The Spinal Cord Mediates Reflex Activity

The spinal cord contains neural circuitry to generate reflexes, stereotypical actions produced in response to a peripherally applied stimulus. One function of a reflex is to generate a rapid response. A familiar example is the rapid, involuntary withdrawal of a hand after touching a danger ously hot object well before the heat or pain is perceived. This type of reflex protects the organism before higher CNS levels identify the problem. Some reflexes are simple, others much more complex. Even the simplest requires coordinated action in which the agonist contracts while the antagonist relaxes. The functional unit of a reflex consists of a sensor, an afferent pathway, an integrating center, an efferent pathway, and an effector. The sensory receptors for spinal reflexes are the proprioceptors and cutaneous receptors. Impulses initiated in these receptors travel along afferent nerves to the spinal cord, where interneurons and motor neurons constitute the integrating center. The final common path, or motor neurons, make up the efferent pathway to the effector organs, the skeletal muscles. The responsiveness of such a functional unit can be modulated by higher motor centers acting through descending pathways to facilitate or inhibit its activation.

Study of the three types of spinal reflexes—the my-otatic, the inverse myotatic, and the flexor withdrawal— provides a basis for understanding the general mechanism of reflexes.

The Myotatic (Muscle Stretch) Reflex. Stretching or elongating a muscle—such as when the patellar tendon is tapped with a reflex hammer or when a quick change in posture is made—causes it to contract within a short time period. The period between the onset of a stimulus and the response, the latency period, is on the order of 30 msec for a knee-jerk reflex in a human. This response, called the my-otatic or muscle stretch reflex, is due to monosynaptic circuitry, where an afferent sensory neuron synapses directly on the efferent motor neuron (Fig 5.8). The stretch activates muscle spindles. Type Ia sensory axons from the spindle carry action potentials to the spinal cord, where they synapse directly on motor neurons of the same (homony-mous) muscle that was stretched and on motor neurons of synergistic (heteronymous) muscles. These synapses are excitatory and utilize glutamate as the neurotransmitter. Monosynaptic type Ia synapses occur predominantly on alpha motor neurons,- gamma motor neurons seemingly lack such connections.

Collateral branches of type Ia axons also synapse on in-terneurons, whose action then inhibits motor neurons of antagonist muscles (see Fig 5.8). This synaptic pattern, called reciprocal inhibition, serves to coordinate muscles of opposing function around a joint. Secondary (type II) spindle afferent fibers also synapse with homonymous motor neurons, providing excitatory input through both monosynaptic and polysynaptic pathways. Golgi tendon organ input via type Ib axons has an inhibitory influence on homonymous motor neurons.

The myotatic reflex has two components: a phasic part, exemplified by tendon jerks, and a tonic part, thought to be important for maintaining posture. The phasic component is more familiar. These components blend together, but either one may predominate, depending on whether other synaptic activity, such as from cutaneous afferent neurons or pathways descending from higher centers, influences the motor response. Primary spindle afferent fibers probably mediate the tendon jerk, with secondary afferent fibers contributing mainly to the tonic phase of the reflex.

Dorsal root

Dorsal root

Reverse Myotatic Reflex

Myotatic reflex circuitry. Ia afferent axons ^BBBlllMII^Ffrom the muscle spindle make excitatory monosynaptic contact with homonymous motor neurons and with inhibitory interneurons that synapse on motor neurons of antagonist muscles. The plus sign indicates excitation, the minus sign indicates inhibition.

Myotatic reflex circuitry. Ia afferent axons ^BBBlllMII^Ffrom the muscle spindle make excitatory monosynaptic contact with homonymous motor neurons and with inhibitory interneurons that synapse on motor neurons of antagonist muscles. The plus sign indicates excitation, the minus sign indicates inhibition.

The myotatic reflex performs many functions. At the most general level, it produces rapid corrections of motor output in the moment-to-moment control of movement. It also forms the basis for postural reflexes, which maintain body position despite a varying range of loads and/or external forces on the body.

The Inverse Myotatic Reflex. The active contraction of a muscle also causes reflex inhibition of the contraction. This response is called the inverse myotatic reflex because it produces an effect that is opposite to that of the myotatic reflex. Active muscle contraction stimulates Golgi tendon organs, producing action potentials in the type Ib afferent axons. Those axons synapse on inhibitory interneurons that influence homonymous and heteronymous motor neurons and on excitatory interneurons that influence motor neurons of antagonists (Fig 5.9).

