Reflex Arc & Reflexes Lab Section 1: What is a reflex? Quick answer



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Reflex Arc & Reflexes Lab
Section 1: What is a reflex?
Quick answer:  a quick movement that occurs in response to something before you even realize you are moving.

Alternative answer:  a local motor response to a local sensation.

    When the doctor taps on your knee, you respond by kicking your leg.  This occurs quickly, before you even realize that you did it.  Another reflex is pulling your hand away from a painful or hot or freezing item quickly, before realizing you did it.   Another is blinking when something touches your eye.  And gagging when you something is placed down your throat (like the doctor telling you to say "ah" while he/she shoves a popsicle stick into the back of your mouth).  All of these reflexes are motor responses to sensory information... where the movement is of the same body region that sensed the stimulus.



    You will see that these reflexes occur because the sensory neurons that bring the stimulus information into the spinal cord synapse directly or nearly directly onto motor neurons located in that area of the spinal cord that the sensory neurons entered. 



Reflex arcs have five essential components:

1. The receptor at the end of a sensory neuron reacts to a stimulus. For example, some receptors in

the skin are sensitive to heat, others to pressure.

2. The sensory neuron conducts nerve impulses along an afferent pathway towards the CNS.

3. The integration center consists of one or more synapses in the CNS.

4. A motor neuron conducts a nerve impulse along an efferent pathway from the integration center to

an effector.

5. An effector responds to the efferent impulses by contracting (if the effector is a muscle fiber) or

secreting a product (if the effector is a gland).
Section 2: Types of Reflexes
Reflexes can be categorized as either autonomic or somatic. Autonomic reflexes are not subject to conscious control, are mediated by the autonomic division of the nervous system, and usually involve the activation of smooth muscle, cardiac muscle, and glands. Somatic reflexes involve stimulation of skeletal muscles by the somatic division of the nervous system.

    Reflexes can also be categorized according to the number of synapses they make. If the sensory neuron (SN) comes in and directly synapses on the motor neuron (MN) that is called a monosynaptic reflex.  Monosynaptic means "one synapse".  So, in the monosynaptic reflex, there is only one synapse that occurs in the spinal cord:  SN -----<  MN (the line with the "<" represents an axon with axon terminals).  This reflex is quick; it only takes as long to respond as it takes for the sensory neuron to carry the information to the spinal cord, cross the synapse to the motor neuron, run back out the motor neuron axon to the muscle, and cause contraction.  Not even one second is necessary.

    A disynaptic reflex requires that one interneuron be interposed between the sensory neuron and motor neuron (SN -----<  IN -----<  MN).  A polysynaptic reflex has more than two synapses in the spinal cord (SN -----<  IN -----<   IN -----<  MN).  The disynaptic and polysynaptic reflexes are the most common. 

Section 3: Importance of Reflex Testing

Reflex testing is an important diagnostic tool for assessing the condition of the nervous system. Distorted, exaggerated, or absent reflex responses may indicate degeneration or pathology of portions of the nervous system, often before other signs are apparent.

If the spinal cord is damaged, then reflex tests can help determine the area of injury. For example, motor nerves above an injured area may be unaffected, whereas motor nerves at or below the damaged area may be unable to perform the usual reflex activities.

Closed head injuries, such as bleeding in or around the brain, may be diagnosed by reflex testing. For example, the oculomotor neuron stimulates the muscles in and around the eyes. If the pressure increases in the cranium (such as from an increase in blood volume due to brain bleeding), then the pressure exerted on the Cranial Nerve III may cause variations in the eye reflex responses.



Section 4: Testing Stretch Reflexes

The primary tool used to test reflex activity is the Taylor Reflex Hammer. Care must be taken to use the proper hammer technique during our lab activities. Improper techniques will not elicit the desired reflexes. The tap stretches a muscle, which stimulates stretch receptors located in the muscle. In response to the increased stretch, which normally would only occur when the muscle load has suddenly increased, the muscle contracts. In normal circumstances the stretch reflex allows muscles to reflexively increase the strength of contraction in response to increased load.





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