NERVOUS SYSTEM

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NERVOUS SYSTEM

• PART 2

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SPINAL CORD

• Major communication link between brain and PNS
  inferior to the head
• Integrates incoming information
• Produces responses through reflex mechanisms

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SPINAL CORD

• Extends from the foramen magnum to the level of
  the second lumbar vertebrae
• Shorter than vertebral column
• Doesnt grow as rapidly as vertebral column
  during development

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SPINAL CORD

• Composed of four segments
• Cervical
• Thoracic
• Lumbar
• Sacral

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SPINAL CORD

• 31 pairs of spinal nerves

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SPINAL CORD

• 31 pairs of spinal nerves
• Exit vertebral column through intervertebral and
  sacral foramina

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SPINAL CORD

• Not uniform in diameter
• Larger at superior end
• Diameter gradually decreases toward inferior end

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SPINAL CORD

• Cervical enlargement
• Inferior cervical region
• Axons supplying upper limbs enter and leave
• Lumbosacral enlargement
• Inferior thoracic, lumbar, superior sacral
  regions
• Axons supplying lower limbs enter and leave

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REFLEXES

• The neuron is the basic structural unit of the
  nervous system
• The reflex arc is the basic functional unit of
  the nervous system
• Smallest, simplest portion capable of receiving a
  stimulus and producing a response

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REFLEXES

• Five basic components of the reflex arc
• Sensory receptor
• Sensory neuron
• Interneuron
• Motor neuron
• Effector organ

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REFLEXES
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REFLEXES

• Reflex
• Autonomic response to a stimulus produced by a
  reflex arc
• Occurs without conscious thought

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REFLEXES

• Reflex
• Action potentials initiated in sensory neurons
• Transmitted to CNS
• Generally synapse with interneurons
• Interneurons synapse with motor neurons
• Send axons out of spinal cord to muscles or glands

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REFLEXES

• Reflexes are homeostatic
• Somatic reflexes protect the body from injury
• Remove the body from painful stimuli that would
  cause tissue damage
• Keep the body from suddenly falling because of
  external forces
• Autonomic reflexes are responsible for
  maintaining relatively constant
• Blood pressure
• CO2 levels
• Water intake

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REFLEXES

• Individual reflexes vary in complexity
• Simple vs. complex pathways
• Individual reflexes vary in their effects
• Excitatory vs. inhibitory

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REFLEXES

• Some reflexes involve simple neuronal pathways
• Monosynaptic reflexes
• Few or no interneurons
• Sensory neurons may synapse directly with motor
  neurons

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REFLEXES

• Some reflexes involve complex pathways and
  integrative centers
• Polysynaptic reflexes
• Sensory neurons synapse with one or more neurons
• Many are integrated within the spinal cord
• Others are integrated within the brain

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REFLEXES

• Some reflexes involve excitatory neurons
• Result in a response
• e.g., Muscle contraction
• Some reflexes involve inhibitory neurons
• Result in inhibition of a response
• e.g., Relaxation of a muscle

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REFLEXES

• Higher brain centers influence reflexes
• May either suppress or exaggerate reflexes
• Major spinal cord reflexes
• Stretch reflex
• Golgi tendon reflex
• Withdrawal reflex
• Crossed extensor reflex

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REFLEXES

• Stretch reflex
• Simplest reflex
• A monosynaptic reflex
• No interneurons involved
• Muscles contract in response to stretching force
  applied to them

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REFLEXES

• Stretch reflex
• Muscle spindle
• Sensory receptor in the stretch reflex
• 3 10 small, specialized skeletal muscle cells
• Contractile only at their ends
• Innervated by
• Gamma motor neurons
• Sensory neurons

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REFLEXES

• Stretch reflex
• Muscle spindle
• Innervated by gamma motor neurons
• Originate from spinal cord
• Control the sensitivity of muscle spindle cells
• Innervated by sensory neurons
• Innervate non-contractile centers of the muscle
  spindle cells
• Synapse directly with alpha motor neurons
• Innervate muscle in which spindle is embedded

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REFLEXES

• Stretch reflex
• Stretching a muscle also stretches muscle
  spindles located among the muscle fibers
• Muscle spindles are the sensory receptors
  involved in the stretch reflex
• Muscle spindles detect the stretch of the muscle

