Spinal Reflexes Are Strongly Influenced By Control Centers Within The Brain
Axons descend from numerous centers within the brainstem and the cerebral cortex and synapse primarily on the spinal interneurons, with some direct input to the motor neurons. This descending control is essential for all conscious command of movement, a topic beyond the scope of this chapter. Less obvious is that the descending pathways can alter the strength of reflexes. For example, to heighten an anxious patient’s stretch reflexes, a neurologist will sometimes ask the patient to perform the Jendrassik maneuver. The patient clasps his or her hands together and pulls while the patient is distracted with that task, the examiner tests the stretch reflexes of the leg. Another example of the brain’s modulation of a stretch reflex occurs when you catch a falling ball. If a ball were to fall unexpectedly from the sky and hit your outstretched hand, the force applied to your arm would cause a rapid stretch reflexcontraction in the stretched muscles and reciprocal inhibition in the antagonist muscles. The result would be that your hand would slap the ball back up into the air. However, if you anticipate catching the falling ball, for a short period around the time of impact , both your stretched muscles and the antagonist muscles contract! This maneuver stiffens your arm just when you need to squeeze that ball to avoid dropping it. Stretch reflexes of the leg also vary dramatically during each step as we walk, thereby facilitating movement of the legs.
Balance In Competing Autonomic Reflex Arcs
The autonomic nervous system is important for homeostasis because its two divisions compete at the target effector. The balance of homeostasis is attributable to the competing inputs from the sympathetic and parasympathetic divisions . At the level of the target effector, the signal of which system is sending the message is strictly chemical. A signaling molecule binds to a receptor that causes changes in the target cell, which in turn causes the tissue or organ to respond to the changing conditions of the body.
After Studying This Chapter You Will Be Able To
- Describe the components of the somatic nervous system
- Name the modalities and submodalities of the sensory systems
- Distinguish between general and special senses
- Describe regions of the central nervous system that contribute to somatic functions
- Explain the stimulus-response motor pathway
The basic withdrawal reflex explained above includes sensory input , central processing , and motor output . Expanding the explanation of the withdrawal reflex can include inhibition of the opposing muscle, or cross extension, either of which increase the complexity of the example by involving more central neurons. A collateral branch of the sensory axon would inhibit another ventral horn motor neuron so that the triceps brachii do not contract and slow the withdrawal down. The cross extensor reflex provides a counterbalancing movement on the other side of the body, which requires another collateral of the sensory axon to activate contraction of the extensor muscles in the contralateral limb.
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Conflict Of Interest Statement
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
The authors sincerely thank and recognize Miss. Jenna Cooper, a student at Vernon Secondary School, for her art work contributions. The authors thank Mr. Jakob Jones, a student at École Beairsto Elementary, and Mrs. Heather Jakobi, Vice Principle Arthur Voaden Secondary School, for comments on an earlier draft.
Perenboom, M. J. L., Van de Ruit, M., De Groot, J. H., Schouten, A. C., and Meskers, C. G. M. 2015. Evidence for sustained cortical involvement in peripheral stretch reflex during the full long latency reflex period. Neurosci. Lett. 584:2148. doi:10.1016/j.neulet.2014.10.034
What Is The Difference Between Reaction And Reflex
Reaction and reflex are two pathways that our nervous system executes in response to stimuli. The reaction is comparatively a slow action that goes via the brain. But, the reflex is a fast action that does not involve the brain. Thus, this is a key difference between reaction and reflex. Furthermore, compared to a reaction, the reflex is an instantaneous action which minimizes the harm.
Moreover, the reaction is a voluntary response while reflex is an involuntary response. Hence, this is another difference between reaction and reflex. The below infographic shows more details on the difference between reaction and reflex.
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Receptors That Provoke The Reflex
The physiology offers an answer as to why these reflexes happen. Well, proprioceptors are located in the muscles, tendons, and ligaments. These specific receptors detect information about the length of the muscle, the tension of the ligaments, the level of the tendon stretching, etc. Specifically, within the skeletal muscles, the receptors are known as muscle spindles, within the joints are the Ruffini corpuscles and free nerve ends, whereas in the ligaments and tendons are the Golgi tendon organs.
All receptors mentioned above inform the central nervous system about the position of the limbs, the strength and the speed of the muscle contraction, and feedback information necessary for the control of movements. The impression that the somatosensory cortex gets after receiving all of this information is actually the awareness of the bodys position in space, which is called kinesthesia.
Spinal Cord And Brain Stem
A sensory pathway that carries peripheral sensations to the brain is referred to as an ascending pathway, or ascending tract. The various sensory modalities each follow specific pathways through the CNS. Tactile and other somatosensory stimuli activate receptors in the skin, muscles, tendons, and joints throughout the entire body. However, the somatosensory pathways are divided into two separate systems on the basis of the location of the receptor neurons. Somatosensory stimuli from below the neck pass along the sensory pathways of the spinal cord, whereas somatosensory stimuli from the head and neck travel through the cranial nervesspecifically, the trigeminal system.
