Spinal Reflexes PDF

PHYSIO 14 – Spinal reflexes 1.​ An independent spinal cord Motor units are the final common path: motoneurons are situated in the spinal cord and are the only ones able to excite the skeletal muscles (in smooth muscles the excitation can be provided also by hormones, catecholamines and molecule...

PHYSIO 14 – Spinal reflexes 1.​ An independent spinal cord Motor units are the final common path: motoneurons are situated in the spinal cord and are the only ones able to excite the skeletal muscles (in smooth muscles the excitation can be provided also by hormones, catecholamines and molecules arriving to the muscles via blood...). The motor system (sensory-motor) is organized in a hierarchical way (spinal cord, brainstem and cortex levels). -​ The cortex is the first level (the hierarchy is not based on position but on the power). The motor areas of the cortex send fibers to the brainstem that then go to the spinal cord but these cortical fibers can also arrive directly to the spinal cord. In the brainstem and in the spinal cord there are circuits for complex functions but it is the cortex that decides when they can work. -​ The spinal cord is able to perform some “independent activities” that do not require sovra-axial commands (spinal reflexes and some automatic movements like locomotion). In the spinal cord are situated sensory neurons, motoneurons and interneurons. -​ The brainstem is also able to do some independent activities since a part of it is the cervical-cranial prolongation of the spinal cord for the sensory-motor neurons of head and neck. But a part of the brainstem instead is considered a structure of the brain with nuclei and structures devoted to complex processes (no sensory or motor action). The reflex and autonomous activity can be organized in the spinal cord and brainstem (it takes over from the spinal cord at the cranial level). 2.​ Reflexes A reflex is an elementary response to either an external or internal sensory stimulus. Reflexes have different features, they are -​ Involuntary: too fast to be decided -​ Stereotyped: every time the elicited response is the same: e.g., pupillary reflex -​ Fast: since the circuit is very restricted -​ innate: people are born with all the circuits but some reflexes present in babies disappear in adults For example, the reflex of the hand of the baby (palmar-prehension) is different from a grasp. The same happens with the plantar surface prehension (functional for babies but dysfunctional for adults). The circuits don’t disappear but are controlled during growth (some reflexes are inhibited and others are maintained). The reflex arc is based on few elements:  ​ peripheral receptor, ​ afferent pathway, ​ integrating centre, ​ efferent pathway ​ peripheral effector. Reflexes can be autonomic (ANS is mostly based on reflexes) and skeleto-muscular reflexes. To study reflexes, decerebrate animal models are used. They are animals with a transverse section between the superior and inferior collicle, causing the spinal cord to be disconnected from the CNS). In this case the reflexes were very enhanced (so they were done in the spinal cord) and some had different features than before (so the descending system has a role on the system). Reflexes can be classified based on the adequate stimulus needed to elicit them: ​ proprioceptive stimulus: o​ stretch reflex (1a fibers in muscle spindle) o​ autogenic inhibition (1b fibers in Golgi tendon organ); ​ stimulus from skin receptors: o​ withdrawal reflex and crossed extensor (nociceptors) o​ plantar reflex (touch) 3.​ Muscle spindle The stretch reflex is assessed hitting with a hammer on the relaxed leg of the patient on the tendon situated over the patella. This indentation stretches the quadriceps (1a fibers respond to stretch). Alpha α Motor Neurons: the alpha motor neurons give rise to large type A alpha (Aα) motor nerve fibers. These fibers branch many times after they enter the muscle and innervate the large skeletal muscle fibers. The stimulation of a single alpha nerve fiber excites from three to several hundred skeletal muscle fibers, which are collectively called the motor unit. Gamma γ Motor Neurons : Along with the α motor neurons, which excite contraction of the skeletal muscle fibers, much smaller γ motor neurons are located in the spinal cord anterior horns. These γ motor neurons transmit impulses through much smaller type A gamma (Aγ) motor nerve fibers, which go to small, special skeletal muscle fibers called intrafusal fibers. These fibers constitute the middle of the muscle spindle, which helps control basic muscle “tone,” a.