Lecture 6: Motor System 1 Flashcards
Motor system functions (2) :
(1) Responds adequately to the
order of integrative & sensory
divisions.
(2) Controls the body motor
activities (muscles):
Skeletal (striated) muscles
Smooth muscles (internal
organs, heart, stomach…
Motor (pyramidal) pathway is composed by 2 order neurons:
1st order neuron: upper motor neuron
2nd order neuron: lower motor neuron
1st order neuron: upper motor neuron starts at:
the motor cortex and ends at the
anterior (ventral) horn of the spinal cord.
1st order neuron: upper motor neuron __ the activity of the lower motor neuron
stimulates and modulates
Longest fibers of the CNS:
1st order neuron: upper motor neuron
2nd order neuron: lower motor neuron starts at:
the anterior horn of the spinal
cord and ends at the neuromuscular
junction.
2nd order neuron: lower motor neuron: __ muscles
Exits muscles
2nd order neuron: lower motor neuron integrated in:
the circuit of motor reflexes.
Spinal cord level: Is the intersection that:
conducts sensory & motor signals from the periphery of body to the brain and vice versa.
Spinal cord is the center of
processing basic sensory & motor
information (i.e. reflexes, can work
without the need of other levels)
What are the two kinds of reflex circuit TYPES that the spinal cord controls?
(1) circuit for motor reflexes (stretch reflex, golgi tendon reflex, withdrawal reflex)
(2) Circuits for reflexes that control internal organ functions (e.g. blood vessels, gastrointestinal, urination, …)
What are example of motor reflexes:
(1) Stretch reflex
(2) golgi tendon reflex
(3) withdrawal reflex
What are example of reflexes that control internal organ functions?
(1) Blood vessels
(2) Gastrointestinal
(3) urination
sensory neurons have their cell bodies in:
dorsal root ganglion
sensory neurons transmit :
somatic senstations (e.g. tactile, proprioceptive, pain) to the CNS
Sensory neurons synapse (2) :
(1) Locally in the spinal cord to be integrated in motor reflexes
(2) Transmit signals to higher centers (somatosensory pathways)
The cell body of motor neurons are located:
in the (ventral (anterior) horn of spinal cord)
Motor neurons directly innervate:
skeletal muscle fibers (LOWER MOTOR NEURON)
Alpha Motor neurons (A-alpha axons) excite:
EXTRAFUSAL MUSCLE FIBERS (motor unit) for MUSCLE CONTRACTION
Gamma Motor neurons (A-gamma axons) excite:
INTRAFUSAL MUSCLE fibers (muscle
sensory receptors) to ADJUST MUSCLE SENSORY INPUTS
Motor neurons are in contact with:
the UPPER MOTOR NEURON (i.e. receives the order to stimulate muscles)
the cell body of inter neurons is located in:
the dorsal horn of the spinal cord
What are interneurons?
Local neurons that have interconnections with sensory & motor neurons
Interneurons synapse with:
motor neurons at the ventral horn of spinal cord
Are interneurons inhibitory or excitatory?
they can be both
interneurons are integrated in:
motor reflexes
Proprioceptive sensation senses:
(1) the state of muscles (starched, contracted, relaxed)
(2) the tension practiced on
the muscle
Sensory receptors of the muscle are:
mechanoreceptors (i.e. stimulated bu mechanical displacement of the muscle)
mechano receptors are fast or slow adapting?
slowly adapting
muscle spindles are distributed throughout:
the muscle belly (INTRAFUSAL FIBERS)
muscle spindles sense (2):
(1) MUSCLE LENGTH (stretched, relaxed, contracted)
(2)rate of change of length
golgi tendon organs are located in the:
muscle tendon
Golgi tendon organs sense (2):
(1) TENSION OF THE TENDON
(2) Rate of change of tension
Signals from muscle spindles and Golgi tendon organs are transmitted to:
higher centers (e.g. cerebral
cortex, cerebellum) through THE DORSAL COLUMN-MEDIAL LEMNISCAL PATHWAY
signals from muscle spindles and golgi tendon organs are integrated in:
motor REFLEXES (spinal cord)
Muscle Spindle is a
stretch receptor
Muscle Spindle is a stretch receptor located in:
the INTRAFUSAL MUSCLE FIBERS (do not have myosin and actin)
intrafusal muscle fibers do not have:
myosin and actin
Muscle Spindle signals (2):
(1) the LENGTH OF MUSCLES
(2) changes IN LENGTH OF MUSCLES
How do muscle spindles work? STRETCHING the muscle:
EXCITES the receptor
and INCREASES the AP FIRING in the fiber.
