Lecture 2: The Nervous System Flashcards
stimulus is detected by a specific _______ that initiates a nervous (electrical) signal
receptor cell
the signal is then sent
(transmitted) via an ______ to the ________
afferent neuron; central nervous system
in spinal cord, this afferent signal is “integrated” by central nervous system neurons
the afferent neuron may
stimulate interneurons which in turn stimulate _____ (outgoing) neurons that transmit signals to
______ such as nerves or glands.
interneurons; efferent; effector organs
effector organs
nerves or glands; receives transmitted signals from efferent neurons (outgoing)
Central Nervous System consists of
the brain and spinal cord
The most primitive part of the
brain
hindbrain
The hindbrain consists of the
medulla oblongata, the pons, and the cerebellum
Contains the control centres for cardiovascular and respiratory function.
medulla oblongata
is an important relay site and is involved in involuntary control
the pons
involved in balance and motor coordination
cerebellum
is primarily concerned with processing auditory and visual information.
midbrain
The size of the
midbrain generally _____ from “lower” to “higher” vertebrates
decreases
contains (amongst
other things) the cerebral hemispheres (cerebrum; cerebral cortex) involved in thought and
consciousness
forebrain
Other important structures include
thalamus, hypothalamus and pituitary
The components of the nervous system that are not part of the central nervous system are referred to as the
Peripheral Nervous System (PNS)
The PNS consists of:
afferent nervous system and efferent nervous system
nervous system that sends
input to the CNS
afferent nervous system
nervous system that carries information from the CNS to the rest of
the body.
efferent NS, which is divided into 2 parts
the efferent nervous system can be divided into
the somatic nervous system (that
regulates skeletal muscle contraction)
and the autonomic nervous system (ANS)
The autonomic nervous system is divided into the
sympathetic nervous system and the parasympathetic nervous
system
Sympathetic and parasympathetic nerves innervate a wide variety of target organs
Important Things to Note about sympathetic and parasympathetic nervous system:
1) the two systems
invariably have the opposite effect on a particular organ. If one system STIMULATES, the other system INHIBITS.
2) Parasympathetic nerves originate predominately from CRANIAL NERVES such as the optic
nerve and the vagus nerve (although there are some parasympathetic nerves that originate from the lower spinal cord).
Sympathetic nerves on the other hand all originate from the SPINAL CORD.
Sensory receptors (sensory cells) come in many forms; there are receptors to detect multiple stimuli (3 main types)
Chemoreceptors
Mechanoreceptors
Photoreceptors
sense some form of chemical stimuli. These can be O2 levels, CO2 levels, pH, ions,
peptides, sugars, etc. The senses of taste and smell involve stimulation of a chemoreceptor cell.
Chemoreceptors
sense some form of physical distortion such as an increase in pressure, the bending
of a hair follicle, stretching of the lung or movement of a muscle.
Mechanoreceptors
sense light (photons).
Photoreceptors
Regardless of the particular stimulus that is being detected, the stimulus is initially sensed by a _________
receptor protein or molecule that is present on the cell membrane.
The interaction of a sensory
stimulus (i.e., a chemical) with the receptor protein/molecule activates some form of signal
transduction pathway within the cell. This can take the form of the opening/closing of ion channels in
the cell membrane or the activation of some form of intracellular second messenger pathway
Ultimately, however, the sensing of the stimulus leads to a change in the membrane potential of the
sensory receptor cell.
In other words, the electrical potential across the receptor cell’s plasma
membrane is altered (usually it becomes more positive).
This leads to stimulation of the afferent nerve that innervates the sensory receptor cell and the initiation of a nerve signal that is then sent to the central nervous system.
A nerve cell is called a ____
neuron
_____ receive sensory information from another cell
dendrites
In other words, when a pre-synaptic cell innervates a post-synaptic cell, the dendrites of the post-synaptic cell receive the information from the pre-synaptic cell.
Simulation of the dendrites of a post-synaptic cell
leads to small changes in the membrane potential within the dendrites of the post-synaptic cell.