​ Anatomy of a muscle spindle A muscle spindle is built around 3 to 12 tiny intrafusal muscle fibers that are pointed at their ends and attached to the glycocalyx of the surrounding large extrafusal skeletal muscle fibers. Each intrafusal muscle fiber is a tiny skeletal muscle fiber. It is formed of a central region and two end regions -​ central region of each of these fibers— that is, the area midway between its two ends—has few or no actin and myosin filaments. Therefore, this central portion does not contract when the ends do but it functions as a sensory receptor. -​ end portions that do contract are excited by small γ motor nerve fibers that originate from small type A gamma (Aγ) motor neurons in the anterior horns of the spinal cord. These gamma motor nerve fibers are also called gamma γ efferent fibers, in contradistinction to the large alpha efferent fibers (type Aα nerve fibers) that innervate the extrafusal skeletal muscle. b.​ Sensory innervation The receptor portion of the muscle spindle is its central portion. The sensory fibers originate in this area and are stimulated by stretching of this midportion of the spindle. One can readily see that the muscle spindle receptor can be excited in two ways: -​ Lengthening the whole muscle stretches the midportion of the spindle and, therefore, excites the receptor. -​ Even if the length of the entire muscle does not change, contraction of the end portions of the spindle’s intrafusal fibers stretches the midportion of the spindle and therefore excites the receptor. Two types of sensory endings, the primary afferent and secondary afferent endings, are found in this central receptor area of the muscle spindle: ​ Primary Ending: In the center of the receptor area, a large sensory nerve fiber encircles the central portion of each intrafusal fiber, forming the primary afferent ending.This nerve fiber is a type Ia fiber and it transmits sensory signals to the spinal cord. ​ Secondary Ending. They are type II fibers that innervate the receptor region on one or both sides of the primary ending. This sensory ending is called the secondary afferent ending. c.​ Division of the Intrafusal Fibers Into Nuclear Bag and Nuclear Chain Fibers—Dynamic and Static Responses of the Muscle Spindle. There are also two types of muscle spindle intrafusal fibers: -​ nuclear bag muscle fibers (one to three in each spindle), in which several muscle fiber nuclei are congregated in expanded “bags” in the central portion of the receptor area. -​ nuclear chain fibers (three to nine), which have nuclei aligned in a chain throughout the receptor area. The primary sensory nerve (Ia) ending is excited by both the nuclear bag intrafusal fibers and the nuclear chain fibers. The secondary ending 5(II) is usually excited only by nuclear chain fibers. d.​ Static response The Primary and the Secondary Endings Both Respond to the Length of the Receptor—“Static” Response. When the receptor portion of the muscle spindle is stretched slowly, the number of impulses transmitted from both the primary and the secondary endings increases almost directly in proportion to the degree of stretching, and the endings continue to transmit these impulses for several minutes. This effect is called the static response of the spindle receptor, meaning that both the primary and secondary endings continue to transmit their signals for at least several minutes if the muscle spindle remains stretched. e.​ Dynamic response The Primary Ending (but Not the Secondary Ending) Responds to Rate of Change of Receptor Length— “Dynamic” Response. When the length of the spindle receptor increases suddenly, the primary ending (but not the secondary ending) is stimulated powerfully. This stimulus of the primary ending is called the dynamic response, which means that the primary ending responds extremely actively to a rapid rate of change in spindle length. Even when the length of a spindle receptor increases only a fraction of a micrometer for only a fraction of a second, the primary receptor transmits tremendous numbers of excess impulses to the Ia sensory nerve fiber, but only while the length is actually increasing. As soon as the length stops increasing, this extra rate of impulse discharge returns to the level of the much smaller static response that is still present in the signal. Conversely, when the spindle receptor shortens, exactly opposite sensory signals occur. Thus, the primary ending sends extremely strong signals, either positive or negative, to the spinal cord to apprise it of any change in length of the spindle receptor. f.​ Gamma Motor neurons Control of Intensity of the Static and Dynamic Responses by the Gamma Motor Nerves. The gamma motor nerves to the muscle spindle can be divided into two types: -​ gamma-dynamic (gamma-d): o​ excites mainly the nuclear bag intrafusal fibers o​ When the gamma-d fibers excite the nuclear bag fibers, the dynamic response of the muscle spindle becomes enhanced, whereas the static response is hardly affected -​ gamma-static (gamma-s). o​ excites mainly the nuclear chain intrafusal fibers o​ stimulation of the gamma-s fibers, which excite the nuclear chain fibers, enhances the static response while having little influence on the dynamic response. RECAP Primary sensory nerve (Ia): Bag and chain Secondary sensory nerve: Chain Static response: slow stretch of muscle spindle causing 1° (Ia) and 2° (II) sensory nerve to fire for minutes. Gamma-static nerve fibers excite nuclear chain (Ia and II) causing the enhancement of static. Dynamic response: rapid stretch causing ONLY 1° sensory nerve (Ia) to fire during the increase in length of the muscle. Gamma dynamic nerve fibers excite nuclear bag (Ia) causing the enhancement of the dynamic. g.