How do muscle spindles work?CONTRACTION of the muscle INHIBITS:
The receptor and decease the AP firing in the fiber
Muscle Spindle signals are transmitted through two fiber types:
(1) Primary sensory ending (Ia) fibers : Transmit information about THE RATE OF CHANGING OF THE MUSCLE LENGTH
(2) SECONDARY SENSORY ENDING (II) fibers: transmit information about THE DEGREE OF STRETCH OF THE MUSCLE.
Muscle Spindle signals are transmitted through two fiber types: Primary sensory ending (Ia) fibers:
Transmit information on THE RATE OF CHANGING OF THE MUSCLE LENGTH
Muscle Spindle signals are transmitted through two fiber types: Secondary sensory ending (II) fibers :
Transmit the information about THE DEGREE OF STRETCH OF THE MUSCLE
What is golgi tendon organ?
Encapsulated sensory receptor through which a bundle of muscle tendon fibers passes
Golgi Tendon Organ Stimulated when
This bundle of muscle
fibers is “TENSED” by the contraction or
the stretching of the muscle (SENSITIVE TO MUSCLE TENSION).
Golgi Tendon Organ Signals are transmitted through:
Ib fibers
Golgi tendon organ : Ib fibers inhibit:
Indirectly
Golgi Tendon Organ: Ib fibers inhibit indirectly (through interneurons) _
ALPHA MOTOR NEURONS at the spinal cord to allow the muscle to relax
and thus to relieve the tension on the
tendon (Golgi Tendon Reflex).
The two types of lower motor
neurons’ (Large A MNs and small Y Mns) cell bodies are located:
in the ventral horn of spinal cord
The cell bodies of large a MNs are located in:
The ventral horn of spinal chord
Large a MNs innervate:
EXTRAFUSAL FIBERS of skeletal muscle that generate the forces needed for
posture and movement (i.e.
CONTRACTION).
The cell bodies of small y Mns are located in:
The ventral horn of the spinal chord
Small y MNs innervate
INTRAFUSAL MUSCLE FIBERS to modulate (adjust) sensory inputs (MUSCLE SPINDLES)
extrafusal
fibers generate:
the forces needed for
posture and movement (i.e. contraction)
intrafusal muscle fibers modulate:
(adjust) sensory inputs (muscle
spindles
Muscle fibres are innervated by:
Nerve fibers (motor neurons) to cause muscle contraction.
Muscle fibers are innervated by nerve fibers (motor neurons) to cause:
Muscle contraction
One nerve fiber (motor neuron) innervates:
Multiple muscle fibers
The motor unit is the:
smallest subunit of muscle fibers that can be controlled by a single motor neuron
SMALL MOTOR UNIT:
SMALL MUSCLES that require precision (e.g. hand, eye, laryngeal) have LOW INNERVATION RATIO (i.e. one nerve innervates 2-3 muscle fibers)
small muscles that require precision (e.g. hand, eye, laryngeal…) have :
low innervation ratio (i.e. one nerve innervates 2-3 muscle fibers)
Large motor unit:
LARGE MUSCLES that do not require precision (e.g. soleus, quadriceps…) have HIGH INNERVATION ratio (i.e. one nerve innervates 80-100 muscle fibers)
The synapse between the motor neuron and the muscle fiber is called:
“neuromuscular junction:
The neuromuscular junction releases:
Acetylcholine
A MUSCLE TWITCH (contraction) is:
The tension developed in response to ONE motor neuron stimulation
Summation means:
The adding together of individual muscle twitches to increase the intensity of overall muscle contraction
summation occurs in two ways:
(1) multiple fiber summation (size principle)
(2) frequency summation
Multiple fiber summation (size principle):
Increasing the NUMBER OF MOTOR UNITS contracting at the SAME TIME (recruitment)
Frequency summation:
Increasing the FREQUENCY OF STIMULATION of ONE motor unit
Size principle:
Depending on the intensity of the stimulation, motor units are recruited in an orderly fashion according to their SIZE
Size principle: smallest motor units vs largest motor unit:
(1) Smallest motor units are recruited first (for weak stimulation)
(2) Largest motor units are recruited last (for strong stimulation)
The size principle allows for:
The gradations of muscle force from small steps (weak contraction) to great steps (strong contraction)
Frequency Summation & Tetanization:
As the frequency of the signal increases:
individual twitchs summate (no
recovery form previous contactions)
Frequency Summation & Tetanization: When the frequency reaches a critical level:
the summated twitchs fuse
together to form one continuous contraction
Tetanization (tonic state) (2):
Contraction reaches a Max level, no response to further stimulations.
