Anatomical Organization of the Nervous System

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Anatomical Organization of the Nervous System
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Anatomical Organization of the Nervous System

• Central nervous system (CNS)
• the brain
• the spinal cord
• major information integration centers of the body
• large masses of neurons
• Peripheral nervous system (PNS)
• nerves that connect the CNS and other locations
  of the body
• neurons propagate APs towards and away from the
  CNS
• ganglions (group of neuronal somas)

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Peripheral Nervous System

• The PNS consists of 12 pairs (left and right) of
  cranial nerves originate from the brain and 31
  pairs (left and right) of nerves are attached to
  the spinal cord
• Sensory (afferent)
• all axons carry impulses from sensory receptors
  via the PNS to the CNS
• Motor (efferent)
• all axons carry impulses via the PNS from CNS
• Mixed
• a mixture of sensory and motor neurons that carry
  impulses via the PNS to and from CNS
• most common type of nerve in the body

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CNS and Nerves of the PNS
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Functions of the Nervous System

• Propagate sensory information (APs) from eyes,
  skin, blood vessels, ears, digestive tract,
  joints, muscles, lungs to the CNS
• Integration
• interpretation of sensory information by the CNS
• type, location and magnitude
• conscious perception (awareness) of some sensory
  information
• Propagate motor information APs from the CNS to
  various effectors

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Structural and Functional Organization of the
Nervous System
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Neuron Types Based on Function

• Sensory (afferent)
• propagate APs from sensory receptors based on
  touch (pressure/stretch), taste, odor, sound,
  vision toward the CNS
• synapse with interneurons
• Interneurons (association)
• receive and interpret APs from sensory neurons
• synapse with motor neurons
• Motor (efferent)
• propagate APs away from the CNS
• synapse with effectors

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Neuron Functions within the Nervous System
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Sensory Neuron Action Potentials

• Stimulation of a sensory neuron creates APs
• The magnitude (strength) of sensory information
  sent to the CNS depends on
• the number of neurons that are firing APs
• the frequency of APs fired per neuron

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Motor Neuron Action Potentials

• Stimulation of a motor neuron creates APs
• The magnitude (strength) of sensory information
  sent to the CNS depends on
• the number of neurons that are firing APs
• the frequency of APs fired per neuron

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Convergence and Divergence of Neuronal Signals
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Reflexes

• A rapid, predictable motor response to a stimulus
• Reflexes can be
• simple
• involve peripheral nerves and the spinal cord
• spinal reflexes
• learned (acquired)
• involve peripheral nerves and the brain
• Following a stimulus, the sensory and motor
  information of a reflex follows a pathway called
  a reflex arc
• in many spinal reflexes, the effector is nearby
  the location of the stimulus

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Reflex Arc

• There are five components of a reflex arc
• Receptor
• respond to stimulus
• Sensory neuron
• transmits the afferent impulse to the CNS
• Integration (control) center
• region within the CNS where synapses (processing
  of sensory info) occur
• Motor neuron
• sends efferent information to an effector
• Effector
• muscle fiber or gland that responds to the
  efferent impulse
• the activity of the effector depends upon the
  magnitude of the stimulus

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Myelination of Neurons of the Nervous System

• All of the neurons in the PNS are myelinated
• Some neurons in the CNS are myelinated, and some
  are unmyelinated
• Areas of the CNS that are made of myelinated
  neurons are called white matter
• represent the locations of long sensory and motor
  neurons that synapse with neurons of the PNS
• Areas of the CNS that are made of unmyelinated
  neurons are called gray matter
• represent the locations of short interneurons
  which make many synapses for integration to
  process sensory information and initiate motor
  information

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White and Gray Matter
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Brain
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Cerebral Cortex

• Largest portion of the brain
• 4 lobes (frontal, parietal, temporal and
  occipital)
• perception of all senses
• memory, emotion, learning
• initiation of voluntary skeletal muscle
  contraction

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Some Functional Areas of Cerebral Cortex

