Ch 11 Flashcards
(111 cards)
1
Q
The nervous system
A
The master controlling and communicating system of the body.
Consists mostly of nervous tissue, with cells densely packed.
2
Q
3 overlapping functions of the nervous system:
A
Sensory Input
Integration
Motor Output
3
Q
Sensory Input/Receptors
A
Millions of sensory receptors monitor changes occurring both inside and outside of the body.
4
Q
Integration
A
The nervous system processes and interprets sensory input and decides what should be done
5
Q
Motor Output
A
The nervous system activates effector organs (muscles and glands) to cause a response.
6
Q
The nervous system is divided into 2 principal parts.
A
Central Nervous System
Peripheral Nervous System
7
Q
Central Nervous System (CNS)
A
Consists of the brain and spinal cord (dorsal body cavity).
This is the integrating and control center of the nervous system.
8
Q
Peripheral Nervous System (PNS)
A
The part of the nervous system outside of the CNS.
Consists of mainly nerves (bundles of axons) that extend from the brain, spinal cord and ganglia.
These peripheral nerves serve as communication lines that link parts of the body to the CNS
9
Q
Ganglia
A
Collections of neuron cell bodies.
10
Q
Spinal nerves
A
Carry impulses to and from the spinal cord
11
Q
Cranial nerves
A
Carry impulses to and from the brain.
12
Q
PNS’ 2 functional subdivisions
A
Sensory/Afferent division
Motor/Efferent division
13
Q
Sensory/Afferent Division of the PNS
A
The sensory/afferent division of the PNS consists of nerve fibers (axons) that convey impulses to the central nervous system from sensory receptors located throughout the body.
Somatic sensory fibers convey impulses from the skin, skeletal muscles and joints.
Visceral sensory fibers - transmit impulses from the visceral organs.
14
Q
Motor/Efferent Division of the PNS
A
Transmits impulses from the CNS to effector organ (muscles and glands). These impulses activate muscles to contract and glands to secrete.
The motor division has 2 main parts:
1. Somatic nervous system
2. Autonomic nervous system
15
Q
Somatic nervous system (of motor division)
A
Somatic motor nerve fibers that conduct impulses from the CNS to skeletal muscles. AKA the voluntary nervous system.
16
Q
Autonomic nervous system (ANS) of the motor division
A
Consists of visceral motor nerve fibers that regulate the activity of smooth muscles, cardiac muscle, and glands. AKA involuntary nervous system.
2 subdivisions of the autonomic nervous system:
The parasympathetic and sympathetic - they work in opposite directions.
17
Q
The nervous system is made up of 2 principal cell types:
A
Neuroglia (glial cells) - small supporting cells that surround and wrap the more delicate neurons.
Neurons - nerve cells that are excitable (respond to stimuli by changing their membrane potential) and transmit electrical signals.
18
Q
Neuroglia
A
Neuroglia are smaller cells that neurons associate closely with.
There are 6 types of Neuroglia (4 in CNS, 2 in PNS).
- Astrocytes
- Microglial Cells
- Ependymal Cells
- Oligodendrocytes
- Satellite cells (PNS)
- Schwann cells (PNS)
19
Q
Astrocytes
A
(Type of Neuroglia in the CNS)
Most abundant/main type of Neuroglia cells.
- Supports and braces the neurons
- Exchange between capillaries in the neurons
- Guides the migration of young neurons
- Chemical environment around the neurons
- Responds to nervous impulse transmitters
- Participation in information processing of the brain.
20
Q
Microglial Cell
A
Defensive cells of the CNS
Deal with injured neurons
Can turn into phagocytes and get rid of debris
21
Q
Ependymal cells
A
Ependymal cells (of the CNS) line cerebrospinal fluid filled cavities.
Can circulate the cerebrospinal fluid (as they are ciliated).
Form a barrier between cerebrospinal fluid in cavities and the tissue beneath it.
22
Q
Oligodendrocytes
A
Have processes that form myelin sheaths around CNS
nerve fibers.
Wrap around central nervous system fibers, forming the myelin sheath.
23
Q
Satellite cells of the PNS
A
-Surround neuron cell bodies of the PNS
- Their function is similar to Astrocytes of the CNS
24
Q
Schwann cells of the PNS
A
Surround all peripheral nerve fibers and form myelin sheaths in thicker nerve fibers.
