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Lab 5. The Nervous System – Somatic &

Visceral Reflexes

LEARNING OBJECTIVES

Instructions: When you have mastered the objective, place a check mark in the box.

At the completion of this exercise, you will be able to:

1. Analyze the structure of monosynaptic and polysynaptic reflex arcs,

by using diagrams to compare and contrast their structural features. ☐ 2. Comprehend somatic and autonomic reflexes, by demonstrating their effects, and

explaining how each structural component of a reflex arc contributes to its function. ☐

3. Analyze the structural and functional features of somatic and autonomic reflex arcs. ☐

INTRODUCTION

Figure 1. Reflex Arc Classifications

A reflex is a fast, predictable, and involuntary motor response to a stimulus. Reflex responses are

typically protective in nature and function to prevent damage to various bodily tissues. Some reflexes

are used to keep physiological variables, such as blood pressure and respiratory rate, within a narrow

homeostatic range. Reflex responses are made possible by a neuronal structure consisting of a series of

neurons, which together are termed a reflex arc.

A simple reflex arc begins with an afferent sensory neuron that detects a specific stimulus. The

afferent sensory neuron carries sensory information, propagated by graded and action potentials, to an

integrating center, usually located within the central nervous system (CNS). In the integrating center,

action potentials are directed to an effector organ through an efferent motor neuron. The effector

organ produces a response that is either protective or involved in maintaining homeostasis. Often, the

effector is skeletal muscle tissue, which produces a variety of reactive movements, such as a jerk

reaction.

There is diversity of reflex responses in the body, and may be classified based on various

characteristics. Innate reflexes are present genetically or during development, such as the Babinski

reflex. Normal infants will respond with an upward extension of the toes when the lateral aspect of the

Development

Innate reflexes

Aquired reflexes

Response

Somatic reflexes

Autonomic reflexes

Circuit complexity

Monosynaptic reflexes

Polysynaptic reflexes

Processing site

Spinal reflexes

Cranial reflexes

Stamp for

Credit

sole of the foot is firmly stroked. Absence of this reflex may indicate abnormal development. Acquired

reflexes are learned through experience and practice and may require more complex response patterns.

For example playing the piano or typing becomes an acquired reflex, but unlike innate reflexes, signals

very complex efferent signals. Somatic reflexes produce a response by the activation of skeletal

muscles through efferent signals propagated by somatic motor neurons. In contrast, autonomic reflexes

result in autonomic motor neurons producing responses in smooth muscle, cardiac muscle, or glands.

The simplest reflex arc structure may contain only two neurons, an afferent sensory neuron and

an efferent motor neuron. Such reflex arcs, lacking interneurons, are termed monosynaptic.

Monosynaptic neurons are called such, because there is only a single synapse in the entire reflex arc.

However, polysynaptic reflex arcs contain one or more interneurons between the afferent sensory

neuron and efferent motor neuron, and so will contain at least two synapses.

In this lab, you will explore the effects of various sensory modalities on production of somatic

and visceral reflexes.

TERMS TO DEFINE BEFORE LAB

1. Afferent sensory neuron: _________________________________________________________

_________________________________________________________________________________

2. Efferent motor neuron: ___________________________________________________________

_________________________________________________________________________________

3. Reflex arc: ____________________________________________________________________

_________________________________________________________________________________

4. Somatic reflex: _________________________________________________________________

_________________________________________________________________________________

5. Autonomic reflex: ______________________________________________________________

_________________________________________________________________________________

LABORATORY PROCEDURES

 In Activity 1, you will create diagrams of mono- and poly-synaptic reflex arcs and compare and

contrast their structures and functions.

 In Activity 2, you will stimulate and observe various somatic reflexes and identify structural

components involved in the reflex.

 In Activity 3, you will stimulate and observe various visceral reflexes and identify structural

components involved in the reflex.

 In Activity 4, you will compare and contrast the structural and functional features of somatic and

visceral reflex arcs.

ACTIVITY 1 – COMPREHEND THE STRUCTURAL SIMILARITIES & DIFFERENCES

BETWEEN MONO- AND POLYSYNAPTIC REFLEX ARCS

1. Draw a diagram of a monosynaptic reflex arc, such as that found in the knee jerk response. Label the

following: stimulus, receptor, quadriceps muscle, sensory neuron, spinal cord, gray matter, white

matter, synapse, motor neuron, effector (quadriceps muscle).

2. Draw a diagram of a polysynaptic reflex arc such as that which might control a response to touching

a sharp object with your index finger. Label the following: pain receptors, sensory (afferent) neuron,

gray matter, white matter, spinal cord, interneuron, motor (efferent) neuron, effector (biceps brachii

muscle).

