Block 5

Dr. C.R.Houser

SPINAL CORD II: REFLEXES OF THE SPINAL CORDGoalsAfter studying your lecture notes and reading assignments, you should be able to:

1. Explain, by making a drawing and using the correct terminology, the circuitry underlying deep tendon reflexes that are tested during most neurological exams.

2. Understand the basic circuitry and functions associated with the Golgi tendon organ and flexor withdrawal reflex.

3. Appreciate the extensive functional circuitry within the spinal cord.

Reading:Reflexes: Nolte, Chapt. 9, Sensory Receptors.., pp. 210-220 / 215-224; Chapt.10, Spinal Cord, pp.234-238 / 238-242.

I. BASIC SPINAL CORD REFLEXES Overview

The circuitry for several important reflexes is present within the spinal cord. While these reflexes can be modulated by descending influences from more rostral brain regions, they do not require the participation of higher centers for their operation. One important class of spinal cord reflexes utilizes only one synapse between the sensory and motor elements (a monosynaptic reflex). However, most reflexes involve one or more interneurons within the pathway (polysynaptic reflexes).

Background information

Two classification schemes have been used to describe and categorize the axons within a peripheral nerve. It is useful to be aware of these systems now because the sensory and motor endings that are utilized in the various spinal reflexes are supplied by fibers of different sizes, and these fibers are identified by the terminology of these classification systems. One classification (by letters, Groups A-C) includes both sensory and motor fibers and considers the degree of myelination as well as the diameter of the fibers. In general, the smallest fibers (Group C) are unmyelinated whereas the larger fibers (Groups A-B) are myelinated and have the highest conduction velocities. A second classification (by Roman numerals, Groups I-IV) considers only the sensory fibers. The following table is provided for reference and not for memorization.

Classification of Peripheral Nerve Fibers (from Nolte, 2002)

REFLEX PATHWAYS AND FUNCTIONS

We will consider 3 basic spinal cord reflexes, each of which is mediated by a different type of sensory receptor and afferent fiber1) the stretch reflex; 2) the Golgi tendon organ reflex; and 3) the flexor withdrawal reflex.

1. Stretch Reflex (Myotatic Reflex or Deep Tendon Reflex)

Tests of the myotatic reflex are a basic part of most neurological exams. These reflexes are often referred to as tendon reflexes or deep tendon reflexes (DTRs) because a tendon is often tapped to elicit the response, but the muscle is really the site of action. An understanding of this reflex must begin with a fascinating receptor, the muscle spindle.

A. Muscle Spindle (Described earlier in your lecture on Skeletal Muscle)

The spindle consists of a connective tissue capsule that encloses several intrafusal muscle fibers.

The muscle spindle receives sensory innervation from two types of nerve fibers, primary and secondary.

The muscle spindle also receives a motor innervation from small gamma motor neurons.

The muscle spindle is located within the regular skeletal muscle and is attached to the muscle fibers by connective tissue. The spindles are arranged in parallel with the regular muscle fibers.

B. Pathway of the Stretch Reflex

The basic circuitry of the stretch reflex is quite straightforward:

1. The reflex is initiated by stretch of the primary endings of the muscle spindle.

2. These Ia fibers form monosynaptic, excitatory connections with the alpha motor neurons supplying the same (homonymous) muscle.

3. Additional connections of the Ia fibers within the spinal cord include:

a. Excitatory connections with motor neurons of synergistic muscles;

b. Disynaptic inhibitory connections with motor neurons of antagonistic muscles.

4. Information also reaches higher centers including the

a. Cerebellum. Fibers from the lower limbs and trunk form synaptic connections with neurons in the dorsal nucleus of Clarke (C8-L2); axons of neurons in this nucleus then travel by way of the dorsal spinocerebellar tract to the cerebellum. Information from the upper limbs ascends to the accessory cuneate nucleus in the medulla; the cuneoerebellar tract then serves as the upper limb equivalent of the dorsal spinal cerebellar tract.b. Thalamus and cortex via the dorsal column pathway (and less well defined paths).

The reflex circuitry and function become a bit more complex when the motor innervation of the spindle is considered. Gamma motor neurons innervate the polar ends of the intrafusal muscle fibers and, thus, can influence the sensitivity of the muscle spindle. Contraction of the distal ends of the intrafusal fibers leads to stretch of the sensory endings that are wrapped around the intrafusal fibers. This can activate the previously described reflex circuitry. Note that the gamma motor neurons do not innervate the regular muscle fibers directly and are not contacted by the primary sensory endings. Instead the gamma motor neurons are innervated by other afferents from the dorsal roots (such as cutaneous afferents) and by descending motor pathways that influence both the alpha and gamma motor neurons.

