Chapter 12 Central Nervous System Flashcards
The nervous system
Brain and spinal cord
Receptors of sense organs (eyes, ears, etc.)
Nerves that connect to other systems
Nervous tissue contains two kinds of cells
Neurons for intercellular communication
Neuroglia (glial cells)
Neuroglia (glial cells)
Essential to survival and function of neurons
Preserve structure of nervous tissue
Anatomical divisions of the nervous system
Central nervous system
Peripheral nervous system
Central nervous system (CNS)
Brain and spinal cord
Consists of nervous tissue, connective tissue, and blood vessels
Functions to process and coordinate sensory data from inside and outside body
Motor commands control activities of peripheral organs (e.g., skeletal muscles)
Higher functions of brain include intelligence, memory, learning, and emotion
Special Sensory Receptors Path
Smell, Taste, Vision, Balance, Hearing - to Afferent Div of the PNS then to brain (CNS)
Visceral Sensory Receptors Path
Monitors internal organs, to Afferent Div of the PNS then to brain
Somatic Sensory Receptors Path
Skeletal, Muscle, Joints, Skin, (External Senses) to Afferent Div of PNS and then to brain
From Brain (CNS) to Skeletal Syst
Brain to Efferent Motor Command then to Somatic Nervous Syst (SNS), to Skeletal syst.
From Brain to Parasympathetic System
Brain, To Efferent Syst, Motor Commands, Autonomic Nervous Syst (ANS), Parasympathetic Syst to Smooth Muscle, Cardiac Muscle and Glands
From Brain to Sympathetic Nervous Syst
Brain, to Efferent Motor Commands, Autonomic Nervous Syst (ANS), Sympathetic Syst. to Smooth Muscle, Cardiac Muscle, Glands and Adipose Tissue
Neurons
Basic functional units of the nervous system
Send and receive signals
Function in communication, information processing, and control
Neurons – Cell body (soma)
Large nucleus and nucleolus
Perikaryon (cytoplasm)
Mitochondria (produce energy)
RER (Rough Endoplasmic Reticulum) and ribosomes (synthesize proteins)
Cytoskeleton of perikaryon
Neurofilaments and neurotubules
Similar to intermediate filaments and microtubules
Neurofibrils
Bundles of neurofilaments that provide support for dendrites and axon
Nissl bodies
Dense areas of RER and ribosomes in perikaryon
Make nervous tissue appear gray (gray matter)
Dendrites
Short and highly branched processes extending from cell body
Dendritic spines
Fine processes on dendrites
Receive information from other neurons
80–90 percent of neuron surface area
Axon
Single, long cytoplasmic process
Propagates electrical signals (action potentials)
Axoplasm
Cytoplasm of axon
Contains neurofibrils, neurotubules, enzymes, and organelles
Structures of the axon
Axolemma - Plasma membrane of the axon and Covers the axoplasm
Initial segment - Base of axon
Axon hillock - Thick region that attaches initial segment to cell body
Structures of the axon pt 2
Collaterals
Branches of the axon
Telodendria
Fine extensions of distal axon
Axon terminals (synaptic terminals)
Tips of telodendria
Neurons - Axonal (axoplasmic) transport
Movement of materials between cell body and axon terminals
Materials move along neurotubules within axon
Powered by mitochondria, kinesin, and dynein
Structural classification of neurons
Anaxonic neurons - May have more than 2 processes
Small and may all be Dendrites
All cell processes look similar, Axons not Obvious
Found in brain and special sense organs
Bipolar neurons - 2 processes, Seperated by Cell Body
Small and rare
One dendrite and one axon
Found in special sense organs (sight, smell, hearing)
Structural classification of neurons pt 2
Unipolar neurons (pseudounipolar neurons)
Single Elongated Process off to the side
Axon and dendrites are fused
Cell body to one side
Most sensory neurons of PNS
Multipolar neurons - have more than 2 processes
Has single long axon and multiple dendrites
Common in the CNS
All motor neurons that control skeletal muscles
Sensory neurons (afferent neurons)
Unipolar
Cell bodies grouped in sensory ganglia
Processes (afferent fibers) extend from sensory receptors to CNS
Somatic sensory neurons
Monitor external environment
Visceral sensory neurons
Monitor internal environment
Ganglia
Nerve cell cluster or a group of nerve cell bodies located in the autonomic nervous system and sensory system
ganglia house the cells bodies of afferent nerves and efferent nerves
Sensory receptors
Interoceptors
Monitor internal systems (e.g., digestive, urinary)
Internal senses (stretch, deep pressure, pain)