The dorsal column system and the spinothalamic tract are two major pathways that bring sensory information to the brain . The sensory pathways in each of these systems are composed of three successive neurons.
Ascending Sensory Pathways of the Spinal CordFigure 1. The dorsal column system and spinothalamic tract are the major ascending pathways that connect the periphery with the brain.
The sensory pathway for gustation travels along the facial and glossopharyngeal cranial nerves, which synapse with neurons of the solitary nucleus in the brain stem. Axons from the solitary nucleus then project to the ventral posterior nucleus of the thalamus. Finally, axons from the ventral posterior nucleus project to the gustatory cortex of the cerebral cortex, where taste is processed and consciously perceived.
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Reflex Action To Stimuli
Reflexes defend the body before you even realise the problem on a conscious level. They happen faster than a reaction to stimuli. For instance, if you dont know something is hot and touch it, a reflex action kicks in, and you withdraw your hand. A doctor taps your knee with a hammer, and it jerks upwards. You didnt decide to do that movement it was a reflex action.
What Is Reflex Action
Sometimes our bodies or parts of our body go through involuntary movements as a result of a particular stimulus. These involuntary movements are not performed consciously, as our bodies undergo these reactions as a result of some action.
These involuntary actions are known as reflex actions. In other words, reflex action can be defined as the entire process of an involuntary response to peripheral nerve stimulation, that is, a response without any thought or conscious effort that involves a part of the central nervous system.
What Happens in a Reflex Action?
A good example of reflex action would be the constriction of pupils when light is flashed on the eyes. This is called the pupillary light reflex, where light is the stimulus. As the lights are flashed, the brain receives impulses via the optic nerve, and the response is sent to the pupillary musculature through automatic nerves that supply the eye.
Reflexes are significant because they protect our body parts, especially the sense organs. They protect us from sudden stimuli in the environment that might be harmful to us. Reflex actions thus take care of the survival of organisms.
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The Structure Of Reflexes
One difference between a somatic reflex, such as the withdrawal reflex, and a visceral reflex, which is an autonomic reflex, is in the efferent branch. The output of a somatic reflex is the lower motor neuron in the ventral horn of the spinal cord that projects directly to a skeletal muscle to cause its contraction. The output of a visceral reflex is a two-step pathway starting with the preganglionic fiber emerging from a lateral horn neuron in the spinal cord, or a cranial nucleus neuron in the brain stem, to a ganglionfollowed by the postganglionic fiber projecting to a target effector. The other part of a reflex, the afferent branch, is often the same between the two systems. Sensory neurons receiving input from the peripherywith cell bodies in the sensory ganglia, either of a cranial nerve or a dorsal root ganglion adjacent to the spinal cordproject into the CNS to initiate the reflex . The Latin root effere means to carry. Adding the prefix ef- suggests the meaning to carry away, whereas adding the prefix af- suggests to carry toward or inward.
Comparison of Somatic and Visceral ReflexesFigure 1.
Your Body’s Natural And Automatic Reflexes
A reflex is an involuntary movement to a stimulus. It is a relatively simple way your body relays information that never reaches conscious awareness. Most of us take a lot of what the body does for us for granted, and thats a good thing. It would be extremely difficult to have to plan and actively execute every tiny movement we make.
As you read this, subtle readjustments are constantly being made between the muscles of your spine and torso to keep you in balance. Your eyes make tiny readjustments for every shift of your head. Your pupils dilate appropriately to adjust to the level of light and to focus on whats in front of you. When you swallow, your throat automatically closes off your airway to prevent saliva from going down the wrong tube. Each breath you take automatically readjusts to provide the right balance of oxygen and carbon dioxide in your blood, as well as taking deeper breaths now and then to keep the lungs open.
These are just a few examples of the automatic responses that keep us functioning every day. Most of the functions that are critical for life are outside of our conscious control. Instead, these functions are governed by reflexes.
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Reflexes Originating From The Lung And Chest Wall
Various reflexes that arise in the rib cage influence the intercostal and phrenic motoneurons. These reflexes are of potential importance in newborns, whose rib cage is compliant and therefore prone to distortion during REM sleep. Rib cage distortion is associated with breathing pattern changes, including decreases in inspiratory time and tidal breathing, prolongation of expiratory time, irregular breathing, and even apnea.310,311
Homayoun Kazemi, Douglas C. Johnson, in, 2002
Events That Precede A Stimulus In Organisms
- The first event begins with the receptor detecting a stimulus from a sensory organ. The stimulus could be in the form of pressure, temperature or chemicals.
- This is followed by the sensory neuron sending a signal to the relay neuron.
- The relay neuron then sends the signal to the motor neuron.
- The motor neuron sends a signal to the organ or a cell that acts to the stimuli, called the effector.
- Finally, the effector produces an instantaneous response, such as a knee-jerk reaction.
Check the video provided below for further reference.