Similar function as Oligodendrocytes.
Vital to regeneration of damaged peripheral nerve fibers.
25
Photo of satellite and Schwann cells
26
Neurons
Neurons are nerve cells.
Are the structural units of the nervous system.
Large, highly specialized cells that conduct impulses.
Special characteristics:
1. Extreme longevity (lasts a person’s lifetime)
2. Amitotic with few exceptions - they don’t have the ability to divide.
3. High metabolic rate: requires continuous supply of oxygen and glucose.
All have a cell body and one or more processes.
27
Neuron cell body
Most neuron cell bodies are located in the CNS.
Nuclei: Clusters of neuron cell bodies in the CNS
Ganglia: Clusters of neuron cell bodies in the PNS.
28
Neuron Processes
Armlike processes that extend from the cell body.
- The CNS contains both neuron cell bodies and their processes.
- The PNS contains chiefly neuron processes.
Tracts are bundles of neuron processes in the CNS.
Nerves are bundles of neuron processes in the PNS
2 types of processes:
1. Dendrites - take the incoming message towards the cell body
2. Axons - carries the message out towards the effector
Each neuron can have 1 axon but can have multiple dendrites.
29
The axon’s functional characteristics…
Axons are the conducting region of the neuron.
Axolemma - the neuron cell membrane
Axon Terminal - impulse goes down axon to the axon terminal.
- Neurotransmitters often released at axon terminal (AcH)
2 direction system:
Anterograde - away from the cell
Retrograde - towards the cell body; toxins, etc move retrograde.
30
Myelin Sheath
Composed of myelin, a whitish protein-lipid substance.
Function of myelin: Protect and electrically insulate the axon; Increase the speed of nerve impulse transmission.
31
Myelinated and nonmyelinated fibers
Myelinated fibers: segmented sheath surrounds most long or large-diameter axons.
Nonmyelinated fibers: do not contain sheaths; conduct impulses more slowly.
32
Myelination in the PNS
Myelin sheath gaps are gaps between the Schwann cells
Nonmyelinated fibers
33
Myelin sheaths in the CNS
-Formed by processes of Oligodendrocytes, not whole cells.
- Each cell can wrap up to 60b axons at once.
- Myelin sheath gap is present
- No outer collar of perinuclear cytoplasm
- Thinnest fibers are unmyelinated, but are covered by long extensions of adjacent Neuroglia
34
White Matter
Densely myelinated fibers (of the brain)
35
Gray Matter
Cell bodies and unmyelinated fibers
36
Classifications of Neurons
There are 3 types of structural classifications grouped by the number or processes:
1. Multipolar - 3 or more processes, 1 axon, and dendrites.
2. Bipolar - 1 axon, 1 dendrite
3. Unipolar - a T like process; 2 axons
Unipolar is AKA pseudounipolar
Peripheral (distal) process: associated with sensory receptor
Proximal (central) process: enters the CNS
37
38
Functional classification of neurons
There are 3 types of neurons grouped by direction in which nerve impulse travels relative to the CNS
1. Sensory - transmit from the sensory receptors towards the the CNS (mostly Unipolar) . Their cell bodies will be in the ganglia of the PNS
2. Motor - carry the impulse from the CNS to the effector (mostly multipolar) Most of their cell bodies are located in the CNS
3. Interneurons - between the motor and the sensory neurons. Most found within the CNS
39
Membrane Potentials
- Like all cells, neurons have a resting membrane potential.
- Unlike most other cells, neurons can rapidly change resting membrane potential.
- Neurons are highly excitable.
40
Basic Principles of Electricity
- Opposite charges are attracted to each other.
- Energy is required to keep opposite charges separated across a membrane.
- Energy is liberated when the charges move toward one another.
- When opposite charges are separated, the system has potential energy (energy waiting to be released).
41
Voltage
A measure of potential energy generated by a separated charge.
- Measured between 2 points in volts (V) or millivolts (mV)
- Called potential difference or potential.
- The charge difference across the plasma membrane results in potential.
- The greater the charge difference between points, the higher the voltage.
42
Current
The flow of electrical charge (ions) between 2 points.
