Spinal Reflexes Flashcards
Where are cell bodies for the alpha motor neurons located within the spinal cord?
- Sensory afferent neurons
- Located in the grey matter of the ventral horn of the spinal cord (motor root)
Explain “the components of a reflex are contained within the spinal cord.”
- Input from the brain is not required to generate a reflex
- Interneurons collect reflex information from sensory afferent nerve fibers
- Information is sent through ventral root to alpha motor neurons (efferent)
- Effector (muscle/gland)
- Central integration is in the spinal cord
Which spinal cord segments correlate to
where the radial n. originates?
C7-T2 Spinal Cord Segments
Triceps reflex (not always present in normal dogs)
Extensor carpi radialis (not always present in normal dogs)
The spinal cord segments C6-C8
corresponds to which spinal nerve?
Musculocutaneous n.
Biceps reflex
Why is it clinically important to know that a reflex does not rely on the brain for its action to occur?
- Reflexes don’t need input from the brain to occur
- Central integration for reflexes remains at the spinal cord
- Spinal reflexes don’t require input from the brain- too fast!
- A elicited reflex doesn’t indicate proper brain function
Spinal reflexes are used to test the function of peripheral nerves
Where is the receptor in a stretch reflex arc?
Myotatic Reflex (Stretched Reflex):
- Fundamental postural mechanism
- Local reflex
- Effects limited to the muscle with the outgoing afferent nerve fiber
- Produces “muscle tone”
Sensory receptor location = Muscle spindle
8-10 Intrafusal fibers arranged parallel to extrafusal fibers
Alpha motor neurons innervate extrafusal fibers (effector organ)
Muscle stretches after tendon of the muscle is stimulated via stretching of neuromuscular spindal
Afferent inpulse carried by sensory neuron to synapse on alpha motor neuron at the spinal cord
Leads to contraction of the muscle
Produces reflex
In a myotatic reflex,
why do we describe it as
monosynaptic?
- Only 1 synapse between the sensory and alpha motor neuron
- Pathway starts when the muscle spindle is stretched in response to a stimulus
- The reflex arc only consists of 2 neurons (1 sensory and 1 motor)
What is the function of a
myotatic reflex?
Elicited by postural changes!!!!!
- Fundamental postural mechanism
- Critical antigravity function at a postural level
- Least amount of delay between sensory input and motor output
- Alpha and gamma motor neurons activated
Function: Correct for changes in muscle length when extrafusal fibers contract (get shorter) or stretch (get longer)
Purpose: Resist changes in muscle length
What is the efferent neuron and effector in the tendon jerk spinal reflex?
Central integration:
- Synapse on extensor alpha motor neuron (EPSP)- excitatory
- Synapse on inhibitory interneuron (IPSP)- inhibitory
- The interneuron synapses on flexor muscle alpha motor neurons
- Reciprocal innervation of agonist (extensor) EPSP and antagonist (flexor) IPSP
Efferent neuron: alpha motor neuron
Effector: signal from alpha motor neuron to shorten striated muscle
Draw a basic reflex arc and
Label 5 components (neurons, etc)
Basic Reflex Arc:
Sensory receptor
Afferent Nerve Fiber: sensory info from receptor to CNS (spinal cord grey matter) dorsal root ganglion
Central integration: Requires atleast 1 synapse with the CNS
Efferent Neuron: Lower motor neuron produces an axon that passes out of the spinal cord to project the efferent structure
Effector: Skeletal muscle, gland or smooth myocardial muscle
What is the difference between a withdrawal reflex and a tendon jerk reflex?
Withdrawl Reflex:
- Detection of a painful stimulus
- Polysynaptic
Withdrawl Reflex Receptor:
- Pain receptor (nociceptor)
- Bare nerve ending
What is the afferent fiber (neuron) in the
nociceptor/withdrawal reflex?
Flexor reflex afferents:
- Composition Bare Nerve Ending
- A-fibers : Fast pain, small, myelinated
- C-fibers: Slow pain, long-lasting unmyelinated
- Reciprocal Innervation
- Crossed Extensor Component (when animal is standing)
What is central integration?
- Synapse of the alpha and/or gamma motor neurons that compose the gray matter in the spinal cord.