3. How are the structures of mono- and polysynaptic reflex arcs similar. How are they different?

Monosynaptic Both Polysynaptic

ACTIVITY 2 – COMPREHEND THE STRUCTURAL COMPONENTS INVOLVED IN

SOMATIC REFLEXES

The patellar reflex and Achilles (ankle) reflex are both stretch receptor reflexes. Stretch

receptor reflexes are involved in maintenance of posture, balance and locomotion. Tapping a tendon

stimulates sensory receptors in the muscle, called muscle spindles. Muscle spindles are receptors deep

inside the belly of a muscle that detect the length of the muscle fiber and will produce contraction via a

reflex arc if the muscle fiber becomes too long.

Experiment 1 – Patellar (Knee) Reflex

The patellar reflex tests the femoral nerve and spinal cord segment, L2-L4. Identify the position of the

L2-L4 vertebrae. This is a monosynaptic stretch reflex. Absence or decrease of the patellar reflex is

called Westphal’s sign.

1. Have your partner sit on a chair or bench top so that their legs hang freely.

2. Feel your partner’s knee for the patella bone or knee-cap.

3. Slide your fingers distally from the patella until you no longer feel bone, this is the patellar

tendon.

4. Using the taller end of the reflex hammer, gently strike the patellar tendon as close to the center

as possible.

5. Record your observations in the space below (flexion/extension, and intensity of the reaction).

Instructions: Answer the following questions.

1. What type of receptor is activated during this reflex?

______________________________________________________________________________

2. What muscle group is the effector in this reflex?

______________________________________________________________________________

3. What might cause this reflex to be absent?

______________________________________________________________________________

Experiment 2 – Achilles (Ankle) Jerk Reflex

This experiment tests spinal cord segments S1 and S2. Identify the position of these segments in the

vertebral column. May be indicative of sciatic nerve pathology and is usually absent in disk herniations

at the L5-S1 level. The achilles jerk reflex is a monosynaptic stretch reflex.

1. Have your partner take off their shoes and socks and kneel on a chair with feet hanging over the

edge of the chair in a relaxed position.

2. Feel for your partner’s achillies tendon, and strike it near the ankle (calcaneous).

3. Record your observations in the space below (flexion/extension, and intensity of the reaction).

Instructions: Answer the following questions.

1. What type of receptor is activated during this reflex?

______________________________________________________________________________

2. What muscle group is the effector in this reflex?

______________________________________________________________________________

3. What might cause this reflex to be absent?

______________________________________________________________________________

Experiment 3 – Jendrassik Maneuver

You may have noticed a lack of response, or a low intensity response during the previous two

experiments, because your partner anticipated the result. This anticipation may cause your partner to

contract muscle groups opposing the reflex, in order to reduce the intensity of the response. The

Jendrassik maneuver is designed to direct your partner’s attention away from the reflex and will prevent

this anticipation. To distract your partner, ask him/her to clasp their hands together and pull them apart

with as much force as they can, while you are testing the reflex.

1. Repeat experiments 1 and 2 while your partner is distracted.

2. Record observations in the space below:

 Patellar reflex

 Ankle jerk reflex

Experiment 4 – Corneal reflex

There is a high density of pain receptors on the cornea of the eye, which serves to protect it from

being scratched. When activated, even by light touch, a response is triggered whereby the eyelids

reflexively close and protect the cornea from damage. This information travels along the ophthalmic

branch of the fifth cranial nerve (V1), called the trigeminal nerve. A failure of this reflex is a very bad

sign, usually indicating lethal damage to the brain stem. It is often assessed in patients who are in a

coma. Stimulation of the reflex in one eye usually results in the closing of both eyelids.

1. Obtain a thin piece of absorbent cotton.

2. Stand on one side of your lab partner, and have them look in the opposite direction from you.

3. Quickly but gently rub the piece of cotton on the corneal region of your partner’s eye. Take care

not to scratch the cornea. This procedure should not be done in a person who is wearing contact

lenses.

4. Record the response below:

Instructions: Answer the following questions.

1. What is the usefulness of this reflex?

___________________________________________________________________________

2. Identify the sensory organ that was activated.

___________________________________________________________________________

3. What effector organs were involved?

___________________________________________________________________________

Experiment 5 – Pharyngeal (Gag) Reflex

The pharyngeal reflex is a somatic, cranial nerve reflex that uses both the hypoglossal nerve (cranial

nerve IX) and the vagus nerve (cranial nerve X). This reflex is initialed by stimulating pressure receptors

around the uvula near the back of the throat. While absence of a pharyngeal reflex may indicate damage

to the glossopharyngeal nerve, the vagus nerve, or brain death, it predictive power is limited by the fact

that aproximately one-third of healthy adults lack a pharyngeal reflex. Therefore, pharyngeal sensation

may be a stronger predictor of problems with swallowing, such as in stroke patients (Davies et al.,

1995).

1. For this exercise, choose one of your labmates with a strong stomach, to avoid inducing

vomiting.

2. Using a tongue depressor, gently stroke the tissue on either side of the uvula.

3. Record the response in the space below:

Instructions: Answer the following questions.