​ Continuous discharge Continuous Discharge of the Muscle Spindles Under Normal Conditions. Normally, when there is some degree of gamma nerve excitation, the muscle spindles emit sensory nerve impulses continuously. Stretching the muscle spindles increases the rate of firing, whereas shortening the spindle decreases the rate of firing. Thus, the spindles can send to the spinal cord either positive signals (increased numbers of impulses to indicate stretch of a muscle) or negative signals (reduced numbers of impuls- es) to indicate that the muscle is unstretched. 4.​ Stretch reflex The simplest manifestation of muscle spindle function is the muscle stretch reflex. Whenever a muscle is stretched suddenly, excitation of the spindles causes reflex contraction of the large skeletal muscle fibers of the stretched muscle and of closely allied synergistic muscles. a.​ Neuronal Circuitry of the Stretch Reflex. The basic circuit of the muscle spindle stretch reflex, showing a type Ia proprioceptor nerve fiber originating in a muscle spindle and entering a dorsal root of the spinal cord. A branch of this fiber then goes directly to the anterior horn of the cord gray matter and synapses with anterior motor neurons that send motor nerve fibers back to the same muscle from which the muscle spindle fiber originated. Thus, this monosynaptic pathway allows a reflex signal to return with the shortest possible time delay back to the muscle after excitation of the spindle. Most type II fibers from the muscle spindle terminate on multiple interneurons in the cord gray matter, and these transmit delayed signals to the anterior motor neurons or serve other functions. b.​ Dynamic Stretch Reflex and Static Stretch Reflexes. The stretch reflex can be divided into two components: -​ the dynamic stretch reflex -​ the static stretch reflex. The dynamic stretch reflex is elicited by potent dynamic signals transmitted from the 1° sensory endings of the muscle spindles, caused by rapid stretch or un-stretch. That is, when a muscle is suddenly stretched or unstretched, a strong signal is transmitted to the spinal cord, which causes an instantaneous strong reflex contraction (or decrease in contraction) of the same muscle from which the signal originated. Thus, the reflex functions to oppose sudden changes in muscle length. The dynamic stretch reflex is over within a fraction of a second after the muscle has been stretched (or unstretched) to its new length, but then a weaker static stretch reflex continues for a prolonged period thereafter. The static stretch reflex is elicited by the continuous static receptor signals transmitted by both 1° and 2° endings. The importance of the static stretch reflex is that it causes the degree of muscle contraction to remain reasonably constant, except when the person’s nervous system spe- cifically wills otherwise. To do a movement the brain needs to know what to expect: it increases the volume of sensory inputs income that are relevant and decreases the inputs that could be dysfunctional. The brain takes into account if a response to the reflex will be functional or not (has to be inhibited). e.g., if a fast movement is performed the velocity of the muscle spindle is more relevant in respect to the stretch: the activity of gamma dynamic will be enhanced. To have a reflex the fibers go to: -​ the muscle that has to perform the activity -​ the antagonist muscle (one will stretch and the other will contract). The stretch reflex is the only monosynaptic and is the fastest one. 1a fibers go to the muscles that have to be stretched and the ones that are synergic on the same joint but they also assure that the antagonist will be inhibited thanks to an inhibitor interneuron to allow reciprocal inhibition. If the dorsal roots are cut there’s still a reaction (delayed reaction in the transient phase) that is the passive force (active only if there are intact dorsal roots). The profiles of 1a fibers and the motorneuron will be perfectly related to acceleration, velocity and intensity. 