Download Nervous from Cyber

Nervous from Cyber-Anatomy Web site and Think Quest
In the nervous system there are two main types of cells in the
nervous system: neurons and supporting cells. Neurons
provide a pathway for messages traveling to and from the
brain and spinal coed. Supporting cells provide structural
reinforcement, protection, insulation, and generally help
neurons.
Neurons are specialized cells for transporting signals from
location to location. There are many different types of
neurons, but they all share common characteristics. First of
all, they all share large cell bodies, and they all have fingerlike extensions called processes. Processes increase the
distance which the cells can conduct messages. There are two types of processes: dendrites and
axons.
Dendrites convey signals towards the cell body. Axons conduct signals away from the cell
body. Dendrites are short and numerous and branch extensively. Most neurons have only a
single axon which is usually very long. Schwann cells are arranged along the length of the
axon. All together these Schwann cells form an insulating layer which is called the myelin
sheath.
Axons can be branched and each branch can terminate in hundreds or thousands of small
branchlets called telodendria which end in a small bulbous end called the synaptic knob.
Synaptic knobs relay messages to other cells via neurotransmitters. Between the synaptic knobs
are gaps called synapses.
In the body cells usually carry a negative charge. Nervous cells are able to alter their charge
in order to conduct signals. Cells that are able to do this are called excitable cells. Neurons are
able to alter their charge largely due to ions.
Cells in which a signal begins are called pre-synaptic cells. Cells which receive the signal
are called post-synaptic cells. There are two types of synapses: electrical and chemical. An
electrical synapse allows the signal to spread directly from the pre-synaptic to the post-synaptic
cells. Chemical synapses allows for cells that so not have an electrical connection to spread their
messages. This happens by converting the electrical signal to a chemical signal that then travels
across the synapse and is then converted back to an electrical signal on the other side of the
synapse. These chemicals are called Neurotransmitters.
Dendrites convey signals towards the cell body. Axons conduct signals away from the cell
body. Dendrites are short and numerous and branch extensively. Most neurons have only a
single axon which is usually very long. Schwann cells are arranged along the length of the
axon. All together these Schwann cells form an insulating layer which is called the myelin
sheath.
Axons can be branched and each branch can terminate in hundreds or thousands of small
branchlets called telodendria which end in a small bulbous end called the synaptic knob.
Synaptic knobs relay messages to other cells via neurotransmitters. Between the synaptic knobs
are gaps called synapses.
In the body cells usually carry a negative charge. Nervous cells are able to alter their charge
in order to conduct signals. Cells that are able to do this are called excitable cells. Neurons are
able to alter their charge largely due to ions.
Cells in which a signal begins are called pre-synaptic cells. Cells which receive the signal
are called post-synaptic cells. There are two types of synapses: electrical and chemical. An
electrical synapse allows the signal to spread directly from the pre-synaptic to the post-synaptic
cells. Chemical synapses allows for cells that so not have an electrical connection to spread their
messages. This happens by converting the electrical signal to a chemical signal that then travels
across the synapse and is then converted back to an electrical signal on the other side of the
synapse. These chemicals are called Neurotransmitters.
At the present time there are about ten molecules that are known to be neurotransmitters.
Dozens more are good candidates and the list of possibilities is growing. Acetylcholine is the
most common neurotransmitter. This chemical triggers the contraction of a muscle cell at the
junction of the motor neuron and a skeletal muscle cell.