1. What is the usefulness of this reflex?

___________________________________________________________________________

2. Identify the sensory organ that was activated.

___________________________________________________________________________

3. What effector organs were involved?

___________________________________________________________________________

ACTIVITY 3 – COMPREHEND THE STRUCTURAL COMPONENTS INVOLVED IN

AUTONOMIC REFLEXES

The following reflexes are known as visceral reflexes, because they all stimulate responses from

visceral effectors (smooth muscle tissue, cardiac muscle tissue, and glands).

Experiment 6 – Gustatory (Salivary) Reflex

Receptors in the mouth detect acid, which may be harmful if it accumulates, and initiate a reflex that

triggers the excretion of more saliva. Releasing more saliva dilutes the acid in the mouth.

1. Place a small piece of pH paper under the tip of your tongue for a few seconds and record the pH

in the table below.

2. Swallow any remaining saliva.

3. For the next 2 minutes and 30 seconds, collect as much of the saliva you normally produce as

you can in a 10 mL graduated cylinder. Record the volume in the table below.

4. Place a few drops of vinegar (or lemon juice) in your mouth and leave it there for about 5

seconds then spit it into the sink.

5. Check the pH of saliva and record it in the table below.

6. For the next 2 minutes and 30 seconds, collect as much of the saliva you normally produce as

you can in a 10 mL graduated cylinder. Record the volume in the table below.

pH Volume of saliva collected (mL)

Initial

After vinegar (lemon

juice)

Instructions: Answer the following questions.

1. What cranial nerves innervate the salivary glands? Is the innervation sympathetic or

parasympathetic? Is saliva release stimulated or inhibited?

______________________________________________________________________________

2. Which thoracic spinal nerves branch and innervate the salivary glands? Is the innervation

sympathetic or parasympathetic? Is saliva release stimulated or inhibited?

______________________________________________________________________________

Experiment 7 – The Pupillary Light Reflex

The pupillary reflex is triggered by photoreceptors (light receptors) in the eye. Photoreceptors in the

retina send afferent signals along the optic nerve (CN II) to the midbrain. Efferent signals leave the

midbrain through the oculomotor nerve (CN III) and innervate the sphincter pupillae (circular muscle) of

the iris. Parasympathetic fibers innervate the sphincter pupillae, and sympathetic fibers innervate the

dilator pupillae (radial muscles). Bright light stimulates pupil constriction and an immediate reduction in

light to the over-stimulated photoreceptors. A reduction in light causes dilation of pupils. Alcohol and

opioids cause pupil constriction, and LSD, MDMA, psilocybin, cocaine, and amphetamines cause pupil

dilation. May be used to determine damage of optic nerve and oculomotor nerve by looking for the

direct or consensual reflexes. Emergency room physicians use the pupillary light reflex to assess brain

stem function.

Part 1: Ipsilateral Reflex (constriction of the eye on the same side as the stimulus)

1. Obtain a pen light.

2. Have your lab partner close her/his eyes for 2 minutes while facing the of the pen light.

3. Have your partner open his/her eyes and cover the right eye, while shining the light into the left

eye.

4. Immediately measure the size of your partner’s left pupil.

5. Repeat the steps above for the right eye.

6. Record your observations in the space below:

Part 2: Contralateral Reflex (constriction of the eye on the opposite side as the stimulus)

1. Obtain a pen light.

2. Have your lab partner close her/his eyes for 2 minutes while facing the of the pen light.

3. Have your partner open his/her eyes. While shining the light into the left eye. Observe the right

eye.

4. Immediately measure the size of your partner’s right pupil.

5. Repeat the steps above for the right eye.

6. Record your observations in the space below:

Instructions: Answer the following questions.

1. When testing the pupillary response of a patient, you observe that when you shine the light into

her right eye, neither the right or left pupil constrict. What can you conclude about the activity of

the cranial nerves? Which nerve is damaged? Explain your conclusion.

______________________________________________________________________________

2. When testing the pupillary response of a patient, you observe that when you shine the light into

her right eye, the right eye does not constrict but the left eye does. What can you conclude about

the activity of the cranial nerves? Which nerve is damaged? Explain your conclusion.

______________________________________________________________________________

ACTIVITY 4 – ANALYZE THE STRUCTURE AND FUNCTION OF SOMATIC AND

VISCERAL REFLEX ARCS

1. Draw a diagram of a visceral (autonomic) reflex arc. Label the following: stimulus, sensory

receptor (in viscera) dorsal root ganglion, spinal cord, gray matter, white matter, preganglionic

neuron, postganglionic neuron, autonomic ganglion.

2. How are the structures of somatic and visceral reflex arcs similar? How are they different?

Somatic Both Visceral

REFERENCES

Davies, A.E., Kidd, D., Stone, S.P., and MacMahon, J. (1995). Pharyngeal sensation and gag reflex in

healthy subjects. Lancet, 345 (8948), 487-488.