Achilles reflex which muscles contract

Vestibulospinal tracts. The two vestibulospinal tracts originate in 2 of the 4 vestibular nuclei Figure 2. The lateral vestibulospinal tract originates in the lateral vestibular nucleus.

It courses through the brainstem and through the anterior funiculus of the spinal cord on the ipsilateral side, before exiting ipsilaterally at all levels of the spinal cord. The medial vestibulospinal tract originates in the medial vestibular nucleus , splits immediately and courses bilaterally through the brainstem via the medial longitudinal fasciculus MLF and through the anterior funiculus of the spinal cord, before exiting at or above the T6 vertebra.

The vestibulospinal tracts mediate postural adjustments and head movements. They also help the body to maintain balance. Small movements of the body are detected by the vestibular sensory neurons, and motor commands to counteract these movements are sent through the vestibulospinal tracts to appropriate muscle groups throughout the body.

The lateral vestibulospinal tract excites antigravity muscles in order to exert control over postural changes necessary to compensate for tilts and movements of the body.

The medial vestibulospinal tract innervates neck muscles in order to stabilize head position as one moves around the world. It is also important for the coordination of head and eye movements. Click on the labels to see the highlighted area. Reticulospinal tracts. The two reticulospinal tracts originate in the brainstem reticular formation , a large, diffusely organized collection of neurons in the pons and medulla Figure 2.

The pontine reticulospinal tract originates in the pontine reticular formation , courses ipsilaterally through the medial longitudinal fasciculus and through the anterior funiculus of the spinal cord, and exits ipsilaterally at all spinal levels.

The medullary reticulospinal tract originates in the medullary reticular formation , courses mainly ipsilaterally although some fibers cross the midline through the anterior funiculus of the spinal cord, and exits at all spinal levels. The reticulospinal tracts are a major alternative to the corticospinal tract, by which cortical neurons can control motor function by their inputs onto reticular neurons. These tracts regulate the sensitivity of flexor responses to ensure that only noxious stimuli elicit the responses.

Damage to the reticulospinal tract can thus cause harmless stimuli, such as gentle touches, to elicit a flexor reflex. The reticular formation also contains circuitry for many complex actions, such as orienting, stretching, and maintaining a complex posture. Commands that initiate locomotor circuits in the spinal cord are also thought to be transmitted through the medullary reticulospinal tract.

Thus, the reticulospinal tracts are involved in many aspects of motor control, including the integration of sensory input to guide motor output. Tectospinal tract.

The tectospinal tract Figure 2. It then courses through the pons and medulla, just anterior to the medial longitudinal fasciculus. It courses through the anterior funiculus of the spinal cord, where the majority of the fibers terminate in the upper cervical levels.

Little is known about the function of the tectospinal tract, but because of the nature of the visual response properties of neurons in the superior colliculus the optic tectum , it is presumably involved in the reflexive turning of the head to orient to visual stimuli.

Voluntary movement. The most distinctive function of the descending motor pathways is the control of voluntary movement. These movements are initiated in the cerebral cortex, and the motor commands are transmitted to the musculature through a variety of descending pathways, including the corticospinal tract, the rubrospinal tract, and reticulospinal tracts.

How voluntary movements are initiated and coordinated by the motor cortex is the subject of the next chapter. Reflex modulation. Another critical function of the descending motor pathways is to modulate the reflex circuits in the spinal cord.

The adaptiveness of spinal reflexes can change depending on the behavioral context; sometimes the gain strength or even the sign extension vs. The descending pathways are responsible for controlling these variables. For example, consider the flexor reflex under two conditions. Gamma bias. Recall from the previous chapter that there are two types of spinal motor neurons.

Alpha motor neurons innervate extrafusal muscle fibers , which provide the force for a muscle contraction. Gamma motor neurons innervate the ends of intrafusal fibers and help to maintain the tautness of muscle spindles , such that they are sensitive to changes of muscle length over a wide range. In order to work adaptively, the activity of alpha and gamma motor neurons must be coordinated.

