Download Mark Time Reflex

Functions of the Vestibular Apparatus
Control of Muscle Function by the Spinal Cord and Brain
Stem
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Motor Signals
Cause the muscle contraction, secretory function,
other motor effects throughout the body.
Sensory Information
• Integrated at all levels of the nervous system.
• Causes appropriate motor responses.
• Cerebrum – where the most complicated responses are
controlled.
Responses:
• Begin in the spinal cord with simple reflexes.
• Extend into the brain stem with still more complicated
responses.
• Extend to the cerebrum.
Motor Functions of the Spinal Cord
• The cord grey matter is the integrative area for the
cord reflexes and other motor functions
• Sensory signals enter the cord through the posterior
(sensory) roots.
Signals travel to two separate destinations.
• One branch of the sensory nerve terminates in the grey matter
of the cord and elicits local segmental reflexes and other
effects.
• Another branch transmits signals to higher levels of the nerve
system – to higher levels in the cord itself- to the brain stem,
or even to the cerebral cortex.
Neurons
Several million neurons are in the grey matter of the
spinal cord between one spinal nerve and the next.
• Sensory relay neurons
• Anterior motor neurons
• Interneurons
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The anterior Motor Neurons
Located on the anterior roots of the cord gray matter
Several thousand of them
Fifty to 100% larger than the others
Give rise to the nerve fibers that leave the cord via the
anterior roots and innervate the skeletal muscle fiber
The neurons are of two types
(1) Alpha motor neurons
• Give rise to Aα nerve fibers that innervate the large skeletal muscle
fibers
• Motor Units- Stimulation of a single nerve fiber excites three to several
hundred skeletal muscle fibers.
(2) The Gamma Motor Neurons
• Smaller
• Transmit impulses through the Aγ fibers to very small
intrafusal fibers
Intrafusal Fibers
• Are part of the muscle spindle
• Help control muscle contraction
Interneurons
• Located in all area of the cord gray matter (dorsal, anterior,
and in between)
• More numerous than anterior motor neurons
• Small and highly excitable firing 1500 times/second
• Have interconnections one with the other
• Innervate the anterior motor neurons
Interneurons
• Interconnections between interneurons and anterior motor
neurons are responsible for the integrative functions of the
spinal cord.
Types of Neuronal Circuits
Neuronal circuits found in the interneuron pool of cells
of the spinal cord include:
– Diverging circuits
– Converging circuits
– Repetitive-discharge circuits
Multisegmental Connections in the Spinal
Cord
The propriospinal fibers:
• Ascending and descending nerve fibers in the spinal
cord
• Fibers that run from one segment of the cord to
another
• Provide pathways for the multisegmental reflexes
(reflexes that coordinate simultaneous movements in
the forelimbs and hindlimbs)
Sensory receptors
• (1) Muscle spindles- distributed throughout the belly of
the muscle; send information to the nervous system
about either the muscle length or rate of change of its
length.
• (2) Golgi tendon organs- located in the muscle tendons;
transmit info about tendon tension.
Receptor Function of the Muscle Spindle
• Each spindle is built of around 3 to 12 intrafusal (small skeletal)
muscle fibers pointed at their ends and attached to the
glycocalyx of the extrafusal skeletal muscle fibers.
• Central portion doesn’t contract as the ends do (few actin and
myosin filaments); It functions as sensory receptor.
• End portions (contract)- excited by small γ motor nerve fibers
(originating from the γ motor neurons) in the anterior horns of
the spinal cord.
Sensory Innervation of the Muscle
Spindle
Receptor area of the spindle has two types of sensory nerve
endings:
• The Primary Ending- A large type Ia sensory fiber innervates
the center of the spindle receptor
• The Secondary Ending- type II nerve fibers innervate the
receptor to one side of the primary ending (The fiber spirals
around the intrafusal fiber; fiber stretches stimulating nerve
ending)
Static Response of Primary and
Secondary Endings
• Receptor portion of the muscle spindle is stretched slowly, the
number of impulses transmitted increases directly proportional
to the degree of stretch, and the endings transmit the impulses
for many minutes.
