NERVOUS SYSTEM AND REFLEXES
Introduction:
The nervous system includes two divisions—the Central Nervous System (CNS) and the Peripheral Nervous
System (PNS). The Central Nervous system includes the brain and spinal cord which processes sensory
information, integrates and determines appropriate motor responses. Whereas, the nerves that connect the rest of
the body to the CNS are the major structures of the Peripheral Nervous System. The nerves of the PNS function
to bring sensory information to the central nervous system and to carry the commands of the CNS to cells of the
body.
Neurons are the cells of the nervous system which conduct electrical impulses which is how the nervous system
carries and processes information. Neurons fall into three general categories: sensory neurons, interneurons and
motor neurons. Sensory neurons monitor particular types of energy or molecules, stimuli, which result in the
neuron conducting an electrical impulse toward the CNS. Interneurons of the CNS interpret the information from
the sensory neuron and can issue a response by signaling a motor neuron, through another electrical impulse, to
carry out an appropriate response, thus maintaining homeostasis.
Macroscopic observation of the brain and spinal cord provides a way to identify myelinated and nonmyelinated
neurons. The myelinated neurons form the white matter and nonmyelinated neurons form the gray matter in
characteristic patterns. The surface of the brain consists of gray matter and the interior of the spinal cord, seen as
an ‘H’ or butterfly in cross section consists of gray matter. The white matter, myelinated neurons function to
conduct electrical impulses at a high speed from one area to another.
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Activity 1: Microscopic Observation of Nervous Tissue
Observation of nervous tissue begins by microscopic examination of the white and gray matter of the spinal cord
in cross section. The outer layer of white matter consists of myelinated neurons running in tracts forming
ascending and descending pathways to carry sensory and motor information. Extending from the white matter are
long processes of the sensory neurons and motor neurons, together forming the spinal nerves. There is an
enlarged portion of the dorsal root where the cell bodies of sensory neurons reside and information is sent to the
interneuron within the spinal cord. Motor neuron cell bodies lie within the spinal cord and their axons extend from
the spinal cord as the ventral root. Both the dorsal and ventral roots merge, to form a mixed nerve, carrying both
sensory and motor information between the body and the spinal cord.
Within the white matter is a ‘butterfly’ shape of gray matter consisting of cell bodies of neurons, interneurons,
which play an important role in the function of reflexes as they are responsible for determining and initiating an
appropriate response to a stimulus.
At the center of the spinal cord, is the central canal, which contains cerebrospinal fluid that is circulated by
ciliated ependymal cells.
Materials:
Compound Light Microscope
Prepared slides: spinal cord (c.s.). neuron smear
Procedure: Observe the spinal cord cross section using scanning power or low power objective lens.
1). Before Lab, prepare your notebook by drawing a diagram in your histology atlas of the spinal cord,
label the different areas observed (bold-faced).
2). In lab, observe the prepared spinal cord slide using your scanning power or low power objective lens
and confirm the identify of the structures labeled in your histology atlas. Make any notes as needed.
3). Review the neuron smear slide as needed. Practice identifying the cell membrane, axon, dendrites,
nucleus, nucleolus and cytoplasm.
4). As time permits, observe the central canal of the spinal cord using your oil immersion lens and
identify the ciliated ependymal cells.
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Activity 2: Observation of the Human Brain Model
Practice identifying the various structures of the brain on the model or diagram and answer the associated
questions in your lab notebook.
Although the laboratory models lack the meninges, membranes that cover the surface of the brain, you should be
able to identify the external anatomical features: right and left cerebral hemispheres of the cerebrum,
cerebellum, and the internal structures of the brainstem including the midbrain, pons, and medulla oblongata.
Depending on the model, you may be able to identify the pituitary gland which includes axons from the
hypothalamus. The axons that form the stalk, connecting the hypothalamus to the pituitary gland is called the
infundibulum.
Most models allow students to view the internal structures and spaces. Play with your model and see if you can
identify the corpus callosum, lateral ventricles, third ventricle (between the corpus callosum and thalamus),
the structures of the diencephalon (thalamus and hypothalamus) and fourth ventricle (between the brain
stem and the cerebellum). The ventricles of the brain are spaces that contain cerebrospinal fluid (CSF). This fluid
is produced by specialized capillaries of the brain called the choroid plexus.
The nervous tissue of the brain may appear ‘white’ or ‘gray’. White matter consists of neurons whose axons are
covered with a myelin (fatty) sheath, whereas, gray matter consists of neurons that lack a myelin cover. Observe
the surface of the cerebrum is composed of gray matter. It is arranged in characteristic ridges, gyri, and grooves
or sulci. Similarly, the cerebrum has an outer area of gray matter and the internal white matter forms a unique
pattern called the arbor vitae (tree-like).
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Several cranial nerves from the PNS can be seen on in inferior surface: olfactory bulb, optic nerves with optic
chiasma and trigeminal nerves. The olfactory bulb is a sensory nerve that transmits information from the nasal
cavity to the brain. The optic nerve is a sensory nerve that transmits visual information from the eye to both sides
of the cerebrum. The optic chiasma functions to separate axons transmitting information from one eye to both
sides. The trigeminal nerve is a mixed nerve that transmits sensory information to the brain, as well as, motor
information to the body from the brain. It has three distinct branches. The mandibular and maxillary division of the
trigeminal nerve conducts sensory impulses from the skin of the face, mouth and nasal mucosa and surface of the
eyes to the brain. The mandibular division contains motor neurons that innervate muscles of the mouth for
chewing.