- Monosynaptic
- Myotatic Reflex
- Polysnaptic
What role do interneurons play in a tendon jerk reflex?
Central integration:
- Synapse on extensor alpha motor neuron
- Synapse on inhibitory interneuron
- Interneuron synapses on flexor muscle alpha motor neurons
Result: reciprocal innervation of agonist (extensor) and antagonist (flexor)
What is reciprocal innervation?
How is it useful to an animal that is standing and has nociception tested in one foot?
Skeletal Muscles are antagonistic pairs
- Contraction of 1 muscle is accompanied by inhibition of an antagonistic muscle
- Extensor/flexor muscles
Withdrawl Reflex (Flexor Reflex):
- Causes a retraction of the foot
- Sensory afferents enter dorsal horn of spinal cord
- Excitatory synapse on interneurons
- Polysynaptic central integration
- Flexors (excited) contract
- Extensors (inhibited)
**Inhibition of extensors & excite the flexors so the foot/paw can withdraw**
*
In the nociceptor/withdrawal reflex, what is happening to the flexor and extensor muscles in the leg which is stimulated?
Sensory afferents –> Excitatory synapse on interneurons in dorsal horn of spinal cord
Ipsilateral flexor motor neurons:
- Excited
- Contract so the foot/paw can withdraw
Extensors: inhibited
Where is central integration for the panniculus reflex?
Interneurons project onto alpha motor neurons in the ventral horn from C8 - T2
Intersegmental, polysynaptic
What is the name of the nerve that is tested by the panniculus reflex?
Nerve: Lateral thoracic n.
Effector: Cutaneous trunchi m.
If an animal was pinched on the left side to test the panniculus reflex, and the animal shivered only on the left side, what does that mean about the reflex arc? What is damaged?
The reflex arc was not damaged.
Collaterals from T1 dermatome to L3 dermatome afferent fibers travel up the cord to synapse on interneurons at the level of C8, T1 and T2 remain ipsilateral (same side of the body).
if the lateral thoracic nerve is damaged on the left side of the animal, tell me what would happen if you stimulated the left and then the right sides of the animal?
Bilateral response
Lateral thoracic n. is a peripheral nerve
Left side wouldn’t show a skin twitch
Right side would show a skin twitch
The perineal region is a good test for ____________spinal cord segments.
Sacral
What are the signs of an appropriate perineal reflex response?
Best test of function of sacral spinal cord segments and associated nerve roots.
Response:
- Contraction of anal sphincter muscle
- Flexion of the tail
S1-S3 Test
In lower motor neuron damage what is the effect on Reflexes?
Atrophy of muscles?
Tone of muscles?
The brachial plexus is made up of peripheral nerves originating from which spinal cord segments?
All thoracic limb & peripheral nerves: C6-T2
Musculotaneous n. : C6-C8
Radial n.: C7-T2
The lumbrosacral plexus is made up of peripheral nerves originating from which spinal cord segments?
Sciatic n.: L6-S1
Femoral n.: L4-L6
Peroneal fibular: L6-L7
Tibial: L7-S1
Lumbrosacral plexus: L4-S3
Testing the tendon jerk reflex function of the triceps is testing the ________peripheral nerve. If the reflex is diminished, the lesion could be located at the ________spinal cord segments.
Radial n.
C7-T2
Where are receptors in the panniculus reflex?
T or F: Spinal reflexes only tell you about the lower motor neuron function (alpha motor neurons).
False
Reflexes test muscles and nerves
Bilateral loss of a reflex = damage to a spinal cord segment
Do you need to test a reflex on both sides of an animal? Why or why not?
Why do we call a tendon jerk reflex “reciprocal innervation”?
Inhibit flexors & excite extensors on 1 leg
Contraction of 1 muscle is accompanied by simaltaneous inhibition of an antagonistic muscle
Are synapses at muscles always excitatory?
EPSP or IPSP
Reciprocal innervation due to inhibitory and excitatory synapses
In a reflex, how are some muscles activated while others are quieted? Where does that message occur?