The function of the inverse myotatic reflex appears to be a tension feedback system that can adjust the strength of contraction during sustained activity. The inverse my-otatic reflex does not have the same function as reciprocal inhibition. Reciprocal inhibition acts primarily on the antagonist, while the inverse myotatic reflex acts on the agonist.

The inverse myotatic reflex, like the myotatic reflex, has a more potent influence on the physiological extensor muscles than on the flexor muscles, suggesting that the two reflexes act together to maintain optimal responses in the antigravity muscles during postural adjustments. Another hypothesis about the conjoint function is that both of these

Dorsal root ganglion cell ibJ2

Inverse Myotatic Reflex
Alpha motor neurons

^Inverse myotatic reflex circuitry. Contraction of the agonist muscle activates the Golgi tendon organ and Ib afferents, which synapse on interneurons that inhibit agonist motor neurons and excite the motor neurons of the antagonist muscle.

Homonymous Motor Neurons

reflexes contribute to the smooth generation of tension in muscle by regulating muscle stiffness.

The Flexor Withdrawal Reflex. Cutaneous stimulation— such as touch, pressure, heat, cold, or tissue damage—can elicit a flexor withdrawal reflex. This reflex consists of a contraction of flexors and a relaxation of extensors in the stimulated limb. The action may be accompanied by a contraction of the extensors on the contralateral side. The axons of cutaneous sensory receptors synapse on interneurons in the dorsal horn. Those interneurons act ipsilaterally to excite the motor neurons of flexor muscles and inhibit those of extensor muscles. Collaterals of interneurons cross the midline to excite contralateral extensor motor neurons and inhibit flexors (Fig. 5.10).

There are two types of flexor withdrawal reflexes: those that result from innocuous stimuli and those that result from potentially injurious stimulation. The first type produces a localized flexor response accompanied by slight or no limb withdrawal; the second type produces widespread flexor contraction throughout the limb and abrupt withdrawal. The function of the first type of reflex is less obvious, but may be a general mechanism for adjusting the movement of a body part when an obstacle is detected by cutaneous sensory input. The function of the second type is protection of the individual. The endangered body part is rapidly removed, and postural support of the opposite side is strengthened if needed (e.g., if the foot is being withdrawn).

Collectively, these reflexes provide for stability and postural support (the myotatic and inverse myotatic) and mo

^FGUREHR^k Flexor withdrawal reflex circuitry. Stimulation of cutaneous afferents activates ipsilateral flexor muscles via excitatory interneurons. Ipsilateral extensor motor neurons are inhibited. Contralateral extensor motor neuron activation provides postural support for withdrawal of the stimulated limb.

bility (flexor withdrawal). The reflexes provide a foundation of automatic responses on which more complicated voluntary movements are built.

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Responses

  • sebastian
    Where are ascending fibers in a reflex located?
    8 years ago
  • mustafa
    Why are cord mediated reflexes faster than others?
    8 years ago
  • mewael petros
    What is the withdrawal reflex when a hand touches something hot?
    8 years ago
  • donna
    What is the agonist and antagonist muscle in knee jerk reflex?
    8 years ago
  • marcel
    How can facilitation and inhibition the flexor withdrawal reflex?
    8 years ago
  • CRUZ
    What are some cordmediatef reflexes?
    8 years ago
  • annikki
    How interneurons function?
    8 years ago
  • CARADOC
    Do motor neurons inhibit or excite skeletal muscles?
    8 years ago
  • shukornia
    What skeleton muscle cells are the effectors of the withdrawal reflex?
    8 years ago
  • sarah loewe
    What is the minimum number of synapses mediating inverse myotatic reflex?
    8 years ago
  • eulalia
    Why interneuron inhibits motor neuron to flexor muscles?
    8 years ago
  • Ulpu Enckelman
    What is the type of reflex that is a quick contraction of the flexor muscle?
    8 years ago
  • FEAVEN
    Why is the myotactic reflex called a monosynaptic reflex/?
    8 years ago
  • rosalia
    What inhibits myotatic relexes?
    7 years ago
  • PATRICIA
    Which reflex activates muscles on the same side of the stimulus?
    7 years ago
  • SARAH
    What are the conclusions of muscular activity on reflex activity?
    7 months ago

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