- Current can be used to do work
- The flow is dependent on voltage and resistance
43
Resistance
A hindrance to charge flow
- insulator: substance with high electrical resistance
- conductor: substance with low electrical resistance.
44
Ohm’s law
Ohm’s law gives the relationship of voltage, current and resistance.
Current (I) = voltage (V) /resistance (R)
- Current is directly proportional to voltage.
- The greater the voltage (potential difference), the greater the current
- There is no net current flow between points with the same potential
- Current is inversely proportional to resistance
- The greater the resistance, the smaller the current.
45
The role of membrane ion channels
In membrane ion channels: Large proteins (inside membranes) serve as selective membrane ion channels (K+ ion channels allow only K+ to pass through)
46
2 main types of ion channels
Leakage (no gated) channels, which are always open.
Gated channels, in which part of the protein changes shape to open/close the channel.
- There are 3 main gated channels:
1. Chemically gated
2. Voltage-gated
3. Mechanically gated.
47
Chemically gated (ligand-gated) channels
These channels open only with the binding of a specific chemical (ex: neurotransmitter)
48
Voltage-gated channels
These channels open and close in response to changes in membrane potential.
49
Mechanically gated channels
These channels open and close in response to physical deformation of receptors, as in sensory receptors.
50
Electrochemical gradient
Electrical and chemical gradients combined.
- Ion flow creates an electrical current, and voltage changes across the membrane.
- Expressed by rearranged Ohm’s law equation: V = IR
51
Generating the resting membrane potential
- A voltmeter can measure potential(charge) difference across the membrane of a resting cell.
- The resting membrane potential of a resting neuron is approximately -70 mV
- The cytoplasmic side of the membrane is negatively charged relative to the outside which will be positively charged (this is referred to as polarized)
- The actual voltage difference varies from -40 mV to -90 mV
52
Differences in ionic composition of the ECF vs ICF
- ECF (extracellular fluid) has a higher concentration of Na than ICF (intracellular fluid)
- This concentration is balanced chiefly by chloride ions (CL)
- ICF has a higher concentration of K than ECF
- there’s negatively charged proteins inside of all cells
- K plays the most important role in membrane potential.
53
Differences in plasma membrane permeability
- The plasma membrane is impermeable to anionic proteins (negatively charged proteins)
- It is slightly permeable to NA (through leakage channels)
- Sodium diffuses into the cell down the concentration gradient.
- 25 times more permeable to K than NA (more leakage channels)
- Potassium diffuses out of the cell, down the concentration gradient.
- Quite permeable to Cl
54
More potassium diffuses out than sodium diffuses in
- As a result, the inside of the cell is more negative
- This establishes the resting membrane potential.
55
The sodium-potassium pump (NA/K ATPase) stabilizes resting membrane potential
- This pump maintains concentration gradients for Na nd K
- 3 Na are pumped out of the cell while 2 K are pumped back in.
56
Changing the Resting Membrane Potential
Membrane potential changes when:
- Concentrations of ions across the membrane change.
- Membrane permeability to ions changes.
Changes produce 2 types of signals:
1. Graded potentials: incoming signals operating over short distances
2. Action potentials: Long-distance signals at axons.
Changes in membrane potential are used as signals to receive, integrate and send information.
57
Terms describing membrane potential changes relative to resting membrane potential:
- Depolarization - a decrease in membrane potential (number moves towards zero and becomes less negative)
- Hyperpolarization (an increase in membrane potential, away from zero, more negative. Inside of the membrane becomes more negative than the resting potential).
58
Graded Potentials
- Short-lived, localized changes in membrane potential (small area)
- The stronger the stimulus, the more voltage changes and the farther the current flows.
- Triggered by stimulus that opens gated ion channels
- Results in depolarization or sometimes hyperpolarization
- Named according to the location and function:
- Receptor potential (generator potential): graded potentials in receptors of sensory neurons
- Postsynaptic potential: neuron graded potential
59
Action Potentials
- Principal way neurons send signals
- A means of long-distance neural communication
- Occur only in muscle cells and axons of neurons
- Brief reversal of membrane potential with a change in voltage of ~100 mV (millivolts)
- APs do not decay over distance the way graded potentials do.
- APs in neurons are also referred to as Nerve impulses
- Involves the opening of specific voltage-gated channels.