Exteroceptors
Monitor external environment (e.g., temperature)
Complex senses (e.g., sight, smell, hearing)
Proprioceptors
Monitor position and movement of skeletal muscles and
joints
Types of sensory receptors pt 2
Proprioceptors (cont)
Carry instructions from CNS to peripheral effectors
Via efferent fibers (axons)
Somatic motor neurons of SNS
Innervate skeletal muscles
Visceral motor neurons of ANS
Innervate all other peripheral effectors
Smooth and cardiac muscle, glands, adipose tissue
Motor neurons
Signals from CNS to visceral effectors cross autonomic ganglia that divide axons into
Preganglionic fibers
Postganglionic fibers
Interneurons
Most are in brain and spinal cord
Some in autonomic ganglia
Located between sensory and motor neurons
Responsible for
Distribution of sensory information
Coordination of motor activity
Involved in higher functions
Memory, planning, learning
Neuroglia
Support and protect neurons
Make up half the volume of the nervous system
Many types in CNS and PNS
Neuroglia
Types of NEUROGLIA
Astrocytes
Ependymal
Oligodendrites
Microgila
Astrocytes (Brain) Star Shaped, anchor to capillaries
Blood Brain Barrier, structural support, regulate ion, neutrient, gas concentrations, absorb recycle neurotransmitters, scar tissue after injury
Have large cell bodies with many processes
Function to
Maintain blood brain barrier (BBB)
Create three-dimensional framework for CNS
Repair damaged nervous tissue
Guide neuron development
Control interstitial environment
Ependymals (Brain) - simple cuboidal epithelial cells that line fluid-
filled passageways within the brain and spinal
cord
line the ventricles + central canal (spinal cord), assist with cerebrospinal fluid
Form epithelium that lines central canal of spinal cord and ventricles of brain
Produce and monitor cerebrospinal fluid (CSF)
Have cilia that help circulate CSF
Oligodendrites (Brain)
structural framework, myelinate sheet like process that surrounds CNS Axons, increases speed of action potentials. Nerves appear white.
Internodes—myelinated segments of axon
Nodes (nodes of Ranvier) lie between internodes
Where axons may branch
White matter
Regions of CNS with many myelinated axons
Gray matter of CNS
Contains unmyelinated axons, neuron cell bodies, and dendrites
Microgila (Brain) - are phagocytes
Smallest and least numerous neuroglia
Have many fine-branched processes
Migrate through nervous tissue
Clean up cellular debris, wastes, and pathogens by phagocytosis
Neural responses to injuries
Wallerian degeneration
Axon distal to injury degenerates
Schwann cells
Form path for new growth
Wrap around new axon
Nerve regeneration in CNS
Limited by astrocytes, which
Produce scar tissue
Release chemicals that block regrowth
All plasma (cell) membranes produce electrical signals by ion movements
Membrane potential is particularly important to neurons
Resting membrane potential
Three important concepts
The extracellular fluid (ECF) and intracellular
fluid (cytosol) differ greatly in ionic composition
Extracellular fluid contains high concentrations of Na+ and
Cl–
Cytosol contains high concentrations of K+ and negatively
charged proteins
Cells have selectively permeable membranes
Membrane permeability varies by ion
Graded potential
Temporary, localized change in resting potential
Caused by a stimulus
Action potential (nerve impulses)
All-or-none principle
Any stimulus that changes the membrane potential to threshold
Will cause an action potential
All action potentials are the same
No matter how large the stimulus
An action potential is either triggered or not
Is an electrical impulse
Produced by graded potential
Propagates along surface of axon to synapse
Propagated changes in membrane potential
Affect an entire excitable membrane
Begin at initial segment of axon
Do not diminish as they move away from source
Stimulated by a graded potential that depolarizes the axolemma to threshold
Threshold for an axon is –60 to –55 mV
Passive processes acting across cell membrane
Current
Movement of charges to eliminate a potential difference
Resistance
How much the membrane restricts ion movement
If resistance is high, current is small
Passive processes acting across cell membrane
Chemical gradients
Concentration gradients of ions (Na+, K+)
Electrical gradients
Charges are separated by cell membrane
Cytosol is negative relative to extracellular fluid
Electrochemical gradient
