Transcript General principle of nervous system

General principle of nervous
system
• Nervous system
– Multi-tasking unit in the body
• Thinking
• Regulation of actions
General design
• Central nervous
system neuron
– Basic functional unit
– 100 billion units
– Signals received by
synapses
• Located in neural
dentrites and cell
bodies
• Few hundreds to
200,000 synaptic
connection
General design
• Central nervous
system neuron
– Outgoing signals
• Axon (branched)
– Special feature
• Signals move forward
only
• Signals travel in a
specific direction for
performing specific
function
Sensory receptors
• Initiation of nervous system activity
– Sensory experience
– Immediate reaction
– Generation of memory
• Somatic sensory
system
– Sensory information
from the entire body
surface and deep
tissues
– Information enters
CNS via the peripheral
nervous system
• Somatic sensory
system
– Information conducted
into multiple sensory
areas
• The spinal cord (all
level)
• The reticular substance
of the lower brain
• Thalamus
• Celebellum
• Cerebral cortex
Motor parts of nervous system
• Motor functions of the nervous system
– Skeletal muscle contraction
– Smooth muscle contraction
– Activity of endocrine and exocrine glands
• Effectors
– Anatomical structures whose functions are
under neural control
• Skeletal motor system
• Autonomic motor system
– Smooth muscles
– Glands
• Level of control
– The spinal cord (all
level)
– The reticular
substance of the lower
brain
– Thalamus
– Celebellum
– Cerebral cortex
• Specific role for specific level of control
– Lower level (lower brain and spinal cord)
• Autonomic
– Higher level
• Thought process
• Deliberate movement
Integrative function
• Processing incoming information
– Generation of appropriate mental and motor
responses
• Brain
– Discards > 99% of all sensory information
• Some relegated to subconscious
– Channels information to proper integrative
and motor region of the brain
• Generation of appropriate responses
• Role of synapses
– Direct pathway of nerve impulses
• Spreading of the signals throughout the nervous system
– Very complex
• Degree of difficulty in passing the impulses
• Input from other areas in the nervous system
– Facilitatory
– Inhibitory
• Selective blocking and passing of impulses
• Selective amplification and channeling of the signals
Memory
• Very small amount of input
– Immediate responses
• Larger portion
– Stored for future motor activity and thought
process
– Majority in cerebral cortex but some in the
lower brain
• Role of synapses
– Increased ability to pass the particular type of
signal
• Facilitation
– Subsequent adaptation of brain to the
particular signal
• Repeated exposure and passing through the
synapses
• Generation of the signal within the brain and
movement of the signal through synapses in the
absence of input
• Importance of memory
– Reference point for the action
– Comparison of new input with previously
stored information
– Generation of new memory or appropriate
reaction
Major level of CNS function
• Spinal cord
• Lower brain/subcortical
• Higher brain/cortical
Spinal cord
• Neural circuits in the cord
– Walking movements
– Withdrawal reflex
– Support reflex against gravity
– Reflex regulating local blood vessel, digestive
tract movement, or urinary excretion
• Information from upper level of control
– Commanding the cord center to perform
specific function
Lower brain/subcortical
• Primitive parts of the brain
• Subconscious activities
– Arterial pressure
– Respiration
– Equilibrium
– Feeding reflex
– Many emotional responses
Higher brain/cortical
• Memory storehouse
• Function in association with lower centers
– Precise regulation of functions dictated by the
lower center
– Thought process
• Use of information from lower centers to trigger
activation of stored information
CNS synapses
• Information
– Action potential/nerve impulses
• Each impulse
– Blocked in its transmission from one neuron
to the next
– Changed from a single impulse to multiple
impulses
– Combined with other impulses to become
more complex/intricate impulses
Types of synapses
• Two types
– Chemical
– Electrical
Types of synapses
• Electrical synapses
– Conduct electricity from one cell to the next
• Direct open fluid channels
• Gap junctions
– Very few in the CNS
– Transmission of information
• Either direction
Types of synapses
• Chemical synapses
– The majority of synapses used in signal
transmission within the CNS
• Neurotransmitters
• One-way conduct
– Chemical synapses
– Signals transmission in only one direction
• Neuron that secretes transmitter (presynaptic)
• Neuron that receives transmitter (postsynaptic)
