Spinal Reflexes

Spinal Reflexes

Yes, the white matterThe outer portion of the spinal cord made of myelinated nerve fibers that transmit signals. of the spinal cordThe central nervous system structure that relays signals between the brain and body. sends action potentials to the brain. It also sends them from the brain. But, the spinal cord makes a lot of decisions for you all the time.  These reactions to stimuliChanges in the environment that are detected by sensory receptors. are called reflexesAutomatic responses to stimuli..  These are spinal reflexes because neuronsThe functional cells of the nervous system that transmit signals. only in the spinal cord participate.

This diagram kinda puts together many of the things we’ve been talking about and drawing.  This blue neuron is a unipolar sensory neuron with its cell bodyThe central part of a neuron containing the nucleus and organelles. in the dorsalRelating to the back side of the body. root ganglia.  Look at how it synapses in gray matterThe inner portion of the spinal cord composed mostly of neuron cell bodies and synapses. of the dorsal horn with a green neuron.  This green neuron is an interneuron that exists only within the CNSComposed of the brain and spinal cord; integrates and processes information. .  It is this interneuron that makes the decision that you are burning your hand. You need to withdraw said hand from the hot and painful stimulus.  Stimulus felt by nociceptorsPain receptors that respond to tissue damage or potentially harmful stimuli. and thermoreceptorsSensory receptors that respond to temperature changes. in your somatic tissues.  This interneuron synapses with this orange neuron. This orange neuron is a somatic motor neuron. It will go all the way to the neuromuscular junctions in your hand muscles. You know each of these components gets a specific name when we talk about spinal reflexes. Officially there are five parts of a spinal reflex. There is a receptorA structure that detects stimuli., such as the nociceptors in our example. There is a sensory neuron, the blue unipolar neuronA neuron with one process that splits into peripheral and central branches, common in sensory neuron in our example. There is an interneuron, the green neuron encased fully in the gray matter of the spinal cord. Then there is an efferent neuron carrying motor information, which is our orange neuron in this example. Finally, there is an effector. Defector is a word that we heard back when we talked about negative feedbackA control mechanism that reverses a change in the body to maintain homeostasis.. It is the effector that reverses the stimulus. In our example, skeletal muscle of the hand can move the hand away from the hot cup. This action stops the stimulus.

Classification

We just looked at a polysynaptic spinal reflex. Polysynaptic spinal reflexes have at least two synapses and at least 3 neurons. There are simpler spinal reflexes which are referred to as monosynaptic these have only two neurons and one synapseThe junction between two neurons where communication occurs.. The example that we used is also an example of an ipsilateralReferring to the same side of the body. spinal reflex. All of the neurons are located on only one side of the spinal cord. There is no communication to the other side. In contrast to that, there are contralateralReferring to the opposite side of the body. spinal reflexes. In these reflexes, white matter decusses across one of the commissures of the spinal cord. This brings information to the other side of the body.

Withdraw Reflex

The withdrawal reflex is a quite simple reflex. It allows you to withdraw yourself from a painful noxious or just simply damaging stimulus. The withdrawal reflex is mono synaptic with only one sensory and one motor neuron. The sensory neuron detects the stimulus or threat. It sends an action potentialA rapid, temporary electrical charge that travels along neurons, allowing signal transmission. into the Gray matter of the spinal segment associated with the threat. This sensory neuron then synapses directly with interneuronsNeurons that process information and connect sensory and motor neurons, found only in the CNS. encase within the gray matter. 

Motor neurons will come out of the spinal segment. They innervateTo invade a tissue with nervous tissue is to innervate it. the flexors that can help you withdraw from the stimulus, such as the biceps brachiiFlexor / Supinator Front of upper arm; bends elbow and turns palm upward. pictured here. This reflex is named for the withdraw action that flexors can create. This reflex can save your life. It allows your body to withdraw from a harmful stimulus. You do not have to consciously register the stimulus.  The flexors are contracting at the same time. Another multipolar neuronA neuron with one axon and multiple dendrites; the most common type in the CNS. inhibits the extensors, such as the triceps brachiiExtensor Back of upper arm; straightens the elbow here.  This inhibition guarantees the strongest and quickest withdraw.

There is one more modification to a withdraw reflexA reflex that moves a limb away from a painful stimulus. that makes it different than the extensors reflex.  Information about the event is carried to the brain via a third interneuron.  This makes you consciously aware of this stimulus.

