Download Ch. 27 notes - The Nervous System

BIO 110
Dr. Ely
Chapter 27 Notes – The Nervous System
I.
Functions of the nervous system
a. Deliver chemical messages from around the body to the brain and spinal cord
b. Deliver chemical messages to muscles and glands to respond to a stimulus
II.
Nervous system response to stimuli
a. Sensory input - specialized receptor cells of sensory organs (e.g. ears) detect a stimulus (e.g.
sound of an alarm clock) and a message travels to the brain and spinal cord
b. Integration - the central nervous system (CNS; brain + spinal cord) determines the nature of the
stimulus and chooses the appropriate response (e.g. “reach over and turn off alarm”)
c. Motor output - a message travels from the CNS to the appropriate muscles and glands to effect
a response (e.g. muscles are stimulated to move the body and turn off the alarm)
III.
Types of neurons - nervous system cells
a. Sensory neuron - carries signals from receptor cells to the CNS (see Sensory input above)
b. Interneuron - neurons in the CNS that process sensory information and determine appropriate
motor output (see Integration above)
c. Motor neuron - carries signals from the CNS to muscles and glands; causes muscles to contract
and glands to secrete (see Motor output above)
IV.
Parts of a neuron
a. Cell body - expanded portion of the neuron that contains the nucleus and other organelles
b. Dendrites - short extensions of the neuron that receive signals from sensory receptor cells and
other neurons
c. Axon - typically the longest extension of the neuron; transmits/conducts the signals gathered
by dendrites to the next cell
i. a myelin sheath covers most axons except some interneurons
1. formed from the membranes of other cells that wrap around the axon
2. speeds up the transmission of the nerve impulse (see below)
V.
The Nerve Impulse
a. Background information
i. A nerve impulse, or action potential, begins as a change in the overall charge of the
cytoplasm in an axon
ii. The inside of an axon is negative due to the maintenance of an ion concentration
gradient by the neuron, and also because of many large proteins in the cell that carry a
negative charge
iii. Certain proteins in the cell membrane are gated ion channels, one for Na+ and one for
K+; these channels open and close to begin and end an action potential
b. Propagation of a nerve impulse along the axon
i. The cytoplasm begins with a negative charge
BIO 110
Dr. Ely
ii. The sodium ion (Na+) channels are stimulated to open by the preceding segment
iii. Na+ rushes in until the cytoplasm at this location reaches a cation concentration
threshold that “flips” the charge to net positive
iv. the sodium ion channels close and the potassium ion (K+) channels open; meanwhile
sodium channels in the next section of the axon open, beginning a new action potential
v. K+ rushes out of the cytoplasm until it flips the charge to net negative again, restoring
resting conditions
vi. the next action potential forms in the adjacent section, then begins to shut down as the
following section forms an action potential
vii. In this way, the action potential is self-propagated along the axon until it reaches the
end. It is described as an “all-or-nothing” event: if the action potential forms, it will
travel the length of the axon.
viii. The myelin sheath speeds the transmission by restricting the opening of the gated
channels to small gaps between individual myelin sheath-forming cells. These gaps are
called the nodes of Ranvier. Since only the nodes of Ranvier can form action potentials,
the nerve impulse “jumps” down the axon much faster.
c. The Synapse
i. The synapse is the place where the axon terminals meet another cell and transmit a
nerve impulse
ii. The cell delivering the nerve impulse is called the pre-synaptic cell; the cell receiving the
nerve impulse is called the post-synaptic cell.
iii. The synaptic cleft is a small gap between the two cells at the site of the synapse.
iv. Nerve impulse transmission at the synapse
1. An action potential arrives at the axon terminal and stimulates the exocytosis of
vesicles filled with chemicals known as neurotransmitters
2. Neurotransmitters like acetylcholine and norepinephrine enter the synaptic cleft
and trigger the opening of sodium ion channels on the post-synaptic cell to
stimulate an action potential (if the cell is another neuron)
3. If the post-synaptic cell is a muscle or gland, other responses will be triggered by
the neurotransmitters that lead to a contraction (muscles) or secretion of a
substance (glands)
d. Organization of the nervous system
i. The Central Nervous System (CNS) consists of the brain and the spinal cord.
ii. The Peripheral Nervous system (PNS) consists of the cranial and spinal nerves that
branch off of the CNS and extend throughout the body
1. The Somatic system is the portion of the PNS that innervates skeletal muscle. All
of the body’s conscious movements, and the unconscious reflexes, are provided
by the somatic system.
2. The Autonomic system is the portion of the PNS that innervates smooth muscle,
such as the muscles of the digestive system.
BIO 110
Dr. Ely
a. The Parasympathetic division of the autonomic system controls these
muscles under relaxed conditions (e.g. normal breathing rate, digestive
system processing)
b. The Sympathetic division of the autonomic system controls these
muscles under emergency conditions, often called the “fight or flight”
response (e.g. increased breathing rate, suspension of digestive system
processing)
3. The nerves of the body are bundles of axons of neurons in the PNS. These
bundles are both sensory neuron axons carrying information from sensory
receptors toward the CNS as well as motor neuron axons carrying information
from the CNS to the muscles and glands.
4. The cell bodies of neurons are located either within the CNS (interneurons and
motor neurons) or in ganglia, which appear as small bulges just outside of the
CNS. Thus, individual axons can be up to one meter long.