Neurophysiology: Reflexes

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Questions and Answers

What is the primary function of the spinal cord in relation to the sensory system?

To process and interpret sensory information

To generate motor responses to stimuli

To store and retrieve sensory information

To receive and distribute information to the PNS (correct)

What is the role of interneurons in the spinal cord's motor system?

To integrate sensory and motor functions for reflex activity

To contain complex neural circuits for motor control (correct)

To generate motor responses to sensory stimuli

To transmit motor signals from the brain to the PNS

What is the characteristic of a monosynaptic reflex?

It is a voluntary motor response

It involves multiple synapses in the CNS

It has a reflex arc involving contact directly between sensory and motor neurons (correct)

It requires the integration of sensory and motor functions

What is the purpose of testing reflexes in a clinical setting?

To evaluate the integrity of different components of the reflex arc and the influence of descending UMN motor pathways Signup and view all the answers

What is the function of the spinal cord in relation to motor control?

To relay efferent/motor information from motor management centers to connect with LMN Signup and view all the answers

What is the definition of a reflex?

An involuntary, qualitatively unvarying motor response of the nervous system to a stimulus Signup and view all the answers

What is the function of the reflex arc?

To direct a motor response to a stimulus Signup and view all the answers

What is the primary characteristic of a segmental reflex arc?

It passes through only a small segment of the spinal cord or a small region of the brainstem. Signup and view all the answers

What is the characteristic of a polysynaptic reflex?

It involves sensory neurons, interneurons, and motor neurons Signup and view all the answers

What is the main action of the patellar reflex?

Muscle contraction Signup and view all the answers

What is the type of effector organ involved in an autonomic reflex?

Cardiac muscle or smooth muscle, or a gland Signup and view all the answers

What is the term for the relaxation of an antagonistic muscle while the other muscle is contracting?

Reciprocal inhibition Signup and view all the answers

Which of the following reflexes is an example of a stretch reflex?

All of the above Signup and view all the answers

What is the primary characteristic of an intersegmental reflex arc?

It passes through multiple segments of the spinal cord or several major brain divisions. Signup and view all the answers

What are the spinal cord segments involved in the patellar reflex?

L4, L5, L6 Signup and view all the answers

What is the term for the reflex that involves afferent axons going from a muscle stretch receptor to directly synapse with spinal cord alpha motor neurons?

Stretch reflex Signup and view all the answers

What is the primary function of the inverse stretch reflex?

To prevent excessive tension on the muscle in response to muscle contraction Signup and view all the answers

What is the effect of the golgi tendon organ reflex on the duration and force of muscle contraction associated with the stretch reflex?

It decreases the duration and force of muscle contraction Signup and view all the answers

What is the secondary action of the inverse stretch reflex?

Reciprocal stimulation of synergistic muscles Signup and view all the answers

What is the role of the receptor in the golgi tendon organ reflex?

It is sensitive to muscle tension and increases its firing rate when the muscle contracts Signup and view all the answers

What is the characteristic of the flexor reflex in response to a noxious stimulus?

The force and duration of the withdrawal reflex are proportional to the intensity of the noxious stimulus applied Signup and view all the answers

What is the mechanism of the flexor reflex?

Alpha motor neurons going to the flexor muscles are stimulated, while those to the extensor muscles are inhibited Signup and view all the answers

What is the main action of the inverse stretch reflex?

The normal stimulation is reduced, and inhibited at the level of the spinal cord Signup and view all the answers

What is the characteristic of reciprocal innervation in the flexor reflex?

The stimulation of alpha motor neurons going to the flexor muscles leads to the inhibition of alpha motor neurons going to the extensor muscles Signup and view all the answers

Which of the following is NOT a primary function of the Upper Motor Neurons (UMNs)?

Regulation of cardiovascular function Signup and view all the answers

What is the result of loss of inhibitory UMN influence on Lower Motor Neurons (LMNs)?

Increased muscle tone and reflexes Signup and view all the answers

Which of the following is a characteristic of Lower Motor Neurons (LMNs)?

Cell bodies located in the central nervous system (CNS) Signup and view all the answers

What is the primary function of the corticomedullary pathway?

Initiation of voluntary movement Signup and view all the answers

Which of the following is a sign of a Lower Motor Neuron (LMN) lesion?

Decreased muscle tone and reflexes Signup and view all the answers

What is the primary location of the Lower Motor Neurons (LMNs) that innervate the limbs?

Cervical and lumbar intumescences Signup and view all the answers

What is the clinical significance of an intact reflex?

It tells the clinician that the lesion does NOT involve that area of the CNS or PNS Signup and view all the answers

Which of the following is a characteristic of Upper Motor Neurons (UMNs)?

