BIOL #23: Nervous System Flashcards

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1
Q

Neuron

A

Most neurons have the same three parts:

1) Dendrites- receive electrical signals from the axons of adjacent cells.
2) Cell body (or soma)- which includes the nucleus and organelles, integrates the incoming signals from the dendrites and generates an outgoing signal.
3) Axon – transmits the outgoing signal to the dendrites of other neurons.

The end of the axon is usually divided into many branches.

  • Each branched end of an axon transmits information to another cell at a junction called a synapse.
  • The part of the axon branch that forms a synapse with another cell is called a synaptic terminal.

Highly branched axons can transmit information to many target cells.

At most synapses, chemical messengers, called neurotransmitters, pass information on to the next cell.

  • The transmitting neuron is called the presynaptic cell.
  • The receiving cell is called the postsynaptic cell.
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2
Q

Membrane Potential

A

Ions are always unequally distributed between the inside of cells and the surrounding environment, such that the inside of the cell is negatively charged relative to its outside.

  • This charge difference (or voltage) is called the membrane potential and is measured in millivolts (mV).
  • Membrane potentials are always expressed as inside-relative-to-outside.
  • When a membrane potential exists, the ions on both sides of the membrane have potential energy. If the membrane were removed, ions would spontaneously move from the area of like charge to the area of unlike charge—causing a flow of charge, called an electric current.
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3
Q

Resting Potential

A

A neuron that is not sending a signal is referred to as resting and its membrane potential during this time is called its resting potential.
- In neurons, the resting membrane potentials are typically between -60 and -80 mV.

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4
Q

How Is the Resting Potential Maintained?

A

Resting potential exists in part because neurons have a high intracellular concentration of K+ and low intracellular concentrations of Na+ and Cl–.

The Na+ and K+ gradients of cells are maintained by sodium potassium pumps in the plasma membrane.

These pumps use the energy of ATP hydrolysis to actively transport Na+ out of the cell and K+ into the cell.

A sodium potassium pump transports three sodium ions out of the cell for every 2 potassium ions transported in, generating a net export of positive charge.

Even though sodium-potassium pumps can create a net negative charge inside the cell, every pump only changes the resting voltage by a few millivolts.

Another mechanisms is necessary to help maintain a voltage difference of 60-80 mV.

Ion channels, pores in the membrane that allows only specific ions to pass through, allow ions to diffuse across the membrane down their concentration gradient.

A resting neuron has a large number of open K+ channels but few open Na+ channels, making neurons more permeable to K+.

Because there is a higher concentration of K+ inside the resting neuron than outside, K+ will diffuse out, further increasing the net negative charge in the neuron.

The build up of the negative charge inside the cell stops when the excess negative charge inside the cell begins to exert an attractive force that opposes the flow of K+ out of the cell.

The net flow of K+ out of a neuron proceeds until the chemical and electrical forces are in balance, called the equilibrium potential for K+ (EK) – this produces a net negative charge in the neuron.

A hypothetical cell with only sodium ion channels would produce a net positive charge for the equilibrium potential of Na+ (ENa) because the higher concentration of Na+ outside the cell would cause a net movement into the cell.
- Because the resting potential of neurons has a net negative charge, it can be deduced that few sodium ions channels are open in this state.

Because the resting potential of a neuron is -60 to -80 mV, neither K+ or Na+ is at equilibrium in a resting neuron (ENa and EK do not match neuron resting potential), thus each ion still has a (potential) net flow.

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5
Q

Generating Nerve Impulses

A

Because neither K+ or Na+ is at equilibrium in a resting neuron, there is potential for membrane potential to move towards either ENa or EK.

Under conditions that allow Na+ to cross the membrane more readily, the membrane potential will move towards ENa and away from EK, which is what happens during the generation of a nerve impulse.

Changes in membrane potential occur because neurons contain gated ion channels, which open or close in response to stimuli.

The opening and closing of gated ion channels alters the membrane’s permeability to particular ions, which alters the membrane potential.

Opening gated K+ channels with a stimulus would increase the membrane’s permeability to K+ .

  • Resulting in net diffusion of K+ out of the cell and shifting of membrane potential towards EK (-90 mV).
  • This produces an increase in the magnitude of the membrane potential (i.e. inside the cell becomes more negative), called hyperpolarization.
  • In a resting neuron, hyperpolarization results from any stimulus that increases the outflow of positive ions (or in the inflow of negative ions).

Opening gated Na+ channels with a stimulus would increase the membrane’s permeability to Na+ .

