THE NERVOUS SYSTEM

1
THE NERVOUS SYSTEM

• HEADING
• VOCABULARY
• IMPORTANT INFO

2
Major Structures of the Nervous System

• Brain, cranial nerves, spinal cord, spinal
  nerves, ganglia, enteric plexuses and sensory
  receptors

3
Nervous System Divisions

• Central nervous system (CNS)
• consists of the brain and spinal cord
• Peripheral nervous system (PNS)
• consists of cranial and spinal nerves that
  contain both sensory and motor fibers
• connects CNS to muscles, glands all sensory
  receptors

4
Subdivisions of the PNS

• Somatic (voluntary) nervous system (SNS)
• neurons from cutaneous and special sensory
  receptors to the CNS
• motor neurons to skeletal muscle tissue
• Autonomic (involuntary) nervous systems
• sensory neurons from visceral organs to CNS
• motor neurons to smooth cardiac muscle and
  glands
• sympathetic division (speeds up heart rate)
• parasympathetic division (slow down heart rate)
• Enteric nervous system (ENS)
• involuntary sensory motor neurons control GI
  tract
• neurons function independently of ANS CNS

5
Neurons

• Functional unit of nervous system
• Have capacity to produce action potentials
• electrical excitability
• Cell body
• single nucleus with prominent nucleolus
• Nissl bodies (chromatophilic substance)
• rough ER free ribosomes for protein synthesis
• Neurofilaments give cell shape and support
• Microtubules move material inside cell
• Cell processes dendrites axons

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The basic unit of the nervous system
neuron?

• Dendrites receive stimuli
• Nerve cell body
• _at_ nucleus transmits the stimuli
• Axon transmits the impulse to another dendrite

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Parts of a Neuron
Neuroglial cells
Nucleus with Nucleolus
Axons or Dendrites
Cell body
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Dendrites

• Conducts impulses towards cell body
• Typically short, highly branched unmyelinated
• Surfaces specialized for contact with other
  neurons
• Contains neurofibrils Nissl bodies

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Axons

• Conduct impulses away from cell body
• Long, thin cylindrical process of cell
• Impulses arise from initial segment (trigger
  zone)
• Side branches (collaterals) end in fine processes
  called axon terminals
• Swollen tips called synaptic end bulbs contain
  vesicles filled with neurotransmitters

Synaptic boutons
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Functional Classification of Neurons

• Sensory (Afferent) neurons
• transport sensory information from skin, muscles,
  joints, sense organs viscera to CNS
• Motor (Efferent) neurons
• send motor nerve impulses to muscles glands
• Interneurons (Association) neurons
• connect sensory to motor neurons
• 90 of neurons in the body

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What is a synapse ?

• Junction of two neurons
• Neurotransmitterconvert the electrical impulse
  into a chemical message

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Axon ending terminal bud

• Transfers the electrical nerve impulse
• By chemical neuron-transmitters
• From one neuron to the next

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The Action Potential Summarized
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Chemical Synapses

• Action potential reaches end bulb and
  voltage-gated Ca 2 channels open
• Ca2 flows inward triggering release of
  neurotransmitter
• Neurotransmitter crosses synaptic cleft binding
  to receptors
• the more neurotransmitter released the greater
  the change in potential of the postsynaptic cell
• Synaptic delay is 0.5 msec
• One-way information transfer

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The Central Nervous System

• 1) Spinal Cord
• 2) Brain
• medulla for
• breathing
• cerebellum for
• balance
• cerebrum for
• higher thinking bw

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Spinal Cord Protection

• By the vertebral column, meninges, cerebrospinal
  fluid, and vertebral ligaments.

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External Anatomy of Spinal Cord

• Flattened cylinder
• 16-18 Inches long 3/4 inch diameter
• In adult ends at L2
• In newborn ends at L4
• Growth of cord stops at age 5
• Cervical enlargement
• upper limbs
• Lumbar enlargement
• lower limbs

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Spinal Cord Spinal Nerves

• Spinal nerves begin as roots
• Dorsal or posterior root is incoming sensory
  fibers
• dorsal root ganglion (swelling) cell bodies of
  sensory nerves
• Ventral or anterior root is outgoing motor fibers

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Spinal Nerves

• 31 Pairs of spinal nerves
• Named numbered by the cord level of their
  origin
• 8 pairs of cervical nerves (C1 to C8)
• 12 pairs of thoracic nerves (T1 to T12)
• 5 pairs of lumbar nerves (L1 to L5)
• 5 pairs of sacral nerves (S1 to S5)
• 1 pair of coccygeal nerves
• Mixed sensory motor nerves

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The Brain and Cranial Nerves

• almost 3 lb.
• Brain functions in sensations, memory, emotions,
  decision making, behavior

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Principal Parts of the Brain

• Cerebrum
• Diencephalon
• thalamus hypothalamus
• Cerebellum
• Brainstem
• medulla, pons midbrain

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Medulla Oblongata

• Continuation of spinal cord
• Ascending sensory tracts
• Descending motor tracts
• Nuclei of 5 cranial nerves
• Cardiovascular center
• force rate of heart beat
• diameter of blood vessels
• Respiratory center
• medullary rhythmicity area sets basic rhythm of
  breathing
• Information in out of cerebellum
• Reflex centers for coughing, sneezing, swallowing
  etc

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Pons

• One inch long
• White fiber tracts ascend and descend
• Pneumotaxic apneustic areas help control
  breathing
• Middle cerebellar peduncles carry sensory info to
  the cerebellum
• Cranial nerves 5-7

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Midbrain

• One inch in length
• Extends from pons to diencephalon
• Cerebral aqueduct connects 3rd ventricle above to
  4th ventricle below

