Transcript Central Nervous System (CNS)

Neurophysiology
The Central Nervous System
Nervous System
• Functions
– Sensory input – monitoring stimuli occurring inside & outside the
body
– Integration – interpretation of sensory input
– Motor output – response to stimuli by activating effector organs
Figure 11.1
Organization of the Nervous System
• Central nervous system
(CNS)
– Brain and spinal cord
– Integration and
command center
• Peripheral nervous
system (PNS)
– Paired spinal and
cranial nerves
– Carries messages to
and from the spinal
cord and brain
Peripheral Nervous System: Afferent Division
• Afferent (sensory) division
– transmits impulses
from receptors to the
CNS.
– Somatic afferent fibers –
carry impulses from skin,
skeletal muscles, and
joints
– Visceral afferent fibers –
transmit impulses from
visceral organs
Peripheral Nervous System: Efferent Division
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Motor (efferent) division –
transmits impulses from the
CNS to effector organs. Two
subdivisions:
– Somatic nervous system –
provides conscious control
of skeletal muscles
– Autonomic nervous system
– regulates smooth muscle,
cardiac muscle, and glands
Sensory
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General somatic senses – receptors are widely spread
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Touch
Pain
Vibration
Pressure
Temperature
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Proprioceptive senses – detect stretch in tendons and muscle
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Special somatic senses
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• Body sense – position and movement of body in space
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Hearing
Balance
Vision
Smell
Visceral sensory
• General visceral senses – stretch, pain, temperature, nausea, and hunger
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Widely felt in digestive and urinary tracts, and reproductive organs
• Special visceral senses - taste
Motor
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General somatic motor – signals contraction of skeletal
muscles
• Under our voluntary control
• Often called “voluntary nervous system”
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Visceral motor
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Regulates the contraction of smooth and cardiac muscle
Makes up autonomic nervous system
Controls function of visceral organs
Often called “involuntary nervous system”
• Autonomic nervous system
Central Nervous
System:
• Brain
• Spinal cord
Organization of the Nervous System
Figure 8-1: Organization of the nervous system
Nerve Tissue
• The two principal cell types of the
nervous system are:
– Neurons – excitable cells that transmit
electrical signals
– Neuroglia - supporting cells
Neuron Classification
• Functional:
– Sensory (afferent) — transmit impulses toward the CNS
– Motor (efferent) — carry impulses away from the CNS
– Interneurons (association neurons) — shuttle signals through CNS
pathways
Neuroglia
Figure 12.6
CNS Protection
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Hair, skin, cranium
Meninges
Cerebrospinal fluid
Blood brain barrier
Meningeal Layers
• Meningeal layer of the brain cushion
and protect delicate neural tissue
Figure 9-4b
Cerebrospinal Fluid
• Shock absorbing
medium
• Provides a optimum
and stable
environment for
generating nerve
impulses
• Provides a medium for
the exchange of
nutrients and wastes
between blood and
nervous tissue
Cerebrospinal Fluid
• Formed by selective transport across ependymal cells
• Volume 125-150 ml and is replaced > 3 times/day, flow
maintained by 10 mmHg pressure gradient
• Path: ventricles  subarachnoid space, reabsorbed into blood
in dural sinuses through arachnoid villi
Blood Brain Barrier
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Extensive capillaries & sinuses
Tight junctions promoted by astrocyte
Limits permeability for most molecules except O2, CO2, alcohol,
steroids, H2O
Protects brain: hormones & circulating chemicals
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Protects CNS from chemical fluctuations
Prevents entry of harmful substances
Prevents entry of molecules that could act as neurotransmitters
Brain receives 15% of blood pumped by heart
Brain responsible for about half of body’s glucose consumption
Membrane transporters move glucose from plasma into the brain
interstitial fluid
Figure 9-6: The blood-brain barrier
Brain Organization
• Trillion interneurons fill the brain
• Up to 200,000 synapses each
• Levels of complexity
– Cerebral cortex
– Basal nuclei
– Thalamus
– Hypothalamus
– Cerebellum
– Brain stem
Brain component
Cerebral cortex
Cerebral cortex
Basal nuclei
(lateral to thalamus)
Basal nuclei
Thalamus
(medial)
Thalamus
Hypothalamus
Hypothalamus
Cerebellum
Cerebellum
Midbrain
Brain stem
Brain stem
(midbrain, pons,
and medulla)
Pons
Medulla
Spinal cord
Brain component
Cerebral cortex
Basal nuclei
Thalamus
Hypothalamus
Cerebellum
Brain stem
(midbrain, pons,
and medulla)
Major Functions
1. Sensory perception