Ca2+ ions are maintained in the sarcoplasm, the contractile state of the muscle is
sustained (myosin & actin stay attached).
During tetanization, contraction:
Reaches a max level, no response to further stimulation
During tetanization (tonic state) Ca+ ions are :
Maintained in the sarcoplasm, the contractile state of the muscle is sustained (myosin and actin stay attached)
Sensory neurons are __ from muscle spindle and golgi tendon
afferent fibers
Cell body of the sensory neurons can be found in:
The dorsal root ganglion
Sensory neurons synapse in the
Spinal cord (directly with motor neurons (monosynaptic) or interneurons (polysynaptic))
Interneurons are considered “polysynaptic” because:
they are involved in neural pathways that have multiple synapses
Motor neurons are monosynaptic when:
they are part of a reflex arc that involves only one synapse between a sensory neuron and a motor neuron.
motor neuron cell bodies are located in:
the ventral horn of the spinal cord
Motor neurons directly innervate two types of skeletal muscle fibers:
(1) Alpha motor neurons
(2) Gamma motor neurons
Alpha motor neurons (A-alpha axons) excite:
Extrafusal muscle fibers (motor unit) for muscle contraction
Gamma motor neurons (A-gamma axons) excite:
INTRAFUSAL MUSCLE fibers (muscle sensory receptors) to ADJUST MUSCLE SENSORY INPUTS
Motor neurons are in contact with:
Descending fibers (i.e. upper motor neuron) ((The descending fibers originate from the brain, primarily the motor cortex, brainstem, and other higher motor centers, and travel down the spinal cord to synapse onto motor neurons in the anterior horn of the spinal cord. These motor neurons then send signals to muscles, facilitating movement.))
What are interneurons :
Local neurons in the spinal cord that have interconnections with sensory and motor neurons
Interneurons synapse with:
motor neurons at the ventral horn of the spinal cord
Interneurons: Inhibitory or excitatory?
Both!
Interneurons __ the activity of motor reflexes
modulate
Spinal cord reflex as a general mechanism is:
A mechanism by which sensory impulse
is automatically converted into a motor response
Spinal cord reflex: voluntary or involuntary?
involuntary
What do we mean when we say that spinal cord reflexes are involuntary?
Rapid (seconds) mechanism that is controlled by the spinal cord
integration center
Spinal cord reflexes are __ mechanism to the body
protective
Ex: Prevent an over stretch/contraction of the muscle, withdrawal from a painful stimulus, keep balance)
What are three types of spinal cord reflexes:
(1) Stretch reflexes
(2) Golgi tendon reflex
(3) Withdrawal reflex
What are two types of stretch reflexes?
(1) Patellar reflex
(2) Biceps reflex
What are two types of withdrawal reflexes?
(1) Withdrawal reflex
(2) Crossed extensor reflex
Mechanism of a stretch reflex:
- Stretching the muscle activates the muscle spindles
-Associated sensory fibers (Ia and II) Transmit impulses to the spinal cordL
1. SENSORY FIBERS synapse DIRECTLY (monosynaptic) with a MOTOR NEURON to cause the CONTRACTION of the STRETCHED MUSCLE (extensor)
2. SENSORY FIBERS synapse with an INTERNEURON that INHIBIT the MOTOR NEURON of the ANTAGONIST MUSCLES (flexors) to prevent it from contracting (i.e. relax)
STRETCH (PATELLAR) REFLEX causes a sudden:
FORWARD KICKING MOVEMENT of the LOWER LEG in response to a SHARP TAP on the PATELLAR LIGAMENT
FUNCTION of the stretch reflex:
to OPPOSE SUDDEN CHANGES IN MUSCLE LENGTH (stretching)
Over stretching = tear
Stretching the muscle in the stretch reflex activates sensory fibers of muscle spindles (2)
(1) Sensory fibers EXCITE monosynpatically (directly) ALPHA MOTOR NEURONS of the STRETCHED MUSCLE (i.e quadriceps) and cause it to CONTRACT (extend of the lower leg)
(2) Sensory fibers EXCITE INTERNEURONS that in turn INHIBIT ALPHA MOTOR NEURONS of the antagonist muscle (i.e. hamstings to prevent it to contract) ( i.e. relax) –> Polysynaptic (indirect inhibition)
Abnormality of the reaction of the stretch reflex:
Suggest that there may be damage to the CNS or PNS
BICEPS REFLEX causes:
A sudden kicking movement of the lower arm in response to a sharp tarp on the BICEPS TANDON
–> this reflex opposes further stretch of the muscle
Stretching the muscle a in the biceps reflex activates:
(1) sensory fibers EXCITE monosynaptically (directly) ALPHA MOTOR NEURONS of the STRETCHED MUSCLES (BICEPS and BRACHIALIS) and cause them to CONTRACT (flexion of the lower arm)
(2) Sensory fibers EXCITE INTERNEURONS that in turn INHIBIT ALPHA MOTOR NEURONS of the antagonist muscle (triceps)
to prevent it from contracting (i.e. relax)
Polysynpatic (indirect) inhibition
Abnormality of the stretch reflex suggest that :
There may be damage to the CNS or PNS
Golgi tendon reflex is the:
OPPOSITE of the STRETCH reflex (inverse stretch reflex) –> Sudden backward movement of the leg
FUNCTION of the golgi tendon reflex:
relieves excess tension on the tendon (i.e. prevent it from tearing)
Contracting of a muscle (e.g. Quadriceps) in the golgi tendon reflex:
creates a TENSION on the TENDON attached to the same muscle and thus ACTIVATES the GOLGI TENDON ORGANS
What are the sensory fibers of the golgi tendon?