• The receptor responds to a change in length of the spindle and
also continues to transmit its signal for a prolonged period of
time.
Dynamic Response of the Primary Ending
• The primary (but not the secondary) ending responds to rapid
changes in length transmitting increased/decreased no. of
impulses into the Ia fiber while the length is
increasing/decreasing
• Primary ending sends strong signals to the spinal cord to
apprise it of any changes in length of the spindle receptor area.
Intrafusal/Extrafusal Muscle Length
Two ways in which the spindle can be stimulated:
• (1) By stretching the whole muscle
• (2) By contracting the intrafusal muscle fiber while the
extrafusal fiber remains at the normal length
The Muscle Spindle is the Comparator
Length of the two types of muscle fibers is compared:
• Length of extrafusal fibers > length of intrafusal
fibers → Spindle excited
• Length of extrafusal fibers < length of intrafusal
fibers → Spindle inhibited
The Stretch (Myotatic) Reflex
has two components
The Dynamic Stretch Reflex
• Rapid stretch of the muscle causes potent signal while the length of the muscle is
increased.
• Signal goes directly to the anterior motor neurons without passing through interneurons.
• This sudden stretch of the muscle causes reflex contraction of the same muscle opposing
further stretch of the muscle.
The Static Stretch Reflex
• After the muscle has been stretched to its new length
(dynamic), a weaker (static) stretch reflex continues
for a long period of time (elicited by static receptor
signals excessive length transmitted from primary and
secondary endings of muscle spindles).
It continues to cause muscle contraction.
The Dumping Function of the Stretch
Reflex
• Signals from other parts of the nervous system are unevenly
transmitted to a muscle in an increasing then decreasing
fashion followed by a change to another intensity level.
The Load Reflex
• Biceps is contracted and forearm is horizontal to the earth.
• Five lb. weight is put in the hand.
• Degree of activity of static muscle fiber reflex determines the
extent of the hand drop.
• Very active static reflex causes strong contraction of the
skeletal muscle fiber in the biceps limiting the degree of fall
and maintaining the forearm in a horizontal position.
The Gamma Efferent System
• The gamma efferent stimulation of the spindle muscle fibers
tenses these fibers and enhances the excitability of the muscle
spindles.
• The bulboreticular facilitatory region and its allied areas of the
brain stem transmit excitatory signals through the gamma
nerve fibers of the muscle spindles; this shortens the ends of
the spindles and stretches the central receptor regions
increasing their signal output.
The Muscle Spindle System Stabilizes
Body Position
• Spindles on both sides of each joint are activated at
the same time increasing reflex excitation of the
skeletal muscle on both sides of the joint.
• Joint becomes stabilized and force tending to move the
joint is opposed by sensitized stretch reflex.
• Movements of the fingers require intricate motor
procedures.
The Golgi Tendon Reflex
• Is an encapsulated sensory receptor through which a small
bundle of muscle tendon fibers pass.
• Ten to 15 muscle fibers are connected in series with each Golgi
tendon organ and the organ is stimulated by the tension
produced by the small bundle of muscle fibers.
• While the spindle detects changes in the muscle length, the
tendon organ detects changes in muscle tension.
The Muscle Tension
• Like the primary receptor of the muscle spindle, the tendon organ has both dynamic and
static responses responding when tension increases (dynamic response) and settling down
to a lower level of steady-state firing (static response).
• When the Golgi organs of a muscle are stimulated by increased muscle tension, signals
are transmitted into the spinal cord causing reflex inhibition in the muscle and
preventing the development of too much tension in the muscle.
• Fibers that exert excess tension become inhibited while those that exert too little
tension become more excited (absence of reflex inhibition), spreading the muscle load
over all the fibers and preventing muscle damage.
Neural Circuit of the Tendon Reflex
• The dorsal spinocerebelar tracts carry instantaneous
information from the muscle spindles and the Glogi
tendon organs to the cerebellum (120m/s).
• Additional pathways transmit similar info into the
reticular regions of the brain stem and to the motor
areas of the cerebral cortex.
