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How does the eagle judge the distance from the ground? How does the rabbit decide which plant to eat? Does the rabbit think the eagle looks beautiful soaring up there? Where do your thoughts come from and how do you put them into words to speak them to someone else? Nerve impulses traveling from receptors in the skin or in the eyes transmit messages to the brain and back to the muscles.
TINY SENSORS CREATE A RADICAL MOLECULAR RESPONSE
What a beautiful summer day at the beach: palm trees swaying, seagulls calling. Taste the salt in the hot air. The water looks cool and refreshing, rippling and glittering in the sun as the waves gently wash onto the sand and splash onto the rocks at the base of the coastal bluffs. Removing your shoes, you blissfully step off the boardwalk into dry, soft, deep sand to find a place for your towel near the water.
Suddenly,
HOT!
HOT!
HOT!
You sprint to the water before the extreme sand temperature can burn holes in your feet, or at least that is what it feels like.
What happened? While the optic nerve behind your eyes and the sensory nerves in your nose sent messages to the pleasure centers in your brain that the scene in front of you could not be more perfect, the skin sensors on the bottom of your feet registered dangerous temperatures. Instantly, all of your body systems went from zero to 60 in under a second.
- Your brain sent a message back to the muscles along motor neurons to lift your feet quickly and to the adrenal glands to produce epinephrine.
- Your heart started racing.
- Your breathing became rapid.
- You began to sweat.
- Your face muscles contracted.
- You lost your appetite.
It all began with a sensory neuron (nerve cell) similar to the one below and the signal sent along the nerve cell.
Learn more about how sensory receptors work.
Nerve cells (neurons), like most other cells, have a nucleus, plasma membrane, mitochondria and the other cell organelles. The part of the neuron where the nucleus and most of the other organelles can be found is called the Cell Body. The cell body has many projections called Dendrites that receive and pass along messages.
Review cell organelles.
If the cell body of a nerve cell with its dendrites sticking out in all directions looks like someone who put their finger in an electrical socket it is an appropriate description. When the message comes in, an electrical impulse fires down the dendrite to the cell body and then zips down the Axon to the next nerve cell (neuron).
When the message arrives at an axon terminal, Synaptic Knob, the message is transferred across the Synaptic Cleft to a dendrite on the next nerve cell via a chemical process.
Read more information on the synapse.
View Enlarged Picture of Neuron/Synapse
Explanation:
- The electrical impulse traveling down the axon is the signal.
- A vesicle inside the synaptic knob containing neurotransmitter molecules responds to the signal.
- The membrane of the vesicle merges with the cell membrane on the synaptic knob.
- The neurotransmitter molecules are released into the synaptic cleft between nerve cells and bind to the ion channel receptor sites.
- The protein that forms the ion channel changes shape in response to the bound neurotransmitter and opens the channel similar to the way a key in a lock that makes the door open.
- Positive sodium ions flow through the open ion channel into the dendrite of the next neuron making the area slightly positively charged and transmitting the impulse on through more neurons to the brain.
Watch an animation of a nerve impulse.
ASSOCIATED MOLECULES
Neurotransmitters are small molecules but the neurotransmitter receptor sites are on giant proteins.
The neurotransmitter Acetylcholine can be represented like this:
A model of the Receptor Protein that creates the sodium channel looks like this:
Image from the Protein Databank
THREE TYPES OF NEURONS
View Enlarged Picture of Three Types of Neurons
Drawing by Ann Marie Wellhouse
Find out more about neurons.
STRUCTURE OF THE NERVOUS SYSTEM
What do living things do?
Move
Breathe - Respire
Eat – Metabolize nutrients
Reproduce
Respond to Stimuli
All of those functions are mediated by the nervous system.
The Peripheral Nervous System (PNS) - the sensory cells and nerves - links the stimuli and response. The peripheral nervous system begins with sensory cells that perceive the stimuli. Sensory nerve cells carry the message to the central nervous system. The Central Nervous System (CNS) - the brain and the spinal cord – interprets the signal and regulates the response to stimuli. Interneurons in the brain and spinal cord link the message to the motor neurons. Motor neurons carry the message back to tell the muscles to move the body.
Take a deep breath. You can adjust your breathing consciously but most of the time you are not aware of your body when it takes in oxygen and sends out carbon dioxide waste. That is your Autonomic Nervous System at work. It makes sure you breathe and your heart beats, that your stomach has the digestive chemicals it needs after you eat, and it also regulates many other activities that you never really think about.
The Autonomic Nervous System includes:
The Sympathetic system which prepares the body to respond; and,
The Parasympathetic system which instructs the body to relax.
Investigate the autonomic system and the fight or flight mechanism.
What an Odd Shape
Inside the head of animals is a strange looking organ called the brain. The brain is the place where the signals of the nervous system are analyzed and managed to produce a response. For example, vision is largely the result of the processing and interpretation going on in the visual cortex of the brain. The visual data from the sensory neurons of the eyes travels via nerve impulses to the occipital lobe of the brain. The occipital lobe is linked to other parts of the brain.
Learn more about the vision pathway.
The mammal brain evolved after the invertebrates, fish, amphibians, reptiles and birds. It includes many homologous structures from those animals. The brainstem, for instance is called the old brain or the reptilian brain because it is a major brain structure in reptiles. Structures in one animal that are similar and develop from the same tissue area as those in a common ancestor are said to be homologous.
The mammal brain is also much more complex than the other animals. It is divided into many parts:
Brain Stem directs the autonomic nervous system;
Cerebellum is where the information is stored to automatically remember how to do something physical such as the movement skills used for swimming;
Cerebral Cortex is the area where more advanced thinking and responding occur - It is divided into lobes;
Limbic System and Thalmus are regulators (e.g. body temperature and heart rate);
Corpus Callosum links the two hemispheres.
View "3-D brain anatomy" and explore the brain by structure and function.
Mammals with an advanced ability to use complex thinking skills, like humans, often have a folded, convoluted cerebral cortex that provides more surface area in the small space of the skull. They also often have a larger cortex in comparison to the size of the brain stem.
Mammals that rely more on instinct may have more brain stem than their relatively smooth cortex.
WHAT CONTROLS THE DEVELOPMENT OF THIS AMAZING STRUCTURE?
Scientists have discovered many genes that are essential for normal brain development to occur. However, brain development is not simply controlled genetically (by “nature”), the environment (“nurture”) also plays an important role. In addition, there is a great deal of flexibility in brain development. Scientists say that the developing brain exhibits plasticity because it can often organize things differently to compensate for damage that would lead to severe limitations if it occurred in an adult brain.
Watch the clip from the video The Developing Brain: An Intricate Blend of Nature and Nurture. In the video Joan Stiles, professor of cognitive sciences at UCSD, describes how the brain of a girl who had a prenatal (before birth) stroke that damaged all of the major language areas compensated for the damage. In adults, the same sort of damage leads to a severe language disorder called aphasia. What were the young girl’s language abilities? How is this possible?
INSERT: Clip: 48 minutes 45 seconds to 55 minutes 54 seconds
From “These are MRI images…” to “But also recruits these additional language resources.”