60
4 main steps to generating AP
1. Resting state: All gated Na and K channels are closed. (Only leakage channels are open which helps maintain the resting potential)
2. Depolarization: Na channels open; Na influx causes more depolarization which opens up the sodium channels so that the ICF becomes less negative. Then you reach threshold (-55 - (-50) mV) which causes all the Na channels to open up. The membrane polarity goes up to a +30 mV
3. Repolarization: Returning back to resting membrane potential… Na channels are inactivating, and K channels open
4. Hyperpolarization: Some K channels remain open, and Na channels reset
61
Pic: All 4 events of AP Generation
62
Pic
63
Threshold and the All-or-None Phenomenon
- Not all depolarization events produce AP
- For an axon to “fire”, depolarization must reach threshold voltage to trigger AP
- At threshold:
- Membrane is depolarized by 15 to 20 mV (-50, -55 mV)
- Na permeability increases
- Na influx exceeds K efflux
- The positive feedback cycle begins (All or none)
64
All or None
An AP either happens completely or does not happen at all.
65
Propagation of an AP
- Propagation allows AP to be transmitted from the origin down the entire axon length toward terminals.
- Na influx through voltage gates in one membrane area causes local currents that cause the opening of Na voltage gates in adjacent membrane areas.
- This leads to depolarization of that area, which in turn causes depolarization in the next area.
66
Once initiated, an AP is self-propogating
- In nonmyelinated axons, each successive segment of the membrane depolarizes, then repolarizes.
- Propogation in myelinated axons differs.
- Since Na channels closer to the AP origin are still inactivated (closed), no new AP is generated there.
- AP occurs only in a forward direction.
67
Coding for Stimulus Intensity
- All action potentials are alike and are independent of stimulus intensity.
- The CNS tells the difference between a weak stimulus and a strong one by the frequency of impulses.
- Frequency is the number of impulses (APs) received per second.
- Higher frequencies of impulses received means a stronger stimulus.
68
Refractory Periods
- Refractory period is the time in which a neuron cannot trigger another AP.
- Voltage gated Na channels are open, so the neuron cannot respond to another stimulus.
There are 2 types of refractory periods:
1. Absolute
2. Relative
69
Absolute refractory period
- The time from opening of Na channels until resetting of the channels.
- Ensures that each AP is an all or none event.
- Enforces one way transmission of nerve impulses
70
Relative refractory period
- Follows the absolute refractory period
- Most Na channels have returned to their resting state.
- Some K channels are still open
- Repolarization is occurring
- Threshold for AP generation is elevated.
- Only exceptionally strong stimuli can stimulate an AP
71
Conduction Velocity
- APs occur only in axons, not other cell areas.
- AP conduction velocities in axons vary widely
- Rate of AP propogation depends on 2 factors:
1. Axon Diameter - larger diameter fibers have less resistance to local current flow, so they have a faster impulse conduction.
2. Degree of myelination: There are 2 types of conduction depending on the presence or absence of myelin (Continuous conduction and Saltatory conduction).
72
Continuous Conduction
A slow conduction that occurs in nonmyelinated axons
73
Saltatory conduction
Occurs only in myelinated axons and is about 30 times faster.
- Myelin sheaths insulate and prevent leakage of the charge.
- Voltage gated Na channels are located at myelin sheath gaps
- APs are generated only at the gaps.
- Electrical signals appear to jump rapidly from gap to gap
74
Nerve fibers are classified according to:
- diameter, degree of myelination and speed of conduction.
These fall into 3 groups
1. Group A fibers
2. Group B fibers
3. Group C fibers
75
Group A fibers
- Largest diameter
- Myelinated somatic sensory and motor fibers of skin, skeletal muscles, and joints.
- Transmit at 150 m/s (~300 mph)
76
Group B fibers
- Intermediate diameter
- Lightly myelinated fibers
- Transmit at 15m/s (~30 mph)
Include ANS visceral motor and sensory fibers that serve visceral organs.
77
Group C fibers
- Smallest diameter
- Unmyelinated
- Transmit at 1 m/s (~2 mph)
Include ANS visceral motor and sensory fibers that serve visceral organs.
