4. Functions of Motor System
The most important functions of the motor
system are to maintain posture and to exhibit
different movements which ultimately
depend upon contractions of skeletal
muscles.
These motor activities occur at conscious as
well as subconscious levels.
5. Motor Neurons
Contraction of skeletal muscles
occurs under the control of a set of
neurons called motor neurons.
Each motor neuron and the muscle
fibers innervated by it constitute a
motor unit.
6. Upper Motor Neurons
Neurons in the higher centers of brain
which influence the lower motor neurons.
They are present in:
a.Cerebral cortex.
b.Basal ganglia.
c.Brainstem.
d.Cerebellum.
7. Lower Motor Neurons
They innervate the skeletal muscles
directly.
They include:
a.Motor nuclei of cranial nerves in
brainstem.
b.Alpha and Gamma motor neurons in
anterior horn of gray matter of spinal cord.
8. Alpha and Gamma Motor Neurons
There are 2 types of motor neurons in
anterior horn of gray matter of spinal cord:
a.Alpha motor neurons – Larger; Innervate
extrafusal muscle fibers.
b.Gamma motor neurons – Smaller;
Innervate intrafusal muscle fibers.
They are always coactivated (alpha-
gamma coactivation).
9. Interneurons
Present in all areas of gray matter of spinal
cord.
They are interconnected to each other as
well as to alpha and gamma motor
neurons.
Most of the signals from sensory spinal
nerves as well as from higher centers
(brainstem, cerebral cortex) are indirectly
transmitted to alpha and gamma motor
neurons via interneurons.
10.
11. Types of Muscle Fibers
Extrafusal muscle fibers – Larger; Constitute the majority
of skeletal muscle; Innervated by alpha motor neurons;
Used to generate force.
Intrafusal muscle fibers – Smaller; Innervated by gamma
motor neurons; Take part in formation of muscle spindles.
a. Nuclear bag fibers – Larger; Nuclei arranged in a
central “bag” region.
b. Nuclear chain fibers – Smaller; Nuclei arranged in
chains; More abundant than nuclear bag fibers.
12.
13.
14. Muscle Spindle
Spindle-shaped organs composed of intrafusal
muscle fibers and innervated by both sensory and
gamma motor nerve fibers.
Arranged “in parallel” with extrafusal muscle fibers.
Muscle spindles are the stretch receptors that
detect the change in length of a muscle.
15.
16. Golgi Tendon Organ
Found in tendons of the muscles.
Arranged “in series” with the extrafusal
muscle fibers.
Detect change in tension of a muscle.
Innervated by sensory nerve fibers.
19. Reflex Arc
Motor responses to specific kind of stimuli without
involvement of higher centers is called reflex.
The neuronal circuit that directs this motor
response is called reflex arc. It includes:
a.Sensory receptor.
b.Sensory nerve.
c.Interneuron in spinal cord.
d.Motor neuron.
e.Effector muscle.
20. Types of Reflexes
a.Monosynaptic – Involves only one
synapse; Example: Stretch reflexes e.g.,
Knee-jerk reflex.
b.Disynaptic – Involves two synapses;
Example: Golgi tendon reflexes e.g.,
Clasp-knife reflex.
c.Polysynaptic – Involves multiple synapses;
Example: Flexor-withdrawal reflexes e.g.,
Withdrawal after touching a hot stove.
21. Stretch Reflex (Monosynaptic)
Muscle is stretched
Increase in the firing rate of sensory nerve fibers
arising from muscle spindles
Sensory nerve fibers enter spinal cord and activate
alpha motor neurons innervating that muscle
Contraction of the muscle that was stretched
22.
23. Golgi Tendon Reflex (Disynaptic)
Contraction of the muscle
Increase in the firing rate of sensory nerve fibers
arising from Golgi tendon organs
Sensory nerve fibers enter spinal cord and
activate inhibitory interneurons
Inhibitory interneurons inhibit the firing of alpha
motor neurons
Relaxation of the muscle
24.
25. Flexor-Withdrawal Reflex (Polysynaptic)
Painful stimulus
Increase in the firing rate of sensory
nerve fibers
Sensory nerve fibers enter spinal cord and
synapse on multiple interneurons
Interneurons activate the alpha motor neurons innervating ipsilateral
flexors and contralateral extensors while inhibit the alpha motor
neurons innervating ipsilateral extensor and contralateral flexors
Flexion (withdrawal) on ipsilateral side and extension on
contralateral side (crossed-extension reflex)
26.
27. Functions of Spinal Cord Reflexes
a.To maintain steady posture.
b.To ensure quick withdrawal from a noxious
stimuli to avoid injury.
c.To prevent overstretching of muscle fibers.
d.To control the tension of an active muscle.
29. Introduction
Motor pathways descend from upper
motor neurons (present in the cerebral
cortex and brainstem) and influence
lower motor neurons in the anterior horn
of spinal cord.
30. Types of Descending Pathways
Descending motor pathways are of 2
types:
a.Pyramidal tracts – From cerebral cortex.
b.Extrapyramidal tracts – From brainstem.
32. Introduction
Brainstem consists of:
a. Medulla oblongata.
b. Pons.
c. Midbrain.
Brainstem influences the motor activities
by means of extrapyramidal tracts taking
origin from brainstem and terminating in
anterior horn of spinal cord.
35. Rubrospinal Tract
Origin – Red nucleus (midbrain).
Function – Facilitation of flexor muscles
and inhibition of extensor muscles.
36. Lateral Vestibulospinal Tract
Origin – Lateral vestibular nucleus
(medulla oblongata).
Function – Facilitation of the extensor
muscles and inhibition of the flexor
muscles.
39. Tectospinal Tract
Origin – Superior colliculi (midbrain).
Function – Movement of head and neck
in response to visual stimuli (spinovisual
reflexes).
