PSYC 100 Chapter 3: Chapter 3
16 Dec 2016
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Chapter 3
Neurons: The Origin of Behavior
- Neurons: cells in the nervous system that communicate with one another to perform
information-processing tasks
- Santiago Ramon y Cajal (1852-1934) stained neurons in order to learn what they look like, and
was able to see that they come in different shapes and sizes
o He also sa that the theads did ot atuall touh othe euos
- He believed that neurons are the information-processing units of the brain, and thought that
despite the gaps, they still manage to communicate with each other
Components of the Neuron
- Cell body (soma): the part of a neuron that coordinates information-processing tasks and keeps
the cell alive
- Dendrites: the part of a neuron that receives information from other neurons and relays it to
the cell body
- Axon: the part of a neuron that transmits information to other neurons, muscles, or glands
o Terminal buttons: small knobs at the end of the axon that secrete neurotransmitters
- Myelin sheath: an insulating layer of fatty material
o The myelin sheath is made up of glial cells: support cells found in the nervous system
o Glial cells can digest parts of dead neurons, provide physical and nutritional support for
the neurons, or form the myelin to help the axon transmit information more efficiently
- Synapse: the junction or region between the axon of one neuron and the dendrites or cell body
of another for communication
o Most adults have between 100 trillion and 500 trillion synapses
Major Types of Neurons
- Sensory neurons: receive information from the external world and convey this information to
the brain via the spinal cord
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2
o E.g., bipolar neurons in retina
- Motor neurons: carry signals from the spinal cord to the muscles to produce movement
- Interneurons: connect sensory neurons, motor neurons, or other interneurons
o E.g., purkinje and pyramidal cells
- Besides these specializations, neurons are also somewhat specialized depending on their
location within the body
Electric Signalling: Communicating Information within a Neuron
- Communication between neurons proceeds in two stages: conduction and transmission
o 1st stage is conduction of an electric signal over relatively long distances within neurons
in a particular order (dendrites, cell body, axon)
o 2nd stage is the transmission of the electric signals between neurons over the synapse
o These stages together are referred to as the electrochemical action of neurons
The ‘estig Potetial: The Oigi of the Neuos Eletial Popeties
- The euos poous embrane allows ions to flow into and out of the cell
- Resting potential: the difference in electric charges between the inside and outside of a
euos ell eae (-70mV)
o I the euos estig state, thee is a high concentration of positive ions [K+] inside the
neuron compared to outside it
o In contrast, there is high concentration of Na+ outside neuron
o The concentration of K+ is controlled by channels in the axon membrane
o When the channels are open, K+ ions flow out, creating energy
- Resting potential is potential energy because it allows for possible electrical impulse
- Depolarization: process during which positively charged sodium (Na+) ions flow into the axon
making it less negatively charged
The Action Potential: Sending Signals over Long Distances
- The neuron maintains its resting potential most of the time
- Action potential: an electric signal that is conducted along an axon to a synapse and causes a
ief hage i a euos electrical charge (+40mV)
o Occurs when the electric shock reaches a certain level, or threshold
o An all-or-none that moves down the entire length of the axon. Increases in the electric
shock above threshold did not increase the strength of an action potential
o When an electric charge is raised to the threshold value, the K+ channels shut down, and
other channels allow positively charged ions, Na+, to flow inside
o After the action potential reaches its maximum, the membrane channels return to their
original state, and K+ flows out until the axon reaches its resting potential
- Refractory period: the time following an action potential during which a new action potential
cannot be initiated
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o The imbalance in ions eventually is reversed by an active chemical pump in the cell
membrane that moves Na+ outside and K+ inside
o Electron charge move down the axon in domino effect. This simple mechanism ensures
it achieves its full intensity regardless of distance travelled
o Myelin facilitates the conduction of action potential,
o Louis-Antoine Ranvier discovered myelin does not cover entire axon; the exposed
points are called the nodes of Ranvier
o When an electric current passes down the length of a myelinated axon, the charge
jups fo ode to ode athe tha haing to traverse the entire axon
o This jumping is called salutatory conduction, and it helps speed the flow of information
down the axon
Chemical Signalling: Synaptic Transmission between Neurons
- Axons end in terminal buttons: knob-like structures that branch out from an axon
o Terminal buttons are filled with vesicles that contain neurotransmitters: chemicals that
transmit information across the synapse to a eeiig euos dedites
- The dendrites of the receiving neuron contain receptors: parts of the cell membrane that
receive the neurotransmitter and initiate a new electric signal
- How do dendrites know which neurotransmitters to accept and which to dismiss? There are two
possible answers:
o Different types of neurotransmitters are prevalent in different parts of the brain, to
avoid confusion between adjacent neurons
o Receptor sites act like a lock-and-key system, so only some neurotransmitters bind to
particular sites
- Neurotransmitters leave the synapse through three processes:
o Reuptake occurs when neurotransmitters are reabsorbed by the terminal buttons of the
presynaptic neuron (where the signal came from) axon
o Neurotransmitters can be destroyed by enzymes in the synapse in a process called
enzyme deactivation
o Neurotransmitters can bind to the receptor sites called autoreceptors on the
presynaptic neurons
▪ Autoreceptors detect how much of a neurotransmitter has been released and
signal the neuron to stop releasing said neurotransmitter
Synaptic Transmission
- Postsynaptic potential (PSP): voltage change at receptor site
- P“Ps ae gaded
o I.e., they increase or decrease the probability of a neural impulse in the receiving cell in
proportion to their size
- Tpes of P“Ps:
o Excitatory PSP
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Document Summary
Neurons: cells in the nervous system that communicate with one another to perform information-processing tasks. Santiago ramon y cajal (1852-1934) stained neurons in order to learn what they look like, and. 1 was able to see that they come in different shapes and sizes: he also sa(cid:449) that the (cid:858)th(cid:396)eads(cid:859) did (cid:374)ot a(cid:272)tuall(cid:455) tou(cid:272)h othe(cid:396) (cid:374)eu(cid:396)o(cid:374)s. He believed that neurons are the information-processing units of the brain, and thought that despite the gaps, they still manage to communicate with each other. Cell body (soma): the part of a neuron that coordinates information-processing tasks and keeps the cell alive. Dendrites: the part of a neuron that receives information from other neurons and relays it to the cell body. Axon: the part of a neuron that transmits information to other neurons, muscles, or glands: terminal buttons: small knobs at the end of the axon that secrete neurotransmitters.