Reciprocal innervation
- Agonist/Antagonist Muscles are either activated or inhibited
- Polysynaptic
- Sensory afferent (nerve) carries info to spinal cord about sensed change
- Central integration @ spinal cord
- Synapse on extensor alpha motor neuron
- Synapse on interneuron (inhibitory)
- Interneuron synapses on flexor muscle alpha motor neurons
- Inhibiton to antagonist muscle via interneuron at specific spinal cord segment
The panniculus reflex:
Sensory receptor =
Sensory afferent pathway =
Where is central integration?
Motor output is alpha motor neuron to_______
You test the panniculus reflex in a dog. The dog does not respond to stimulus at the level of L2 but can respond to stimulus at the level of T11.
a. Can you identify a likely location for the lesion I told you that the lack of response happened on both the left and right sides?
b. What if I told you the lack of response was only on the left side? Then where would you place the lesion?
What is a dermatome?
Dermatome:
- Area of skin innervated by by cutaneous branches of 1 spinal nerve
- Sensory afferent information from 1 spinal nerve
- Enters spinal cord through 1 dorsal root
- Adjacent dermatomes typically overlap
What is an autonomous zone? How does it differ from a dermatome?
Autonomous Zone:
- Area of skin innervated by 1 peripheral nerve
- No overlap between adjacent peripheral nerves
- Not all peripheral nerves have this zone
- Used to narrow down a lesion/problem area
1 peripheral nerve = multiple spinal nerves
Dermatome:
- Area of skin innervated by by cutaneous branches of 1 spinal nerve
- Sensory afferent information from 1 spinal nerve
- Enters spinal cord through 1 dorsal root
- Adjacent dermatomes typically overlap
What happens to reflexes when an alpha motor neuron is damaged? What is another name for an alpha motor neuron?
Alpha motor neuron = Lower motor neuron
Damage to alpha motor neuron results in weak or absent response to a stimulus
Alpha motor neurons are always excitatory
Known as Hypoactive (or absence) Reflexes
Myotatic Reflex (Stretch Reflex)
(Postural Changes)
**Doesn’t enter dorsal horn**
- Sensory receptor: Muscle spindle afferents
- Normally maintains posture: tonicall fires
- Stretch the muscle: sensed by spindle afferents
- Afferent nerve fiber:
- Large myelinated fibers
- Increase firing input to spinal cord when muscle is stretched
- Stretch evoked reflex
- Central integration:
- 1 synapse within the CNS (monosynaptic reflex)
- Efferent neuron:
- alpha motor neuron assosicated with the same muscle
- Effector:
- Signal from alpha motor neuron to shorten the striated muscle
Hypoactive reflexes
- Always refers to damage to a lower motor neuron (aka alpha motor neuron)
Bilateral loss of a reflex is indicative of:
- Damage to a specific spinal cord segment
- Won’t see a reflex on neither the left or right side
- Won’t see any reflexes in the spinal cord segments moving cranial from the initial test point where the reflex was lost
What are spinal reflexes used to test for?
- Function of peripheral nerves
- Deficits in a peripheral nerve that correspond to specific spinal cord segments allow for the identification of a narrower region for that particular lesion
Damage to a peripheral nerve is indicative of:
Peripheral nerve: collection of spinal nerves
Damage = unilateral loss of function
- Reflex will be absent in either the left or right side of the animal in the spinal cord segments associated with the damaged peripheral nerve
- Reflexes may be seen in the surrounding peripheral nerves
Correlation of Spinal Cord Segments and Vertebrae in the Dog
- C1-C7 spinal nerves exit cranial to their associated vertebrae
-
C8 spinal nerve exits caudal to the C7 vertebrae
- No C8 spinal cord segment
- All remaining spinal nerves exit caudal to their associated vertebrae
Myotatic Reflex (How it Works):
- Increase length of muscles (via stimulus) = extrafusal and intrafusal fibers get stretched
- Sensory neuron around the intrafusal fibers is activated (stimulated)
- Via change in length or velocity
- Sensory signal is carried to the spinal cord
- Alpha and gamma motor neurons are activated
- Alpha motor neuron sends AP to extrafusal fibers telling them to contract
-
Gamma motor neuron sends AP to intrafusal fibers telling them to be sensitive
- Golgi tendon organs are in series with the extrafusal fibers
- They stretch when the extrafusal fibers shorten due to contraction
