Unit Eleven: The Nervous System: C. Motor and Integrative Neurophysiology

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Unit Eleven The Nervous System C. Motor and
Integrative Neurophysiology

• Chapter 54 Motor Functions of the Spinal Cord
  the Cord Reflexes

Guyton and Hall, Textbook of Medical Physiology,
12 edition
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Organization of the Spinal Cord for Motor
Functions
Fig. 54.1
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Organization of the Spinal Cord for Motor
Functions

• Anterior Motor Neurons
• Located in the anterior horn of the spinal cord
• Two types alpha and gamma directly innervate
• skeletal muscle fibers
• Interneurons- present in all areas of the cord
• gray matter

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Organization of the Spinal Cord for Motor
Functions
Fig. 54.2 Peripheral sensory fibers and
anterior motor neurons
innervating skeletal muscle
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Organization of the Spinal Cord for Motor
Functions

• Interneurons (cont.)
• Responsible for most of the integrative functions
  of
• the spinal cord
• All types of neuronal circuits are found in the
• interneuronal pool

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Muscle Sensory Receptors

• Muscles and Tendons with Two Types of Sensory
• Receptors
• Muscle spindles-distributed throughout the belly
• of the muscle information about muscle length
• or rate of change of length
• Golgi tendon organs-located in the tendons and
• transmit information about tendon tension or
• rate of change of tension

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Muscle Sensory Receptors

• Muscles and Tendons with Two Types of Sensory
• Receptors
• Signals are for intrinsic muscle control
• Operate at a subconscious level

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Muscle Sensory Receptors

• Receptor Function of the Muscle Spindle

Fig. 54.3 Muscle spindle
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Muscle Sensory Receptors

• Receptor Function of the Muscle Spindle
• Structure and motor innervation- 3-12 intrafusal
  fibers
• surrounded by large extrafusal fibers
• The central portion does not contract
• when the ends do
• 2. End portions are excited by gamma motor fibers

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Muscle Sensory Receptors

• Receptor Function of the Muscle Spindle
• b. Sensory innervation of the muscle spindle
• Lengthening the whole muscle stretches the
• point of the spindle and excites the receptor
• Even if the length of the muscle does not
• change, you get the receptor excited
• 3. Primary and secondary endings

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Muscle Sensory Receptors
Fig. 54.4 Details of nerve connections from the
nuclear bag and nuclear chain
muscle spindle fibers
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Muscle Sensory Receptors

• Receptor Function of the Muscle Spindle
• c. Division of the intrafusal fibers
• Nuclear bag muscle fibers
• Nuclear chain fibers
• Static Response-when the receptor is stretched
• slowly, the number of impulses transmitted from
• both primary and secondary endings increases
• in proportion to the stretching and can continue
  for
• several minutes

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Muscle Sensory Receptors

• Receptor Function of the Muscle Spindle
• e. Dynamic Response- when the length of the
  spindle
• receptor increases suddenly, only the primary
  ending
• is stimulated the primary responds actively to
  a
• rapid rate of change
• f. Gamma motor nerve control of static and
  dynamic
• responses gamma-s and gamma-d excite specific
• fibers
• G. Continuous discharge of impulses to the
  spinal cord

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Muscle Sensory Receptors

• Muscle Stretch Reflex- monosynaptic pathway
• Neuronal circuitry

Fig. 54.5 Neuronal circuit of the stretch reflex
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Muscle Sensory Receptors

• Muscle Stretch Reflex- monosynaptic pathway
• Dynamic stretch reflex-responds to the primary
• sensory endings of the muscle spindles caused by
• rapid stretch or unstretch functions to oppose
• sudden changes in muscle length
• Static stretch reflex- weaker and elicited by
• continuous static receptor signals transmitted
  by
• primary and secondary endings

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Muscle Sensory Receptors

• Damping Mechanism in Smoothing Muscle
  Contraction

Fig. 54.6
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Muscle Sensory Receptors

• Damping Mechanism in Smoothing Muscle
  Contraction-
• signal averaging function of the muscle spindles
• Role of the Muscle Spindle in Voluntary Motor
  Activity
• Coactivation of alpha and gamma neurons
• Keeps the length of the receptor portion of the
• muscle spindle from changing during the course
  of
• a contraction
• Maintains the proper damping function of the
  muscle
• spindle

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Muscle Sensory Receptors

• Brain Areas for Control of the Gamma Motor
  System
• Excited specifically by signals from the
  bulboreticular
• facilitatory region
• Secondarily by impulses transmitted into the
  bulbo-
• reticular area from the cerebellum, basal
  ganglia,
• and the cerebral cortex
• Muscle Spindle System Stabilizes Body Position
• During Tense Action

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Muscle Sensory Receptors

• ClonusOscillation of Muscle Jerks

Fig. 54.7
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Muscle Sensory Receptors

• Golgi Tendon Reflex
• Detects muscle tension not changes in muscle
  length
• Has both primary and static responses
• Transmits signals into the spinal cord and on to
  the
• cerebellum and cerebral cortex
• Local cord signal stimulates a single inhibitory
• interneuron that inhibits the anterior motor
  neuron
• Is a negative feedback that prevents too much
  tension
• on the muscle

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Muscle Sensory Receptors

• Golgi Tendon Reflex
• Tendon reflex equalizes contractile forces among
• the muscle fibers
• Apprise higher motor control centers of
• instantaneous changes occurring in the muscles

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Flexor Reflex and the Withdrawal Reflexes

• Neuronal Mechanism of the Flexor Reflex

Fig. 54.9 Flexor reflex, crossed extensor
reflex, and reciprocal inhibition
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Flexor Reflex and the Withdrawal Reflexes

• Neuronal Mechanism of the Flexor Reflex
• Involves the following basic types of circuits
• 1. diverging circuits to spread the reflex to
  the
• necessary muscles for withdrawal
• 2. circuits to inhibit the antagonistic
  muscles
• (reciprocal inhibition)
• 3. circuits to cause afterdischarge lasting
  after
• the stimulus stops

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Flexor Reflex and the Withdrawal Reflexes

• Neuronal Mechanism of the Flexor Reflex

Fig. 54.10 Myogram of the flexor reflex showing
rapid onset, an interval of fatigue,
and finally, afterdischarge when the
stimulus is over
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Crossed Extensor Reflex

• Neuronal Mechanism

Fig. 54.11 Myogram of a crossed extensor reflex
showing slow onset but prolonged afterdischarge
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Crossed Extensor Reflex

• Reciprocal Inhibition and Reciprocal
  Innervation

Fig. 54.12 Myogram of a flexor reflex showing
reciprocal inhibition
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Reflexes of Posture and Locomotion

• Reciprocal Inhibition and Reciprocal
  Innervation
• Positive supportive reaction
• Stepping and walking movements