Thus, whenever motor commands are sent by descending pathways to alpha motor neurons, the appropriate compensating commands are sent to gamma motor neurons. This coordination of alpha-gamma motor commands is called alpha-gamma coactivation , and the adjustment of spindle sensitivity by gamma activation is called gamma bias. Consider the following two examples:. Plays a major role in the fine control of distal musculature. Arises exclusively from the primary motor cortex.

Is an uncrossed pathway. Terminates primarily in the posterior dorsal horn. In this lab, students will record electromyograms EMGs for the fibers in a muscle, and use them to determine the time between the stretch of a tendon and the arrival of a motor impulse at the muscle.

Surface electromyography EMG measures the electrical activity in a muscle by placing recording electrodes on the skin over the muscle. To be more precise, an EMG measures fluctuations in the electrical activity of muscles due to muscle cell action potentials.

The EMG produces an electromyogram, which records both the relative amplitude relative strength and timing of muscle contractions. The EMG displays the electrical potentials generated by the muscle cells on a computer screen as a series of peaks and troughs defining the EMG wave Fig. In this lab, two monosynaptic reflexes in a human subject will be studied: the Achilles tendon reflex, and the patellar tendon knee-jerk reflex.

The effect of pre-existing tension in the effector muscle, or motor activity in other muscle groups, upon reflex responses will be measured. The coordination of motor activity in antagonistic muscles may also be studied. Stretch reflexes are protective reflexes that ensue to avoid damage due to over-stretching a muscle.

In this lab, students will determine the response time, conduction velocity speed , and amplitude strength of two stretch reflexes: the Achilles reflex at the ankle and the patellar knee-jerk reflex. The velocity of a reflex informs us about the health of the receptors, neurons, and muscles involved in a reflex and can help to diagnose neuromuscular damage or disease. Conduction velocity cannot be measured directly and must be calculated using path length and reaction time data.

Objective: To determine the conduction velocity of the Achilles tendon reflex arc. For the purpose of this exercise, assume that the sensory-motor synapse is at spinal segment S1, which is near the iliac spine, or the upper rim, of the hip bone.

Multiply this measurement by 2 to determine the total length of the reflex path. Note the result in meters or mm. Objective: To determine the amplitude and conduction velocity of the patellar tendon reflex arc under different conditions. The withdrawal reflex nociceptive or flexor withdrawal reflex is a spinal reflex intended to protect the body from damaging stimuli. It is polysynaptic, and causes the stimulation of sensory, association, and motor neurons.

When a person touches a hot object and withdraws his hand from it without thinking about it, the heat stimulates temperature and danger receptors in the skin, triggering a sensory impulse that travels to the central nervous system. The sensory neuron then synapses with interneurons that connect to motor neurons. Some of these send motor impulses to the flexors to allow withdrawal. Some motor neurons send inhibitory impulses to the extensors so flexion is not inhibited—this is referred to as reciprocal innervation.

Although this is a reflex, there are two interesting aspects to it:. Golgi tendon organ : The Golgi tendon organ, responsible for the Golgi tendon reflex, is diagrammed with its typical position in a muscle left , neuronal connections in spinal cord middle , and expanded schematic right. The tendon organ is a stretch receptor that signals the amount of force on the muscle and protects the muscle from excessively heavy loads by causing the muscle to relax and drop the load.

Privacy Policy. Skip to main content. Peripheral Nervous System. Search for:. Components of a Reflex Arc A reflex arc defines the pathway by which a reflex travels—from the stimulus to sensory neuron to motor neuron to reflex muscle movement.

Learning Objectives Describe the components of a reflex arc. Key Takeaways Key Points Reflexes, or reflex actions, are involuntary, almost instantaneous movements in response to a specific stimulus. Reflex arcs that contain only two neurons, a sensory and a motor neuron, are considered monosynaptic.

Examples of monosynaptic reflex arcs in humans include the patellar reflex and the Achilles reflex. Most reflex arcs are polysynaptic, meaning multiple interneurons also called relay neurons interface between the sensory and motor neurons in the reflex pathway.

Key Terms motor neuron : A neuron located in the central nervous system that projects its axon outside the CNS and directly or indirectly control muscles. The absence of or a slight Achilles reflex reaction may indicate a medical condition, e.

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