AP2 1.5: Reflexes

Question 1

Reflexes

Answer 1

Reflexes are nearly instantaneous, automatic, and involuntary motor responses within the nervous system. Reflexes start from stimuli occurring inside or outside of the body. Reflexes come in a variety of forms. Subconscious reflexes occur within the body, such as the regulation of blood sugar by the hormones. Other reflexes are noticeable, such as shivering in response to body temperature dropping. A more obvious reflex occurs when you touch a very hot object and involuntarily withdraw your hand.

Question 2

brainstem

Answer 2

Some reflexes, such as blinking the eye, involve the brainstem. The brainstem regions include the midbrain, pons, and the medulla. Other reflexes, such as the flexor-withdrawal reflex, only involve the spinal nerves and spinal cord in an action known as the reflex arc. A reflex arc refers to the neural pathway that a nerve impulse travels. Certain reflexes can be tested to examine the function of the nervous system because the reflex arc passes through the central nervous system.

Question 3

dorsal root

Answer 3

Sensory information travels into the spinal cord via the dorsal root of a nerve

Question 4

ventral root

Answer 4

Motor information travels out of the ventral root of the spinal cord

Question 5

gray matter

Answer 5

The gray matter of the spinal cord contains the cell bodies of neurons, and it is also where neurons synapse with other neurons (such as interneurons).

Question 6

white matter

Answer 6

The white matter of the spinal cord contains the axons of neurons. The spinal cord is located centrally, with white matter surrounding the gray matter

Question 7

interneurons

Answer 7

Recall interneurons are located only within the central nervous system to connect neurons to each other.

Question 8

dorsal root ganglion (DRG)

Answer 8

The dorsal root ganglion (DRG) contains the cell bodies of sensory neurons

Question 9

dorsal root

Answer 9

The dorsal root contains the axons of sensory neurons. Sensory neurons synapse on cells within the posterior horn of the spinal cord in the gray matter (Figure 1.23). Sensory neurons synapse on interneurons within the gray matter of the spinal cord. Interneurons then synapse on motor neurons.

Question 10

posterior horn

Answer 10

Sensory neurons synapse on cells within the posterior horn of the spinal cord in the gray matter

Question 11

anterior horn

Answer 11

Motor neuron cell bodies are located within the anterior horn of the gray matter.

Question 12

spinal nerve

Answer 12

A spinal nerve contains both sensory and motor neurons from the ventral and dorsal roots

Question 12

ventral root

Answer 12

Motor neurons (axons) leave the spinal cord via the ventral root

Question 13

Spinal reflexes

Answer 13

Spinal reflexes occur faster than a conscious decision to move coming from the brain. Spinal reflexes are faster not only because they involve fewer neurons, but also because the electrical signal does not have to travel to the brain and back. Spinal reflexes only travel to the spinal cord and back, which is a much shorter distance. Several examples of spinal reflexes are the flexor reflex (withdrawal of your hand from a very hot object) and the stretch reflex on an opposing muscle to prevent overstretching of its antagonist.

To help an organism avoid injury, a reflex arc provides an immediate withdrawal from dangerous stimuli. While all sensory information does eventually reach the brain, the advantage of the reflex arc is production of a response by way of the spinal cord without the need to wait for processing by the brain. In this way, a response occurs in the body even before it is consciously perceived by the brain. A reflex arc refers to the neural pathway that a nerve impulse travels, typically consisting of five components:

Question 14

five components:

Spinal reflexes

Answer 14

1. The receptor at the end of a sensory neuron reacts to a stimulus.
2. The sensory (afferent) neuron conducts nerve impulses along an afferent pathway towards the central nervous system (CNS).
3. The integration center consists of one or more synapses in the CNS.
4. A motor (efferent) 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 (a muscle) or secreting a product (a gland).

Question 15

Stretch reflexes

Answer 15

Stretch reflexes are a special type of muscle reflex that protect the muscle against increases in length that may tear or damage muscle fibers. Stretch reflexes are also very important in maintaining upright posture. Muscle spindles are specialized muscle cells that are constantly monitoring the amount of stretch in a muscle. If the muscle becomes overstretched, it triggers a reflex to occur, which contracts the muscle back to its appropriate length.

Question 16

patellar reflex

Answer 16

The patellar reflex is one example of a stretch reflex. The patellar reflex is also called the knee-jerk reflex used in physicians’ offices. The patellar reflex tests the stretch reflex of the quadriceps femoris muscle in the thigh. The primary purpose of the patellar reflex is to prevent the overstretching of the quadriceps.

Question 17

patellar tendon

Answer 17

The patellar tendon attaches the quadriceps muscle to the tibia bone of the lower leg (follow the mapped neural pathway in Figure 1.24 below). Recall the quadriceps muscle is an extensor muscle of the knee. It raises the lower leg as it contracts, thereby extending the angle of the knee joint. Tapping the patellar tendon when the lower leg is suspended off a table stretches the tendon, which also stretches the quadriceps muscle.

When the muscle spindle detects the stretch, a signal is sent along the afferent neuron, through the dorsal root ganglion (DRG) to the spinal cord via the dorsal root. The afferent (sensory) neuron synapses in the gray matter of the spinal cord directly onto an efferent (motor) neuron in the anterior horn of the gray matter. The efferent neuron travels out the ventral root and through the nerve to synapse on the same muscle. The effect of the motor signal is to contract the quadriceps muscle to prevent it from overstretching.

Question 18

flexor withdrawal reflex

Answer 18

The flexor withdrawal reflex (Figure 1.25) is a second example of a reflex with a more complex neural pathway. If a very hot object is touched, a pain receptor in the skin generates nerve impulses, which move along the dendrite of a sensory neuron toward the cell body and the central nervous system in the same manner as sensory information travels as described above. From the cell body, the impulses travel along the axon of the sensory nerve and synapse on many interneurons (in green in Figure 1.25, below) within the gray matter of the spinal cord

Question 19

Excitatory interneurons

flexor withdrawal reflex

Answer 19

Excitatory interneurons send excitatory signals to motor neurons, causing muscular contraction

Question 20

Inhibitory interneurons

flexor withdrawal reflex

Answer 20

Inhibitory interneurons send inhibitory signals that prevent muscular contraction.

Question 21

flexor withdrawal reflex, many interneurons are involved

Answer 21

In the flexor withdrawal reflex, many interneurons are involved. Interneurons synapse on two different motor neurons. One excitatory interneuron synapses on a motor neuron to the biceps, causing the biceps to contract. Simultaneously, an inhibitory interneuron synapses on a motor neuron to inhibit the triceps, preventing a contraction. Contraction of the biceps causes elbow flexion, withdrawing the hand from the hot object. The triceps receive an inhibitory signal to prevent contraction, so a co-contraction of the triceps and biceps does not occur. If the biceps and triceps contracted simultaneously, the elbow would remain extended, leaving the hand fixed over the hot object causing increased damage.

Question 21

whole series of responses occurs almost instantaneously

Answer 21

This whole series of responses occurs almost instantaneously because the sensory neuron stimulates several interneurons. More interneurons are involved in this reflex process, some of which also synapse on neurons eventually reaching the brain. The brain makes the person conscious of the stimulus and the reaction that occurred involuntarily in their body.