  • Resulting in net diffusion of Na+ into the cell and shifting of membrane potential towards ENa (+62 mV).
  • This produces a reduction in the magnitude of the membrane potential (i.e. inside the cell becomes less negative), called depolarization.
  • In a resting neuron, depolarization results from any stimulus that increases the inflow of positive ions (or in the outflow of negative ions).
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6
Q

Graded Potentials

A

A graded potential is a simple shift in membrane potential in response to hyperpolarization or depolarization.

The magnitude of this shift corresponds to the magnitude of the stimulus.

Induces a small electrical current that leaks out of the neuron as it moves along the membrane, thus the signal decays with distance from the source.

These are NOT the nerve signals that travel along axons.

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7
Q

Action Potentials

A

An action potential is a massive change in membrane voltage due to sufficiently strong depolarization shifts in membrane potential.

Unlike graded potentials, action potentials have a constant magnitude and can regenerate in adjacent regions of the membrane, thus action potentials can spread along axons making them well-suited for transmitting a signal over long distances.

Action potentials arise because some of the ion channels in neurons are voltage-gated ion channels.

  • These channels open and close when the membrane potential passes a particular level.
  • If a depolarization event opens voltage-gated sodium channels, the resulting flow of Na+ into the neuron will result in further depolarization, causing more voltage-gated sodium channels to open
  • This positive feedback mechanism triggers a fast, large magnitude response.

The particular level of membrane potential that voltage-gated ion channels must encounter for depolarization to occur is called the threshold.
- For mammalian neurons, the threshold of membrane potential is about -55 mV.
- Action potentials either occur fully or not at all, representing an all-or-none response to stimuli because of its positive feedback nature.
- Regardless of the size of the stimulus, the magnitude of the action potential does not change, thus it is the frequency of action potentials—rather than their size— that is the meaningful signal for information being transmitted within the nervous system.
+ e.g. in hearing, a louder sound results in more frequent action potentials being transmitted than a softer sound.

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8
Q

Andrew Huxley & Alan Hodgkins

A

During the 1940s and 1950s, Andrew Huxley & Alan Hodgkins performed experiments using giant neurons in squid to model how action potentials are generated, earning them a Nobel Prize.

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9
Q

Modeling Action Potentials

A

Membrane depolarization opens both Na+ and K+ channels, but they respond independently and sequentially:

  • Sodium channels open first, creating depolarization and initiating an action potential.
  • As the action potential proceeds, the sodium channel become inactivated due to conformation changes (inactivation loop).
  • Sodium channels remain inactivated until the membrane returns to the resting potential and the channels close.
  • Potassium channels open more slowly than sodium channels, but remain open and functional until the end of the action potential.
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10
Q

Modeling Action Potentials: Step 1

A

Resting State: When the membrane of the axon is at the resting potential, voltage-gated sodium and potassium channels are closed. Ungated channels remain open to maintain the resting membrane potential.

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11
Q

Modeling Action Potentials: Step 2

A

Depolarization: A stimulus opens some of the voltage-gated sodium channels. Na+ flows through those channels depolarizing the membrane. If the depolarization reaches the threshold, it triggers an action potential.

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12
Q

Modeling Action Potentials: Step 3

A

Rising phase of the action potential: Once the threshold is crossed, the positive-feedback cycle brings the membrane potential very close to ENa (Na+ influx makes the inside of the membrane positive with respect to the outside).

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13
Q

Modeling Action Potentials: Step 4

A

Falling phase of the action potential: Most sodium channels become inactivated, blocking the Na+ inflow. Most potassium channels open, permitting K+ outflow, which makes the inside of the cell negative again.

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14
Q

Modeling Action Potentials: Step 5

A

Undershoot: The sodium channels close but the potassium channels remain open. As the potassium channels close and the sodium channels become unblocked but remain closed, the membrane returns to the resting state. This stage is called ‘undershoot’ because the membrane potential is briefly closer to EK than it is to its resting membrane potential.

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15
Q

Refractory Period

A

The sodium channels remain inactivated during the falling phase and the early part of the undershoot phase.

  • As a result, if a second depolarizing stimulus occurs during this period, it will be unable to trigger an action potential – this downtime, when a second action potential cannot be initiated, is called the refractory period.
  • The refractory period sets a limit on the frequency at which action potential can be generated and ensures that all signals travel down the axon in a single direction.
  • For most neurons, the interval between the onset of an action potential and the end of the refractory period is only 1-2 milliseconds (msec) – because action potentials are so brief, a neuron can produce hundreds per second.
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16
Q

How Is the Action Potential Propagated?

A

At the site where an action potential is initiated (usually closest to the cell body), Na+ inflow during the rising phase creates an electrical current that depolarizes the neighboring region of the axon membrane.