Flexor Reflex
• Also called nociceptor/pain reflex
• Painful stimulus is applied to the hand and flexor muscle of the upper arm
becomes excited.
• Flexor reflex eliciting pathway: first into the interneuron pool and then to
the anterior motor neurons.
Signals traverse many types of circuits
• Diverging circuits to spread the reflex to the
necessary muscle for withdrawal
• Circuits to inhibit the antagonist muscle (reciprocal
inhibition circuits)
• Circuits to cause a prolonged repetitive afterdischarge
(after the stimulus is over)
Reciprocal Inhibition
• Excitation of a group of muscles is associated with inhibition of
another group.
• When a stretch reflex excites one muscle, it inhibits the
antagonist muscle at the same time.
• Reciprocal innervation refers to the neural circuit that causes
the reciprocal relationship.
• Reciprocal relationships exist between the two sides of the
cord (ex. flexor and extensor reflexes).
The Positive Supportive Reaction:
• The pressure on the foot pads of an animal whose spinal cord
was transected will by reflex stiffen the limbs sufficiently to
support the weight of the body.
• Complex circuit exists in the interneurons.
• The locus of the pressure on the foot pad determines the
direction in which the limb is extended.
The Rhythmical Stepping Reflex
• When the lower portion is separated from the remainder of the
spinal cord and a longitudinal section is made down the center
of the cord to block neuronal connections between the two
limbs, each hind limb can still perform individual stepping
functions.
• Cycle of forward reflection of the limb followed by backward
extension takes place.
• Every time stepping occurs forward in a limb, the opposite limb
steps backward (reciprocal innervation between the two limbs).
The “Mark Time Reflex”
• Stepping reflex that involves all four limbs occurs diagonally between the
forelimbs and hindlimbs (reciprocal innervation occuring the entire
distance up and down the cord between the fore- and hind limbs).
The Galloping Reflex
• Both forelimbs move backward while both hindlimbs
move forward.
• If stretch or pressure stimuli are applied (1) equally to
opposite limbs at the same time- a galloping reflex will
result.
(2) unequally on one side- diagonal walking reflex is
elicited.
Abdominal Spasm in Peritonitis
• Is the type of local muscle spasm caused by a cord reflex
resulting from irritation of the parietal peritoneum by
peritonitis.
• Same type of spasm occurs during surgical operation. Pain
impulses from the parietal peritoneum cause the abdominal
muscle to contact and squeeze the abdomen and extrude the
intestines through the surgical wound.
• Deep anesthesia is required for intra-abdominal operation.
Muscle Cramp
• A small amount of initial irritation causes more and more
contraction until a muscle cramp ensues.
• Severe cold, lack of blood flow to the muscle, or exercise of
the muscle elicit sensory impulses (pain) that are transmitted
from the muscle to the spinal cord causing reflex muscle
contraction.
• Contraction stimulates sensory receptors more and a positive
feedback mechanism occurs.
Spinal Shock
• Spinal cord is transected in the neck and all cord functions (including cord reflexes) are
blocked.
• Normal activity of the cord neurons depend on continual signals from higher centers
transmitted through:
- The vestibulospinal tract
- The reticulospinal tract
- The corticospinal tract
Spinal Functions Affected during Spinal Shock:
• The arterial blood pressure falls immediately (40 mmHg) illustrating that activity in the
sympathetic nerves to the blood vessels and heart is blocked.
• All skeletal muscle reflexes are blocked.
• The sacral reflexes for control of bladder and colon emptying are suppressed.
Brain Stem Anatomy
- Medulla
- Pons
- Mesencephalon
It is an extension of the spinal cord upward into the cranial cavity because it contains
motor and sensory nuclei that perform motor and sensory functions for the face and
head regions in the same way the anterior and posterior gray horns of the spinal cord do
(from the neck down).
Brain stem provides special control functions:
- Control of respiration
- Control of cardiovascular system
- Control of gastrointestinal function of the body
- Control of stereotyped movements
- Control of equilibrium
- Control of eye movement