Sum of chemical and electrical forces acting on an ion across the membrane
A form of potential energy
Equilibrium potential
Membrane potential at which there is no net movement of a particular ion across cell membrane
K+ = –90 mV
Na+ = +66 mV
Plasma membrane is highly permeable to K+
Explains similarity of equilibrium potential for K+ and resting membrane potential (–70 mV)
Resting membrane’s permeability to Na+ is very low
So Na+ has a small effect on resting potential
Resting membrane potential exists because:
Cytosol differs from extracellular fluid in chemical and ionic composition
Plasma membrane is selectively permeable
Membrane potential changes in response to temporary changes in membrane permeability
Results from opening or closing of specific membrane channels
In response to stimuli
Na+ and K+ are the primary determinants of membrane potential
Passive ion channels (leak channels)
Are always open
Permeability changes with conditions
Active ion channels (gated ion channels)
Open and close in response to stimuli
At resting membrane potential, most are closed
Chemically Gated Ion Channel - (Active Channel)
Called Ligand - Gated Ion Channel.
Opens when it binds to specific Chems (ie ACh)
Found on cell bodies and dendrites of neurons
Voltage-gated ion channels
Respond to changes in membrane potential
Found in axons of neurons and sarcolemma of skeletal and cardiac muscle cells
Activation gate opens when stimulated
Inactivation gate closes to stop ion movement
Three possible states
Closed but capable of opening
Open (activated)
Closed and incapable of opening (inactivated)
Mechanically gated ion channels
Respond to membrane distortion
Found in sensory receptors that respond to touch, pressure, or vibration
Graded potentials (local potentials) -
Characteristics
Membrane potential is most changed at site of stimulation; effect
decreases with distance
Effect spreads passively, due to local currents
Graded change in membrane potential may involve depolarization
or hyperpolarization
Stronger stimuli produce greater changes in membrane potential
and affect a larger area
Changes in membrane potential
That cannot spread far from site of stimulation
Produced by any stimulus that opens gated channels
Example: a resting membrane is exposed to a chemical
Chemically gated Na+ channels open
Sodium ions enter cell
Membrane potential rises (depolarization)
Graded potentials - Repolarization
When the stimulus is removed,
membrane potential returns to normal
Hyperpolarization
Results from opening potassium ion channels
Positive ions move out, not into cell
Opposite effect of opening sodium ion
channels
Increases the negativity of the resting potential
Sodium ions move parallel to plasma membrane
Producing local current
Which depolarizes nearby regions of plasma membrane (graded
potential)
Change in potential is proportional to stimulus
Often trigger specific cell functions
Example: exocytosis of glandular secretions
ACh causes graded potential at motor end plate at neuromuscular junction
Resting membrane with closed chemically gated sodium ion channels
-70mv
Membrane exposed to chemical that opens the sodium ion channels
-65mv
Generation of action potentials
Step 1: Depolarization to threshold
Step 2: Activation of voltage-gated Na channels
Na+ rushes into cytosol
Inner membrane surface changes from negative to positive
Results in rapid depolarization
Generation of action potentials
Step 3: Inactivation of Na channels and activation of K+ channels
At +30 mV, inactivation gates of voltage-gated Na+ channels close
Voltage-gated K+ channels open
K+ moves out of cytosol
Repolarization begins
Characteristics of graded potentials (Action Potentials)
Membrane potential is most changed at site of stimulation; effect
decreases with distance
Effect spreads passively, due to local currents
Graded change in membrane potential may involve depolarization
or hyperpolarization
Stronger stimuli produce greater changes in membrane potential
and affect a larger area
Generation of action potentials
Step 1: Depolarization to threshold
Step 2: Activation of voltage-gated Na channels
Na+ rushes into cytosol
Inner membrane surface changes from negative to positive
Results in rapid depolarization
Depolarization to Threshold (-60mv)
The stimulus that initiates an action
potential is a graded depolarization
large enough to open voltage-gated
sodium channels. The opening of the
channels occurs at a membrane
potential known as the threshold.