Ib
Golgi tendon sensory fibers (Ib) transmit impulses to the:
Spinal cord
Golgi tendon sensory fibers (Ib) transmit impulses to :
Spinal cord
Golgi tendon sensory fibers (Ib) transmit impulses to spinal cord:
(1) Sensory neurons synapse with INHIBITORY INTERNEURONS that INHIBIT ALPHA MOTOR NEURONS of the CONTRACTED muscle (i.e. quadriceps and cause it to relax)
(2) Sensory neurons synapse with EXCITATORY INTERNEURONS that EXCITE ALPHA MOTOR NEURONS of ANTAGONIST MUSCLES (e.g. Hamstrings) and cause it to CONTRACT
The withdrawal reflex is initiated by
a painful stimulus that causes
automatic withdrawal of the threatened body part.
Pain sensory neurons synapse with:
a pool of spinal interneurons:
withdrawal reflex: Pain sensory neurons synapse with a pool of spinal interneurons:
(1) EXCITE motor neurons of the FLEXOR muscle (e.g. Biceps) and cause the muscle to CONTRACT
(2) INHIBIT motor neurons of the EXTENSOR muscle (e.g. Triceps) and cause the muscle to RELAX
Withdrawal and Crossed Extensor Reflex
- Neuronal Circuits: two reflexes:
Withdrawal &
Crossed Extensor reflexes:
Painful stimulus elicits:
(1) FLEXOR REFLEX (i.e. withdrawal reflex) in AFFECTED LIMB (limb withdrawal)
(2) EXTENSOR REFLEX (i.e. crossed extensor reflex) in the OPPOSITE LIMB (limb extension)
FLEXOR REFLEX (i.e. withdrawal
reflex) in AFFECTED LIMB (limb
withdrawal):
CONTRACTION of the FLEXOR muscle and RELAXATION of the EXTENSOR muscle
EXTENSOR REFLEX (i.e crossed extensor reflex) in the OPPOSITE LIMB (LIMB EXTENSION)
CONTRACTION of the EXTENSOR muscles and RELAXATION of the FLEXOR muscle
Withdrawal and Crossed Extensor Reflex - Neuronal Circuits: This series of reflexes serves to:
Push body away from the stimulus, but also to shift weight to the opposite limb (i.e prevents falls)
Abnormality in the stretch reflexes response suggests that:
there may be
damage to the LOWER MOTOR NEURON or UPPER MOTOR NEURON
Hyporeflexia:
WEAK (or absent) reflex response
In hyporeflexia, the problem is in:
the LOWER MOTOR NEURON
Hyporeflexia is the disruption of
either afferent or efferent fibers (PNS) in the stretch reflex loop (e.g.,
cut in the fibers, neuropathies, nerve or root compression…).
Hyporeflexia: localisation of spinal segment:
(1) Patellar reflex corresponds to L4
(2) BIceps reflex corresponds to C5-C6
Hyperreflexia:
STRONG reflex response
in Hyperreflexia, the problem is in:
the upper motor neuron –> the motor neuron has an inhibitory control on the stretch reflex
Loss of central command on the spinal cord leads to
an enhanced stretch reflex
(e.g., CNS lesions affecting the motor pathways).