Stretch (Extensor) Reflex

One of the most common extensor reflexes that is tested is called the patellar reflexThe knee-jerk reflex.. There is a tiny sensory structure near here to tibial tuberosityRough bump below kneecap on anterior surface for patellar ligament attachment. that contains what is called a golgi tendon receptor. When stimulated with a reflex hammer, this receptor sends the sensation via a unipolar sensory neuron. The sensation is sent to the spinal cord, where it synapses with interneurons.  These interneuron process the incoming information and determine an output.  These interneurons then synapse with the multipolar motor neuronsNeurons that carry outgoing information from the CNS to effector organs. that will leave the spinal cord via the spinal nerve.

The motor neurons innervate the muscle attached to the tendon. The tendon contains the golgi apparatus. This setup activates the rectus femorisExtensor / Flexor Front of thigh; straightens the knee and helps lift the leg at the hip., which is a knee extender. The rectus femoris contraction allows the knee to extend outward. At the same time, the knee flexors on the posterior side of the thigh are inhibited. Muscles such as the semitendinosis and semimembranosusExtensor / Flexor Deep to semitendinosus; extends hip and flexes knee on the inner thigh experience this inhibition. This reflex is kind of like if you wanted to kick somebody in the shins as they attack you.

Cross Extensor Reflex

The cross extensor reflex is a reflex that has both an ipsilateral and a contralateral response. It controlsVariables that remain constant to ensure a fair test. muscles on both sides of the body.  This reflex doesn’t just activate muscles, it also inhibits muscles too.  And, finally, we are going to discover that this is definitely a polysynaptic reflex with lots of neurons.

Let’s start with this sensory neuron in the right arm over here that has a tactile receptorsProteins located on the surface or inside cells that bind specific molecules (e.g., neurotransmitter on it.  It senses something in the outside environment. It generates an action potential. It sends information through a unipolar neuron to the spinal cord.  For instance, suppose your spouse is going into the crawl space to find a suspected rabid raccoon. Not that this has ever happened to my spouse.  Your spouse reaches out with their right hand and is promptly bitten by said raccoon.  Right away, your spouse pulls their right arm away from the rabid raccoon.  That’s an ipsilateral response because the stimulus was on the right side. That is also that life-saving withdraw reflex. Your spouse extends their left arm backward to brace themself as they fall. It is too late. As your spouse falls backward, they twist their outstretched left wrist.  Now, your spouse not only has rabies, but also has a broken wrist.  But, the outstretched left arm was a contralateral response. It involved the left arm. The stimulus was on the right or the opposite side. 

Now that we have the contralateral and ipsilateral aspects of this reflex established, let’s look at how it activates muscles. Let’s also examine how it inhibits muscles.  On the side ipsilateral to the stimulus, we are going to have a withdraw reflex. In our example, this is the right side. Your spouse is pulling their arm away from the rabid raccoon.  In the ipsilateral arm, we’d want to activate flexors and inhibit extensors.  Since this is an arm, that means that we will activate the biceps brachii.  Let’s draw a neuron for that.  We also want to inhibit the triceps brachii which is the main extensor of the arm.  We need another neuron for that.  By inhibiting the extensors, we get a more powerful withdraw. 

On the contralateral side to the stimulus, we are going to activate the extensors. This is on the left side in our example. We will also inhibit the flexors.  We’re still dealing with the same 2 muscles of the arm. The biceps brachii is the flexor. The triceps brachii is the extensor. A neuron comes from the spinal cord. It activates the extensors for the triceps brachii eye. It inhibits the flexors, which is the biceps brachii. By doing this, the left arm can reach out to the back. It stabilizes your body as you withdraw from the stimulus. We’ve done this example with arms. Now, I want you to think about what might happen if you stepped on a piece of glass. You would withdraw the foot that stepped on the glass. You would also activate the extensors on the opposite leg. This braces you from falling down. My spouse reaching out with their left arm was a reflex trying to stabilize them from falling.  This is why he broke his left wrist. 

This reflex is quite complicated. That is why it is used as an example here. It is also included in the quiz associated with this material.  This reflex addresses the terms contralateral and ipsilateral.  This reflex addresses activation and inhibition.  And, finally, this reflex is polysynaptic involving a lot more than two neurons.