Activation of Sodium Ion Channels and Rapid Depolarization (+10mv)
When the sodium channel activation gates
open, the plasma membrane becomes
much more permeable to Na+. Driven by the
large electrochemical gradient, sodium
ions rush into the cytosol, and rapid
depolarization occurs. The inner membrane
surface now has more positive ions than
negative ones, and the membrane potential
has changed from −60 mV to a positive
value.
Generation of action potentials
Step 3: Inactivation of Na channels and activation of K+ channels
At +30 mV, inactivation gates of voltage-gated Na+ channels close
Voltage-gated K+ channels open
K+ moves out of cytosol
Repolarization begins
Inactivation of Sodium Ion Channels and Activation of Potassium Ion Channels Starts Repolarization
As the membrane potential approaches +30 mV, the inactivation gates of the voltage-gated sodium channels close. This step is known as sodium channel inactivation, and it coincides with the opening of voltage-gated potassium channels. Positively charged potassium
ions move out of the cytosol, shifting the membrane potential back toward the resting level. Repolarization now begins
Generation of action potentials
Step 4: Return to resting membrane potential Voltage-gated K+
channels begin to close
As membrane reaches normal resting potential
K+ continues to leave cell
Membrane is briefly hyperpolarized to –90 mV
After all voltage-gated K+ channels finish closing
Resting membrane potential is restored
Action potential is over
Time Lag in Closing All Potassium Ion Channels Leads to Temporary
Hyperpolarization
The voltage-gated sodium channels remain inactivated until the membrane has repolarized to near threshold level. At this time, they regain their normal status: closed but capable of opening. The voltage-gated potassium channels begin closing as the membrane reaches the normal resting membrane potential (about −70 mV). Until all of these potassium channels have closed, potassium ions continue to leave the cell. This produces a brief hyperpolarization.
After all the voltage-gated potassium channels close, the membrane potential returns to the normal resting level. The action potential is now over, and the membrane is once again at the resting membrane potential.
Refractory period
From beginning of action potential To return to resting state
During which the membrane will not respond normally to additional stimuli
Absolute refractory period
All voltage-gated Na+ channels are already open or inactivated
Membrane cannot respond to further stimulation
Relative refractory period
Begins when Na+ channels regain resting condition
Continues until membrane potential stabilizes
Only a strong stimulus can initiate another action potential
Depolarization results from influx of Na+
Repolarization involves loss of K+
Sodium–potassium exchange pump
Returns concentrations to prestimulation levels
Maintains concentration gradients of Na+ and K+ over time
Uses one ATP for each exchange of two extracellular K+ for three
intracell ular Na+
Propagation
Moves an action potential along an axon in a series of steps
Continuous propagation of action potentials
Occurs in unmyelinated axons
Affects one segment of an axon at a time
Step 1: Action potential develops at initial segment Depolarizes
membrane to +30 mV
Step 2: Local current develops
Depolarizes second segment to threshold
1) As an action potential develops at the initial segment, the membrane
potential at this site depolarizes to +30 mV
2) A local current then develops as the sodium ions entering at 1 spread away from the open voltage-gated channels.
A graded depolarization quickly brings the axon membrane (axolemma) in segment 2 to threshold.
