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Lecture 6
Spinal Reflexes &
Neuronal Integration
Reflex = an inherent, subconscious, relatively consistent responses to a particular stimulation. In contrast... Reaction = an inherent, subconscious, relatively consistent responses to a particular stimula- tion, involving the cerebellum and cerebral cortex; e.g., hopping reaction & tactile placing reaction.
Examples of brainstem reflexes include: — eyelids close when the cornea is touched (corneal reflex) — lip moves in response to a noxious stimulation (pin prick)
Examples of spinal reflexes, involving spinal nerves and the spinal cord, include: — extensor thrust: paw proprioceptors trigger limb extension — panniculus reflex: pricking skin triggers contraction of cutaneus trunci (panniculus) m. — myotatic reflex: muscle stretch is resisted by reflex contraction of the muscle — withdrawal reflex: limb flexes to withdraw from a noxious stimulus
NOTE: Reflex responses are determined by interneurons which “hard-wire” afferent input to efferent output. Interneurons organize efferent neurons (motor units) into meaningful movement components, which can be utilized by either spinal input or descending pathways. Also, interneurons form pattern generators for repetitive movements. Locomotor pattern generators exist in the spinal cord (e.g., on a treadmill, hind limbs exhibit stepping even in a cat that has its spinal cord transected in the thoracic region, i.e., isolated from the brain). Since "voluntary movement" and "involuntary reflex/reaction" compete for control of the same interneurons circuits, they cannot be independent on one another. Thus, brain activity will influence spinal reflex responses, making clinical reflex evaluation an interpretive art.
BACKGROUND INFORMATION ABOUT PROPRIOCEPTION
Proprioceptors are mechanoreceptors, located in muscles/tendons & joint capsules/ligaments.
Proprioceptors provide:
- subconscious feedback about the status of muscles & joints,
- conscious kinesthesia (sense of position & movement), and
- pain Joint receptors:
- free nerve endings that respond to extreme movement or inflammation (pain)
- encapsulated receptors: — tonic: signal joint position — phasic: respond to rate of change in joint position (largely subconscious)
Muscle & tendon proprioceptors: free nerve endings: pain (Golgi) tendon organs: located in series with muscle fibers (tension detector) muscle spindles: located in muscle belly (length detector)
Lecture 6
Spinal Reflexes &
Neuronal Integration
Reflex = an inherent, subconscious, relatively consistent responses to a particular stimulation. In contrast... Reaction = an inherent, subconscious, relatively consistent responses to a particular stimula- tion, involving the cerebellum and cerebral cortex; e.g., hopping reaction & tactile placing reaction.
Examples of brainstem reflexes include: — eyelids close when the cornea is touched (corneal reflex) — lip moves in response to a noxious stimulation (pin prick)
Examples of spinal reflexes, involving spinal nerves and the spinal cord, include: — extensor thrust: paw proprioceptors trigger limb extension — panniculus reflex: pricking skin triggers contraction of cutaneus trunci (panniculus) m. — myotatic reflex: muscle stretch is resisted by reflex contraction of the muscle — withdrawal reflex: limb flexes to withdraw from a noxious stimulus
NOTE: Reflex responses are determined by interneurons which “hard-wire” afferent input to efferent output. Interneurons organize efferent neurons (motor units) into meaningful movement components, which can be utilized by either spinal input or descending pathways. Also, interneurons form pattern generators for repetitive movements. Locomotor pattern generators exist in the spinal cord (e.g., on a treadmill, hind limbs exhibit stepping even in a cat that has its spinal cord transected in the thoracic region, i.e., isolated from the brain). Since "voluntary movement" and "involuntary reflex/reaction" compete for control of the same interneurons circuits, they cannot be independent on one another. Thus, brain activity will influence spinal reflex responses, making clinical reflex evaluation an interpretive art.
BACKGROUND INFORMATION ABOUT PROPRIOCEPTION
Proprioceptors are mechanoreceptors, located in muscles/tendons & joint capsules/ligaments.
Proprioceptors provide:
- subconscious feedback about the status of muscles & joints,
- conscious kinesthesia (sense of position & movement), and
- pain Joint receptors:
- free nerve endings that respond to extreme movement or inflammation (pain)
- encapsulated receptors: — tonic: signal joint position — phasic: respond to rate of change in joint position (largely subconscious)
Muscle & tendon proprioceptors: free nerve endings: pain (Golgi) tendon organs: located in series with muscle fibers (tension detector) muscle spindles: located in muscle belly (length detector)
Withdrawal Reflex = Flexor (Crossed Extensor) Reflex
Features of the reflex (diagrammed above) include... — primary afferent neuron (1) participates in both reflexes (2) and ascending pathways (3); — divergent interneuronal circuit propagates to several segments and right and left sides (B); — positive feedback prolongs the reflex beyond the time of the stimulus (A); — individual interneurons are either excitatory or inhibitory (black cells) in their effect; — antagonists are inhibited while agonists are excited (reciprocal innervation) (D); — descending pathways (C) modify reflex circuit (reflex is not independent of brain control). NOTES:
- As the reflex is tested clinically, the crossed extension component disappears after the first 3 weeks of age as descending pathways mature to inhibit the extension; but later in life, the normally inhibited crossed extension reappears if “upstream” damage to descending fibers removes the inhibition.
