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PSYC 1010 Chapter Notes -Brainstem, Substance P, Peripheral Nervous System

Course Code
PSYC 1010
Rebecca Jubis

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November 22 2011 - The Nervous System pp. 177 to 203
NERVOUS SYSTEM: the body’s neuron-based master control system; it works together with the endocrine
- 90% of the nervous system consists of glia cells which help maintain the proper concentrations of
vital ions in the fluid around neurons and assist in the formation of connections between brain
Neurons are the communication lines of the nervous system
SENSORY NEURONS: a neuron that collects information about a stimulus and relays it to the spinal cord
and brain
INTERNEURON: a neuron in the spinal cord or brain that receives sensory input, processes it, and sends
signals to other neurons
MOTER NEURONS: a neuron that relays signals from interneurons to muscles and glands effectors that
carry out responses
DENDRITE: a short, branching extension of a neuron that receives incoming signals (“input zones”)
AXON: the extension of a neuron that conducts signals away from the neuron
Endings are known as “output zones” where messages are sent to other cells
Properties of a neuron’s plasma membrane allow it to carry signals
- Neurons can response to certain stimuli by producing an electrical signal
- In a resting neuron, the gated sodium channels are closed and the plasma membrane allows only a
little sodium to leak inward. The membrane is more permeable to potassium; therefore there is s a
concentration gradient across the membrane. Following the rules of diffusion, sodium tends to
move in and potassium tends to move out.
- The cytoplasm next to the membrane is more negative than the fluid outside of it. The steady
charge difference across the membrane is -70 millivolts. The minus indicates that the cytoplasm
side of the membrane is more negative than the outer side of the membrane (known as resting
membrane potential)
Nerve impulses = Action potentials
- When an adequate signal reaches a resting neuron’s input zone the sodium gates open and sodium
rushes into the neuron. As they flow in, the cytoplasm next to the plasma membrane become less
negative. When the voltage difference across the neuron plasma membrane shifts by a minimum
amount called the threshold, the result is an action potential
- When the threshold level is reached the opening of sodium gates doesn’t depend on the strength
of the stimulus anymore
Action potentials travel away from their starting point
- Each action potential propagates itself, moving away from its starting point. This occurs because
the changes in membrane potential leading to an action potential don’t lose strength.
A neuron can’t “fire” again until ion pumps restore its resting potential
- The area of the cell’s plasma membrane can’t receive another signal until its resting membrane
potential is restored
- Sodium is always leaking into the neuron (down an electrochemical gradient) and potassium is
always leaking out (down its concentration gradient) because the inside of the cell is more

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negative than the outside and the negatively charged proteins in the cytoplasm help create this
electric gradient
SODIUM POTASSIUM PUMP: a carrier protein through which active transport moves potassium ions into
a neuron and sodium ions outward with the energy from ATP
Action potentials are “all-or-nothing”
- Every action potential in a neuron spikes to the same level above threshold as an all-or-nothing
- If threshold is not reached, the disturbance to the plasma membrane will fade away as soon as the
stimulus is removed
- Each spike lasts for about a millisecond.
- Halfway through the action potential, potassium channels open so they can flow out and restore
the original voltage difference across the membrane.
- After the resting membrane potential has been restored, most potassium gates close and sodium
gates are in their initial state (ready to be opened when a stimulus arrives)
How neurons communicate
1. Action potentials flow along the axon of a motor neuron to neuromuscular junctions, where an
axon terminal forms a synapse with a muscle fibre
2. The axon terminal stores chemical signalling molecules called neurotransmitters inside synaptic
3. Arrival of an action potential causes exocytosis of synaptic vesicles, and neurotransmitter
molecules enter the synapse
4. The plasma membrane of the muscle fibre has receptors for the neurotransmitter
5. Binding of the neurotransmitter opens a channel through the receptor. The opening allows ions to
flow into the receiving cell
Neurotransmitters can excite or inhibit a receiving cell
- Exciting signals help drive the membrane toward an action signal
- Inhibiting signals have the opposite effects
- Neuromodulators can magnify or dampen the effects of a neurotransmitter
o Endorphins inhibit nerves from releasing substance P, which conveys information about
Examples of effects
Acetylcholine (Ach)
Causes skeletal muscle contractions;
affects mood and memory
Epinephrine and norepinephrine
Speed heart rate; dilate the pupils and
airways to lungs; slow GI tract
contractions; increase anxiety
Reduces excitatory effects of other
neurotransmitters; roles in memory
learning, fine motor coordination
Elevates mood; has a role in memory
Inhibits the release of other
Competing signals are “summed” up
- Signals compete for control of the membrane potential at the trigger zone
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