Continuous propagation
Step 3: Action potential occurs in second segment
Initial segment begins repolarization
Step 4: Local current depolarizes next segment Cycle repeats
Action potential travels in one direction (1 m/sec)
Saltatory propagation of action potentials
Occurs in myelinated axons
Faster than continuous propagation
Requires less energy
Myelin prevents continuous propagation
Local current “jumps” from node to node
Depolarization occurs only at nodes
Axon diameter affects propagation speed of action potentials
The larger the diameter, the lower the resistance and faster the speed
Types of axons based on diameter, myelination, and propagation speed
Type A fibers
Type B fibers
Type C fibers
Type A fibers
Myelinated
Large diameter
Transmit information to and from CNS rapidly
(120 m/sec), for example
Sensory information such as position and balance
Motor impulses to skeletal muscles
Type B fibers
Myelinated
Medium diameter
Transmit information at intermediate speeds (18 m/sec)
Type C fibers
Unmyelinated
Small diameter
Transmit information slowly (1 m/sec)
Example: most sensory information
Messages carried by nerves are routed according to priority
Critical information is transmitted through Type A fibers, for example
Sensory information about things that threaten survival
Motor commands that prevent injury
Synapse
Specialized site where a neuron communicates with another cell
Presynaptic neuron
Sends the message
Postsynaptic neuron
Receives message
Types of synapses
Electrical synapses
Direct physical contact between cells
Presynaptic and postsynaptic membranes are locked together at
gap junctions
Ions pass between cells through pores Local current affects both
cells Action potentials are propagated quickly Found in some
areas of brain, the eye, and ciliary ganglia
Chemical synapses
Signal transmitted across a gap by neurotransmitters
Chemical synapses
Most common type of synapse between neurons
Only type of synapse between neurons and other cells
Cells are separated by synaptic cleft
Presynaptic cell sends the message
Postsynaptic cell receives the message
Types of chemical synapses
Neuromuscular junction
Synapse between neuron and skeletal muscle cell
Neuroglandular junction
Synapse between neuron and gland cell
Neurotransmitters
Chemical messengers contained within synaptic vesicles in axon
terminal of presynaptic cell
Released into synaptic cleft
Affect receptors of postsynaptic membrane
Broken down by enzymes
Reabsorbed and reassembled by axon terminal
Function of chemical synapses
Axon terminal releases neurotransmitters that bind to postsynaptic
plasma membrane
Produces localized change in permeability and graded potentials
Action potential may or may not be generated in postsynaptic cell, depending on
Amount of neurotransmitter released
Sensitivity of postsynaptic cell
Cholinergic synapses
Release acetylcholine (ACh) at
All neuromuscular junctions involving skeletal muscle fibers
Many synapses in CNS
All neuron-to-neuron synapses in PNS
All neuromuscular and neuroglandular junctions in parasympathetic division of ANS
Release acetylcholine (ACh) at
All neuromuscular junctions involving skeletal muscle fibers
Many synapses in CNS
All neuron-to-neuron synapses in PNS
All neuromuscular and neuroglandular junctions in parasympathetic division of ANS
Events at a cholinergic synapse
Action potential arrives at axon terminal and depolarizes membrane
Extracellular calcium ions enter axon terminal and trigger exocytosis of ACh
ACh binds to receptors on postsynaptic membrane and depolarize it
ACh is removed from synaptic cleft by acetylcholinesterase (AChE)
AChE breaks ACh into acetate and choline
Synaptic delay
A synaptic delay of 0.2–0.5 msec occurs between
Arrival of action potential at axon terminal
And effect on postsynaptic membrane
Mostly due to time required for calcium ion influx and neurotransmitter release
Fewer synapses lead to faster responses
Some reflexes involve only one synapse
Synaptic fatigue
Occurs when neurotransmitter cannot be recycled fast enough to meet demands of intense stimuli
Response of synapse weakens until ACh is replenished
Classes of neurotransmitters
Excitatory neurotransmitters
Cause depolarization of postsynaptic membranes
Promote action potentials
Inhibitory neurotransmitters
Cause hyperpolarization of postsynaptic membranes
Suppress action potentials
The effect of a neurotransmitter on postsynaptic membrane
Depends on the properties of the receptor
