Chapter #11: Fundamentals of Nervous System & Nervous Tissue Flashcards
Sensory Input
-allows us to respond to stimuli
-monitors changes that occur inside and outside the body
What is responsible for sensory input
several different sensory receptors
Integration
processing & interpretation of input information –> the nervous system “decides” what response to make
What is responsible for integration?
CNS; usually the brain, but sometimes the spinal cord
Motor Output (motor response)
response is carried out –> travels to effector organ
What is responsible for motor output?
muscles/glands are the most common
Functions of the nervous system
sensory input, integration, and motor output
Components of the nervous system
Central Nervous System (CNS) and Peripheral Nervous System (PNS)
CNS
composed of the brain and spinal cord
What is the function of the CNS?
is responsible for interpreting sensory input and deciding motor output
PNS
Composed of nerves that extend from the CNS to the rest of the body
What is the function of the PNS?
allows information to be sent between the CNS and the rest of the body
Neurons
-nerve cells that can respond to stimuli & transmit electrical signals
-cells of the nervous system specialized to generate or transmit electrical signals (nerve impulses)
Why are neurons important?
-create and send messages to various parts of the body
-integration and motor output could not occur without neurons
Neuroglia (glial cells)
provide support and maintenance to neurons
Why are neuroglias important?
they make sure neurons remain healthy, alive, and functional
Types of neuroglia
astrocytes, microglial cells, ependymal cells, satellite cells, oligodendrocytes, and schwann cells
Astrocytes (CNS)
-most abundant, support & protect neurons in CNS
-Star-shaped, with projections connecting to and wrapping around neurons, nerve endings, and surrounding blood capillaries
What are the functions of astrocytes?
- Provide nutrient supply for neuron cells
- Allows migration of young neurons
- “Clean up” outside neuron cells (leaked K+ ions, neurotransmitters; important for resting membrane potentials)
Microglial Cells (CNS)
-contact nearby neuron cells to monitor neuron health
-migrate toward injured neurons & transform into a macrophage and phagocytize the neuron
Why are microglial cells important?
If a cell is dying off or neuron is damages, they will transform into macrophages because you do not want dead cells taking up space
Ependymal Cells (CNS)
-most ependymal cells have cilia
-lines central cavities of CNS to circulate cerebrospinal fluid within cavities
Satellite cells (PNS)
-Support & protect neuron cell in PNS
-similar to astrocytes in the CNS
Oligodendrocytes (CNS) & Schwann Cells (PNS)
-Wrap around thicker nerve fibers in CNS & PNS
-Function: myelin sheath creates an insulating covering for neurons
How is the insulation covering for neurons useful?
allows you to speed up the rate at which electrical signals can be sent; makes messages faster
General structures of a neuron
Cell body, dendrites (found in cell body), and axons (found in cell body)
Characteristics of neurons
longevity, amitotic, and metabolism
What is longevity?
neurons last entire lifetime
What is amitotic?
Neurons do not divide and are never replaced
What is metabolism?
Neuronal activity is extremely high
Cell body of neuron
-portion of cell containing the nucleus
-Function: plasma membrane can receive information from surrounding neurons
-Most cell bodies are found in the CNS & are protected by bone
-Clusters of cell bodies in CNS are called nuclei, those in PNS are called ganglia
Why is it helpful to have cell bodies surrounded by bone?
makes it a little less susceptible to damage
Dendrites
-main receptive region of neuron
-A single neuron can have dozens of dendrites
-Function: provide increased surface area for incoming signals, convey incoming messages toward the cell body
What would happen if there were no dendrites?
the dendrite is the main receptive region of neuron, so the neuron would become less receptive to incoming information
Axon
-single, long “nerve fiber” extending from the cell body
-The axon is the conducting region of the neuron
-Function: generates and transmits nerve impulses away from the cell body
-Bundles of axons in the CNS are called tracts, those in PNS are called nerves
-Axon branches at the end to form terminal branches & axon terminals
What is the function of axon branches?