Ultimately, this leads to stimulation of the region of the neuron (cell) called the
axon hillock (or trigger zone or spike-initiation zone).
The axon hillock then generates an electrical signal called a(n)
action potential
The action potential travels down the ____ and ultimately leads to the stimulation of _____
The action potential travels down the AXON and ultimately leads to the stimulation of ANOTHER CELL (NEURON OR EFFECTOR ORGAN) via SYNAPTIC TRANSMISSION
branches of an axon from the pre-synaptic cell form ____
this is the site of nervous (synaptic) transmission from one neuron to another
synapses with the dendrites of the post-synaptic cell
a collection of axons from many different neurons
nerve
cell bodies of all these neurons are found together in
ganglia or nuclei
are support cells for neurons
Glial cells
Three major types of glial cells:
microglia - removes waste products
astrocytes - keep neurons in place and also from blood-brain barrier (physical barrier to the movement of certain substance from the blood into the brain tissue)
Oligodendrocytes (within the central nervous system) and Schwann cells (within the peripheral
nervous system) form myelin.
form myelin
Oligodendrocytes and Schwann cells
is an insulating sheath that covers axons. It is formed by Oligodendrocytes and Schwann cells.
Myelin
The _________ wraps around the axon multiple times creating a myelin
sheath. The myelin sheath has a very ___ permeability to ions making it a(n) ____ electrical insulation.
plasma membrane of the Schwann cells
low permeability
excellent
Is all of the axon covered in myelin?
NO - Not all of the axon is covered in myelin.
There are gaps in the myelin sheath called ______
This allows for very rapid conduction of nerve impulses by a process called ______
Nodes of Ranvier
saltatory conduction
In order for one neuron to send a nervous signal to another neuron or for a nerve to send a signal to an
effector organ, electrical activity generated in the cell body (of a neuron) must be sent down the ____ to the ____.
axon to the synapse
This electrical activity is generated in the axon hillock and is called
action potential (or spike).
The action potential travels down the axon to the synapse (the site at which a pre-synaptic neuron sends a signal to a post-synaptic neuron or a cell of an effector organ). The transmission of the electrical signal from one neuron to another neuron is called ______. This can be
accomplished by electrical or chemical means (see below).
synaptic transmission
The plasma membrane of a cell (any cell not just a neuron) has an electrical potential (i.e., a difference
in electrical charge) across it. If a voltmeter were to be used to measure the electrical potential across a cell membrane, it would measure that the inside of the cell is negative with respect to the outside of the cell.
The difference in the electrical potential between the inside of the cell and the outside of the cell is
called the ____
membrane potential
the abbreviation for membrane potential is Em
An “excitable” cell such as a
neuron or muscle
For an “excitable” cell such as a neuron or a muscle cell, membrane potential will change depending upon whether the cell is at “rest” or is in the process of being excited (stimulated by a nerve or sensory input). The membrane potential that exists when a cell is at rest (is not being excited) is called the ________
resting membrane potential
The value for membrane potential is usually in the range of ______ millivolts (mV). The actual value depends upon the particular type of cell in question.
-80 to -60
A cell is (CHOOSE: negative or positive) inside compared to the outside because of the distribution of charged particles.
NEGATIVE
Inside a cell there are lots of ______ charged anions such as proteins and amino acids.
The extracellular fluid (the outside of the cell) essentially has no (or very few) negatively charged anions such as proteins or amino acids.
negatively
The inside of the cell (cytosol) has a HIGH concentration of ______ ions and a LOW concentration of ____ ions.
The outside of the cell (the extracellular fluid) has the opposite.
high [K+] and low [Na+]
An electrical signal in a neuron; large and rapid transient increase in membrane potential
action potential
An action potential is generated as a result of…
Na+ ions moving INTO the neuron through voltage-gated Na+ channels
and K+ ions moving OUT of the neuron through voltage-gated K+ channels
In both cases, the inward movement of Na+ and the outward movement of K+, these ions are moving in response to
concentration and electrical gradients
Na+ that moves into the cell and K+ that moves out of the cell are returned to the extracellular fluid and cytosol, respectively via the action of the
Na+/K+ ATPase (pump)
An action potential is a large and rapid transient increase in membrane potential.
