Lecture 2: Spinal Cord Somatic Reflexes PDF

Lecture 2: Spinal cord somatic reflexes ● Reflex Arc: 1. Receptor activation 2. Sensory info transmission (thru DRG) 3. Processing of information 4. Motor neuron activation 5. Execution ● Basic neuronal circuit for a motor control: ○ Motor neurons are located in the ventral (anterior) gray horn o...

Lecture 2: Spinal cord somatic reflexes ● Reflex Arc: 1. Receptor activation 2. Sensory info transmission (thru DRG) 3. Processing of information 4. Motor neuron activation 5. Execution ● Basic neuronal circuit for a motor control: ○ Motor neurons are located in the ventral (anterior) gray horn of SC ○ Receive sensory information from muscle spindle ○ Send motor commands to muscles ● Muscle Receptors: ○ The muscle spindle signals the length of a muscle and changes in the length of a muscle. ○ The Golgi tendon organ signals the amount of force being applied to a muscle ○ Two major structures: check table below ■ Golgi tendon organ → muscle tension ■ Muscle spindle → Muscle length & rate of change ● Complex sensory receptors found in SM. ● They lie parallel to extrafusal muscle fibers so that they can detect changes in muscle length ● Has 3 parts: ○ Specialized intrafusal muscle fibers: contain non-contractile region ■ At the center, not involved in contraction, just surrounded by spinal nerves ○ Sensory fibers (Ia) that terminate in non contractile region of the intrafusal fibers ■ The one sending sn & got a branch that goes all the way to the neuron → two types (Ia & Ib) ○ Motor axons (gamma) that terminate in the polar regions of the intrafusal fibers ● Motor innervations of the muscle: ○ Two types of motor neurons: Alpha: Aα → for contraction ● Extrafusal innervation ● Large diameter axons = fast transmission ● A motor unit consists of a single αmn & all the muscle fibers it innervates Gamma: γ ● Part of the muscle spindle ● Intrafusal innervation ● Contraction of Intrafusal fibers does not directly cause sig changes in muscle tension or length; however, when level of tension in these fibers is adjusted, γmn influence the sensitivity of the muscle spindle to stretch ● two types of gamma motor neurons: dynamic & static ● Modulate sensory information in a dynamic & static fashion ● Significance of linkage between alpha & gamma motor neurons: (insert pic) ○ Sustained tension elicits steady firing in the Ia sensory fiber ■ Neither alpha nor gamma MN are activated ○ A characteristic pause occurs in the ongoing discharge of Ia fiber when αMN alone is stimulated. ■ The Ia fiber stops firing bcz spindle is unloaded by the resulting contraction ■ Contraction will lead to slackness of the non contractile muscles, they will be unloaded(absent activity) ■ The alpha is activated electrically ■ Gamma is inactive ○ If a γMN to the spindle is also stimulated, the spindle is not unloaded during contraction & the pause in discharge of the Ia fiber is filled in ■ Alpha & gamma are activated ○ Thus :Gamma MN enables spindle to signal length changes over the full range of muscle length ● Dynamic & static Gamma-motor innervation of the muscle spindle (insert pic) ● Static vs.dynamic responses of Ia sensory fibers (insert pic) ● Gamma motor neurons: Static & Dynamic responses: (insert pic) ○ Stretch alone (no gamma stimulation): ■ Dynamic response is small ■ Steady state firing is moderate ○ Stimulated static gamma motor neuron: ■ Dynamic response ↓ ■ Steady state response ↑ ○ Stimulated dynamic gamma motor neuron: ■ Dynamic response markedly ↑ ■ Steady state response is close to original level ● Golgi tendon organ: (insert pic) ○ located in the tendons of muscles & are thus arranged in series with the muscle. ○ They are supplied by group Ib afferent fibers ○ Their in-series relationship means that tendon organs can detect the force level generated by the muscle, whether it is due to passive stretch or to active contraction of the muscle. ● Golgi tendon organ: ○ Afferent fibers Ib: large diameter myelinated axons which is demyelinated when entering capsule. Small ending intertwined with the collagen fibers. ○ Muscle contraction stretching tendon organ, Ib are compressed firing of AP ○ Average level of activity of Ib gives a good idea of the force exerted on the muscle ○ Synapses to an inhibitory interneuron in the spinal cord ● Autogenic inhibition reflex: ○ Tension applied to a muscle ○ The group Ib fibers that innervate Golgi tendon organ are activated ○ Ib afferent fibers synapse onto interneuron: Ib inhibitory interneuron ○ Synaptic inhibition of alpha motor neuron that innervates the same muscle that caused Ib afferent to fire ● Reciprocal excitation in the autogenic inhibition reflex: ○ Ib afferent fiber bifurcates in the spinal