Anatomy Nervous System
Anatomy Nervous System
Anatomy Nervous System
Coordination
Animals must be able to sense and respond to the environment in which they live if they are to survive. They need to
be able to sense the temperature of their surroundings, for example, so they can avoid the hot sun. They must also be
able to identify food and escape predators.
The various systems and organs in the body must also be linked so they work together. For example, once a predator
has identified suitable prey it has to catch it. This involves coordinating the contraction of the muscle so the predator
can run, there must then be an increased blood supply to the muscles to provide them with oxygen and nutrients. At
the same time the respiration rate must increase to supply the oxygen and remove the carbon dioxide produced as a
result of this increased activity. Once the prey has been caught and eaten, the digestive system must be activated to
digest it.
The adjustment of an animals response to changes in the environment and the complex linking of the various
processes in the body that this response involves are called co-ordination. Two systems are involved in
co-ordination in animals. These are the nervous and endocrine systems. The first operates via electrical impulses
along nerve fibres and the second by releasing special chemicals or hormones into the bloodstream from glands.
The Neuron
Neurons are cells that have been adapted to carry nerve impulses. A typical neuron has a cell body containing a
nucleus, one or more branching filaments called dendrites which conduct nerve impulses towards the cell body and
one long fibre, an axon, that carries the impulses away from it. Many axons have a sheath of fatty material called
myelin surrounding them. This speeds up the rate at which the nerve impulses travel along the nerve (see diagram
14.1).
Diagram 14.2 - The relationship between sensory, relay and motor neurons
Reflexes
A reflex is a rapid automatic response to a stimulus. When you accidentally touch a hot object and automatically jerk
your hand away, this is a reflex action. It happens without you having to think about it. Animals automatically blink
when an object approaches the eye and cats twist their bodies in the air when falling so they land on their paws.
(Please dont test this one at home with your pet cat!).
Swallowing, sneezing, and the constriction of the pupil of the eye in bright light are also all reflex actions.
The path taken by the nerve impulses in a reflex is called a reflex arc. Most reflex arcs involve only three neurons
(see diagram 14.4). The stimulus (a pin in the paw) stimulates the pain receptors of the skin, which initiate an
impulse in a sensory neuron. This travels to the spinal cord where it passes, by means of a synapse, to a connecting
neuron called the relay neuron situated in the spinal cord. The relay neuron in turn makes a synapse with one or more
motor neurons that transmit the impulse to the muscles of the limb causing them to contract and remove the paw
from the sharp object. Reflexes do not require involvement of the brain although you are aware of what is happening
and can, in some instances, prevent them happening. Animals are born with their reflexes. You can think of them as
being wired in.
Conditioned Reflexes
In most reflexes the stimulus and response are related. For example the presence of food in the mouth causes the
salivary glands to release saliva. However, it is possible to train animals (and humans) to respond to different and
often quite irrelevant stimuli. This is called a conditioned reflex.
A Russian biologist called Pavlov carried out the classic experiment to demonstrate such a reflex when he
conditioned dogs to salivate at the sound of a bell ringing. Almost every pet owner can identify reflexes they have
conditioned in their animals. Perhaps you have trained your cat to associate food with the opening of the fridge door
or accustomed your dog to the routines you go through before taking them for a walk.
Summary
The neuron is the basic unit of the nervous system. It consists of a cell body with a nucleus, filaments known as
dendrites and a long fibre known as the axon often surrounded by a myelin sheath.
A nerve is a bundle of axons.
Grey matter in the brain and spinal cord consists mainly of brain ells while white matter consists of masses of
axons.
Nerve Impulses travel along axons.
Adjacent neurons connect with each other at synapses.
Reflexes are automatic responses to stimuli. The path taken by nerve impulses involved in reflexes is a reflex arc.
Most reflex arcs involve 3 neurons - a sensory neuron, a relay neuron and a motor neuron. A stimulus, a pin in
the paw for example, initiates an impulse in the sensory neuron that passes via a synapse to the relay neuron
situated in the spinal cord and then via another synapse to the motor neurone. This transmits the impulse to the
muscle causing it to contract and remove the paw from the pin.
The nervous system is divided into 2 parts: the central nervous system, consisting of the brain and spinal cord
and the peripheral nervous system consisting of nerves connected to the brain and spinal cord. The autonomic
nervous system is considered to be part of the peripheral nervous system.
The brain consists of three major regions: 1. the fore brain which includes the cerebral hemispheres (or
cerebrum), hypothalamus and pituitary gland; 2. the hindbrain or brain stem containing the medulla
oblongata and 3. the cerebellum.
Protective membranes known as the meninges surround the brain and spinal cord.
There are 12 pairs of cranial nerves that include the optic, olfactory, accoustic and vagus nerves.
The spinal cord is a cable of nerve tissue surrounded by meninges passing from the brain to the end of the tail.
Spinal nerves emerge by a ventral and dorsal root between each vertebra and connect the spinal cord with
organs and muscles.
The autonomic nervous system controls internal body functions not under conscious control. It is divided into 2
parts with 2 different functions: the sympathetic nervous system that is involved in the flight and fight response
including increased heart rate, bronchial dilation, dilation of the pupil and decreased gut activity. The
parasympathetic nervous system is associated with decreased heart rate, pupil constriction and increased gut
activity.
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Worksheet
Nervous System Worksheet [2]
Test Yourself
1. Add the following labels to this diagram of a motor neuron.
cell body | nucleus | axon | dendrites | myelin sheath | muscle fibres
2. What is a synapse?
3. What is a reflex?
4. Rearrange the parts of a reflex arc given below in the order in which the nerve impulse travels from the sense
organ to the muscle.
sense organ | relay neuron | motor neuron | sensory neuron | muscle fibres
5. Add the following labels to the diagram of the dogs brain shown below.
cerebellum | cerebral hemisphere | cerebral cortex | pituitary gland | medulla oblongata
6. What is the function of the meninges that cover the brain and spinal cord
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Websites
http://en.wikipedia.org/wiki/Neuron Wikipedia. Lots of good information here but as usual a warning that
there are terms and concepts that are beyond the scope of this course. Also try reflex action ; autonomic nervous
system ;
http://images.google.co.nz/imgres?imgurl=http://static.howstuffworks.com/gif/brain-neuron.gif&
imgrefurl=http://science.howstuffworks.com/brain1.htm&h=296&w=394&sz=17&hl=en&start=5&
tbnid=LWLRI9lW_5PZhM:&tbnh=93&tbnw=124&prev=/
images%3Fq%3Dneuron%26svnum%3D10%26hl%3Den%26lr%3D%26sa%3DN How Stuff Works. This site is
for the neuron but try neuron types, brain parts and balancing act too.
http://www.bbc.co.uk/schools/gcsebitesize/flash/bireflexarc.swf Reflex Arc. Nice clear and simple
animation of a reflex arc.
Glossary
Link to Glossary [3]
References
[1] http:/ / flickr. com/ photos/ royalty-free-images/ 139764662/
[2] http:/ / www. wikieducator. org/ Nervous_System_Worksheet
[3] http:/ / en. wikibooks. org/ wiki/ Anatomy_and_Physiology_of_Animals/ Glossary
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License
Creative Commons Attribution-Share Alike 3.0 Unported
http:/ / creativecommons. org/ licenses/ by-sa/ 3. 0/
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