PreLab Activity:
Draw a diagram of the human brain. Label the external structures clearly on your diagram as indicated by the
bold-faced print above. Practice your identifications before lab.
Draw a diagram of the structures seen from a midsagittal section (see diagram at top of previous page) of the
human brain. Label the internal structures clearly on your diagram as indicated by the bold-faced print above.
Practice your identifications before lab.
In-Lab Activity:
Observe the models of the human brain. Observe the preserved human brain specimens. Practice your
identifications on the specimens.
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Activity 3: Observation of Spinal Cord Model and the Reflex Arc
The spinal cord and spinal nerves function below the level of consciousness. The spinal cord is responsible for
thousands of reflex arcs, automatic responses to a stimulus, without communicating with the brain.
Spinal nerves are paired, extending laterally from the spinal cord. Surrounding the spinal cord is the bone
vertebral column. Each spinal nerve is composed of sensory neurons and motor neurons (mixed nerves). The
sensory neurons carry information to the spinal cord and attach from the dorsal side of the spinal cord where it is
called the dorsal root ganglion. The ganglion, enlarged region of the dorsal root, houses the sensory neuron
cell bodies. Motor neurons extend from the spinal cord on the ventral side forming the ventral root before coming
together to form a mixed nerve.
Each reflex arc is composed of three different neurons, all carrying out a specific function in the reflex:
1. Sensory neuron. The sensory neuron monitors environmental stimuli (energy or molecules) and when
stimulated sends a nervous impulse to the spinal cord.
2. Interneuron. Interneurons lie entirely within the CNS. These neurons are responsible for integrating incoming
information and formulating a response based on current conditions and past experience. The interneurons
determine an appropriate response and stimulate a motor neuron through the generation of a nervous
impulse.
3. Motor neuron. The motor neuron carries out the orders for a response. The nervous impulse travels the
length of the motor neuron to an effector cell. An effector cell will carry the response. Effector cells include
muscle cells which contract or glands which secrete a substance.
PreLab Activity:
Draw a diagram and label the parts of the cross section of a spinal cord. Draw a diagram identifying the different
components of a reflex arc. Be sure to label the direction of the reflex and beside each component, briefly
describe its role in the function of the reflex.
In-Lab Activity:
Observe the model of the cross section of the spinal cord. Observe how the sensory and motor nerves exit
individually first and then converge into a single mixed nerve.
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Activity 4: Patellar Reflexes
Physicians use reflex tests to assess the condition of the nervous system. The patellar reflex is commonly used
for this purpose. When the patellar tendon is struck just below the kneecap with a reflex hammer, the reflex action
is a slight, instantaneous contraction of the large muscle (quadriceps femoris) on the front of the thigh that
extends the lower leg. Striking the patellar tendon causes the quadriceps femoris muscle to be slightly stretched
for an instant. This stretching of the muscle causes impulses to be formed and carried along a sensory neuron to
the spinal cord where they are passed to a motor neuron that carries the impulses back to the muscle, thereby
causing a weak, brief contraction. Work in pairs to perform the reflex test.
Materials: Reflex Hammer
Procedure and Observations:
1) The subject should sit on the edge of a table with his/her legs hanging over the edge, but not touching the
floor.
2) Strike the patellar tendon with the small end of the reflex hammer and observe the response.
3) Divert the subject’s attention by having the subject interlock the fingers of both hands and pull the hands
against each other while you strike the patellar tendon again.
4) Test both legs and record the results. Rate the strength of your patellar response as (3) strong, (2) moderate,
(1) weak, or (0) none and record in your lab notebook.
5) Answer the accompanying questions in your notebook.
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Activity 5: PHOTOPUPILLARY REFLEX
The photopupillary reflex enables a rapid adjustment of the size of the pupil of the eye to the existing light
intensity. This reflex is coordinated by the brain. The reflex is most easily observed in persons with light-colored
eyes. When a bright light stimulates the retina of the eye, impulses are carried to the brain by sensory neurons. In
the brain, the impulses are transmitted to interneurons which determine an appropriate response which is carried
out by motor neurons that cause the muscles of the iris to contract. Contraction of the iris muscles decreases the
size of the pupil and controls the amount of light entering the eye.
Materials (per pair of students): Penlight
Procedure and Observations:
1. You should repeat the procedure so each partner gets a turn at being the ‘subject’.
2. Have the subject sit with the eyes closed, facing a darkened part of the room for 1-2 minutes. When the
subject opens his or her eyes, note any change in pupil size. Describe your observations and explain these
observations in your lab notebook.
3. While the subject is looking into the darkened area, shine a pen light in his/her eyes (from about 3 feet away).
Describe your observations of the subject's pupils. Explain your observations in your lab notebook.
4. To observe the effect of unilateral (uni=one, lateral=side) stimulation, have the subject look into the darkened
area and, while you observe the subject’s right eye, shine a penlight in the left eye. Note any response.
Repeat with the left eye. Describe your observations of the subject's pupils. Explain your observations in your
lab notebook.
After completing all lab activities, write a conclusion in the space provided in your lab notebook.
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