It begins with the
membrane potential at its resting level (aka ____) which is then increased to a _____
resting potential
threshold potential
From resting potential to threshold potential
A large and rapid increase in membrane potential called a
depolarization
The membrane potential reaches a peak and then begins to decrease again. This decrease is called a
repolarisation
At the end of the repolarisation phase, the membrane potential actually goes below the resting membrane potential. This is called a ___
Membrane potential then rises again to the resting potential level.
hyperpolarisation.
The changes in membrane potential during an action potential are the result of
Na+ moving into the cell through a voltage-gated Na+ channel and K+ moving out of the cell through a voltage-gated K+ channel.
In order for Na+ to move from the extracellular fluid (ECF) through the Na+ channel into the cytosol, it
must pass through two gates within the Na+ channel: first it must move through the _____ and then through the ______.
activation gate
inactivation gate
The activation gate is _____ when the membrane potential is at the resting level. As the membrane potential increases from the resting potential to the threshold potential, activation gates in some Na+
channels begin to open. Once the membrane potential reaches the threshold potential, the activation gates in all Na+ channels _____ (very rapidly).
Na+ activation gate
RESTING: closed —> THRESHOLD: open
Inactivation gate - when is it open/closed?
RESTING: open —> THRESHOLD: close (slowly)
The inactivation gate in the Na+ channel is open at the resting membrane potential. Once the membrane
potential reaches threshold potential and the depolarisation phase begins, the inactivation gate begins to ____ (they close slowly)
In order for K+ to leave the cell it must pass through the activation gate in the K+
channel.
Note that there is only an activation gate in the K+ channel; there is no inactivation gate in the K+ channel.
The K+ channel’s activation gate is closed at the resting membrane potential. It opens slowly during the depolarisation phase.
From Resting Potential to Threshold Potential
We begin at the resting membrane
potential.
The action potential begins with a relatively slow increase in the membrane potential from the resting membrane potential to the threshold potential. This slow increase in membrane potential is
due to Na+ moving into the cell through the Na+ channels that have their activation gates open.
The stimulus that allows some of these Na+ channel activation gates to be open comes from sensory receptors (or stimulation from another neuron). At this point the inactivation gates are all open. The K+channel activation gate is closed at this stage so there is no movement of K+out of the cell.
From Threshold to “Mid-Depolarisation”
Once the membrane potential reaches the
threshold potential, the activation gates in all of the Na+ channels open. This allows for a large movement of Na+ into the cell.
Since Na+ is a positive ion, it carries its positive charge into the cell and causes an increase in the membrane potential. At this stage the Na+ channel inactivation gates are still open and the K+ channel activation gates are still closed.
From “Mid-Depolarisation” to the Peak of the Action Potential
As the membrane potential increases from the mid stages of the depolarisation phase to the peak of the action potential, the Na+ channel inactivation gates begin to close.
Also during this time, the activation gates in the K+ channels begin to open. Once the peak of the action potential is reached all of the Na+ channel inactivation gates are closed and all of the K+ channel activation gates are open.
Therefore, at this point there is no more movement of Na+ into the cell (because the inactivation gates are closed) and K+ is now moving out of the cell (through the open activation gates in the K+
channel).
From the Peak of the Action Potential into the Repolarisation Phase
At this point, the Na+ channel inactivation gates are closed so there is no more movement of Na+ into the cell.
The membrane potential begins to decrease because K+ is leaving the cell through the open activation gates in the K+ channel. Note that although the activation gates of the Na+ channel are open, there is no movement of Na+
into the cell through the Na+ channel because the inactivation gates are closed.
Membrane Potential Falls from the Peak Level Back below the Threshold Potential
Membrane potential continues to fall during the repolarisation phase as K+ continues to move
out of the cell through the open K+ channel. As membrane potential falls (but is still above threshold), the inactivation gates in the Na+ channel begin to open and the activation gates in the K+ channels begin to close. Once membrane potential falls below the threshold potential the activation gates in the Na+ channels close.
