Question

Describe the most important specific reflex patterns of movement provided by spinal cord. (Explain in detail and give examples)

Answer 1

A major part of the spinal cord function is regulated by the brain. However, many functions of the spinal cord are also executed independently from the brain, such as a spinal reflex. Many of the protective functions necessary for survival are embedded within the spinal reflexes.This allows for faster reflex actions to occur by activating spinal motor neurons without the delay of routing signals through the brain. The brain will receive the sensory input while the reflex is being carried out and the analysis of the signal takes place after the reflex action.

A reflex is an involuntary, stereotypical response of the effector tissue from the stimulation of receptors. These reflexes are executed by the successive activation of a certain number of neurons that are mutually connected. The last neuron generally innervates the effector tissue, which is usually a muscle. These neurons and the effector tissue form relations that are called the reflex arc, which is the basic unit of a reflex.

There are two types of reflex arcs:

1. autonomic reflex arc - affecting inner organs
2. somatic reflex arc - affecting muscles

Based on how many neurons participate in one arc, the reflexes can be:

1. monosynaptic - When a reflex arc consists of only two neurons, one sensory neuron, and one motor neuron
2. polysynaptic - When one or more interneurons connect afferent (sensory) and efferent (motor) signals

The most important specific reflex patterns of movement provided by the spinal cord are:

1. Stretch Reflex - This (myotatic reflex) is a muscle contraction in response to stretching within the muscle. This reflex has the shortest latency of all spinal reflexes. It is a monosynaptic reflex that provides automatic regulation of skeletal muscle length. An example of the stretch reflex is the knee-jerk test performed by physicians. When the patellar tendon is tapped with a small hammer, or other device, it causes a slight stretch in the tendon, and consequently the quadriceps muscles.

2. Golgi Tendon Reflex - This (inverse stretch reflex) operates as a protective feedback mechanism to control muscle tension by causing muscle relaxation before muscle force becomes so great that tendons might be torn. Besides protecting against too much tension on the muscle and tendon, this reflex may help spread muscle load throughout the muscle fibers, thereby preventing damage to isolated fibers. Whereas the stretch reflex regulates muscle length, the tendon reflex helps regulate muscle force. For example, when a load is placed on the muscle, the afferent neuron from the Golgi tendon organ fires into the central nervous system and the motor neuron from the spinal cord is inhibited via a synaptic impulse and the muscle relaxes.

3. Crossed Extensor Reflex - This is a withdrawal reflex. The reflex occurs when the flexors in the withdrawing limb contract and the extensors relax, while in the other limb, the opposite occurs. The crossed extensor reflex is contralateral, meaning the reflex occurs on the opposite side of the body from the stimulus. For example, when a person steps on a nail, the leg that is stepping on the nail pulls away, while the other leg takes the weight of the whole body.

4. Withdrawal Reflex - This (nociceptive or flexor withdrawal reflex) is a spinal reflex intended to protect the body from damaging stimuli. It is polysynaptic, and causes the stimulation of sensory, association, and motor neurons. For example, when a person touches a hot object and withdraws his hand from it without thinking about it, the heat stimulates temperature and danger receptors in the skin, triggering a sensory impulse that travels to the central nervous system. The sensory neuron then synapses with interneurons that connect to motor neurons to send motor impulses to the flexors to allow withdrawal.