Typically originate in the brain and project to the spinal cord Signup and view all the answers

What is the effect of a lesion at C1-C5 on the muscle tone of the thoracic limb?

Normal to increased muscle tone Signup and view all the answers

A lesion at T3-L3 affects the muscle tone of which limb?

Neither thoracic nor pelvic limb Signup and view all the answers

What is the characteristic posture of Schiff-Sherrington posture?

Rigid forelimb extension with normal hindlimb function Signup and view all the answers

What is the cause of Schiff-Sherrington posture?

Interruption of inhibitory neuron input from the lumbar spinal cord Signup and view all the answers

A lesion at L4-S3 affects the muscle tone of which limb?

Pelvic limb only Signup and view all the answers

What is the effect of a lesion at CD1-CD5 on the muscle tone of the thoracic limb?

Fully normal muscle tone Signup and view all the answers

What is the prognosis of Schiff-Sherrington posture in underlying disease?

No association with prognosis Signup and view all the answers

What is the typical cause of spinal trauma leading to Schiff-Sherrington posture?

Being hit by a car Signup and view all the answers

Study Notes

Spinal Cord Function

The spinal cord receives and distributes sensory information to the peripheral nervous system (PNS) and relays afferent/sensory information to brain centers.
The spinal cord integrates sensory and motor functions for reflex activity, both within a limb and between limbs.
The spinal cord relays efferent/motor information from motor management centers to connect with lower motor neurons (LMN) and contains interneuron and complex neural circuits for motor control.

Reflex Activity

A reflex is an involuntary, qualitatively unvarying motor response of the nervous system to a stimulus.
A reflex arc contains five fundamental components: a receptor, sensory neuron, one or more synapses in the CNS, a motor neuron, and a target organ.
A reflex always returns some condition of the body back to the desired state.

Reflex Classification

Reflexes can be classified as monosynaptic or polysynaptic, based on the presence or absence of interneurons.
Reflexes can be classified as ipsilateral or contralateral, based on the location of the reflex arc.
Reflexes can be classified as segmental or intersegmental, based on the extent of the spinal cord or brainstem involved.
Reflexes can be classified as somatic or autonomic, based on the type of effector organ involved.

Stretch Reflex

A stretch reflex is a monosynaptic reflex that responds to muscle stretching, causing muscle contraction.
The stretch reflex involves afferent axons going from a muscle stretch receptor to directly synapse with spinal cord alpha motor neurons, which cause contraction of that same muscle.
The patellar reflex is an example of a stretch reflex, testing the stretch reflex mediated by the femoral nerve.

Inverse Stretch Reflex

The inverse stretch reflex, also known as the golgi tendon organ reflex, has the major goal of preventing excessive tension on the muscle, in response to muscle contraction.
The receptor is sensitive to muscle tension, increasing its firing rate when the muscle contracts, which can excite an inhibitory interneuron in the spinal cord, inhibit the alpha motor neuron, and lead to muscle relaxation.

Flexor Reflex

The flexor reflex is a coordinated polysynaptic reflex in which all the flexor muscles of the limb contract in response to a noxious stimulus.
The flexor reflex involves alpha motor neuron stimulation, while those to the extensor muscles are inhibited, due to reciprocal innervation.

Upper Motor Neurons (UMN)

UMN's initiate, regulate, modify, and terminate the activity of lower motor neurons (LMN).
UMN's may inhibit or facilitate LMN.
Loss of inhibitory UMN results in increased muscle tone and spinal reflexes.
Loss of facilitatory UMN results in paresis or paralysis.

Lower Motor Neurons (LMN)

LMN have their cell bodies in the CNS (brainstem or spinal cord), while their axons project into the PNS via cranial or spinal nerves, connecting with skeletal muscle at the neuromuscular junction.
When stimulated, LMN's induce muscle contraction.
Muscle tone and bulk depend on LMN function.
Loss of LMN's results in paresis/paralysis, with decreased to absent muscle tone and reflexes.

UMN vs LMN: Signs

Clinically, an intact reflex, be it normal or exaggerated, tells the clinician that the lesion does NOT involve that area of the CNS or PNS.

Spinal Cord Lesion Localization: Exception

Schiff-Sherrington posture occurs due to interruption of inhibitory neuron input from the lumbar spinal cord, characterized by rigid forelimb extension with normal function, with concurrent hindlimb paresis and paralysis.
This posture is NOT associated with prognosis of the underlying disease.

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Description

Test your knowledge on the roles of the spinal cord in receiving and distributing sensory information, integrating sensory and motor functions, and relaying motor information.