity of the muscle spindles(so that they respond appropriately as the extrafusal fibers contract and
shorten)

Gamma Motor neurons

8

Types of Muscle Fibers

- Extrafusal Fibers
- Intrafusal Fibers

9

- make up the bulk of muscle
- innervated by alpha motor neurons
- provide the force for muscle contraction

Extrafusal Fibers

10

- smaller than extrafusal muscle fibers
- are innervated by gamma motor neurons
- encapsulated in sheaths to form muscle spindles
- are too small to generate significant force

Intrafusal Fibers

11

- central role in skeletal muscle control
- cell bodies are topographically arranged within the ventral horn of the spinal cord
- axons innervate skeletal muscle fibers
- cell bodies receive numerous synaptic connections from:
o proprioceptors
higher levels of the CNS including the brainstem, basal ganglia, cerebellum, and motor cortex

Alpha Motor Neuron

12

Topographic Arrangement

- muscles of the trunk are medial
- muscles of the extremities are lateral
- limb flexors are dorsal
- limb extensors are ventral

13

¥ synapse with the pool of motor neurons by which they are stimulated
¥ predominantly inhibitory
¥ bring about recurrent or feedback inhibition

Renshaw Cells

14

What type of synaptic arrangement is exemplified by Renshaw cells?

one to many

15

What neurotransmitter is released by Renshaw cells?

Glycine

16

What type of neuronal circuit is exemplified by Renshaw cells?

Divergent

17

sense of awareness of:
- position of the body in space
- progress of the movement by sensory receptors within the muscles and joints

Proprioception

18

- mechanoreceptors within muscles and joints
- provide the CNS with information regarding muscle length, position and tension (force)

Proprioceptors

19

More than half of all the nerve fibers that ascend and descend in the spinal cord are __.

Propiospinal fibers

20

two major proprioceptors:

O Muscle Spindle
O Golgi Tendon Organ

21

- small, encapsulated intrafusal fibers
- lie in parallel with extrafusal muscle fibers
- send information to the nervous system about muscle length or rate of change of length
- innervation is as follows:
1. efferents via gamma motor neurons - regulates sensitivity of the spindles
2. afferents via group Ia (primary or annulospiral endings) and group II fibers (secondary endings) - respond to muscle stretch

Muscle Spindles

22

True or False

The finer the movement required, the greater the number of muscle spindles in a muscle.

True

23

Types of Intrafusal Fibers in Muscle Spindles

1. Nuclear Bag Fibers
o detect the rate of change in muscle length (fast, dynamic changes)
o innervated by group Ia afferents
o have nuclei collected in a central “bag” region

2. Nuclear Chain Fibers
o detect static changes in muscle length
o innervated by group II afferents
o more numerous than nuclear bag fibers
o have nuclei arranged in rows

24

Role of Muscle Spindles

- comparators for maintenance of muscle length
- important during goal-directed voluntary movements
o voluntary changes in muscle length are initiated by motor areas of the brain
o includes changes to the set-point of the muscle spindle system

25

- simultaneous activation of extrafusal fibers (by alpha motor neurons) and intrafusal fibers (by gamma motor neurons)
- readjusts the sensitivity of muscle spindles continuously as the muscle shortens
- allows the muscle spindles to be functional at all times during a muscle contraction

Co-activation

26

- mechanoreceptors that lie within the tendons of muscles immediately beyond their attachments to the muscle fibers
- respond to degree of tension within muscles
- group Ib afferent fibers relay this information to the CNS (in particular the spinal cord and cerebellum)

Golgi Tendon Organ

27

- rapidly executed, automatic, and stereotyped response to a given stimulus
- simplest form of irritability associated with the nervous system

Reflex

28

- neurons participating in a reflex form a reflex arc, which includes:
o receptor
o afferent neuron that synapses in the CNS
o efferent neuron that sends impulses to an effector
o interneurons may be present between the afferent and efferent neurons

Reflex arc

29

Afferent vs Efferent

Remember SAME!

Sensory = Afferent
Motor = Efferent

30

Classification of Neural Reflexes

1. Efferent division that controls the effector
2. Integrating region within the Nervous system
3. Time at which the reflex develops
4. The number of neuron in the reflex pathway