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REFLEXES

• Stretch reflex
• The stretch stimulates the sensory neurons
  innervating the center of each muscle spindle
• Sensory neurons conduct action potentials to the
  spinal cord

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REFLEXES

• Stretch reflex
• Sensory neurons synapse directly with alpha motor
  neurons
• No interneurons are involved

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REFLEXES

• Stretch reflex
• Alpha motor neurons transmit action potentials to
  the skeletal muscle
• Causes rapid contraction of the stretched muscle
• Contraction opposes the stretch of the muscle

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REFLEXES

• Stretch reflex
• Describe the stretch reflex in terms of the five
  basic components of the reflex arc
• Sensory receptor
• Sensory neuron
• Interneuron
• Motor neuron
• Effector organ

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REFLEXES
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REFLEXES
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REFLEXES

• Stretch reflex
• Sensory neurons of the muscle spindles possess
  collateral axons
• Synapse with additional neurons in ascending
  nerve tracts
• Brain perceives that muscle has been stretched
• Descending neurons within spinal cord synapse
  with neurons of stretch reflex
• Activity of these neurons is modified
• Important in maintaining posture, coordinating
  muscular activity

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REFLEXES
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REFLEXES

• Stretch reflex
• Muscle contraction could result in decreased
  sensitivity to stretch
• Skeletal muscle contracts
• Spindles passively shorten as muscle shortens
• Tension on centers of muscle spindles decreases
• Decreased tension decreases sensitivity to stretch

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REFLEXES

• Stretch reflex
• Sensitivity to stretch is maintained
• Gamma motor neurons stimulate muscle spindles to
  contract
• Contraction of muscle fibers at ends of spindles
• Contraction pulls on center of spindle
• Proper tension is maintained

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REFLEXES
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REFLEXES

• Stretch reflex Knee-jerk reflex
• Patellar reflex
• Classic example of the stretch reflex
• Used to determine whether higher CNS centers
  normally influencing this reflex are functional

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REFLEXES

• Stretch reflex Knee-jerk reflex
• Patellar ligament is tapped
• Tendons and muscles of quadriceps femoris group
  are stretched
• Spindle fibers within these muscles are also
  stretched
• Stretch reflex is activated
• Contraction of the muscles extends the leg

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REFLEXES

• Stretch reflex Knee-jerk reflex
• Altered stretch reflex indicates a problem
• Greatly exaggerated stretch reflex
• Neurons within the brain innervating gamma motor
  neurons and enhancing stretch reflex are overly
  active
• Absent or suppressed stretch reflex
• Reflex pathway may not be intact

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REFLEXES
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REFLEXES

• Golgi tendon reflex
• Prevents contracting muscles from applying
  excessive tension to tendons
• Tendons connect muscle to bone

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REFLEXES

• Golgi tendon reflex
• Golgi tendon organs
• Located within tendons near the muscle-tendon
  junction
• Encapsulated nerve endings
• Numerous terminal branches
• Small swellings associated with bundles of
  collagen fibers in tendons

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REFLEXES

• Golgi tendon reflex
• Tendons stretch as muscle contracts
• Increased tension in tendon
• Increased tension stimulates action potentials in
  sensory neurons from Golgi tendon organs
• Golgi tendon organs are sensitive only to intense
  stretch

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REFLEXES

• Golgi tendon reflex
• Sensory neurons enter spinal cord
• Synapse with inhibitory interneurons
• Interneurons synapse with alpha motor neurons
• Inhibitory neurotransmitters released
• Fewer action potentials produced
• Alpha motor neurons innervate the muscle to which
  the Golgi tendon organ is attached
• Muscle relaxes

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REFLEXES
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REFLEXES

• Golgi tendon reflex
• Muscle relaxation reduces tension applied to the
  muscle and tendons
• Protects them from damage
• Tension applied to muscles and tendons in legs
• An athletes Golgi tendon reflex is sometimes
  inadequate to protect
• Large muscles and sudden movement make an athlete
  particularly vulnerable to damage
• Hamstring pulls, Achilles tendon injuries

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REFLEXES

• Withdrawal reflex
• Flexor reflex
• Removes a limb or other body part from a painful
  stimulus

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REFLEXES

• Withdrawal reflex
• Action potentials from painful stimuli are
  conducted by sensory neurons to the spinal cord
• Sensory neurons synapse with excitatory
  interneurons
• Interneurons synapse with alpha motor neurons
• Alpha motor neurons stimulate muscles that remove
  limb from the source of painful stimulus
• Usually flexor muscles

