A reflex is one involuntary (automatic) solution to stimulus that conveniently returns the body to homeostasis. Over there are several kinds of reflexes. Instances are shivering in an answer to low main point body temperature; or withdrawing your hand indigenous a hot stove when temperature and pain receptors in your hand register the stimulus.

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A reflex arc refers to the neural pathway that a nerve advertise follows. The reflex arc generally consists of five components:

A receptor, and also independent sensory cell, or an ending of a sensory neuron, reacts to a stimulus (e.g., a big receptor).The sensory, or afferent, neuron sends a nerve impulse through an afferent pathway to the central nervous system.An integration center consists of one or an ext synapses in the CNS (typically the spinal cord) whereby the incoming information and also outgoing solution are integrated.A motor, or efferent, neuron sends out a nerve impulse follow me an efferent pathway native the integration center to an effector cell.The effector cabinet responds come efferent impulses (for example, through contracting, if the effector is a muscle fiber).

Because integrating center processing may occur at the level that the spinal cord rather than inquiry impulses to travel to the brain, reflex responses have a fairly short path length and, thus, a rapid reaction time contrasted to voluntary or mindful body movements.


Reflexes need a minimum of 2 neurons, a sensory neuron (input) and also a motor neuron (output) (see number 1). The sensory neuron detects stimuli and also sends a signal towards the CNS. The sensory neuron synapses v a engine neuron, which innervates one effector tissue, such as skeletal muscle in order to pull away from painful stimuli. This kind of reflex is a “withdrawal” reflex and is monosynaptic, definition only one synapse has to be crossed between the sensory neuron and the engine neuron. This is the simplest reflex arc, and also the integrating facility is in the spinal cord. Polysynaptic reflexes are much more complex, however also much more common. Castle involve interneurons, discovered in the CNS, i beg your pardon further process stimulus and output information. Beyond straightforward reflexes v integrating centers in the spinal cord, more facility reflexes have actually integration centers in the brainstem or also in the cerebrum.

A stretch reflex is a kind of muscle reflex, i m sorry protects muscle against increases in length that have the right to tear and damage muscle fibers. The major purpose that the patellar reflex – the stretch reflex the the quadriceps femoris muscle – is come prevent extreme stretching of the quadriceps. The patellar reflex is portrayed in number 1.

Figure 1. The monosynaptic patellar reflex. A tap to the patellar tendon follow me the quadriceps muscle (1) leading to activation the the muscle spindle (2). The afferent neuron the the muscle spindle, detecting stretch, sends a signal come the spinal cord (3) and also synapses straight with a engine neuron (4) that causes the quadriceps muscle to contract (5). In parallel, an inhibitory blog post is sent out via one interneuron come the hamstrings (6, 7), resulting in hamstrings relaxation (8). The reflex in ~ the hamstrings is polysynaptic. This work-related by Cenveo is licensed under an imaginative Commons Attribution 3.0 United says https://creativecommons.org/licenses/by/3.0/us/ .

The patellar tendon attaches the quadriceps muscle to the tibial tuberosity of the lower leg. The quadriceps is an extensor muscle: once it contracts it extends the edge of the knee share by elevating the lower leg indigenous a bend position. Tapping the patellar tendon pulls and also stretches the quadriceps muscle and also causes the sensory receptor of the muscle, referred to as a muscle spindle fiber, come send a signal along the afferent neuron to the spinal cord (Figure 2). This reasons the efferent neuron to return a signal to the quadriceps muscle come contract and also lift the lower leg. This activity resists the initial stretch and is a classic example of an adverse feedback.

Figure 2. A muscle spindle. loved one motor and sensory innervation presented for the intrafusal fibers within the spindle. Keep in mind the relative placement the extrafusal (contractile) fibers approximately the spindle. This work-related is license is granted under creative Commons Attribution united state 1.0. Https://creativecommons.org/licenses/by/1.0/

To far better consider the events that result in the “sensation” the muscle stretch, we must very first define muscle fiber types. Muscle spindles, sensory structures which save intrafusal muscle fibers, room innervated by sensory neurons and are arranged in parallel to common (contractile), extrafusal, muscle fibers. Intrafusal fibers room not contractile where they associate v the sensory neuron. Intrafusal fibers respond to tension in the tissue by depolarizing a sensory neuron. The sensory neuron synapses with a motor neuron in the spinal cord the innervates contractile extrafusal fibers. The convulsion of the extrafusal fibers, that is, contraction of the belly of the muscle, releases stress on the intrafusal fibers, to decrease stimulation come neuron.