1a fibers communicate directly to motoneurons. In the graph there are 2 different muscular stretch recordings. When the stretch is performed very fast (1st case), there’s a fast rate of firing at the beginning and a fast adaptation to the steady state. Also, in the EPSP there’s a summation that creates a steep post synaptic potential going to the axon hillock that reacts with the dynamic and static sensitivity of the motoneuron. So, the profile of the force parallels the stretch: perfectly tuned to the acceleration of the stretch (the 1a fibers and the motoneurons have the same properties). The stretch reflex is a way to adequate the properties of the muscles to the ones of the neurons. If a muscle is stimulated while fixing its length, the force is different from the case in which the length can change: in the contractile portion, there’s the best result if the sarcomere is at rest position, in the belly/whole muscle the performance is better if there’s a stretch (passive force adds to the active one). If the length is changed without stimulation there’s a passive force. In the upper graph there’s no univocal relation between the force and the length. So, the brain has to receive the actual length of the muscle to compute the right frequency of discharge to get that force (the mechanical properties of the muscle should be taken into account). The 1a fibers communicate this to the brain automatically since they have the same properties of the motoneurons: they directly adjust the frequency of discharge of the motoneurons without having to change the frequency in the brain. In the second graph there’s homogeneity in the length-tension curve since the reflex adjusts automatically the machine to the actual length. The 2nd advantage is the velocity. The graph represents a situation in which a muscle is exposed to a stretch by changing the lengthening (velocity). On the left (case without stretch reflex): the force goes up but then drops down by a lot until it is partially recovered to reach the last final value. On the right (the reflex is present): there’s no drop but there’s a sudden response paralleling the velocity of the stretch and continuing to counteract the stretch (gives elasticity to the muscle) obtaining a reflex gain. This reflex is very good in dynamic phases: it is able to counterbalance the perturbations in fast movements. As soon as the stretch happens, much more force is asked to the muscle and the force is increased: this prevents the drops in force. Reflexes are the fundamental unit of movement. They are not only useful in clinical practice (by assessing them is possible to understand the state of some parts of the CNS) but they are also part of mechanisms of voluntarily movements. 5.​ Golgi tendon organ The Golgi tendon organ is an encapsulated sensory receptor through which muscle tendon fibers pass. About 10 to 15 muscle fibers are usually connected to each Golgi tendon organ, and the organ is stimulated when this small bundle of muscle fibers is “tensed” by contracting or stretching the muscle. Thus, the major difference in excitation of the Golgi tendon organ versus the muscle spindle is that the spindle detects muscle length and changes in muscle length, whereas the tendon organ detects muscle tension as reflected by the tension in itself. The tendon organ, like the primary receptor of the muscle spindle, has both a dynamic response and a static response, reacting intensely when the muscle tension suddenly increases (the dynamic response) but settling down within a fraction of a second to a lower level of steady-state firing that is almost directly proportional to the muscle tension (the static response). Thus, Golgi tendon organs provide the nervous system with instantaneous information on the degree of tension in each small segment of each muscle. A.​ Transmission of Impulses From the Tendon Organ Into the Central Nervous System. Signals from the tendon organ are transmitted through large, rapidly conducting type Ib nerve fibers. These fibers, like those from the primary spindle endings, transmit signals into local areas of the cord and, after synapsing in a dorsal horn of the cord, through long fiber pathways such as the spinocerebellar tracts into the cerebellum and through still other tracts to the cerebral cortex. The local cord signal excites a single inhibitory interneuron that inhibits the anterior motor neuron. This local circuit directly inhibits the individual muscle without affecting adjacent muscles. b.​ The Tendon Reflex Prevents Excessive Tension on the Muscle. When the Golgi tendon organs of a muscle tendon are stimulated by increased tension in the connecting muscle, signals are transmitted to the spinal cord to cause reflex effects in the respective muscle. This reflex is entirely inhibitory. Thus, this reflex provides a negative feedback mechanism that prevents the development of too much tension on the muscle. When tension on the muscle—and therefore on the tendon—becomes extreme, the inhibitory effect from the tendon organ can be so great that it leads to a sudden reaction in the spinal cord that causes instantaneous relaxation of the entire muscle. This effect is called the lengthening reaction; it is probably a protective mechanism to prevent tearing of the muscle or avulsion of the tendon from its attachments to the bone. RECAP They elicit a particular reflex: autogenic inhibition. The fibers go into the spinal cord and there an interneuron inhibits the neuron directed to the muscle (as soon as a force is developed the motoneuron is switched off). This is an actual inhibition of the resting membrane potential that is well controlled by converging fibers coming from descending pathways. This mechanism can be seen well in pathological condition (lesion of the descending system): there’s an immediate drop of force. 