The Structure of the Nervous System
The human nervous system is composed of two major parts:
The Central Nervous System (CNS)
- made up of the brain and spinal cord
The Peripheral Nervous System
- includes the nerves that lead into and out of the CNS
- consists of the autonomic nervous system and somatic nervous
system
The Anatomy of a Neuron
The nervous system is composed of specialized cells called
neurons. Neurons are specialized for the transmission of
information at rapid rates. Maintenance of homeostasis depends
on this transmission and response.
A neuron consists of the following basic parts:
 Dendrites - thread-like projections that pick up information
from other neurons
 Cell body - contains nucleus and other cell organelles
necessary for proper cell functioning
 Axon
- long stalk through which impulse travels
 Myelin sheath - some neurons possess a fatty covering
over the axon which allows impulse to be
transmitted much faster
 Schwann cells - present in myelinated neurons
- cells which produce myelin
- specialized neurological cells which
provide nutrients for neurons
Types of neurons:
1.
2.
3.
Sensory neurons - receptors; detect change in environment
Association neurons - connect sensory neurons to motor
neurons
Motor neurons - activate muscles and glands
Transmission of a Nerve Impulse
 Information is picked up by the dendrites and is transmitted
towards the axon and AWAY from the dendrites of the neuron
by a wave of polarization or difference in electrical potential.
 At rest neuron contains high conc. of K+ ions inside and a high
conc. of Na+ ions outside
- cell expends energy to do this at rest
(ACTIVE TRANSPORT) (RECALL: Na+/ K+ pump)
- 3 Na+ sent out for every 2 K+ in
- this creates a "resting potential"
- cytoplasm has many negatively charged molecules
such as Cl- ions (inside is negatively charged at rest)
- difference in charge causes voltage of -70 mV
 Nerve impulse is transmitted once the resting potential voltage
is altered or "depolarized"
- this is called an "action potential"
- in response to a stimulus, membrane permeability is altered
- voltage-sensitive protein channels in membrane open and
admit Na+ ions in a small area
- K+ ions flow out as a result causing repolarization
- takes milliseconds to occur
 there is a minimum level of stimulus required to cause a
depolarization (threshold potential)
- an “all or nothing” response
 Resting period of several milliseconds before neuron is ready to
“fire again” or produce another nerve impulse called the
refractory period (milliseconds)
 Saltatory conduction occurs in myelinated neurons
- action potential generated only at Nodes of Ranvier
- impulse “skips” from one node to the next
- speeds of up to 270 mph or 120 m/s !!!