Nerves of Steal

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Nerves of Steal

• Jessica, Leah, Alex

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Topics

1. Motor Neurons
2. Nerve Spinal Pathways
3. Somatic Reflexes

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Motor Neurons

• Jessica

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What are Neurons?

• A neuron is the scientific name of a nerve cell
• Consists of small fibers called dendrites which
  branch out in all directions, attached to it,
  there is a long fiber called the axon
• Axon carries the impulses for it to reach the
  next neuron
• Myelin sheath- an insulated covering, protects
  the nerve impulse, so that there is no
  interference in the message.
• (femail.com.au).

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Motor Neurons

• Motor neuron -nerves located in the Central
  Nervous System (CNS)
• Project their axons outside the CNS and into the
  muscles, through impulses which can reach up to
  435 km an hour
• Often said to be efferent neurons, which means
  they carry nerve impulses away from the CNS to
  the muscles or glands
• Mostly involved in muscle control

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Variety

• There are 3 types of motor neurons
• Somatic Motor Neurons
• Special Visceral Motor Neurons
• General Visceral Motor Neurons

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Somatic Motor Neurons

• Somatic Motor Neurons - directly supply nerves
  and stimulation to skeletal muscles involved in
  the ability to move
• For example
• Muscles in the limbs
• Muscles in the abdomen
• Intercostals muscles (muscles that run between
  the ribs which help form the chest wall and play
  a major role in the mechanical aspect of
  breathing)

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Subdivisions of Somatic Motor Neurons

• Somatic Motor Neurons are subdivided into
• Alpha efferent neurons
• Gamma efferent neurons.

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Special Visceral Motor Neurons General Visceral
Motor Neurons

• Special Visceral Motor Neurons innervate
  bronchial muscles that motorize the face and the
  neck.
• General Visceral Motor Neurons innervate cardiac
  muscle and smooth muscles of the viscera.

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Function

• The interface between a motor neuron and muscle
  fiber is a specialized synapse called the
  neuromuscular junction. Upon adequate
  stimulation, the motor neuron releases a flood of
  neurotransmitters that bind to postsynaptic
  receptors and triggers a response in the muscle
  fiber. (wikipedia.org).

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• Neuromuscular junction is the synapse of the
  axon terminal of a motor neuron with the motor
  end plate.
• The motor end plate is the highly-excitable
  region of muscle fiber plasma membrane which
  initiates action potentials through the muscle's
  surface, causing the muscle to contract.

www.daviddarling.info
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Axon terminal
www.psiwebsubr.org
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Somatic Motor Neurons
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Nerve Pathways

• Leah Enders

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Spinal Tracts

• Divided into two sections
• Ascending Pathways-carry information up cord
• Descending Pathways-conduct motor impulses down
  cord
• Sometimes decussation occurs- tracts cross over
  sides as they go down or up
• Contralateral - origin and destination same side
• Ipsilateral origin and destination diff sides

Anatomy Physiology The unit of Form and
Function Saladin 2004
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Descending
Ascending
Both pathways occur on both sides of spinal cord
http//knol.google.com/k/-/-/yvSo4t-D/JfZatw/515px
-Medulla_spinalis_-_tracts_-_English.svg5B15D.pn
g
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Ascending Tracts

• Sensory signals pass 3 types of neurons on trip
  from origin to brain
• 1st order neuron detects stimulus and
  transmits signal to spinal cord and brain
• 2nd order neuron only goes as far as thalamus
  at upper end of brainstem
• 3rd order neuron- carries signal rest of way
  from thalamus to cerebral cortex (sensory region)

Anatomy Physiology The unit of Form and
Function Saladin 2004
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Major Ascending Tracts

• Gracile Fasciculus-
• Signals from midthorasic (below T6) and lower
  parts of body
• 1st order neurons, travel ipsolateral up
  dorsal column or spine, decussation occurs at
  medulla
• Vibration, visceral pain, deep touch, transmits
  proprioreception from legs and trunk
• Cuneate fasciculus
• Same sensations, type, place as gracile
  fasciculus, joins it at T6
• Medial lemniscus
• 2nd order that picks up where gracile cuneate
  leave off
• Goes to thalamus where 3rd-order take over
• Ultimately goes to contralateral cerebral
  hemisphere

Anatomy Physiology The unit of Form and
Function Saladin 2004
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Major Ascending Tracts

• Spinothalamic tract
• Forms anterolateral system anterior lateral
  side of spinal column
• Carries signals for pressure, pain, temperature,
  pressure, tickle, itch, light touch
• 1st - order ends at dorsal horn _at_ entry of
  spinal cord , 2nd- order decussate to other side
  of cord take it up to thalamus,
• 3rd- order go to cerebral cortex (spatial
  pereception)
• Dorsal Ventral spinocerebellar tracts
• Lateral column travel, proprioreception from
  limbs trunk to cerebellum (motor control)
• 1st order start at muscles and end at dorsal
  horn spinal cord
• 2nd order goes to cerebellum
• Ventral crosses contralateral, but then cross
  again so both end up on ipsolateral side