Not on the nature of the neurotransmitter
Major classes of neurotransmitters include
Biogenic amines
Amino acids
Neuropeptides
Dissolved gases
Biogenic amines
Norepinephrine (NE)
Released by adrenergic synapses
Excitatory and depolarizing effect
Widely distributed in brain and portions of ANS
Dopamine
A CNS neurotransmitter
May be excitatory or inhibitory
Involved in Parkinson’s disease and cocaine use
Biogenic amines
Serotonin
CNS neurotransmitter
Affects attention and emotional states
Gamma-aminobutyric acid (GABA)
Inhibitory effect
Functions in CNS are not well understood
Neuromodulators
Chemicals released by axon terminals that alter
Rate of neurotransmitter release
Or response by postsynaptic cell
Effects are long term and slow to appear
Responses involve multiple steps and intermediary compounds
Affect presynaptic membrane, postsynaptic membrane, or both
Released alone or with a neurotransmitter
Dissolved gases
Are important neurotransmitters
Nitric oxide (NO)
Carbon monoxide (CO)
Neurotransmitters and neuromodulators may have
A direct effect on membrane potential
By opening or closing chemically gated ion channels
Example: ACh, glutamate, aspartate
An indirect effect through G proteins
Example: E, NE, dopamine, serotonin, histamine, GABA
An indirect effect via intracellular enzymes
Example: lipid-soluble gases (NO, CO)
Indirect effects by second messengers
G protein links
First messenger (neurotransmitter)
And second messengers (ions or molecules in cell)
G proteins include an enzyme that is activated when an extracellular
compound binds
Example: adenylate cyclase
Produces the second messenger cyclic-AMP (cAMP)
Indirect effects by intracellular enzymes
Lipid-soluble gases (NO, CO)
Diffuse through lipid membranes
Bind to enzymes inside of brain cells
Information processing
Response of postsynaptic cell (integration of stimuli)
At the simplest level (individual neurons)
Many dendrites receive neurotransmitter messages simultaneously
Some excitatory, some inhibitory
Net effect on axon hillock determines if action potential is produced
Indirect effects by intracellular enzymes
Lipid-soluble gases (NO, CO)
Diffuse through lipid membranes
Bind to enzymes inside of brain cells
Postsynaptic potentials
Graded potentials developed in a postsynaptic cell
In response to neurotransmitters
Types of postsynaptic potentials
Excitatory postsynaptic potential (EPSP)
Graded depolarization of postsynaptic membrane
Inhibitory postsynaptic potential (IPSP)
Graded hyperpolarization of postsynaptic membrane
Information Processing
A neuron that receives many IPSPs
Is inhibited from producing an action potential
Because the stimulation needed to reach threshold is increased
To trigger an action potential
One EPSP is not enough
EPSPs (and IPSPs) combine through summation
Temporal summation
Spatial summation
Temporal summation
Rapid, repeated stimuli at a single synapse
Spatial summation
Simultaneous stimuli arrive at multiple synapses
A neuron becomes facilitated
As EPSPs accumulate
And raise membrane potential closer to threshold
Until a small stimulus can trigger action potential
Summation of EPSPs and IPSPs
Neuromodulators and hormones
Can change membrane sensitivity to neurotransmitters
Shifting balance between EPSPs and IPSPs
Axoaxonic synapses
Synapses between axons of two neurons
Presynaptic inhibition
Decreases the rate of neurotransmitter release at presynaptic
membrane
Presynaptic facilitation
Increases the rate of neurotransmitter release at presynaptic
membrane
Information Processing -
Information may be conveyed simply by the frequency of action potentials received
Depends on degree of depolarization above threshold
Holding membrane potential above threshold
Has the same effect as a second, large stimulus
Maximum rate of action potentials is reached when relative refractory period is eliminated
Summary
Information is relayed in the form of action potentials
Neurotransmitters released at a synapse may have excitatory or inhibitory effects
Neuromodulators can alter rate of neurotransmitter release or response of a postsynaptic neuron
Neurons may be facilitated or inhibited by chemicals other than neurotransmitters or neuromodulators
Summary
Response of postsynaptic neuron can be altered by
Neuromodulators or other chemicals that cause facilitation or inhibition
Activity under way at other synapses
Modification of rate of neurotransmitter release through facilitation or inhibition