• Importance of one-way conduction
– Movement of signal toward specific goal
• Focused transmission of signals from peripheral
neurons toward the specific areas of the CNS
Synapses anatomy
• Components of
anterior motor neuron
– Soma
• Main body
– Axon
– Dendrites
• Projections of soma
– Presynaptic terminals
• Cover surface of
dendrites
• End of nerve fibrils
• Neurons in the brain
– Different from anterior motor neuron
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Size of the cell
Dendrite length, number, and size
Length and size of axon
Number of presynaptic terminals
Presynaptic terminal
• Excitatory/inhibitory
• Structures
– Synaptic cleft
– Transmitter vesicles
• Neurotransmitters
– Mitochondria
• ATP production
– Receptor proteins
Role of Ca ion
• Presynaptic membrane
– Large concentrations of voltage-gated Ca
channels
• Arrival of action potential
– Large influx of Ca ions via opening of Ca
channels
– Release of neurotransmitters
• Unclear mechanism
Receptor proteins
• Surface of the postsynaptic neuron
membrane
– Binding site (extracellular)
– Ionophore component (intracellular)
• Ion channel
• Secondary messenger activator
• Ion channels
– Cation channels
• Activated by excitatory neurotransmitters
• Prevents influx of anions (Cl)
– Negatively charged canal
– Anion channels
• Activated by inhibitory neurotransmitters
• Prevent influx of cations
– Smaller diameter
• Secondary messenger system
– Prolonged excitation/inhibition
• Changes in long-term response characteristics of
the neuron
– G-protein coupled receptors
• Most common
– Opening of the ion channels
– Activation of cAMP/cGMP
– Activation of intracellular enzymes
– Activation of gene transcription
Excitation vs. inhibition
• Additional dimension to nervous function
• Excitation
– Opening of Na channels
• Increased membrane potential
– Depressed conduction
• K channels and/or Cl channels
• Decreased diffusion of cations/anions
• Increased positive charge
– Various changes in the intracellular
metabolism
Excitation vs. inhibition
• Inhibition
– Opening of Cl channels
– Increased K conductance out of the neuron
– Inhibition of cellular metabolism
Neurotransmitters
• Two classes
– Small-molecule, rapidly acting transmitters
• Cause most acute responses
• Acetylcholine, norepinephrine, dopamine, glycine,
GABA, glutamine, serotonin, nitric oxide
– Neuropeptides
• Slow acting molecules
• Larger in size
• Prolonged action that results in long-term changes
of neurons
Electrical events
• Resting potential
– Around -65 mV
– Important for positive
and negative control
• Sensitive to changes
• Charges evenly
distributed throughout
soma
– Highly conductive
intracellular fluid
– Large diameter
Excitation
• Transmitters
– Increased Na permeabiity
• Large influx of Na ions
– Increased membrane potential
• Excitatory post synaptic potential (EPSP)
• -65 mV to -45 mV
– Generation of action potential
• Excitation requires simultaneous
discharge of many terminals (Summation)
Inhibition
• Inhibitory transmitters
– Opening of chloride channels
• Influx of Cl ions into the inside
– Opening of K channels
• Removal of K from inside
– Hyperpolarization (-70 mV)
• Inhibitory postsynaptic potential (IPSP)
Inhibition
• Presynaptic inhibition
– Release of inhibitory neurotransmitters
• Opening of anion channels on the terminal fibril
• Inhibition of synaptic transmission
Summation
• Spatial summation
– Generation of ESPS
• Cannot be done by a single presynaptic terminal
(0.5 to 1 mV ESPS)
– Simultaneous stimulation of many terminals
• Summation of ESPS to reach threshold of firing
• Spatial Summation
Summation
• Temporal summation
– Release of transmitters
• Opening of postsynaptic channels for a very short
period
– A second opening of the channels
• Increased ESPS
– Addition of successive rapid discharge
• Summation of successive ESPS
Facilitation of neurons
• Summated postsynaptic potential
– Not enough for threshold
– Neurons become facilitated
• Membrane potential reaches near the threshold
but not enough for firing
• Quicker response to the stimuli
Role of dendrites
• Signal reception
– Summation of signals from many separate
presynaptic nerve fibers
– Excitation triggered by the signals from
dendrites
• Electronic conduction
– No action potential conduction because of low
voltage-gated Na channels and high threshold
• Decremental conduction
– Location of presynaptic terminals
• Closer to the soma, greater the negative
membrane potential becomes
• Loss of ESPS from dendrites before reaching the
soma
– Permeable to K and Cl
• Further the excitatory synapses are, greater the
amount of decrease in ESPS