Essay #8 - Free Response Question 2007

Cephalization and the development of a brain were very important steps in animal evolution.

a) Discuss the evolutionary origin and adaptive significance of cephalization in animal phyla (anterior end most effective place to concentrate sense organs, first to enter new environment, short distance from receptor to integration, radial to bilateral symmetry, enhanced directional movement)
b) Describe the development of the nervous sytem in the vertebrate embryo (ectodermal origin, neuralation, neural crest, brain & spinal cord, notochord support & protection)
c) At the sound of shattering glass, people quicklyturn their heads. Discuss how the human nervous system functions to produce this type of response to an external stimulus. (reception of stimulus - sensors, sensory neuron to CNS, reflex arc, interneuron delivery to brain, processing, response signal to motor neurons mediating response, autonomic nervous system increase heart, action potential, neurotransmitters, synapse)

Using your text book and the Internet for help, answer the questions below.
You may want to check one or more of the many animations available at or Visual Content for Health Care for additional support.for additional support.

Nervous System :

The nervous system is composed of specialized cells responsible for sensing both the internal and external environment, relaying stimuli to a control center (Central Nervous System), and transmitting electrochemical messages to organs and tissues to initiate an appropriate response to the stimuli. Because the communication has an electrical component the messages can travel very quickly (33 m/sec). Reception of stimuli, analysis of stimuli, and intitiation of a response to the stimuli can happen in a fraction of a second.

The more complex the arrangement of tissues in an organism the more complex the nervous system will be. Sponges, the lowest form of animal life, have no nervous system. Cnidarians (jelly fish, coral, hydra) have a nerve net. Nerve fibers branch througout the the body with limited organization. No brain or central ganglia exists so evaluation of the stimuli is almost non existant. When touched the entire organism will respond rather than just the region of the body where the stimulus had been received

Echinoderms (starfish, sea urchins, sand dollars) have radila symmetry and therefore no anterior end in which a brain might be located. They have ganglia which offer control for a given region of

the body. All other invertebrates have a brain. The complexity of the brain they possess is once again a measure of the complexity of tissues that lie within. The more complex the sensory organs the more complex the integration of information will need to be requiring a more comlex brain.

The most well developed nervous system is seen in mammals. We will look at the human nervous system as an example.

The nervous system is composed of two major divisions:

  • Central Nervous System (brain, nerve cord) that is involved in integration (evaluation of stimuli) and relay of nerve impulses
  • Peripheral Nervous System (everything outside the brain and nerve cord) that is involved in sensory (receiving stimuli) and motor function (inducing a response to stimuli)

The motor component of the peripheral nervous system is divided once again into two divisions:

  • Autonomic (mostly involuntary control of smooth & cardiac muscle)
  • Somatic (voluntary control of skeletal muscle)

The Autonomic nervous system is divided into two divisions:

Sympathetic - stimulate organs to prepare for stressfull circumstances (increase available energy to tissues needed for protecting the body: increases heart rate, increase respiratory rate, decrease blood flow to digestive organs, increase blood pressure to brain, pupil dialation, increased conversion of glycogen to glucose.....) - primary neurotransmitter is norepinephrine

Parasympathetic - stimulate organs to prepare for rest and relaxation (decrease need for energy consumption: decrease heart rate, decrease respiratory rate, increase circulation to digestive organs - primary neurotransmitter is acetylcholine

1- Would you expect every gland and organ in the body to have both sympathetic and parasypathetic neurons (nerve cells) associated with them? Explain!

2- What would a person be attempting to do with their nervous system if they were to meditate in an effort to reduce stress?

3- A new organism is discovered near thermal vents at the bottom of the ocean. It is able to move and respond to touch. The organism displays radial symmetry. What component of its nervous system would you expect would be responsible for integration (evaluation of stimuli & initiation of response)?

4- What might be the significance of the sympathetic and the parasympathetic neurons releasing different neurotransmitters (chemicals discharged by a neuron onto the membrane of effector cells).

Reflex Arcs

Some nerve pathways are set up to bypass brain intervention. These pathways do send stimuli to the brain but a response is induced before the brain evaluates what the stimulus means. This type of nerve pathway is called a Reflex Arc. The stimuli moves from receptors to the nerve cord in the spine. The circuitry of a reflex arc immediately directs the stimulus signal through an interneuron to motor neurons to initiate a respons without requiring conscious control. The Knee Jerk reaction is an excellent example of a reflex arc neural pathway.

When the leg is bent the patella (knee cap) rests high on the knee and leaves tendons and ligaments exposed and vulnerable. A quick contraction of the quadricep muscle would rapidly straighten the leg and minimize injury if something were to strike the exposed tissues. Not requiring brain interaction saves time in intiating a response and may limit damage.

5- What generalization might be made about the purpose of reflex arcs?

6- Provide at least two more examples of reflex arcs in the human nervous system.

The human brain represents the utimate accomplishment of biological evolution. The ability to sort through millions of signals per second and inititiate the appropriate responses for each is awe inspiring. Modeling brain function has enabled elctrical engineers to make significant breakthroughs in the area of artificial intelligence but they still have a LONG WAY to go before they can come remotely close to modeling what the human brain can do.

Sensing changes in body temperature, changes in blood chemistry, light, sound, touch, and taste all at the same time and using the information to establish an ideal internal environment requires special areas of control in the brain.

3 major parts of the brain:

  • Brain Stem - senses the internal environment and intiates response through the Autonomic nervous system (heart rate, respiratory rate, blood pressure, food movement through the digestive system, secretions from glands....)
  • Cerebellum - coordinates impulses originating from the crebrum and destined for voluntary muscles (coordination)
  • Crebrum - thought, dreams, ideas, control of voluntary muscle, special senses (touch, taste, smell, sight, hearing)

The cerebrum is divided into two halves called cerebral hemispheres. The left side controls the right side of the brain and the right half controls the left. specific regions of control. The two hemispheres are not equal in the tasks that they control. Specialization and dominance of cerebral hemispheres may play a large role in determining which skills in an individual are strong. While some individuals have the ability to communicate well through verbal speech, as well as through written speech, others have difficulty wording phrases or speaking in sequential or logical sentences. Individuals with good verbal and logic skills most likely have a dominant left hemisphere. Some individuals are more capable of emotional and artistic expression such as music, painting, drawing, sculpting, and decorating, while others have little or no creative tendencies.these people are most likely have a dominant right cerebral hemisphere.

The Corpus Calossum relays information recieved by one hemisphere to the other side. This insures proper communication between the two halves and efficient evaluation of stimuli received. Individuals that have had brain surgery and the corpus callosum cut may show diminished function in their non dominant hemisphere.

Folds in the cerebral cortex offers greater surface area and a larger number of cell body connections. Recognizeable lobes can be seen in the cortex with specific areas of control. Subjecting a patient to specific stimuli and using heat sensitive probes to show areas of the brain that have enhanced circulation (seen as red regions on the thermo graph) have revealed the primary functions of these lobes.

A stroke is when circulation to a specific region of the brain has been blocked (blood clot, embolism, arteriosclerosis) or redirected (aneurysm). when this occurs the region of the brain impacted will dictate the consequences of such an event.

7- What is an aneurysm?

8- People suffering from a stroke often show loss of muscle control on one side of the body. If you noticed that a stroke victim showed only a half smile on the right side of their face and are unable to use their left hand, what could you assume about the location of the problem?


[Nervous System][Neuron][Sensory Organs]