- The withdrawal reflex is often elicited to assess depth of anesthesia.
Withdrawal Reflex
flexor
extensor
flexor
extensor
DL F.
DL Sulcus
1
2
3
A
B
C
D D
flexor
extensor
flexor
extensor
DL F.
DL Sulcus
A
B
C
D D
Muscle Spindle and Myotatic Reflex
Muscle spindles are:
- elaborate proprioceptors positioned in parallel with muscle fibers;
- designed to signal muscle length
- about 3mm long & 0.5 mm wide.
Morphologically, a muscle spindle consists of a connective tissue capsule enclosing: — two kinds of mechanoreceptors, — two kinds of intrafusal muscle fibers, — two kinds of gamma efferent neurons.
Intrafusal muscle fibers:
vs. extrafusal (typical) muscle fibers
- very small, anchored in endomysium
- do not contribute anything to whole muscle tension
- center of each fiber is packed with nuclei & lacks myofilaments
- polar regions are striated and innervated by gamma neurons
- two kinds of intrafusal muscle fibers: nuclear bag fibers — central region is dilated; fiber extends beyond the capsule; nuclear chain fibers — smaller, central region contains chain of nuclei.
Mechanoreceptors within muscle spindle :
They are activated by stretch of the central region, which is stretched either
- by contraction of polar regions of intrafusal muscle fibers, or
- by passive stretch of the whole muscle (including the intrafusal fibers)
1] primary (annulospiral) endings — spiral around central (nuclear) regions;
they are endings of large nerve fibers (type I (^) A ); initially AP frequency reflects rate of stretch; then steady AP frequency reflects degree of stretch
2] secondary endings — "flower-spray" formations adjacent to nuclear chain regions;
they are endings of type II nerve fibers; AP frequency is proportional to degree of stretch.
Types of nerve fibers found in a muscle nerve
Muscle & Tendon Receptors
Myotatic Reflex
Clinically, a myotatic reflex is elicited by abruptly tapping a tendon (e.g., the patellar tendon). Suddenly deforming/displacing a tendon effectively stretches the associated muscle. When a whole muscle is suddenly stretched (as a result of tendon deformation), annulospiral receptors in muscle spindles are simultaneously excited, triggering a volley of action potentials in I (^) A afferent axons. Within the CNS, the axons activate excitatory synapses on alpha motor neurons that innervate the muscle that was stretched. Also, alpha motor neurons to antagonistic muscles are inhibited via interneurons. As a result, the stretched muscle immediately contracts. Thus, the myotatic reflex functions to oppose muscle stretch. Since interneurons are by-passed in eliciting the contraction, the response is rapid, localized, and relatively resistant to hy- poxia, fatigue, drugs, etc.
Ib
Ia
II
III
IV
A
slow pain nociceptors thermoreceptors
secondary spindle endings encapsulated receptors in joints and skin
tendon organs
annulospiral spindle endings
hair follicle receptors free ending mechanoreceptors pricking pain receptors
GVE Postganglionic
Extrafusal muscle fibers —large motor units
Intrafusal muscle fibers
Extrafusal muscle fibers —small motor units
ALPHA
BETA
GAMMA
DELTA
GVE Preganglionic
C
B
S ize R ange of P eripheral Nerve F ibers
Efferent Nerve Fibers Fiber Diameter Afferent Nerve Fibers
20 μ — — — 16 μ — — — 12 μ — — — — — 6 μ — — 3 μ — — 1 μ non-myelinated 0.2 μ
Two classification schemes for peripheral nerve fibers:
1] Based solely on nerve fiber diameter (I—IV)...
commonly applied to afferent fibers.
2] Derived from the compound action potential:
C ompound A c tion P otential (hypothetic al)
A B C
α β γ δ
NOTE: Nerve fiber = axon + myelin for myelinated fibers and axon for nonmyelinated fbers. Conduction velocity (m/sec) = fiber diameter ( μ m) X 6 (approximately). Thus, a 20 μ m fiber conducts at approximately 120m/s = 270 mph.
