Coordination
In multicellular organisms, such as plants and animals, it is essential that cells can
communicate with each other. This allows them to coordinate their activities
appropriately. Organisms have specialised cells or molecules, called receptors, which
are sensitive to changes in their internal or external environment. These trigger events in
the organism that bring about coordinated responses to the environmental changes.
1) Nervous and endocrine systems as communication systems
a. The basic similarities of 2 systems:
– Provide the body with methods to communicate with its internal and external
environments in order to coordinate responses.
– Employ chemicals to transmit messages and respond to stimulus caused by changes
in their environments.
b. The differences in response times and how they work.
– The nervous system responds to stimuli by sending electrical action potentials along
neurons, which in turn transmit these action potentials to their target cells using
neurotransmitters, the chemical messenger of the nervous system. This response to
stimuli is near instantaneous.
– Hormones are synthesized at a distance from their target cells, and travel through the
bloodstream or intercellular fluid until they reach these cells. Upon reaching their target
cell, the hormones act on the cell to increase or decrease the expression of specific
genes. This process takes significantly longer, as hormones must first be synthesized,
transported to their target cell, and enter or signal the cell. Then, the target cell must go
through the process of transcription, translation, and protein synthesis before the
intended action of the hormone is seen. Although hormones act more slowly than a
nervous impulse, their effects are long lasting. Additionally, target cells can respond to
minute quantities of hormones and are sensitive to subtle changes in hormone
concentration.
c. The nervous and endocrine systems work together to maintain homeostasis.
The endocrine and nervous systems work independently to carry out unique functions
by different methods with some similar elements. However, they do work together to
control and co-ordinate the internal environment of the animal.
The nervous system responds rapidly to short-term changes by sending electrical
impulses.
The endocrine system brings about longer-term adaptations by sending out chemical
messengers (hormones) into the bloodstream.
2) Nerve cells/Neurones
The nervous system composed of nerve cells, or neurones. A neurone has a cell
body with extensions leading off it. Several dendrons carry nerve impulses towards the
cell body, while a single long axon carries the nerve impulse away from the cell body.
Axons and dendrons are only 10µm in diameter but can be up to 4m in length in a large
animal (a piece of spaghetti the same shape would be 400m long)! A nerve is a discrete bundle of several thousand neurone axons.
Nerve impulses are passed from the axon of one neurone to the dendron of another at
a synapse. Numerous dendrites provide a large surface area for connecting with other
neurones.
Most neurones also have many companion cells called Schwann cells, which are
wrapped around the axon many times in a spiral to form a thick lipid layer called
the myelin sheath. The myelin sheath provides physical protection and electrical
insulation for the axon, which greatly speeds up the transmission of action
potentials. There are gaps in the sheath, called nodes of Ranvier. Not all neurones are
myelinated.
Humans have 3 types of neurone:
Sensory neurones have long dendrons and transmit nerve impulses from
sensory receptors all over the body to the central nervous system.
Motor neurones have long axons and transmit nerve impulses from the central
nervous system to effectors (muscles and glands) all over the body.
Interneurones (also called connector neurones or relay neurones) are much
smaller cells, with many interconnections. They comprise the central nervous system.
99.9% of all neurones are interneurones.
Neurones are highly spectalised cells that are adapted for the rapid transmission of
electrical impulses, called action potentials, from one part of the body to another.
Information picked up by a receptor is transmitted to the central nervous system (brain
or spinal cord) as action potentials travelling along a sensory neurone. These neurones
have their cell bodies in small swellings, called ganglia, just outside the spinal cord.
