Lecture 2: The Nervous System

Question 1

stimulus is detected by a specific _______ that initiates a nervous (electrical) signal

Answer 1

receptor cell

Question 2

the signal is then sent

(transmitted) via an ______ to the ________

Answer 2

afferent neuron; central nervous system

Question 3

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.

Answer 3

interneurons; efferent; effector organs

Question 4

effector organs

Answer 4

nerves or glands; receives transmitted signals from efferent neurons (outgoing)

Question 5

Central Nervous System consists of

Answer 5

the brain and spinal cord

Question 6

The most primitive part of the

brain

Answer 6

hindbrain

Question 7

The hindbrain consists of the

Answer 7

medulla oblongata, the pons, and the cerebellum

Question 8

Contains the control centres for cardiovascular and respiratory function.

Answer 8

medulla oblongata

Question 9

is an important relay site and is involved in involuntary control

Answer 9

the pons

Question 10

involved in balance and motor coordination

Answer 10

cerebellum

Question 11

is primarily concerned with processing auditory and visual information.

Answer 11

midbrain

Question 12

The size of the

midbrain generally _____ from “lower” to “higher” vertebrates

Answer 12

decreases

Question 13

contains (amongst
other things) the cerebral hemispheres (cerebrum; cerebral cortex) involved in thought and
consciousness

Answer 13

forebrain

Question 14

Other important structures include

Answer 14

thalamus, hypothalamus and pituitary

Question 15

The components of the nervous system that are not part of the central nervous system are referred to as the

Answer 15

Peripheral Nervous System (PNS)

Question 16

The PNS consists of:

Answer 16

afferent nervous system and efferent nervous system

Question 17

nervous system that sends

input to the CNS

Answer 17

afferent nervous system

Question 18

nervous system that carries information from the CNS to the rest of
the body.

Answer 18

efferent NS, which is divided into 2 parts

Question 19

the efferent nervous system can be divided into

Answer 19

the somatic nervous system (that
regulates skeletal muscle contraction)

and the autonomic nervous system (ANS)

Question 20

The autonomic nervous system is divided into the

Answer 20

sympathetic nervous system and the parasympathetic nervous
system

Sympathetic and parasympathetic nerves innervate a wide variety of target organs

Question 21

Important Things to Note about sympathetic and parasympathetic nervous system:

Answer 21

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.

Question 22

Sensory receptors (sensory cells) come in many forms; there are receptors to detect multiple stimuli (3 main types)

Answer 22

Chemoreceptors

Mechanoreceptors

Photoreceptors

Question 23

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.

Answer 23

Chemoreceptors

Question 24

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.

Answer 24

Mechanoreceptors

Question 25

sense light (photons).

Answer 25

Photoreceptors

Question 26

Regardless of the particular stimulus that is being detected, the stimulus is initially sensed by a _________

Answer 26

receptor protein or molecule that is present on the cell membrane.

Question 27

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

Answer 27

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.

Question 28

A nerve cell is called a ____

Answer 28

neuron

Question 29

_____ receive sensory information from another cell

Answer 29

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.

Question 30

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

Answer 30

axon hillock (or trigger zone or spike-initiation zone).

Question 31

The axon hillock then generates an electrical signal called a(n)

Answer 31

action potential

Question 32

The action potential travels down the ____ and ultimately leads to the stimulation of _____

Answer 32

The action potential travels down the AXON and ultimately leads to the stimulation of ANOTHER CELL (NEURON OR EFFECTOR ORGAN) via SYNAPTIC TRANSMISSION

Question 33

branches of an axon from the pre-synaptic cell form ____

this is the site of nervous (synaptic) transmission from one neuron to another

Answer 33

synapses with the dendrites of the post-synaptic cell

Question 34

a collection of axons from many different neurons

Answer 34

nerve

Question 35

cell bodies of all these neurons are found together in

Answer 35

ganglia or nuclei

Question 36

are support cells for neurons

Answer 36

Glial cells

Question 37

Three major types of glial cells:

Answer 37

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.