• Motor areas initiate action potential for
  skeletal muscle contraction (voluntary movement)
• Somesthetic areas perceive sensory information
  from the skin, muscles and joints
• Broca and Wernicke areas function in learning
  language and speech
• Olfactory area perceives sensation of smell

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Lateralization of the Cerebral Cortex

• The cortex is divided anatomically into right an
  left hemispheres
• connected in the middle by the corpus callosum

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The Cerebellum

• Located dorsal to the pons and medulla
• Protrudes under the occipital lobes of the
  cerebrum
• Makes up 11 of the brains mass
• Modifies the motor information leaving the motor
  cortex
• provides precise timing and appropriate patterns
  of skeletal muscle contraction to maintain
  balance and coordination
• Cerebellar activity occurs subconsciously

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Thalamus

• Major relaying center for both sensory (afferent)
  and motor (efferent) information as it passes
  between the brain and the PNS

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Brain Stem

• Comprised of the pons and the medulla oblongata
• Clusters of neurons (brain centers) in regions of
  the pons and medulla control the basic life
  functions
• heart rate
• controlled by the cardioacceleratory and
  cardioinhibitory centers in the medulla
• blood pressure
• controlled by the cardioacceleratory,
  cardioinhibitory, and vasomotor centers in the
  medulla
• breathing rate
• controlled by the inspiratory and expiratory
  centers in the medulla and pons, respectively
• Control of effectors occurs through the Autonomic
  Nervous System

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Hypothalamus and Pituitary

• Hypothalamus and Pituitary
• 2 glands located inferior to the thalamus
• Secrete hormones which regulate a large number of
  metabolic processes
• metabolic rate
• sex hormone levels in the blood
• growth
• water balance
• blood nutrient levels
• The hypothalamus secretes hormones which in turn
  cause the pituitary to secrete hormones, thus the
  hypothalamus controls the function of the
  pituitary

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Spinal Cord

• The spinal cord is attached to the brain and
  extends to the lumbar region of the vertebral
  column
• Functions include
• integration of basic stimuli presented to the
  body below the neck through simple reflex arcs
• withdrawal reflex in response to pain
• myotatic reflex in response to skeletal muscle
  stretch
• sending sensory and motor information to and from
  the brain

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Spinal Cord Anatomy

• Dorsal (posterior) horns (left and right)
• sensory neurons enter the cord on the dorsal
  aspect where they synapse with interneurons or
  motor neurons
• extend into dorsal roots and ganglia
• Ventral (anterior) horns (left and right)
• motor neurons exits the cord on the ventral
  aspect where they control effectors (muscle or
  glands)
• extend into motor roots
• Dorsal and ventral roots merge together to form
  spinal nerves

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Spinal Cord Anatomy
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Spinal Nerve
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Sensory Division of the PNS

• Sensory division
• made of afferent neurons
• somatic
• sensory neurons send APs from skin, skeletal
  muscles, and joints
• visceral
• sensory neurons send APs from organs within the
  abdominal and thoracic cavities
• heart, lungs, digestive organs, blood vessels,
  kidneys, reproductive organs

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Motor Division of the PNS

• Motor division
• made of efferent neurons that innervate effectors
• somatic
• motor neurons send APs to voluntary skeletal
  muscle
• visceral
• motor neurons send APs to involuntary cardiac
  muscle, smooth muscle and glands
• a.k.a. the Autonomic Nervous System (ANS)
• 2 antagonistic (opposing) divisions
• Sympathetic
• Parasympathetic
• the two divisions control the same effectors
  (with few exceptions) but create opposite
  responses in the effectors

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Autonomic Nervous System

• Visceral motor neurons of the PNS control the
  activity of involuntary effectors such as cardiac
  muscle, smooth muscle and glandular secretion
  affecting
• heart rate
• breathing rate
• sweating
• digestion
• blood pressure
• Action potentials in these motor neurons are
  initiated in the medulla oblongata and the pons
• these motor neurons exit the brain by
• descending tracts of the spinal cord
• exit spinal cord via spinal nerves
• cranial nerves

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Function of the Sympathetic Division