6.​ Flexor reflex The stimulus is noxious (nociceptor receptors are needed) and the information acts on 3 joints (activates all the groups of flexors of the 3 joints). The integration center is very complex and this mechanism is linked directly with posture. If a leg is withdrawn, the other one has to stay extended to stay upright. So, this reflex promotes the flexors in one leg and the extensors in the other one. FRA are flexor reflex afferents that in pathological conditions could elicit a flexor reflex (usually they are inhibited and used in locomotion but if there’s a disconnection in the spinal region they could be activated). A.​ Neuronal Mechanism of the Flexor Reflex. The left-hand portion of the figure shows the neuronal path- ways for the flexor reflex. In this instance, a painful stimuLus is applied to the hand; as a result, the flexor muscles of the upper arm become excited and withdraw the hand from the painful stimulus. The pathways for eliciting the flexor reflex do not pass directly to the anterior motor neurons but instead pass first into the spinal cord interneuron pool of neurons and only secondarily to the motor neurons. The shortest pos- sible circuit is a three- or four-neuron pathway However, most of the signals of the reflex traverse many more neurons and involve the following basic types of circuits: (1)​ diverging circuits to spread the reflex to the necessary muscles for withdrawal; (2)​ circuits to inhibit the antagonist muscles, called reciprocal inhibition circuits; (3)​ circuits to cause afterdischarge that lasts many fractions of a second after the stimulus is over. 7.​ Crossed extensor reflex About 0.2 to 0.5 second after a stimulus elicits a flexor reflex in one limb, the opposite limb begins to extend. This reflex is called the crossed extensor reflex. E xtension of the opposite limb can push the entire body away from the object that is causing the painful stimulus in the withdrawn limb. a.​ Neuronal Mechanism of the Crossed Extensor Reflex. The right-hand portion of the figure shows the neuronal circuit responsible for the crossed extensor reflex, demonstrating that signals from sensory nerves cross to the opposite side of the cord to excite extensor muscles. Because the crossed extensor reflex usually does not begin until 200 to 500 milliseconds after onset of the initial pain stimulus, it is certain that many interneurons are involved in the circuit between the incoming sensory neuron and the motor neurons of the opposite side of the cord responsible for the crossed extension. After the painful stimulus is removed, the crossed extensor reflex has an even longer period of afterdischarge than does the flexor reflex. Again, it is presumed that this prolonged afterdis- charge results from reverberating circuits among the in- terneuronal cells. 8.​ Plantar reflex The plantar reflex is obtained stimulating the plantar surface of the foot from posterior to anterior (this permits the doctor to observe plantar flexion). This is due to the fact that the touch fibers enter the spinal cord going both to the dorsal and plantar flexors but in default conditions, the descending system/cortex facilitates the plantar and inhibits the dorsal. If there’s a lesion in the corticospinal tract (e.g., stroke) there could be a dorsal flexion: Babinski sign. This is not present in babies/neonates since in their case the corticospinal tract is still immature. 9.​ Jaw jerk The jaw jerk is a stretch reflex applied on the head due to the contraction of the masseter muscle. There are no Golgi tendon organ or muscle spindle in the head (only in the jaw elevator and masseter). This reflex is useful during running so that the mouth remains closed (when sleeping the reflexes are depressed so the mouth can open). The jaw jerk is elicited to check the functional integrity of the V cranial nerve. 10.​Neonates reflex Neonates have different reflexes: ​ Moro reflex: used to assess the vestibule (gravity stimulation to have the extension of the upper limbs and flexion of lower limbs); ​ Automatic locomotion: (in neonates, it disappears in 4-6 weeks) if the baby touches a table with the feet he will mimic locomotion in place (it is dysfunctional); ​ Suction and swallowing reflex: (linked to maternal milk) the baby turns the head as soon as the areas close to the lips touch something; ​ Plantar prehension (disappears in 9-10 months to walk); ​ Palmar prehension (not a grasp); ​ Physiological dorsal foot flexion (10 years are needed to develop the corticospinal tract).