neurotransmitter released at axon terminal to pass the impulse to the next neuron
Myelin Sheaths
-Functions: protects and electrically insulates long and/or large nerve fibers to increase speed at which impulses are transmitted
-Found only on axon portion of the neuron
-Not all axons are myelinated
What happens to transmission speed for unmyelinated axons?
message will be sent slower
Myelin Sheaths in the PNS (Schwann Cells)
-Multiple Schwann cells on the axon form the myelin sheath
-BUT none of the Schwann cells contact each other
Myelin Sheath gaps
region of axon that is “exposed” due to absence of Schwann cell covering
Functional Classification of neurons
- Sensory (afferent) neuron
- Motor (efferent) neuron
- Interneuron
Sensory (afferent) neuron
-afferent neurons transmit signals from the body to the CNS
-Receptive endings of this neuron type can function as actual sensory structure, or are associated with larger sensory receptors
Motor (efferent) neuron
-efferent neurons transmit motor response from CNS to the body
-Impulses travel to effector organs (muscle + glands)
Interneuron
-lie between sensory and motor neurons
-Function: pass signals through CNS pathways where integration occurs
-Can connect to other interneurons; can communicate with neighbors
Why do all cells have to maintain a resting membrane potential of -70mV?
the inside of the cell is more negatively charged than the outside
Membrane Potentials of neurons
-Neurons can change their resting membrane potential faster than other cell types
-Neurons can “communicate” with other neurons when the resting membrane potential changes
Why is communication between interneurons necessary or helpful?
necessary for more complex actions & decisions needed to be made
What does changing the resting membrane potential do?
Changing the resting membrane potential allows a neuron to receive, integrate, and send information
What is the cause of a change in resting membrane potential of neurons?
Changing the permeability of the plasma membrane to one (or more) ions
What ion is not allowed to cross the membrane at the resting membrane potential?
Na+ (sodium)
Types of signals that can be produced by a change in resting membrane potential
- Graded potential
- Action potential
Ion channels & membrane potentials
Selective proteins in plasma membrane allow passage of ions into/out of cell; they are necessary for a change in membrane potential to take place!!
Types of proteins
- Leakage (non-gated) channels
- Gated proteins
Leakage (non-gated) channels
always open, allow free flow of ions
Gated proteins
part of protein forms a “gate” that must be opened before ions can move
Types of gated proteins
- Chemically gated
- Voltage-gated
- mechanically gated
Chemically gated
only open when a certain chemical (neurotransmitter) binds to protein
Voltage-gated
open & close in response to changing membrane potentials
Mechanically gated
open in response to physical deformation of receptor
Change in membrane potential voltage due to opening of ion channels can result in:
- Depolarization
- Hyperpolarization
Depolarization
-decrease in membrane potential
-The inside of the membrane becomes less negative than resting potential; potential becomes more POSITIVE
-Excitation of a neuron
Does excitation make the neuron more or less likely to send a message?
makes the neuron more likely to generate an electrical impulse to be sent to neighboring neurons
Hyperpolarization
-increase in membrane potential
-The inside of the membrane becomes more negative than resting potential; potential becomes more NEGATIVE
-Inhibits a neuron
Does inhibition of a neuron make it more or less likely to send a message?
makes the neuron less likely to generate an electrical impulse
Graded potentials
-“Graded”: magnitude varies directly with stimulus strength
-Strong stimulus = strong graded potential
-Graded potentials only occur over short distances
-Current dies off quickly
-Can be depolarizing or hyperpolarizing
-Function: graded potentials are necessary to initiate an action potential
Where are graded potentials most likely to occur on a neuron?
on dendrites
Action potentials (nerve impulses)
-Action potentials (AP): a very brief reversal of membrane potential (from -70 mV to +30 mV)
-Only produced by neurons and muscle cells
-Action potentials have a consistent strength and are long distance
-APs originate at the beginning of axon arising from cell body (“trigger point”); Change in membrane potential from graded potential causes voltage-gated channels to open at this point
Where does an action potential occur on a neuron?