2. Voluntary control of movement
3. Language
4. Personality traits
5. Sophisticated mental events, such as thinking memory,
decision making, creativity, and self-consciousness
1. Inhibition of muscle tone
2. Coordination of slow, sustained movements
3. Suppression of useless patterns of movements
1. Relay station for all synaptic input
2. Crude awareness of sensation
3. Some degree of consciousness
4. Role in motor control
1. Regulation of many homeostatic functions, such as temperature
control, thirst, urine output, and food intake
2. Important link between nervous and endocrine systems
3. Extensive involvement with emotion and basic behavioral patterns
1. Maintenance of balance
2. Enhancement of muscle tone
3. Coordination and planning of skilled voluntary muscle activity
1. Origin of majority of peripheral cranial nerves
2. Cardiovascular, respiratory, and digestive control centers
3. Regulation of muscle reflexes involved with equilibrium and posture
4. Reception and integration of all synaptic input from spinal cord;
arousal and activation of cerebral cortex
5. Role in sleep-wake cycle
Brain Overview
Cerebrum
• Highly developed
• Makes up about 80% of total brain weight
(largest portion of brain)
• Inner core houses basal nuclei
• Outer surface is highly convoluted cerebral
cortex
– Highest, most complex integrating area of the brain
– Plays key role in most sophisticated neural functions
Cerebral Cortex
• Three specializations
– Sensory areas - sensory input translated into perception
– Motor areas - direct skeletal muscle movement
– Association areas - integrate information from sensory and
motor areas, can direct voluntary behaviors
Cerebral Cortex
• Each half of cortex divided into four major lobes
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Occipital lobe - carries out initial processing of visual input
Temporal lobe - initial reception of sound sensation, taste, smell
Parietal lobe - somatosensory processing
Frontal lobe responsible for
• Voluntary motor activity
• Speaking ability
• Elaboration of thought
Primary Somatosensory Cortex
• Located in the postcentral
gyrus, this area:
– Receives information from
the skin and skeletal
muscles
– Exhibits spatial
discrimination
• Somatosensory homunculus
– caricature of relative
amounts of cortical tissue
devoted to each sensory
function
Primary Motor Cortex
• Located in the precentral
gyrus
• Composed of pyramidal
cells whose axons make up
the corticospinal tracts
• Allows conscious control of
precise, skilled, voluntary
movements
• Motor homunculus –
caricature of relative
amounts of cortical tissue
devoted to each motor
function
Language
•Primary areas of cortical specialization for language
–Broca’s area governs speaking ability
–Wernicke’s area
•Concerned with language comprehension
•Responsible for formulating coherent patterns of speech
Figure 9-23: Cerebral processing of spoken and visual language
Functional Areas of the Cerebral Cortex
Figure 9-15
Brain Function: Cerebral Lateralization
• Each lobe has special functions
Figure 9-16
Cerebral Cortex
• Schematic Linking of
Various Regions of the
Cortex
Basal Nuclei
• Act by modifying ongoing
activity in motor pathways
• Primary functions
– Regulates muscle tone
throughout the body
– Selecting and maintaining
purposeful motor activity while
suppressing useless or
unwanted patterns of
movement
– Helping monitor and
coordinate slow, sustained
contractions, especially those
related to posture and support
– Controls large automatic
movement
Thalamus
• Final relay point for
ascending sensory
information
• Coordinates the
activities of the
cerebral cortex and
basal nuclei
• Domain-specific
information
processing
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Hypothalamus
• Receives indirect sensory
inputs from all sensory
systems
• Sends neural outputs to
various motor control
nuclei
• Sends neural outputs to
sympathetic and
parasympathetic nervous
systems
• Sends both neural and
hormonal outputs to
pituitary
Hypothalamus
• Controls somatic motor activities at the
subconscious level
• Controls autonomic function
• Coordinates activities of the endocrine and nervous
systems
• Secretes hormones
• Produces emotions and behavioral drives
• Coordinates voluntary and autonomic functions
• Regulates body temperature
• Coordinates circadian cycles of activity
• 4Fs: feeding, fighting, fleeing, and reproductive
behavior
Limbic System
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Cingulated gyrus
– Coordinates sensory input with
emotions
– Emotional responses to pain
– Basic, inborn behavioral patterns
related to survival and
perpetuation of the species
– Regulates aggressive behavior
Hippocampus - sends memories out
to the appropriate part of the cerebral
hemisphere for long-term storage and
retrieving them when necessary,
Plays important role in motivation and
learning
Amygdala - involved in emotional
responses, hormonal secretions, and
memory,
Cerebellum
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Basic functions: coordination, balance, motor learning, etc.