78
The synapse
-The nervous system works because information flows from neuron to neuron
- Neurons are functionally connected by synapses, junctions that mediate information transfer
- From one neuron to another neuron
- Or from one neuron to an effector cell
- Presynaptic neuron
- Postsynaptic neuron
79
Synaptic connections
- Axodendritic: between axon terminals of one neuron and dendrites of others
- Axosomatic: between axon terminals of one neuron and soma (cell body) of others.
- Less common connections:
- Axoaxonal (axon to axon)
- Dendrodendritic (dendrite to dendrite)
- Somatodendritic (dendrite to soma)
- Two main types of synapses:
- Chemical synapse
- Electrical synapse
80
Chemical Synapses
- Most common type of synapse
- Specialized for release and reception of chemical neurotransmitters
- Typically composed of 2 parts:
- Axon terminal of presynaptic neuron: contains synaptic vesicles filled with neurotransmitter
- Receptor region on Postsynaptic neuron’s membrane: receives neurotransmitter (usually on dendrite or cell body)
- These two parts are separated by fluid filled synaptic cleft
- Electrical impulse changed to chemical across synapse, then back into electrical.
81
Chemical synapse: transmission across the synaptic cleft
- The synaptic cleft prevents impulses from directly passing from one neuron to the next.
- A chemical event has to occur
- Depends on the release, diffusion and receptor binding of neurotransmitters.
- Ensures unidirectional communication between neurons.
82
6 steps of information transfer across chemical synapses:
1. AP arrives at axon terminal or presynaptic neuron
2. Voltage gated Ca channels open, and Ca enters axon terminal
- Ca flows down electrochemical gradient from ECF to inside of axon terminal
3. Ca entry causes synaptic vesicles to release a neurotransmitter
4. The neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the Postsynaptic membrane
5. Binding of the neurotransmitter opens ion channels, creating graded potentials.
6. The neurotransmitter effects are terminated.
83
Synaptic delay in chemical synapses
- Synaptic delay is the time needed for the neurotransmitter to be released, diffuse across the synapse, and bind to receptors.
- this can take anywhere from .3 to 5 M’s
- Synaptic delay is a rate limiting step. Of neural transmission
- Transmission of AP down an axon can be very quick, but the synapse slows the transmission to the Postsynaptic neuron down significantly
- not noticeable because they are still very fast.
84
Electrical Synapses
- Less common than chemical synapses
- Neurons are electrically coupled
- Joined by gap junctions that connect the cytoplasm of adjacent neurons
- Communication is very rapid and may be unidirectional or bidirectional.
- Electrical synapses are found in some brain regions responsible for eye movements or the hippocampus in areas involved in emotions and memory.
- Most abundant in embryonic nervous tissue.
85
Postsynaptic Potentials
- Neurotransmitter receptors cause graded potentials that vary in strength based on:
- The amount of neurotransmitter released.
- The time the neurotransmitter stays in the cleft.
- Depending on the effect of the chemical synapse, there are 2 types of Postsynaptic potentials:
1. EPSP: excitatory Postsynaptic potential
2. IPSP: inhibitory Postsynaptic potential
86
Excitatory Synapses and EPSPs
- The neurotransmitter binding opens chemically gated channels.
- This allows simultaneous flow of Na and K in opposite directions.
- Na influx is greater than K efflux, resulting in a local net graded potential depolarization called excitatory Postsynaptic potential (EPSP)
- EPSPs trigger AP if EPSP is of threshold strength
- This can spread to axon hillock and trigger the opening of the voltage gated channels, causing the AP to be generated.
87
Inhibitory Synapses and IPSPs
- A neurotransmitter binding to a receptor opens chemically gated channels that allow the entrance/exit of ions that cause hyperpolarization.
- This makes the Postsynaptic membrane more permeable to K and Cl
- If K channels open, it moves out of the cell.
If Cl channels open, it moves into the cell.
- This reduces the Postsynaptic neuron’s ability to produce an action potential
- Moves the neuron farther away from threshold (makes it more negative).
88
Integration and Modification of Synaptic Events
- Summation by the Postsynaptic neuron
- A single EPSP cannot induce an AP, but EPSPs can summate (add together) to influence a Postsynaptic neuron.
- IPSPs can also summate
- Most neurons receive both excitatory and inhibitory inputs from thousands of other neurons.