cord ○ One branch innervates the Ib inhibitory interneuron ○ The other branch innervates an excitatory interneuron ○ Excitatory interneuron innervate alpha motor neuron that control the antagonist muscle ● State-dependent reflex reversal: ○ In passive animals: ■ Ib fibers from extensor muscles have an inhibitory effect on homonymous MN ○ During locomotion: ■ These same Ib fibers produce an excitatory effect on those same motor neurons ○ Reason: ■ The neural transmission in the disynaptic inhibitory pathway is depressed ● Stretch reflex: ○ Tap on patella tendon below the knee: also called knee jerk reflex ○ Uncontrollable forward movement of the leg ○ Used in clinic ○ Monosynaptic: contain only two neurons, a sensory & motor neuron ○ Use it to know: ■ 1. At which level is the SCI ■ 2. What type is SCI ● Stretch reflex (myotatic reflex): ○ Tapping on Patella: tendon below the knee ○ Extensor (E) muscle stretches & activates sensory neuron ○ Sensory neuron synapse directly on motor neuron ○ Excited motor neuron transmits signal to extensor muscle ○ E muscle contracts ○ Berne: reflex includes ■ (1) a monosynaptic excitatory pathway from group Ia afferent fibers in muscle spindles to αmn that supply the same & synergistic muscles ■ (2) a disynaptic inhibitory pathway to antagonistic motor neurons ● Reciprocal innervation (inhibition): ○ Antagonist muscle (flexor or F) needs to be relaxed: ease movement ○ Sensory neuron bifurcates: direct synapse on motor neuron synapse to interneuron ○ Interneuron is inhibitory ○ Activity of motor neuron projecting to flexor is inhibited ○ Flexor muscle relaxes ○ Note that reciprocal innervation is polysynaptic ● Inhibitory interneurons can produce either feedforward or feedback inhibition: (INSERT PIC) ○ Feedforward inhibition: enhances effect of active pathway by suppressing activity of other, opposing, pathways ○ Feedback Inhibition: dampens activity within stimulated pathway & prevents it from exceeding a certain critical maximum (GTO) ● Flexor Reflex: (pain receptors are nociceptors) *polysynaptic ○ Case: stepping on sharp object ○ Sensory neuron synapse onto 2 interneurons (IN) ○ INs are either excitatory or inhibitory ○ Inhibitory IN synapse with motor neurons innervating E muscle ○ Excitatory IN synapse with motor neuron innervating F muscle ○ E relaxes & F contracts in ○ the ipsilateral (same side) of the stimulus ○ Foot is moved up away from the sharp object ● Crossed extensor reflex: ○ Axon of IN crosses the spinal cord & innervate 2 IN then 2 MN ○ MN are either stimulatory (+) or inhibitory (-) ○ The E muscle is stimulated: contracts ○ Opposite effect occurs to F muscle ○ Leg is not flexed: standing on one foot ● More on flexion reflex: recruiting of more spinal cord segments: ○ More spinal cord segments are recruited when a reflex is initiated with a strong stimulus ○ Thus: Reflexes are not simply repetitions of a stereotyped movement pattern; they are modulated by properties of the stimulus ● Changes in synaptic properties can modulate strength of a spinal reflex: Effect of UMNs ○ Increased tonic input to a lower motor neuron induces depolarization ○ Lower motor neuron becomes highly sensitive to reflex (sensory) input Muscle spindle Golgi tendon organ Structure ● Intrafusal muscle fibers (modified skeletal muscle fibers) and nerve fibers surrounded by a capsule ● Collagen strands & nerve fibers surrounded by a capsule Location ● Within skeletal muscle; parallel to extrafusal muscle fibers ● Within tendons of skeletal muscle Perception ● Muscle length & change of muscle length (ex: muscle stretching) ● Muscle tension & muscle force Reflex arc ● Myotatic stretch reflex: ○ Alteration of muscle length activates muscle spindles ○ Afferent signals travel via type Ia fibers & type II fibers and the dorsal root ganglion to the spinal cord. ○ Simultaneous activation of: ■ Alpha motor neurons (LMNs) that innervate the agonist muscle (monosynaptic) ■ Inhibitory interneurons that inhibit the alpha motor neurons of the antagonist muscle (disynaptic) ○ See tendon reflexes for more information ● Inverse myotatic stretch reflex ○ Tension of the muscle activates Golgi tendon organs ○ Afferent signals travel via type Ib sensory fibers and the dorsal root ganglion to the spinal cord. ○ Activation of inhibitory interneurons that inhibit the alpha motor neurons of the agonist muscle (disynaptic) ○ Relaxation of the agonist muscle (prevents exhaustive tension) Purpose ● Causes stretched muscle to contract ● Controls tendon tension of activated muscle ● Causes muscle relaxation of agonist muscle before tendon damage occurs Example ● Tapping of reflex hammer on tendon ● Sudden relaxation when a weightlifter uses extremely heavy weights