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REFLEXES
Withdrawal reflex
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REFLEXES

• Withdrawal reflex
• Reciprocal innervation
• Associated with the withdrawal reflex
• Reinforces its efficiency

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REFLEXES

• Withdrawal reflex Reciprocal innervation
• Sensory neurons carry action potentials from pain
  receptors
• Collateral axons of these neurons synapse with
  inhibitory interneurons in the spinal cord
• Inhibitory interneurons synapse with alpha motor
  neurons of extensor muscles
• Extensor muscles relax
• Flexor muscles are contracting
• Extensor muscles relax
• Reduces resistance by extensor muscles

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REFLEXES
Withdrawal reflex with reciprocal innervation
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REFLEXES

• Reciprocal innervation is also involved in
  stretch reflex
• Stretch reflex causes muscle to contract
• Reciprocal innervation causes opposing muscles to
  relax
• e.g., Patellar reflex
• Quadriceps femoris muscle contracts
• Hamstring muscles relax

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REFLEXES

• Withdrawal reflex Crossed extensor reflex
• Sensory neurons activate interneurons
• Interneurons stimulate alpha motor neurons
• Results in withdrawal of a limb
• Collateral axons of these interneurons activate
  alpha motor neurons innervating extensor muscles
  on opposite side of body
• (Or perhaps collateral axons of sensory neurons
  activate two groups of interneurons)

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REFLEXES

• Withdrawal reflex Crossed extensor reflex
• One limb in withdrawn
• The opposite limb is extended
• Weight is shifted to the unaffected limb
• When you step on a nail with your left foot
• You withdraw your left foot
• You simultaneously shift your weight to your
  right foot

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REFLEXES
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REFLEXES

• Reflexes do not operate as isolated entities
• Diverging branches in a reflex arc are sent along
  ascending nerve tracts to the brain
• Responses to a pain stimulus
• Withdrawal reflex removes affected part of body
• Result of action potentials sent to spinal cord
• Perception of the pain sensation
• Result of action potentials sent to brain

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REFLEXES
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SPINAL CORD INJURY

• Damage to spinal cord
• Damage to ascending tracts
• Loss of sensation
• Damage to descending tracts
• Loss of motor functions

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SPINAL CORD INJURY

• Classified according to
• Vertebral level at which the injury occurred
• Most occur in the cervical or thoracolumbar
  region
• Whether the entire cord is damaged at that level
  or only a portion of the cord
• Most are incomplete
• Mechanism of injury
• e.g., Concussion, contusion, laceration
• Most are acute contusions

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SPINAL CORD INJURY
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SPINAL CORD INJURY

• Two types of damage occur
• Primary mechanical damage
• Secondary tissue damage
• Begins within minutes of the primary damage
• Caused by ischemia (reduced blood supply), edema,
  ion imbalances, release of excitotoxins, and
  inflammatory cell invasions
• Affects a larger region than primary damage

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SPINAL CORD INJURY

• Primary mechanical damage
• Prevention vs. treatment
• Stem cells may afford treatment
• How well does our country support this research?
• Secondary tissue damage
• Target of current treatment
• Reduction of inflammation and edema

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SPINAL CORD INJURY

• Most neurons of the adult spinal cord survive
  injury
• Begin to regenerate
• Regrow into site of damage
• Regress to inactive, atrophic state
• Greater regenerative abilities in fetuses and
  newborns

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SPINAL CORD INJURY

• Scar formation is the major block to adult spinal
  cord regeneration
• Specifically, myelin within the scar
• Stimulation of regeneration
• Implantation of
• Peripheral nerves
• Schwann cells
• Fetal CNS tissue
• Administration of some growth factors

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THE BRAIN

• Brainstem
• Three parts
• Medulla oblongata
• Pons
• Midbrain
• Connects spinal cord to remainder of the brain
• Responsible for many essential functions
• e.g., Many reflexes essential for survival
• Damage to brainstem is often fatal
• Damage to other regions of brain less likely fatal

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THE BRAIN

• Medulla oblongata
• 3 cm long
• Most inferior part of the brainstem
• Continuous with the spinal cord
• Contains centers for some reflexes
• Regulation of heart rate, blood vessel
  diameter, respiration, swallowing, vomiting,
  etc.