- Only if EPSPs predominate and bring to threshold will an AP be generated
- Two types of summations: Temporal and Spatial
89
Temporal Summation
- One or more presynaptic neurons transmit impulses in rapid fire order
- First impulse produces EPSP and before it can dissipate another EPSP is triggered, adding on top of the first impulse.
90
Spatial Summation
- Postsynaptic neuron is stimulated by a large number of terminals simultaneously.
- Many receptors are activated, each producing EPSPs, which can then add together.
91
Synaptic Potential
92
Presynaptic inhibition
Mechanisms that suppress the release of neurotransmitters from axons.
93
Pic: Comparison of graded potentials and APs #1
94
Pic: Comparison of graded potentials and APs #2
95
Pic: Comparison of graded potentials and APs #3
96
Pic: Comparison of graded potentials and APs #4
97
Neurotransmitters
- Are the language of the nervous system
- 50 or more neurotransmitters have been id’d
- Most neurons make 2 or more neurotransmitters
- neurons can exert several influences
- Usually released at different stimulation frequencies
- Classified by
1. Chemical structure
2. Function
98
Chemical structure classification of neurotransmitters
- Acetylcholine (ACh)
- Biogenic amines
- Catecholamines
- Dopamine
- Indolamines
- Serotonin: made from the amino acid tryptophan
- Histamine: made from the amino acid histidine.
- Amino Acids: make up all the proteins; some are neurotransmitters
- Peptides (neuroleptides) - string of amino acids that connect as neurotransmitters.
- Purines
- Gases and lipids
- Endocannabinoids
99
Classification of neurotransmitters by function
- Neurotransmitters exhibit a great diversity of functions
- Functions can be grouped into 2 classifications:
- Effects: excitatory vs inhibitory; ACh is excitatory for the NMJ; ACh can inhibit if it attaches to a cardiac muscle.
- Actions: direct actions where the neurotransmitter binds directly to and opens ion channels; indirect actions where the neurotransmitter acts through intracellular second messengers
100
Neurotransmitter receptors
- Channel linked receptors: Lygand gated channels with immediate and brief action; excitatory receptors are channels for small cat ions; inhibitory receptors allow Cl influx that causes hyperpolarization
- G protein linked receptors: their responses are indirect; they’re slow and prolonged.
Involve transmembrane protein complexes
101
Neural Integration
- Neural Integration: neurons functioning together in groups
- Groups contribute to broader neural functions
- There are billions of neurons in CNS
- Must have integration so that the individual parts fuse to make a smoothly operating whole.
102
Neuronal pool
A Neuronal pool is the functional groups of neurons.
- Neuronal pools integrate incoming information received from receptors or other neuronal pools.
- These pools forward processed information to other destinations.
103
Organization of neurons into neuronal pools
- Simple neuronal pool:
- Single presynaptic fiber branches and synapses with several neurons in pool.
- Discharge zone: neurons closer to incoming fiber are more likely to generate an impulse.
- Facilitated zone: neurons on periphery of pool are farther away from incoming fiber; usually not excited to threshold unless stimulated by another source.
104
Serial processing (not usually tested on this)
Serial processing:
- Input travels along one pathway to a specific destination
- One neuron stimulates the next one, which stimulates the next one, etc.
- The system works in an all or none manner to produce a specific, anticipated response.
- The best example of serial processing is a spinal reflex.
105
Serial processing: Reflexes (need to know this)
Reflexes:
- Rapid automatic responses to stimuli
- Particular stimulus always causes same response
- Occurs over pathways called reflex arcs that have 5 components**:
- Receptor
- Sensory neuron
- CNS integration center
- Motor neuron
- Effector
106
Parallel processing
Parallel processing:
- Input travels along several pathways
- Different parts of the circuitry deal simultaneously with info:
- One stimulus promotes numerous responses
- Important for higher-level mental functioning
- Example: A sensed smell may remind one of an odor and any associated experiences
107
Types of Circuits
Circuits: patterns of synaptic connections in neuronal pools
- 4 types of circuits
1. Diverging
2. Converging
3. Reverberating
4. Parallel after-discharge
108
Diverging circuit
109
Converging circuit
110
Reverberating circuit
111
Parallel after-discharge circuit