Question 38

form myelin

Answer 38

Oligodendrocytes and Schwann cells

Question 39

is an insulating sheath that covers axons. It is formed by Oligodendrocytes and Schwann cells.

Answer 39

Myelin

Question 40

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.

Answer 40

plasma membrane of the Schwann cells

low permeability

excellent

Question 41

Is all of the axon covered in myelin?

Answer 41

NO - Not all of the axon is covered in myelin.

Question 42

There are gaps in the myelin sheath called ______

This allows for very rapid conduction of nerve impulses by a process called ______

Answer 42

Nodes of Ranvier

saltatory conduction

Question 43

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 ____.

Answer 43

axon to the synapse

Question 44

This electrical activity is generated in the axon hillock and is called

Answer 44

action potential (or spike).

Question 45

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).

Answer 45

synaptic transmission

Question 46

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 ____

Answer 46

membrane potential

the abbreviation for membrane potential is Em

Question 47

An “excitable” cell such as a

Answer 47

neuron or muscle

Question 48

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 ________

Answer 48

resting membrane potential

Question 49

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.

Answer 49

-80 to -60

Question 50

A cell is (CHOOSE: negative or positive) inside compared to the outside because of the distribution of charged particles.

Answer 50

NEGATIVE

Question 51

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.

Answer 51

negatively

Question 52

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.

Answer 52

high [K+] and low [Na+]

Question 53

An electrical signal in a neuron; large and rapid transient increase in membrane potential

Answer 53

action potential

Question 54

An action potential is generated as a result of…

Answer 54

Na+ ions moving INTO the neuron through voltage-gated Na+ channels

and K+ ions moving OUT of the neuron through voltage-gated K+ channels

Question 55

In both cases, the inward movement of Na+ and the outward movement of K+, these ions are moving in response to

Answer 55

concentration and electrical gradients

Question 56

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

Answer 56

Na+/K+ ATPase (pump)

Question 57

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 _____

Answer 57

resting potential

threshold potential

Question 58

From resting potential to threshold potential

A large and rapid increase in membrane potential called a

Answer 58

depolarization

Question 59

The membrane potential reaches a peak and then begins to decrease again. This decrease is called a

Answer 59

repolarisation

Question 60

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.

Answer 60

hyperpolarisation.

Question 61

The changes in membrane potential during an action potential are the result of

Answer 61

Na+ moving into the cell through a voltage-gated Na+ channel and K+ moving out of the cell through a voltage-gated K+ channel.

Question 62

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 ______.

Answer 62

activation gate

inactivation gate

Question 63

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).

Answer 63

Na+ activation gate

RESTING: closed —> THRESHOLD: open

Question 64

Inactivation gate - when is it open/closed?

Answer 64

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)

Question 65

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.

Answer 65

The K+ channel’s activation gate is closed at the resting membrane potential. It opens slowly during the depolarisation phase.

Question 66

From Resting Potential to Threshold Potential

Answer 66

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.

Question 67

From Threshold to “Mid-Depolarisation”

Answer 67

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.

Question 68

From “Mid-Depolarisation” to the Peak of the Action Potential

Answer 68

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).

Question 69

From the Peak of the Action Potential into the Repolarisation Phase

Answer 69

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.

Question 70

Membrane Potential Falls from the Peak Level Back below the Threshold Potential

Answer 70

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.

Question 71

Membrane Potential Falls below the Resting Potential and then Returns to the Resting Potential

Answer 71

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.

Question 72

Action potentials move very rapidly along axons because of the presence of the

Answer 72

myelin sheath

Question 73

The myelin sheath is not continuous along the entire length of the axon. There are gaps in the myelin called

Answer 73

Nodes of Ranvier

Question 74

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.

Answer 74

saltatory conduction

Question 75

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.

Answer 75

absolute refractory period

Question 76

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.

Answer 76

relative refractory period.

Question 77

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

Answer 77

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

Question 78

Chemical neurotransmitters can be either

Answer 78

excitatory or inhibitory

Question 79

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.

Answer 79

Examples!