Chapter 1: Nerve Cells and Nerve Impulses Flashcards
What is mind-brain and mind-body problem?
How mind relates to body and why does consciousness exist
What is biological psychology?
study of physiological, evolutionary, and developmental mechanisms of behavior and experience. Emphasizes that the goal is to relate biology to issues of psychology (P5). Also a point of view, it holds that we think and act as we do b/c of brain mechanisms b/c ancient animals built this way survived and reproduced (P6)
Dorsal view
View from the top of the brain
Ventral view
View from the bottom of the brain
2 kinds of cells in brain
Neuron and glia
Neurons
convey messages to one another and to muscles and glands, vary enormously in size, shape and functions
Glia
smaller than neurons, have many functions but don’t convey info over great distances
How many neurons in cerebral cortex
Around 16 billion
How many neurons in cerebellum
69 billion
How many neurons in spinal cord
1 billion
What do neurons do?
Receive info and transmit it to other cells
How many cells does average brain have
89 billion
How did we learn that cells were separated individually
By staining cells, which allowed us to study individual cells
What is membrane of cell
Surface of cell, structure that separates inside from the outside environment
What can cross cell membrane
Most chemicals can’t cross membrane, but protein channels in membrane permit controlled flow of water, oxygen, potassium, sodium, chloride and other important chemicals
What cells tend to have a nucleus
Except for mammalian red blood cells, all animal cells have a nucleus, the structure that contains chromosomes
Mitochondria
performs metabolic activities, providing energy that the cell uses for all activities. Mitochondria are genetically unique from one another
What happens if mitochondria overheats or is less active
Overactivity leads to overheating and burning fuel rapidly. Less active can lead to depression and pain. Mutated mitochondria can lead to autism (P19)
Ribosomes
sites within cell that synthesize new protein molecules
Endoplasmic reticulum
network of thin tubes that transport newly synthesize proteins to other locations (P19). Other ribosomes float freely in cells
What do most cells have?
Cell body (Soma), dendrites, axons and presynaptic terminals (P19)
Motor neuron
Efferent (Exit) sends signal
Sensory neuron
afferent (admits) Receives info (P19)
Dendrites
Branching fibers that get narrower near their ends. Surface is lined with specialized synaptic receptors, at which the dendrite receives info from other cells (P20)
Dendritic spines
increases surface area of dendrite, increase sensory capacity (P20)
Soma/cell body
contains ribosomes, nucleus, and mitochondria (P20). Cell body covered w/ synapses on its surface
Axon
conveys impulse to other neurons, organs, muscles
Myelin sheath
insulating material for axons which allow for messages to travel faster more safely (P21)
Nodes of ranvier
interruptions in myelin sheath (P21)
presynaptic terminal
end of each axon branch, aka end bulb or bouton. Releases chemicals from axon that cross junction b/w neuron and another cell (P21)
afferent axon
admit info
efferent axon
exit info (P21)
Glia
Performs many functions. Cells that enhance and modify the activity of neurons in many ways
astrocytes
type of glia. Important for rhythmic activity like breathing. Wraps around synapses of functionally related axons and shields it from chemicals circulating around
microglia
part of immune system, removes viruses and fungi from brain. Proliferate after brain damage removing dead or damaged neurons. Also contribute to learning by removing weakest synapses
oligodendrocytes
in the brain and spinal cord, build myelin sheath that surround and insulate certain vertebrate axons
Schwann cells
in periphery of body and build myelin sheath that surround and insulate certain vertebrate axons
radial glia
guide migration of neurons and their axons and dendrites during embryonic development (P22)
blood brain barrier
depend on endothelial cells that form walls of capillaries. Most large molecules and electrically charged molecules can’t cross, but small uncharged molecules like oxygen and carbon dioxide cross easily. Active transport system pumps glucose and amino acids across membrane
Active transport
protein-mediated process that expends energy to pump chemicals from the blood into the brain. Includes glucose (main energy source of brain), amino acids (building blocks of protein)
What do vertebrate neurons run on for fuel
Glucose, only nutrient to be able to cross blood brain barrier in large quantities. Requires large supply of oxygen
What is the basis of neuron function?
Location, structure and activities
electrical gradient (polarization)
Difference in electrical charge b/w inside and outside of cell. The electrical potential inside the membrane is slightly nega- tive with respect to the outside, mainly because of negatively charged proteins inside the cell
Reason why signal reaches the brain when sensory input from other parts of body are perceived
the axon regenerates an impulse at each point. Imagine a long line of people holding hands. The first person squeezes the second person’s hand, who then squeezes the third person’s hand, and so forth. The impulse travels along the line without weakening because each person generates it anew.