C. Functions of the Muscle Spindle

It is important to understand the effects of stretch and contraction on the discharge rate of the Ia afferent fibers from the muscle spindle:

1. Stretch of the muscle leads to stretch of the spindle, and this increases the discharge rate of the Ia fiber.

2. Contraction of the muscle decreases the stretch of the spindle (allows it to go slack) and reduces the discharge rate of the afferent fibers.

In the situation just described, the muscle spindle is silenced during muscle contraction and would be essentially nonfunctional during this time. However, in real life, the sensitivity of the spindle can be adjusted by the CNS via gamma motor neurons. The effects of the gamma motorneurons may be understood by studying the response of the Ia fiber discharge

1. During muscle contraction (produced by alpha motor neuron stimulation) without gamma motor neuron activation; and

2. During muscle contraction elicited by alpha motor neuron stimulation in conjunction with stimulation of gamma motorneurons.

When the gamma motor system is active during muscle contraction, the silent period of the Ia afferents is filled in by a continued discharge in response to gamma motor neuron activation of the polar ends of the spindle.

(Purves 16.11)

During most normal movements, both alpha and gamma motor neurons are activated at the same time (alpha-gamma coactivation). Under these circumstances, the muscle spindle remains functional and can respond to changes in load during the movement.

D. Roles of the Muscle Spindle in Motor Control

Despite the elegance of this receptor, its precise function in motor control remains unclear. It is certainly the basic substrate for the stretch reflex. Normally, it serves as a negative feedback system that monitors muscle length. The muscle spindle and its circuitry are likely to:

1. Participate in automatic adjustments of the body to maintain posture;

2. Compensate for changes in load during motor activity; and

3. Contribute to normal muscle tone.

4. Contribute to sense of limb position and movement (proprioception and kinesthesia).E. Summary:

1. Muscle spindles are located in parallel with regular muscle fibers;

2. Primary afferent fibers of the spindle respond to

a. Stretch of the regular muscle fibers; andb. Contraction of the polar ends of the spindle (intrafusal muscle fiber).

3. Stimulation of the primary afferent fibers results in

a. Facilitation of the motor neurons to that same muscle through a monosynaptic reflex; and

b. Inhibition of the motor neurons of the antagonists.

4. Gamma motor neurons receive input from descending (supraspinal) motor systems and cutaneous afferents. These motor neurons exert their effects through the stretch reflex pathway but do not contribute directly to muscle tension.

2. Golgi Tendon Organ Reflex A. Golgi tendon organs are found near the tendinous ends of skeletal muscles and are located in series with regular muscle fibers. Information from these receptors is conveyed to the spinal cord by Ib afferent fibers.

B. The circuitry involves:

1. disynaptic inhibition of motor neurons to homonymous and synergistic muscles; and2. disynaptic excitation of motor neurons to the antagonists.

C. The Golgi tendon organ responds to muscle tension. It is very sensitive to muscle contraction but also may respond to extreme muscle stretch.D. The Golgi tendon organ and its circuitry provide a feedback system that monitors and maintains muscle force.

E. Comparisons of Muscle Spindle and Golgi Tendon Organ (Purves 16.12)

3. Flexion Withdrawal Reflex (Flexor reflex; Withdrawal reflex)

A. The withdrawal reflex is mediated by somatic afferents that carry nociceptive information (sensation associated with tissue damage; often used interchangeably with pain) and have small diameters. This reflex may affect the muscles at several different joints and, thus, involves multiple segments of the spinal cord (intersegmental reflexes). Upon entering the spinal cord, the afferent fibers may course rostrally and caudally for several segments within the dorsolateral fasciculus (Lissauers tract) and terminate among spinal interneurons at several different levels.

B. There is

1. Polysynaptic excitation of motor neurons to the appropriate muscles for withdrawal of the limb from the stimulus (often flexor muscles);

2. Polysynaptic inhibition of motor neurons of the antagonistic muscles; and

3. An opposite pattern on the contralateral side leading to a crossed extension response.

STRETCH REFLEX AND GAMMA LOOP

Purves, 16.13

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