• The sympathetic division is called the fight or
  flight system
• activated when the body needs to expend energy
• Involves E activities
• exercise, excitement, emergency, and
  embarrassment
• Promotes necessary changes during these
  activities
• increases heart rate, blood pressure, respiration
  rate, blood flow to skeletal muscles, glucose
  metabolism
• decreases the activity of and blood flow to the
  digestive system organs
• Its activity is illustrated by a person who is
  threatened

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Function of the Parasympathetic Division

• The parasympathetic nervous system is called the
  rest and digest system
• activated when the body needs to conserve energy
• Involves the D activities
• digestion, defecation, and diuresis (urination)
• Promotes necessary changes during these
  activities
• decreases heart rate, blood pressure, respiration
  rate, blood flow to skeletal muscles, glucose
  metabolism
• increases the activity of and blood flow to the
  digestive system organs
• Its activity is illustrated in a person who
  relaxes after eating a meal

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Motor Pathways of the Somatic Nervous Division
vs. Autonomic Nervous Division

• Motor pathways of the Somatic division consist of
  a single motor neuron that extends between the
  brain or spinal cord and the innervated skeletal
  muscle
• Motor pathways of the ANS consist of a two-neuron
  chain between the brain or spinal cord and the
  effector
• the preganglionic begins in the CNS and extends
  along a nerve to a ganglion and synapses with
• the postganglionic neuron which extends from the
  ganglion to an effector organ

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Motor Pathways of the Somatic Nervous Division
vs. Autonomic Nervous Division
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Motor Pathways of the Sympathetic Division
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Motor Pathways of the Parasympathetic Division
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Motor Pathways of the Somatic Nervous Division
vs. Autonomic Nervous Division

• All somatic motor neurons exocytose ACh
• ACh binds to nicotinic acetylcholine receptors on
  the skeletal muscle fiber leading to its
  contraction
• All preganglionic motor neurons exocytose ACh
• ACh binds to nicotinic acetylcholine receptors on
  the postganglionic neuron creating an AP
• All parasympathetic postganglionic motor neurons
  exocytose ACh
• ACh binds to muscarinic acetylcholine receptors
  on the effector tissue/organ causing a response
• All sympathetic postganglionic motor neurons
  exocytose NE
• NE binds to adrenergic receptors on the effector
  tissue/organ causing a response

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Sympathetic vs. Parasympathetic
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Effects of Neurotransmitters of the Autonomic
Nervous System

• The cells of each organ controlled by the ANS
  have membrane receptors to BOTH ACh and NE
• organs are dually controlled
• The response of the organ is determined by the
  identity of the neurotransmitter released
• the binding of ACh to its receptor will cause the
  effector to respond in one way
• the binding of NE to its receptor will cause the
  effector to respond in the opposite way
• The effect of ACh and NE on an effector can be
  either stimulatory or inhibitory (effector
  specific)
• NE increases heart rate, ACh decreases heart rate
• NE decreases the secretion of saliva, ACh
  increases the secretion of saliva

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Dual Control by the Sympathetic and
Parasympathetic Systems
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The Adrenal Glands

• Adrenal glands (toward kidney)
• pyramid-shaped glands on top of each kidney
• Structurally and functionally, they are two
  glands
• Adrenal medulla (inside)
• nervous tissue that is the hormonal branch of the
  sympathetic nervous system (fight/flight)
• Adrenal cortex (outside)
• glandular (epithelial) tissue

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Adrenal Medulla

• Made up of chromaffin cells
• modified sympathetic neurons
• secrete catecholamines (monoamines) of the
  sympathetic nervous system into circulation
• epinephrine (adrenaline)
• norepinephrine (noradrenaline)
• Secreted into circulation in response to the
  activation of the sympathetic nervous system
  during short term stress (lasing
  seconds/minutes/hours))
• Targets various body tissues causing
• an increase in blood glucose levels via
  glycogenolysis and gluconeogenesis in the liver
• the vasoconstriction of blood vessels
• an increase in heart rate and stroke volume