Membrane Potential Falls below the Resting Potential and then Returns to the Resting Potential
At the end of the repolarisation phase, the membrane potential falls below the
resting potential. This is called a hyperpolarisation. It results from a slight excess of K+
movement out
of the cell during the repolarisation phase. In other words, “too many” positive charges left the cell (on
K+) so the membrane potential fell below rest. At this stage the Na+
that entered the cell during the
previous phases of the action potential) is removed from the cell by the Na+/K+ ATPase (pump).
Similarly, the K+ that left the cell during the action potential is moved back into the cell by this pump.
This causes the membrane potential to return from the hyperpolarized state back to resting potential.
Action potentials move very rapidly along axons because of the presence of the
myelin sheath
The myelin sheath is not continuous along the entire length of the axon. There are gaps in the myelin called
Nodes of Ranvier
When an action potential is generated in the axon hillock of the cell body, the positive charge that is now inside the neuron spreads down the axon. When it reaches the first Node of Ranvier it causes an action potential to be generated in that region of the plasma membrane. The positive charge that enters the cell as a result of this action potential then spreads down the axon to the
next Node of Ranvier and causes an action potential in that region of the plasma membrane
This continues such that an action potential essentially jumps along a neuron from Node to Node. This mode of conduction is called _______ and allows for very rapid transmission of an action
potential along an axon. The myelin sheath acts as an insulator and prevents current (charge) leak
across the membrane in areas where it is present. This allows for an efficient spread of charge inside
the neuron from one Node to the next.
saltatory conduction
Once a neuron has generated an action potential there is a period during the depolarisation phase and during the initial stages of the repolarisation phase in which no amount of stimulation can cause this neuron to generate a second action potential.
This is called the
In the depolarisation phase, all of the Na+ channels are open so no amount of
stimulation can cause any more to open.
During the initial stages of repolarisation the Na+
inactivation gates are closed. The Na+
inactivation gates close with depolarisation so another depolarising stimulus would
only push the membrane potential higher and ensure that these gates remain closed. They need a further reduction in membrane potential before they can open.
absolute refractory period
Immediately following the absolute refractory period is the
During this period a strong stimulus can produce a second action potential before the first action
potential has finished. During the initial stages of the relative refractory period, the inactivation gates in the Na+ channel are now beginning to open. At this point, a strong depolarising stimulus could cause the Na+ channel activation gates to open and allow Na+ to enter the cell.
relative refractory period.
When an action potential reaches the end of an axon it enters the axon terminal. In order for a nerve signal to be sent from a pre-synaptic cell to a post-synaptic cell, the depolarisation from this action potential must move from one cell to the next.
This can be accomplished in two ways
First, there can be direct electrical connections between the two cells such that electrical current (i.e., the positive charge carried by Na+
entering the cell) can move directly from one cell into another. This is called an
ELECTRICAL SYNAPSE. In this case, electrical current moves through channels called gap junctions that connect the pre-synaptic cell to the post-synaptic cell.
Second, the action potential that arrives in the pre-synaptic terminal causes the release of neurotransmitters from the pre-synaptic cell. The neurotransmitters enter the space between the two cells (called the synaptic cleft) and binds to neurotransmitter receptors on the cell membran of the post-synaptic cell. Binding leads to generation of action potential in this cell or the inhibition of action potential generation; CHEMICAL SYNAPSE
Chemical neurotransmitters can be either
excitatory or inhibitory
In the case of an excitatory
neurotransmitter, it causes the opening of ion channels on the post-synaptic cell that allow positive ions (such as Na+) to enter the cell and cause the membrane to depolarise.
In the case of an inhibitory neurotransmitter, it causes the opening of ion channels on the post-synaptic cell that allow negative ions (such as Cl-) to enter the cell and cause the membrane to hyperpolarize. If a membrane hyperpolarizes then it becomes harder for any further stimulus to increase its membrane potential to the threshold potential required for action potential generation.
Examples!