Axon
Generating Action Potentials
Generation of an AP involves opening of voltage-gated ion channels in membrane in response to changing membrane potential
How many gates does and Na+ channel have and what are they?
2 gates
1. Activation gate: voltage-sensitive, opens at depolarization
2. Inactivation gate: blocks channel to prevent Na+ movement
K+ gate
only has 1 gate that opens slowly at repolarization
Steps to generating action potentials
- All voltage-gated channels are closed at the resting state (-70 mV)
- Depolarization: voltage-gated Na+ channels open at the axon
- Repolarization
- Hyperpolarization: excess K+ leaves cell
Step 1 of generating actions potentials: All voltage-gated channels are closed at the resting state (-70 mV)
-Leakage channels are still open here!!!
-Is the permeability of sodium high or low during this stage? low
-Is the permeability of K+ high or low during this stage? high (-70)
Step 2 of generating action potentials: Depolarization: voltage-gated Na+ channels open at the axon
-voltage-gated, respond to change in potential
-Effect: Na+ freely enters the cell; Inside of the cell becomes less negative b/c of Na+
-Membrane will reach a threshold voltage (-55 mV) as more Na+ enters the cell; At this voltage, depolarization becomes self-generating
-More Na+ channels open to make inside of cell significantly less negative (30 mV at its peak)
-nothing will happen if this threshold is not reached!
Step 3 of generating action potentials: Repolarization
-This is when the action potential ends to stop ongoing messages from being sent
-Na+ gates close, Na+ permeability drops rapidly, Net influx of Na+ into cell stops completely; this causes AP to stop rising
-Voltage-gated K+ channels open; K+ leaves the cell: restores (-) internal charge of cell
-dips a bit lower to ~ -90mV
Step 4 of generating action potentials: Hyperpolarization: excess K+ leaves cell
-Result: inside of the cell becomes more negative than resting membrane potential (dips down to ~ -90mV)
-While this happens: Na+ activation gates have closed, inactivation gates reopen
-Na+-K+ pump works to re-establish normal Na+ & K+ concentrations outside and inside the cell
-membrane potential returns to -70mV
Action Potentials vs. Stimulus Strength
-APs are independent of stimulus strength
-every action potential is always the same, they never change
How does the nervous system discriminate between a strong stimulus and weak stimulus?
-How frequently nerve impulses are generated!!
-Strong stimuli: impulses are sent more frequently
-weak stimuli: impulses sent less frequently
Refractory Period in Action Potentials
a period of time in which a second AP cannot be generated at an axon
Types of refractory periods
- Absolute Refractory Period
- Relative Refractory Period
Absolute Refractory Period
-Begins when Na+ gated channels open, continues until Na+ channels reset to their original state
-During this time, another AP cannot be generated in the area, no matter how strong the stimulus is b/c all Na+ channels are open
-Ensures each AP is a separate, all-or-none event
-Enforces one-way transmission of the AP
Relative Refractory Period
-Occurs after the absolute refractory period (after repolarization)
-Stimuli that are relatively weak cannot stimulate an AP, but an exceptionally strong stimulus can
-Hyperpolarization causes mV to be more negative: need stronger stimulus to reach threshold
Action Potentials: Conduction Speed
-Impulses can be conducted quickly (ex: postural changes) or more slowly (ex: GI tract, etc.)
What 2 factors is conduction speed dependent on?