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Cerebellum

• 2 cerebellar hemispheres and Vermis (central
  area)
• Function
• correct voluntary muscle contraction and posture
  based on sensory data from body about actual
  movements
• sense of equilibrium

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Thalamus

• 1 inch long mass of gray mater in each half of
  brain (connected across 3rd ventricle by
  intermediate mass)
• Relay station for sensory information on way to
  cortex
• Crude perception of some sensations

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Hypothalamus

• Dozen or so nuclei in 4 major regions
• mammillary bodies are relay station for olfactory
  reflexesinfundibulum suspends the pituitary
  gland
• Major regulator of homeostasis
• receives somatic and visceral input, taste, smell
  hearing information monitors osmotic pressure,
  temperature of blood

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Functions of Hypothalamus

• Controls and integrates activities of ANS which
  regulates smooth, cardiac muscle and glands
• Synthesizes regulatory hormones that control the
  anterior pituitary
• Contains cell bodies of axons that end in
  posterior pituitary where they secrete hormones
• Regulates rage, aggression, pain, pleasure
  arousal
• Feeding, thirst satiety centers
• Controls body temperature
• Regulates daily patterns of sleep

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Epithalamus

• Pineal Gland
• endocrine gland the size of small pea
• secretes melatonin during darkness
• promotes sleepiness sets biological clock
• Habenular Nuclei
• emotional responses to odors

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Cerebrum (Cerebral Hemispheres)

• Cerebral cortex is gray matteroverlying white
  matter
• 2-4 mm thick containing billionsof cells
• grew so quickly formed folds(Gyri) and grooves
  (Sulci or Fissures)
• Longitudinal fissure separates left right
  cerebral hemispheres
• Corpus Callosum is band of white matter
  connecting left and right cerebral hemispheres
• Each hemisphere is subdivided into 4 lobes

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Right versus left Cerebrum
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Limbic System

• Emotional brain--intense pleasure intense pain
• Strong emotions increase efficiency of memory

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2 Types of Nervous Responses?

• A. Voluntary
• the brain
• spinal cord
• B. Involuntary or
• Autonomic System
• Sympathetic
• Parasympathetic

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Somatic Sensory Pathways

• First-order neuron conduct impulses to brainstem
  or spinal cord
• either spinal or cranial nerves
• Second-order neurons conducts impulses from
  spinal cord or brainstem to thalamus--cross over
  to opposite side before reaching thalamus
• Third-order neuron conducts impulses from
  thalamus to primary somatosensory cortex
  (postcentral gyrus of parietal lobe)

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Somatic Motor Pathways

• Control of body movement
• motor portions of cerebral cortex
• initiate control precise movements
• Basal Ganglia help establish muscle tone
  integrate semivoluntary automatic movements
• Cerebellum helps make movements smooth helps
  maintain posture balance
• Somatic motor pathways
• direct pathway from cerebral cortex to spinal
  cord out to muscles
• indirect pathway includes synapses in basal
  ganglia, thalamus, reticular formation
  cerebellum

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The Autonomic Nervous System

• Regulate activity of smooth muscle, cardiac
  muscle certain glands
• Structures involved
• general visceral afferent neurons (Sensory)
• general visceral efferent neurons (Motor)
• integration center within the brain
• Receives input from limbic system and other
  regions of the cerebrum

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The Autonomic Nervous System
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Autonomic versus Somatic NS

• Autonomic NS
• unconsciously perceived visceral
  sensations
• involuntary inhibition or excitation of
  smooth muscle, cardiac muscle or glandular
  secretion
• 2 neurons needed to connect CNS to organ
• Preganglionic and Postganglionic neurons

• Somatic NS
• consciously perceived sensations
• excitation of skeletal muscle
• one neuron connects CNS to organ

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Autonomic versus Somatic NS

• Notice that the ANS pathway is a 2 neuron pathway
  while the Somatic NS only contains one neuron.

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Divisions of the ANS

• 2 major divisions
• Parasympathetic
• Sympathetic
• Dual innervation (2 nerve supplies)
• 1 speeds up organ
• 1 slows down organ
• Sympathetic NS increases heart rate
• Parasympathetic NS decreases heart rate

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Sympathetic Responses

• Dominance by the sympathetic system is caused by
  physical or emotional stress
• E situations emergency, embarrassment,
  excitement, exercise
• Alarm Reaction Flight or Fight Response
• dilation of pupils, increase of heart rate, force
  of contraction BP
• decrease in blood flow to nonessential organs
• increase in blood flow to skeletal cardiac
  muscle
• airways dilate respiratory rate increases
• blood glucose level increase
• Long lasting due to lingering of NE in synaptic
  gap and release of norepinephrine by the adrenal
  gland

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Parasympathetic Responses

• Enhance Rest-and-Digest activities
• Mechanisms that help conserve and restore body
  energy during times of rest
• Normally dominate over sympathetic impulses
• SLUDD Type Responses salivation, lacrimation,
  urination, digestion defecation and 3
  decreases--- decreased HR, diameter of airways
  and diameter of pupil
• Paradoxical Fear when there is no escape route or
  no way to win
• causes massive activation of parasympathetic
  division
• loss of control over urination and defecation

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Control of Autonomic NS

• Not aware of autonomic responses because control
  center is in lower regions of the brain
• Hypothalamus is major control center
• Input emotions and visceral sensory information
• smell, taste, temperature, osmolarity of blood,
  etc
• Output to nuclei in brainstem and spinal cord
• posterior lateral portions control sympathetic
  NS
• increase heart rate, inhibition GI tract,
  increase temperature
• anterior medial portions control
  parasympathetic NS
• decrease in heart rate, lower blood pressure,
  increased GI tract secretion and mobility