Selective permeability
Some chemicals pass through it more freely than others do. Oxygen, carbon dioxide, urea, and water cross freely through channels that are always open. Several biologically important ions, including sodium, potassium, calcium, and chloride, cross through membrane channels (or gates) that are sometimes open and sometimes closed. When the membrane is at rest, the sodium and potassium channels are closed, permitting almost no flow of sodium and only a small flow of potassium. Certain types of stimulation can open these channels, permitting freer flow of either or both ions.
Sodium-potassium pump
The sodium–potassium pump is an active transport that requires energy. As a resultof the sodium–potassium pump, sodium ions are more than 10 times more concentrated outside the membrane than inside, and potassium ions are more concentrated in- side than outside.
The sodium–potassium pump is effective only because of the selective permeability of the membrane, which prevents the sodium ions that were pumped out of the neuron from leaking right back in again. When sodium ions are pumped out, they stay out. However, some of the potassium ions in the neuron slowly leak out, carrying a positive charge with them. That leakage increases the electrical gradient across the mem- brane.
What two forces act on sodium and potassium ions
- Electrical gradient: Sodium positively charged, cell negatively charged, opposites attract, pulling them towards each other.
- Concentration gradient: The difference in distribution of ions across the membrane. Sodium is more concentrated outside than inside, so just by the laws of probability, sodium is more likely to enter the cell than to leave it. Given that both the electrical gradient and the concentra- tion gradient tend to move sodium ions into the cell, sodium would enter rapidly if it could. However, because the sodium channels are closed when the membrane is at rest, almost no sodium flows except for what the sodium–potassium pump forces out of the cell.
Potassium is subject to competing forces. Potassium is positively charged and the inside of the cell is negatively charged, so the electrical gradient tends to pull potassium in. However, potassium is more concentrated inside the cell than outside, so the concentration gradient tends to drive it out.
Typical voltage level inside cell
-70mV
Resting potential
prepares body to act rapidly
Action potential
messages sent by axons:
1. When an area of the axon membrane reaches its threshold of excitation, sodium channels and potassium channels open.
2.At first, the opening of potassium channels produces little effect.
3. Opening sodium channels lets sodium ions rush into the axon.
4. Positive charge flows down the axon and opens voltage- gated sodium channels at the next point.
5. At the peak of the action potential, the sodium gates snap shut. They remain closed for the next millisecond or so, despite the depolarization of the membrane.
6. Because voltage-gated potassium channels remain open, potassium ions flow out of the axon, returning the membrane toward its original depolarization.
7. A few milliseconds later, the voltage-dependent potassium channels close.
hyper polarization
change in charge in neuron. Exaggeration of negative charge. When the stimulation ends, the charge returns to its original resting level
Depolarization
Decrease in negative charge (increase in positive charge). If cell charge reaches threshold, leads to action potential
Threshold of excitation
produces a massive depolarization of the membrane. When the potential reaches the threshold, the membrane opens its sodium channels and lets sodium ions flow into the cell.
Sub threshold stimulation
produces a small response that quickly decays. Any stimulation beyond the threshold, regardless of how far beyond, produces a big response
all or none law
amplitude and velocity of an action potential are independent of the intensity of the stimulus that initiated it, provided that the stimulus reaches the threshold. By analogy, imag- ine flushing a toilet: You have to make a press of at least a certain strength (the threshold), but pressing harder does not make the toilet flush faster or more vigorously.
Molecular basis of action potential
- At the start, sodium ions are mostly outside the neuron, and potassium ions are mostly inside.
- When the membrane is depolarized, sodium and potas- sium channels in the membrane open.