The depolarization of the neighboring region along the axon is large enough to reach the threshold causing an action potential to initiate.

This process is repeated, such that all five steps of the action potential would be propagated along the length of the axon.

The refractory period stops the signal from propagating in the opposite direction.

17
Q

Axon Diameter Affects Transmission Speed

A

Much in the same way a wide hose offers less resistance to flow compared to a narrow hose, a wide axon provides less resistance to current associated with the action potential than does a narrow axon.

  • As a result, large diameter neurons can transmit action potentials much faster than small diameter axons can.
  • Many invertebrates have giant neurons that have wide axons, which enable rapid behavioral responses (e.g. squid).
18
Q

Myelination Affects Speed

A

Vertebrate neurons are typically narrower than invertebrate neurons.

An adaptation of vertebrate neurons that allow for fast signal conduction is electrical insulation, which causes the depolarizing current associated with the action potential to spread farther along the axon interior, bringing more distant regions to the threshold sooner.

The electrical insulation that surrounds vertebrate axons is a myelin sheath, which is composed of a multi-layered protective lipid coating.
- The myelin sheath is produced by supportive glial cells surrounding neurons – particularly the oligodendrocytes in the CNS and the Schwann cells in the PNS.

In myelinated axons, voltage-gated sodium channels are restricted to gaps in the myelin sheath called nodes of Ranvier.

As a result, action potentials are only generated at the nodes, which are in contact with the extracellular fluid.

The advantage of this morphology is that the time-consuming process of opening and closing ion channels is restricted to a limited number of sites along a axon.

This mechanism for action potential propagation is called saltatory conduction, as the action potential appears to jump along the axon from node to node.

19
Q

Synapses

A

In most cases, action potentials are not transmitted from neurons to other cells.

Information is transmitted between neurons at synapases:

  • Electrical synapses contain gap junctions that allow the electrical current to flow directly from neuron to neuron. (relatively uncommon, e.g. the retina)
  • Chemical synapses make up the majority of synapses and involve the release of a chemical transmitted by the presynaptic (sending) neuron to the postsynaptic (receiving) neuron.
20
Q

Chemical Synapses

A

At each terminal, the presynaptic neuron synthesizes the neurotransmitter and packages it in multiple membrane-bound compartments, called synaptic vesicles.

1) An action potential arrives at the synaptic terminal, depolarizing the presynaptic membrane.
2) The depolarization opens voltage-gated channels, triggering an influx of Ca2+.
3) The elevated Ca2+ concentration causes synaptic vesicles to fuse with the presynaptic membrane, releasing neurotranmitter into the synaptic cleft.
4) The neurotransmitter binds to ligand-gated ion channels in the postsynaptic membrane.
5) The ligand-gated ion channels open.

At many chemical synapses, the receptor protein that binds and responds to neurotransmitters is a ligand-gated ion channel, often called an ionotropic receptor.

Binding of the neurotransmitter (the receptor’s ligand) to a particular part of the receptor opens the channel and allows specific ions to diffuse across the postsynaptic membrane.

The result is a postsynaptic potential which is a graded potential in the postsynaptic cell.

When the ligand-gated ion channel is permeable to both K+ and Na+, the membrane will depolarize to a value roughly in between EK and ENa.
- This is called an excitatory postsynaptic potential (EPSP) because such depolarization brings the membrane potential towards the threshold.

When the ligand-gated ion channel is selectively permeable for either K+ or Cl-, the membrane will become hyperpolarized (highly negative inside the cell).
- This is called an inhibitory postsynaptic potential (IPSP) because it moves the membrane potential further from the threshold.

21
Q

Postsynaptic Potential

A

The magnitude of a postsynaptic potential at any one synapse varies with a number of factors, including the amount of neurotransmitter released by the presynaptic neuron.

Since it is a graded response, the postsynaptic potential becomes smaller with distance from the synapse – by the time a single EPSP reaches the axon, the signal is usually too small to trigger a response. Thus multiple potentials must typically be received to trigger a response (summation).

22
Q

Postsynaptic Potential: Summation

A

Post synaptic potentials can be reached due to:

Temporal summation: when two EPSPs occur at a single synapse in rapid succession, allowing the postsynaptic neuron to reach the threshold for the action potential.

Spatial Summation: when two EPSPs occur at different synapses but almost simultaneously, allowing the postsynaptic neuron to reach the threshold for the action potential.

Similar temporal and spatial summation rules apply for IPSPs, in order to counter the effect of EPSPs and inhibit a neuron response.

23
Q

Neurotransmitters

A

Neurotransmitter act as chemical signals that become transformed into electrical signals.