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Chemical Signaling In The Autonomic Nervous System
Where an autonomic neuron connects with a target, there is a synapse. The electrical signal of the action potential causes the release of a signaling molecule, which will bind to receptor proteins on the target cell. Synapses of the autonomic system are classified as either cholinergic, meaning that acetylcholine is released, or adrenergic, meaning that norepinephrine is released. The terms cholinergic and adrenergic refer not only to the signaling molecule that is released but also to the class of receptors that each binds.
The adrenergic system also has two types of receptors, named the alpha -adrenergic receptor and beta -adrenergic receptor. Unlike cholinergic receptors, these receptor types are not classified by which drugs can bind to them. All of them are G proteincoupled receptors. There are three types of -adrenergic receptors, termed 1, 2, and 3, and there are two types of -adrenergic receptors, termed 1 and 2. An additional aspect of the adrenergic system is that there is a second signaling molecule called epinephrine. The chemical difference between norepinephrine and epinephrine is the addition of a methyl group in epinephrine. The prefix nor- actually refers to this chemical difference, in which a methyl group is missing.
Autonomic VaricositiesFigure 4. Fight or Flight? What About Fright and Freeze?
Spinal Reflex/the Reflex Arc
A reflex is an involuntary and nearly instantaneous movement in response to a stimulus. The reflex is an automatic response to a stimulus that does not receive or need conscious thought as it occurs through a reflex arc. Reflex arcs act on an impulse before that impulse reaches the brain.
Relex arcs can be
- Monosynaptic ie contain only two neurons, a sensory and a motor neuron. Examples of monosynaptic reflex arcs in humans include the patellar reflex and the Achilles reflex.
- Polysynaptic ie multiple interneurons that interface between the sensory and motor neurons in the reflex pathway.
Illustration of the reflex arc.
Video of reflex arc
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The Spinal Cord Possesses Other Reflexes And Includes Locomotor Pattern Generators
The spinal reflexes discussed above are only part of the story. There are a variety of other reflexes such as the placing reflex, which helps maintain posture and support. The placing reflex refers to the reaction to tactile stimuli applied to the back of the paw of lower animals. The reaction is to flex the limb and then swing it forward and extend it. For example, a twig touching the dorsum of the foot during its swing phase results in the foot being lifted over the twig. In humans, this is the reaction upon stubbing ones toe: the affected limb is flexed and then rapidly swung forward and extended to catch the falling body. This is a far more complicated reflex than what we have already discussed. It involves a complex sequence of motor neuron activation and inhibition.
Experiments with lower animals show that part of locomotion control resides in pattern generators located in the spinal cord. When the thoracic spinal cord of a cat is cut, severing all connections to the lower motor neurons from higher centers in the brain, and the cat is suspended in a sling above a treadmill, the animal can still raise and place the hind legs, and this motion will keep pace as the treadmill is accelerated. Thus, the basic circuitry for movement of the hind legs resides in the spinal cord.
Which Part Of The Nervous System Controls Reflexes
The part of the brain that controls reflexes is the cerebellum. The cerebellum regulates motor reflexes and is also involved in the synchronization of balance and muscles. The brainstem links and transmits messages to the spinal cord from the brain, regulating functions such as respiration, heart rate, and alertness.
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Reflex Action And Reflex Arc
The neural pathway that controls reflexes and acts on an impulse way before it reaches the brain is called the reflex arc. There are two types of reflex arcs. They are:
- Autonomic Reflex Arc that affects the functioning of inner organs.
- Somatic Reflex Arc that affects the functioning of muscles.
Reflex Action and Reflex Arc
The reflex pathway or the reflex arc is composed of five primary components. These are:
- Receptor It receives information and assists in generating impulses
- Sensory Nerve Its function is to carry information from the receptor to the interneurons present in the spinal cord
- Interneuron Processing of information and generating effective responses is done here
- Motor Nerve It carries the processed information from the spinal cord to the effector
- Effector Organ This is an organ in the body that receives the information from effector neurons and conducts the appropriate response or reflex action.
Is Brain Involved In Reflex Action
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Likewise, why is the brain not involved in the reflex action?
It is because most sensory neurons do not pass directly into the brain but synapse in the spinal cord which allows reflex actions to occur relatively quickly by activating spinal motor neurons without the delay of routing signals through the brain, although the brain receives sensory input while the reflex action
Secondly, which part of the brain controls reflex actions? The brain stem, which consists of the medulla , pons and midbrain . The brain stem controls the reflexes and automatic functions , limb movements and visceral functions .
One may also ask, what is the role of brain in reflex action?
The purpose of reflex action is to provide quick response in situations where the organism does not have enough time to think. Reflex arcs are formed in the spinal cord, although the information inputs also reach the brain. This helps the brain to record this event and remember it for future use.
How does a reflex action occur?
Reflex actions happen through the reflex arc, which is a neural pathway that controls the reflexes. It acts on an impulse even before it reaches the brain. Immediately, the spinal cord sends back signals to the muscle through the motor neuron. The muscles attached to the sense organ move the organ away from danger.
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