In the case of the monosynaptic knee-jerk reflex, hitting the patellar tendon v a mallet stretches the intrafusal yarn of the spindles in the quadriceps muscle, leading to contraction. In parallel, one inhibitory advertise is sent from the spinal cord to cause relaxation of the hamstring muscles, via a polysynaptic pathway.

It is important to keep in mind that, also with the easiest of reflexes, there are multiple inhibitory and also stimulatory influences that can affect the excitability the the motor neuron. These can amplify or suppress a reflex response and may somewhat differ from subject to subject. Lesions that damage the sensory or motor fibers, or damage to the spinal cord, typically diminish a reflex uneven the spinal cord has been totally transected. In the latter case, spinal cord damage or damage to “upper engine neurons,” engine neurons in the brain, may at some point lead to overly reactive reflexes (hyperreflexia).

Additionally, neural task at other sites in the body may affect a reflex arc. Facilitation (reinforcement) may enhance the loved one strength (relative amplitude) and/or rate (reaction time, in milliseconds) of a reflex solution due to maximal isometric contraction of muscle in a remote component of the body – because that example, through clenching the jaw or locking the fingers of the two hands and pulling (the Jendrassik maneuver). The Jendrassik practice (JM) is a special technique for reinforcement the is used in the clinical setup when the is challenging to begin a reflex in a patient. The JM most likely amplifies reflexes by diminish inhibitory signal to the alpha engine neurons that are responsible for muscle convulsion at the neuromuscular junction (NMJ) (Passmore & Bruno 2012). Sometimes, though, external of the clinical setting, experimental outcomes with the JM exercise can result in no result or inhibition that a reflex. various results when implementing the JM might be due to differences in the physiology of healthy speculative subjects versus patients (in the clinical setting) who space being assessed because that nerve problems.

Reflex testing is of clinical value. Experimentation of the patellar response indicates:

The relative health and wellness of the muscle spindle, afferent (sensory) and also efferent (motor) neurons, neuromuscular junctions, and also the extrafusal (contractile) muscle fibers.An appropriate balance the excitatory and inhibitory regulation native the main nervous system.The verity of the L2-L4 vertebral segment of the spinal cord.

Tests for straightforward muscle reflexes, such as the patellar reflex, are basic to any type of physical exam once motor nerve or spinal damage is suspected. The tests help to situate neural damage: engine nerves synapsing in the spinal cord over the damages site aren’t affected, but nerves that originate in ~ or below the injury will many often produce abnormal reflexes. To recognize this, medical professionals may also test large reflexes in the triceps muscle and the Achilles tendon (ankle-jerk reflex) and also compare results.

The Achilles reflex is a monosynaptic stretch reflex comparable to the patellar reflex. In the Achilles reflex, the hammer taps the Achilles tendon while the foot is dorsiflexed, and the foot, in response, must jerk toward the plantar surface. The Achilles reflex originates in the S1 and also S2 nerve roots. Once comparing reflexes from different sites that the body, the locations of the corresponding nerve roots follow me the spinal cord should be considered in order come determine possible sites the injuries and differences in reflex route lengths.

This Week’s laboratory Technique: surface Electromyography

In this lab, students will document electromyograms (EMGs) for the fibers in a muscle, and also use them to recognize the time between the stretch the a tendon and the arrival of a motor impulse at the muscle. Surface electromyography (EMG) actions the electrical task in a muscle by placing recording electrodes on the skin over the muscle. Come be much more precise, one EMG actions fluctuations in the electrical activity of muscles as result of muscle cell activity potentials.

The EMG produces an electromyogram, which documents both the relative amplitude (relative strength) and also timing of muscle contractions. The EMG display screens the electric potentials produced by the muscle cells on a computer screen together a collection of peaks and also troughs specifying the EMG tide (Fig. 3, below).

Figure 3. regular EMG recording because that a stretch reflex in LabScribe.

In this lab, 2 monosynaptic reflexes in a human subject will certainly be studied: the Achilles tendon reflex, and also the patellar tendon (knee-jerk) reflex. Average reflex reaction times, the it was observed time because that the reflex solution measured in seconds, and also reflex conduction velocities, the velocity of infection in the nerve measured in meters/second, because that each reflex arc will be determined and also compared. The impact of pre-existing tension in the effector muscle, or motor activity in other muscle groups, top top reflex responses will be measured. The coordination the motor activity in antagonistic muscles may additionally be studied.

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Some measurements we will certainly make in the lab experiments:

Response time, path length, and also conduction velocity that the patellar and Achilles reflexes.Voluntary vs reflex muscle activity reaction times.Reflex amplitude (strength) and speed with and also without reflex reinforcement.

Can friend think of some hypotheses for the experiment in this week’s laboratory?