- Axon Diameter: larger axon = faster conduction
- Degree of myelination: more myelination = faster conduction
Types of Conduction
- Continuous conduction
- Saltatory conduction
Continuous conduction
-propagation in unmyelinated fibers
-Voltage-gated ion channels are adjacent for the entire length of the axon
Saltatory conduction
-propagation in myelinated fibers
-Voltage-gated ion channels found ONLY in myelin sheath gaps
-AP generated in myelin sheath gap
-AP jumps large distances along length of axon
Transmission of Signals
Signals are transmitted between neurons at a synapse
Synapse
Junction between two neurons that sends information from one neuron to the next
Presynaptic neurons
-conduct impulses toward the synapse
-The neuron that is “sending” the message
Postsynaptic Neurons
-conduct signal away from the synapse
-The neuron that is “receiving” the message
Synaptic Cleft
-neurons are separated by synaptic cleft
-fluid filled space
Process - Transmission of Action Potentials from One Neuron to Another: Chemical Synapses
- Action potential arrives at axon terminal of presynaptic neuron
- Voltage-gated Ca2+ channels in terminal open in response to AP (Ca2+ enters the axon terminal of presynaptic neuron)
- Synaptic vesicles in axon terminal fuse with membrane in response to Ca2+ influx (synaptic vesicles contain neurotransmitters)
-neurotransmitter enters the synaptic cleft - Neurotransmitter crosses cleft, binds to proteins on postsynaptic neuron
- Neurotransmitter binds receptors on the postsynaptic neuron membrane (binds to dendrites)
-Binding causes ions channels to open
-Ion flow generates a graded potential - Neurotransmitter in synaptic cleft is disposed of (prevent prolonged response)
Neurotransmitter can be disposed of by:
- Reuptake
- Degradation
- Diffusion
Reuptake
Reuptake of neurotransmitter by an astrocyte or by the pre-synaptic neuron
Degradation
Degradation of neurotransmitter by an enzyme
Diffusion
Diffusion of neurotransmitter out of the synapse
Postsynaptic Potentials
-the temporary change in membrane potential (i.e., a graded potential) of the postsynaptic neuron
-postsynaptic potential observed in dendrites
-effect of neurotransmitter can be either excitatory or inhibitory
Neurotransmitter binding cause graded potentials that vary in strength according to:
- Amount of neurotransmitter released
-larger amount of neurotransmitter = stronger potential - How long neurotransmitter stays in synaptic cleft
-longer it sits in synaptic cleft = stronger potential
Excitatory postsynaptic potential (EPSP)
-Binding of neurotransmitter causes the membrane to depolarize
-Increases the chance of generating an action potential
-A single EPSP cannot induce an AP alone (several will summate or be added together to generate AP)
Types of summation
- Temporal summation
- Spatial summation
Temporal summation
-the postsynaptic neuron receives multiple EPSPs in rapid-fire order
-not received by the postsynaptic neuron at the same time; closely packed together
Spatial summation
-postsynaptic neuron receives multiple EPSPs at the same time
-EPSPs are “added together” simultaneously
Inhibitory postsynaptic potential (IPSP)
-Binding of neurotransmitter causes the membrane to hyperpolarize
-K+ channels or Cl- channels open, making inside of cell more negative
-decreases the chance of generating an action potential
Neurotransmitters
-chemical signals produced in the cell body & is transported to the axon terminal
-Most neurons produce at least 2 types
-Neurons can release one or more neurotransmitters simultaneously
-Effects: Can be excitatory, inhibitory, or can be either depending on the receptor type they bind
Two types of neurotransmitter receptors
- Channel-linked receptors
- G-Protein Coupled Receptors
Channel-linked receptors
-mediate fast synaptic transmission
-Receptors are ligand-gated ion channels
-When ligand binds: channel opens
-Na+ influx: depolarization
-Cl- influx: hyperpolarization
G-Protein Coupled Receptors
Response is indirect & prolonged
General process of G-protein Coupled Receptors
- Neurotransmitter binds to receptor
- G-protein is activated inside the neuron
- G-protein activates adenylate cyclase
- Adenylate cyclase produces cyclic AMP (cAMP)
