.png)
We recently posted a spinal cord dissection (which you can check out here), where we looked into the anatomy and functions of the spinal cord– which is in my opinion the most underrated player of the nervous system. While many of the spinal cord’s functions are dictated and controlled by the brain, there are also numerous functions that the spinal cord carries out independently. The most important of such functions would be the spinal reflex, which is a collective term for any sort of reflex action mediated through the center of the spinal cord.
A reflex, by definition, is a stereotypical, involuntary response of the effector tissue from the stimulation of receptors. Reflexes are carried out through the successive activation of a certain number of neurons that are interconnected with each other. The last neuron is usually the one that innervates the effector (muscle) tissue. The relation formed by all of these neurons and the effector tissue is collectively referred to as the ‘reflex arc.’ A reflex that consists of just two neurons is known as a monosynaptic reflex, while a reflex that involve the participation of 1+ interneurons (neuron that transmits impulses between other neurons) is known as a polysynaptic reflex. Spinal reflexes can also be subdivided into four major types, each of which we’ll take a look at below.
(Diagram of the Stretch Reflex)
Stretch Reflex: The stretch reflex, also known as the myotatic reflex, refers to a muscle contraction that is in response to stretching within the muscle. The function of this reflex is to maintain the (skeletal) muscles at a constant length, and this monosynaptic reflex is known for having the shortest latency of all spinal reflexes. When a muscle lengthens, the muscle spindles are stretched and their nerve activity increases. This consequently leads to an increase in alpha motor neuron activity, making the muscle fibers contract and resist stretching. A secondary set of neurons also makes the opposing muscles relax.
(Diagram of the Crossed Extensor & Withdrawal Reflex)
Crossed Extensor Reflex: The crossed extensor reflex is a withdrawal reflex where the contralateral (opposite side) limb makes up for the loss of support created when the ipsilateral (same side) limb withdraws from a painful external stimulus. When this reflex is occurring, flexors in the withdrawing limb would contract and extensors relax, while the opposite would occur in the contralateral limb. An example of a crossed extensor reflex would be when someone steps on a nail– the leg that stepped on the nail would withdraw, while the other leg supports the weight of the whole body.
Withdrawal Reflex: The withdrawal reflex, also known as the flexor withdrawal reflex or nociceptive flexion reflex, is a spinal reflex that functions in protecting the body from damaging stimuli. This reflex is polysynaptic, and involves the stimulation of association, sensory, and motor neurons. The reflex quickly coordinates the contractions of all the flexor muscles and relaxation of all the extensor muscles in the limb that is in touch with the damaging stimulus, leading to the limb’s rapid withdrawal. An example of the withdrawal reflex in play would be when someone touches a hot surface, and withdraws their hands quickly without any active thinking. The heat is stimulating the danger/temperature receptors in the skin, triggering a sensory response impulse that travels to the CNS. The sensory neuron then forms a synapse with the interneurons that are connected to the motor neurons, and they send motor impulses to the flexor muscles allowing quick withdrawal of the hand.
(Diagram of the Golgi Tendon Reflex)
Golgi Tendon Reflex: The golgi tendon reflex, also known as the tendon reflex or the inverse stretch reflex, is a component of the reflex arc of the PNS. This reflex functions as a negative feedback mechanism to control muscle tension by inducing muscle relaxation before the muscle forces become so extreme that tendons might tear. Although the golgi tendon reflex is much less sensitive than the stretch reflex, it can override the stretch reflex when tensions are extremely high. (e.g. causing someone to drop a very heavy weight)
Sources:
https://med.libretexts.org/Bookshelves/Anatomy_and_Physiology/Book%3A_Anatomy_and_Physiology_(Boundless)/12%3A_Peripheral_Nervous_System/12.10%3A_Reflexes/12.10C%3A_Spinal_Reflexes
https://www.apsubiology.org/anatomy/2010/2010_Exam_Reviews/Exam_4_Review/CH_13_Proprioception_and_Types_of_Reflexes.htm
Photo Credits:
https://stretchcoach.com/articles/myotatic-stretch-reflex/
https://www.apsubiology.org/anatomy/2010/2010_Exam_Reviews/Exam_4_Review/CH_13_Proprioception_and_Types_of_Reflexes.htm