Research has characterized more than 100 neurotransmitters belonging to five groups: acetylcholine, amino acids, biogenic amines, neuropeptides, and gases.

To qualify as a neurotransmitter, a molecule must:

  • Be present at the synapse and released in response to an action potential.
  • Bind as a ligand to a ligand-gated ion channel receptor on a postsynaptic cell.
  • Be taken up or degraded.

The response triggered by each neurotransmitter depends on the particular kind of receptor expressed by the postsynaptic cell.
- A specific neurotransmitter may bind specifically to more than a dozen different receptors, each producing a different response (e.g. excitatory or inhibitory).

List of Neurotransmitters:

  • Acetycholine
  • Monoamines: Norepinephtine, Dopamine, Serotonin
  • Amino Acids: Glutamate, Gamma-aminobutyric acid (GABA)
  • Peptides: Endophins, enkephalins, substance P
24
Q

Animal Nervous Systems

A

Animals with very simply body plans (e.g. cnidarians) have simple structures of interconnected nerve cells that form a nerve net.

Animals with more complex body plans often have clusters of neurons that perform specialized functions, called nerves (e.g. echinoderms, flatworms, vertebrates).

Animals with active lifestyles typically have cephalization, which is a clustering of sensory neurons and nerves near the anterior (front) of the head – forming rudimentary or complex brain structures and sensory organs (e.g. eyespots or eyes). These anterior nerve clusters often communicate with nerve cords extending along the body and ganglia, which are segmentally arranged clusters of nerves (e.g. planaria and vertebrates).

25
Q

Nervous System: Information Processing

A

Information processing by a nervous system occurs in three stages: sensory input, integration, and motor output.

Sensory neurons transmit information from eyes and other sensors that detect external stimuli or internal conditions.

Processing centers include the brain or ganglia, where signals are integrated (i.e. analyzed, processed).

Motor neurons transmit signals from processing centers to cells, such as muscle cells, to respond to a stimulus.

26
Q

The Vertebrate Nervous System

A

In vertebrates, the brain and the spinal cord form a central nervous system (CNS) and the nerves and ganglia form a peripheral nervous system (PNS).

Examining the anatomy of the vertebrate nervous system allows us to look at electrical signaling at the level of tissues, organs, and organ systems.

27
Q

Peripheral Nervous System

A

The peripheral nervous system (PNS) is made up of neurons outside the CNS.

Its function is to transmit information to and from the CNS, playing a large role in regulation of movement and internal environment.

The PNS consists of two systems with distinct functions:

1) The afferent division consists of neurons that transmit sensory information to the CNS.
2) The efferent division consists of neurons that carry commands (typical motor or secretory) from the CNS to the body (e.g. glands, muscles).

Neurons in the afferent division monitor conditions inside and outside the body. Once information from afferent neurons has been processed in the CNS, neurons in the efferent division carry signals that allow the body to respond to changed conditions in an appropriate way.

The afferent and efferent divisions carry out sensory and motor functions, respectively.

The efferent division is divided into two systems:

1) The motor system controls movement of skeletal muscle, which can be voluntary; typically in response to external stimuli.
2) The autonomic system controls internal processes such as regulation of smooth and cardiac muscle, typically in response to internal stimuli; this system generally carries out involuntary responses, which are not under conscious control.

Maintaining homeostasis often relies on cooperation between these two systems.

The autonomic system is further divided into three divisions:

1) The parasympathetic division promotes relaxation and digestion.
2) The sympathetic division prepares organs for stressful situations (e.g. fight-or-flight responses).
3) The enteric division involves the pancreas, gallbladder and digestive tract and promotes peristalsis and secretion.

28
Q

Functional Anatomy of the CNS

A

The spinal cord, which is made up of many nerves, serves as an information conduit. Much of the information that travels to or from the spinal cord is sent to the brain for processing.

The spinal cord can act independently of the brain as part of the simple nerve circuits that produce reflexes, the body’s automatic responses to certain stimuli.

The spinal cord has white matter (myelinated axons) on the outside and gray matter (neuron cell bodies) on the inside, reflecting its function in linking the CNS to sensory and motor neurons of the PNS.

The brain provides the integrative power that underlies the complex behavior of vertebrates.

The brain consists of four structures: the cerebrum, the cerebellum, the diencephalon, and the brain stem.

Interneurons are the neurons which form the local circuits within the brain.

White matter (myelinated axons) is predominately on the inside of the brain and gray matter (neuron cell bodies) on the outside, reflect the need for the brain to communicate between the different regions to allow for processing of sensory information and learning.