Vestibulocerebellum – balance and control of eye movement
Spinocerebellum – enhances muscle tone and coordinates skilled
voluntary movement – important in synchronization and timing
– Receives input concerning desired action from motor cortex
– Receives feedback concerning actual action from proprioceptors, vestibular
apparatus, eyes
– Compares inputs and sends adjustments or corrective signals to motor tracts
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Cerebrocerebellum – planning and initiation of voluntary activity by
providing input to the cortical motor areas also involved in procedural
memories
Brain Stem: Midbrain, Pons & Medulla
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An important link between spinal cord and higher brain levels, relays motor and
sensory impulses between other “higher” parts of the brain and spinal cord
Midbrain – eye movement control
Pons/Medulla
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Signal relay
Involuntary functions
Many cranial nerves enter
Pyramids – nerve tracts crossover
Cranial Nerves
Table 9-1: The Cranial Nerves
Pons
• Sensory and motor nuclei for four cranial nerves
• Nuclei that help control respiration
• Nuclei and tracts linking the cerebellum with the
brain stem, cerebrum and spinal cord
Medulla oblongata
• Contains relay stations and reflex centers
– Cardiovascular and respiratory rhythmicity centers
• Cardiovascular center - regulates rate and force of heartbeat
and vasoconstriction/dilation
• Respiratory center - regulates basic breathing rhythm
• Reticular formation begins in the medulla oblongata and
extends into more superior portions of the brainstem
Reticular Activating System
• Network in brain stem
• Arousal, sleep, pain, &
muscle tone
• Ascending fiber sends
signals upward
• Arouses and activates
cerebral cortex
• Controls overall degree
of cortical alertness or
level of consciousness:
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maximum alertness
wakefulness
sleep
coma
ElectroEncephaloGram -EEG
• Records electrical activity within
cerebral cortex from EPSP, IPSPs
• Used for
– Diagnose cerebral dysfunction
– Brain death
– Sleep Patterns
Consciousness - Sleep
Functions of Sleep
• “Catch up” time – restore biochemical
and physiological processes
• Role of adenosine
– Increased levels while awake
– Inactivates RAS
– Caffeine blocks adenosine receptors
• Shift gears – long term structural and
chemical changes required to
consolidate memory and learning
Learning and Memory
• Learning has two broad types
– Associative – conditioning, linking two events
together
– Nonassociative
• Habituation
• Sensitization
• Memory has several types
– Short-term and long-term
– Reflexive and declarative
Memory
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Storage of acquired knowledge for later recall
Memory trace - Neural change responsible for retention or storage of
knowledge
Short-term memory - Lasts for seconds to hours
Long-term memory - Retained for days to years
Consolidation - Process of transferring and fixing short-term memory
traces into long-term memory stores
Working memory - Temporarily holds and interrelates various pieces of
information relevant to a current mental task
Long-Term vs Short-Term Memory
Short Term Memory
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Seconds to hours
Limited capacity
Rapid retrieval
Synaptic alterations
– Changes in ion channels
– Presynaptic facilitation cAMP
– Long term potentiation (LTP)
Basic Learning - Behavior
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Habituation – decreased responsiveness to
stimulus
1. Closure of Ca++ channels leads
2. Reduced neurotransmitter release
3. Decrease EPSP
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Sensitization – increase responsiveness
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Release of serotonin from interneuron
Increases cAMP in presynaptic neuron
Blocks K+ channels and prolongs AP
Ca++ channels are open longer
Increasing neurotransmitter output
LTP
• Glutamate acts as NT, binds to:
– AMPA receptors elicit EPSPs
– NMDA receptors open Ca++ channels > 2nd messenger system
• Increases # of AMPA receptors / releases nitric oxide (NO)
• NO causes presynaptic neuron to increase neurotransmitter release
Long Term Memory
• Last days to years
• Unlimited capacity
• Permanent structural changes
– Formation of new synapses between
existing neurons
– Increased dendritic surface area
– Increase in neurotransmitter receptors
– Changes in neurotransmitter synthesis
Long Term Memory
• Consolidation is affected by:
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Amount of rehearsal
Association of new & old data
Level of excitement/importance of information
CREB – regulatory proteins that activate genes important in
long term memory storage
Memory Processing
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Hippocampus (limbic system)
– Short term memory and consolidation
– Declarative memory (facts), specific objects
– Requires conscious recall
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Cerebellum – “how to” memories
– Motor skills (procedural)
– Subconscious recall
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Prefrontal cortex - working memory
– Processes new and retrieved information
– Temporary storage
– Problem solving, planning, organizing
Spinal Cord Regions
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Cervical
Thoracic
Lumbar
Sacral
Figure 9-4a: ANATOMY SUMMARY: The Central Nervous System
Spinal Cord Organization
• Gray matter: mostly cell bodies
– Dendrites & terminals
– Spinal reflex integrating center
• White matter
– Bundles of myelinated axons
• Ascending tracts – sensory
• Descending tracts – motor
– Dorsal roots
– Ventral roots
Spinal Cord Organization
Figure 9-7: Specialization in the spinal cord
Spinal Cord: Integrating Center
Figure 9-8
Neural Reflexes: Overview
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Stimulus
Sensory receptor
Sensory (afferent) neuron
CNS integration
Efferent (motor) neuron
Effector (target tissue)
Response (movement)
Feedback to CNS
Neural Reflex
Figure 13-1: Monosynaptic and polysynaptic somatic motor reflexes
Patellar Tendon Reflex:
Stretch & Reciprocal Inhibition
Figure 13-7: The knee jerk reflex
Cross Extensor
Reflex/Withdrawal Reflex
Figure 13-8: Flexion reflex and the crossed extensor reflex