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THE BRAIN

• Medulla oblongata
• Near inferior end, most descending nerve tracts
  cross to the opposite side
• Each half of the brain controls the opposite half
  of the body
• Two rounded oval structures protrude from
  anterior surface
• Olives
• Nuclei involved in balance, coordination, and
  modulation of sound from the inner ear

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THE BRAIN
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THE BRAIN

• Pons
• Portion of the brainstem just superior to the
  medulla oblongata
• Contains ascending and descending nerve tracts
• Functions
• Relays information from the cerebrum to the
  cerebellum
• Work with respiratory centers in the medulla to
  help control respiratory movements
• etc.

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THE BRAIN

• Midbrain
• Mesencephalon
• Smallest region of the brainstem
• Just superior to the pons
• Functions
• Involved in hearing
• Visual reflexes
• Receive input from eyes, skin, and cerebrum
• Unconscious regulation and coordination of motor
  activities
• Maintaining muscle tone
• Coordinating movements
• etc.

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THE BRAIN

• Reticular formation
• Several loosely packed nuclei (gray matter)
• Scattered throughout the length of the brainstem
• Receives axons from many sources
• Especially from nerves innervating the face
• Functions involve cycles such as sleep-wake cycles

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THE BRAIN

• Cerebellum
• Attached to brainstem posterior to the pons
• Control of muscle movement and toneregulats the
  extent of intentional movement
• Involved in learning motor skills

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THE BRAIN
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THE BRAIN
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THE BRAIN

• Diencephalon
• Part of brain between brainstem and cerebrum
• Main components
• Thalamus
• Subthalamus
• Epithalamus
• Hypothalamus

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THE BRAIN

• Thalamus
• Largest part of the diencephalon
• 80 of its mass
• Much of the sensory input projects to the
  thalamus
• Also influences mood and actions associated with
  strong emotions
• e.g., Fear, rage, etc.

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THE BRAIN
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THE BRAIN

• Subthalamus
• Small area immediately inferior to the thalamus
• Involved in controlling motor functions

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THE BRAIN

• Epithalamus
• Small area superior and posterior to the
  thalamus
• Involved in emotional and visceral responses to
  odors
• Contains pineal gland
• Possible role in controlling the onset of puberty
• Possible role in regulating biorhythms
• e.g., Sleep-wake cycle

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THE BRAIN

• Hypothalamus
• Most inferior portion of the diencephalon
• Mammillary bodies are involved in olfactory
  reflexes and emotional responses to odor
• May be involved in memory and sense of direction
• Infundibulum connects hypothalamus to posterior
  pituitary gland
• Hypothalamus plays an important role in
  controlling endocrine system
• Regulates pituitary glands secretion of hormones

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THE BRAIN

• Hypothalamus
• Sensory neurons provide input from
• Internal organs
• Taste receptors
• Limbic system (responses to smell)
• Specific cutaneous areas
• Nipples and external genitalia
• Eyes
• Prefrontal cortex of cerebrum
• Carries information relative to mood through
  the thalamus

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THE BRAIN

• Hypothalamus
• Controls temperature by stimulating sweating or
  shivering
• Promote or inhibit eating or drinking
• Important in many functions related to mood,
  motication, and emotion
• e.g., Sexual pleasure, feeling relaxed after a
  meal, rage, fear, etc.
• Interacts with reticular activating center in
  brainstem to coordinate sleep-wake cycle

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THE BRAIN
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THE BRAIN
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THE BRAIN

• Cerebrum
• Accounts for largest portion of brain weight
• Divided into right and left hemispheres
• Each is divided into lobes
• Frontal lobe
• Parietal lobe
• Occipital lobe
• Temporal lobe

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THE BRAIN
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THE BRAIN
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THE BRAIN

• Frontal lobe
• Voluntary motor function
• Motivation
• Aggression
• Sense of smell
• Mood
• Parietal lobe
• Major center for reception and evaluation of
  sensory information
• Smell, hearing, and vision are exceptions

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THE BRAIN

• Occipital lobe
• Not distinctly separate from the other lobes
• Reception and integration of visual input
• Temporal lobe
• Receives and evaluates input for smell and
  hearing
• Important role in memory
• Involved in abstract thought and judgment

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THE BRAIN