Anatomy Physiology The unit of Form and
Function Saladin 2004
20
http//faculty.irsc.edu/FACULTY/TFischer/AP1/ascen
ding20pathways.jpg
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Descending Tracts

• Carry signals down brainstem and spinal cord,
  involves 2 neuron types
• Upper motor neuron starts at soma in cerebral
  cortex or brainstem and has axon that goes to a.
• Lower motor neuron in brainstem or spinal cord
  and goes rest of way to muscle or whatever target

Anatomy Physiology The unit of Form and
Function Saladin 2004
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Major Descending Tracts

• Vestibulospinal Tracts
• Starts at brainstem vestibular nucleus, receives
  impulses from balance from inner ear
• Goes thru ventral column of spinal cord
• Controls limb muscles that maintain balance
  posture
• Tectospinal tract
• Starts midbrain region crosses sides in
  brainstem, branches at medulla forming lateral
  medial tracts upper spinal cord
• Reflex movments of head, escpecially responding
  to visual and auditory stimuli

Anatomy Physiology The unit of Form and
Function Saladin 2004
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Major Descending Tracts

• Lateral Medial Reticulospinal tracts
• Begin at reticular formation of brainstem
• Control muscles of upper lower limbs,
  especially maintaining posture balance
• Contain descending analgesic pathways that reduce
  transmission of pain signals to brain
• Corticospinal tracts
• Send motor signals from cerebral cortex
• 1.) Decussate in lower medulla, form lateral
  corticospinal tract , or
• 2.) Dont cross form ventral corticospinal
  tract (decussate lower but still control
  contralateral side)
• Precise finely coordinated limb movements

Anatomy Physiology The unit of Form and
Function Saladin 2004
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http//www.youtube.com/watch?v9BaWBGRVxp8
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Somatic Reflexes

• Relating to the somatic nervous system
• Alex

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Reflexes

• Require stimulation
• Are quick
• Are involuntary
• Are stereotyped
• Mediated by brainstem and spinal cord
• Result in involuntary contraction of a muscle

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Reflex Arc

• Pathway
• Somatic receptors in skin, muscle or tendon
• Afferent nerve fibers carry information from
  receptors to spinal cord
• Interneurons (usually)
• integrate info
• Efferent nerve fibers
• carry motor impulses
• to skeletal muscles
• Skeletal muscles carry
• out response

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Muscle Spindles (Stretch Receptors)

• Proprioceptors that provide cerebellum with
  feedback needed to regulate tension in skeletal
  muscles
• 100 per gram of muscle in foot
• hand, less or none elsewhere
• 4-10 mm long
• Made up of
• 3-12 Intrafusal fibers
• modified muscle fibers
• only ends have sarcomeres and can
• contract
• A few nerve fibers
• Primary afferent fibers coil around middle of
  intrafusal fibers, react to onset of stretch
• Secondary afferent fibers wrap around ends,
  respond to prolonged stretch
• Gamma motor neurons spinal cord to contractile
  ends of intrafusal fibers, stimulate contraction
  to keep muscle spindles taut and responsive

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Stretch (Myotatic) Reflex

• Mediated primarily by the brain
• Muscles contract when stretched
• Maintains equilibrium and posture
• Smooth/dampen muscle action
• Muscle stretches, stimulates muscle spindle
• Sends afferent signal to cerebellum by way of
  brainstem
• Cerebellum integrates info and relays to cerebral
  cortex
• Cortex sends signal back, muscle contracts
  (usually a set of synergistic and antagonistic
  muscles)

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Tendon Reflex (e.g. knee-jerk)

• Similar to stretch reflex, but occurs if muscle
  is stretched suddenly
• Spinal component
• more pronounced
• Reciprocal inhibition
• Inhibits antagonistic
• muscles from
• contracting

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Flexor (Withdrawal) Reflex

• Contraction of flexor muscles to withdraw limb
• Utilizes reciprocal inhibition
• Uses more complex neural pathways than tendon
  reflex
• Polysynaptic reflex arc
• Some signals arrive quickly, some take longer
• Sustained contraction (until consciously aware)

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Crossed Extensor Reflex

• Shift center of gravity to maintain balance
• Contraction of extensor muscles in limb opposite
  from one withdrawn
• Occurs simultaneously with flexor reflex

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Golgi Tendon Reflex

• Golgi tendon organs
• proprioceptors located on tendon
• near junction with muscle
• Nerve endings encapsulated in collagen fibers
• 1 mm long
• When relaxed, no pressure on nerve endings
• When taut, pressure applied to nerve endings
• Limits force produced by muscles in order to
  prevent injury to tendon
• Spreads workload evenly across whole muscle