- At the peak of the action potential, the sodium channels close.
voltage gated channels
axon channels regulating sodium and potassium permeability depends on the voltage difference across the membrane. At the resting potential, the sodium channels are fully closed and the potassium channels are almost closed, allowing only a little flow of potassium. As the membrane becomes depo- larized, both the sodium and the potassium channels begin to open, allowing freer flow. At first, opening the potassium channels makes little difference, because the concentration gradient and electrical gradient are almost in balance anyway. However, opening the sodium channels makes a big difference, because both the electrical gradient and the concentration gradient tend to drive sodium ions into the neuron. When the depolarization reaches the threshold of the membrane, the sodium channels open wide enough for sodium to flow freely. Driven by both the concentration gradient and the electrical gradient, the sodium ions enter the cell rapidly, until the electrical potential across the membrane passes beyond zero to a reversed polarity. Of the total number of sodium ions near the axon, less than 1 percent cross the membrane during an action potential. Even at the peak of the action potential, sodium ions continue to be far more concentrated outside the neuron than inside. Because of the persisting concentration gradient, sodium ions still tend to diffuse into the cell. However, at the peak of the action potential, the sodium gates snap shut.
Depolarizing the membrane also opens potassium channels. At first, opening those channels made little difference. However, after so many sodium ions have crossed the membrane, the inside of the cell has a slight positive charge instead of its usual negative charge. At this point both the concentration gradient and the elec- trical gradient drive potassium ions out of the cell. As they flow out of the axon, they carry with them a positive charge. Because the potassium channels remain open after the sodium channels close, enough potassium ions leave to drive the membrane beyond its usual resting level to a temporary hyper polarization.
At the end of this process, the membrane has returned to its resting potential, but the inside of the neuron has slightly more sodium ions and slightly fewer potassium ions than before. Eventually, the sodium–potassium pump restores theoriginal distribution of ions, but that process takes time. After an unusually rapid series of action potentials, the pump cannot keep up with the action, and sodium accumulates within the axon. Excessive buildup of sodium can be toxic to a cell.
Local anesthetic drugs
attach to the sodium channels of the membrane, preventing sodium ions from entering.
propagation of action potential
describes the transmission of an action potential down an axon.
Back-propagation
when action potential back propagates into dendrite, dendrite becomes more susceptible to structural changes responsible for learning
myelin sheaths
insulating material composed of fats and proteins.
myelinated axons
axons in vertebrates covered in fat and protein
Where does action potential start
Nodes of ranvier
Saltatory conduction
Jumping of action potentials from node to node. Conserves energy
refractory period
sodium gates shut, resists production of further action potentials
Absolute refractory period
Membrane cannot produce another action potential regardless of stimulation
relative refractory period
stronger than usual stimulus needed to initiate an action potential
What does refractory period depend on
- Sodium channels are closed
- Potassium flowing out of cell at faster than usual rate
local neurons
have no axon, can only exchange info with neighbors, all or none rule doesn’t apply, can have graded potential
Importance of inhibiting and activating neurons
Inhibiting neurons just as important as activation
Does action potential lose intensity over distance
No, cost is a delay between the stimulus and its arrival in the brain
tripartite synaptic hypothesis
tip of an axon releases chemicals that cause neighboring astrocyte to release chemicals of its own, magnifying or modifying the message to the next neuron (possible contributor to learning and memory)
Adenosine triphosphate (ATP
provides energy, runs sodium-potassium pump
temporal summation
additive effect can pass threshold of postsynaptic neuron from one neuron
spatial summation
summed of multiple neurons at same time
EPSP
excitatory graded potential (depolarization)
IPSP
Inhibitory graded potential (hyper polarization)
Process of synaptic transmission
- Action potential: Used to send electrical chemical signals (ions). Sodium moves into cell (depolarizes neuron). Once it reaches threshold, sodium channel opens (voltage gated sodium channel). This further depolarizes cell, and leads to next sodium channel opening and more sodium coming into neuron. Once sodium channels all open and sodium enters cell, they all shut. This leads to potassium channels opening, which causes potassium to rush out of cell. This causes neuron to become more polarized again, becoming hyperpolarized (unable to fire again until it returns to baseline).
Phases of action potential:
a. Sodium comes in (depolarization)
b. Potassium goes out (hyperpolarization)
c. Sodium-potassium pump throws 3 sodium out and 2 potassium in (resets to baseline - Neurotransmitter release: Excitatory or inhibitory transmitters. Released into gap b/q 2 cells. Chemicals need to diffuse into synaptic cleft.
- Neurotransmitter binding: Chemical binds to receptor specifically for it.
- Postsynaptic potential: Graded depolarization caused by sodium ions entering neuron. If it is excitatory, it will bind to sodium or calcium receptor channels (depolarization). Triggers influx of ions. Inhibitory neurotransmitter will bind to potassium channels or chloride channels (due to hyperpolarization).
- Integration
- Neurotransmitter reuptake
