Glossary of Circuits in the Nervous System – somatic Reflexes

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Explain how reflexes are classified.
Somatic (involving skeletal mm)

autonomic (rarely involve skeletal muscle endpoints)

Visceral (involve fibers from both the autonomic nervous system and the somatic nervous system)

Superficial (having to do with skin or mucus membraines e.g., pain) versus Deep (having to do with mm spindles or near bone e.g., stretch reflexes)

Normal versus Pathological (e.g., babinski's reflex)

Cord Level
Explain the characteristic features of reflexes
A. Reflexes, like more complex functions of the nervous system, have PARALLEL and HIERARCHICAL organization. A hierarchy of reflexes controls which reflex will occur when two reflex pathways are stimulated simultaneously.

B. Reflexes are GRADED RESPONSES that reflect the intensity of the stimulus.

C. Reflexes are characterized by a fixed spatial relationship between the site of stimulation and the particular muscles that contract; this relationship is called the LOCAL SIGN.

D. Unlearned reflexive responses to complex environmental stimuli are more properly called FIXED ACTION PATTERNS, e.g., smiling, grimaces, speech sounds, etc.
Describe the difference in circuitry between myotatic and inverse myotatic reflexes
myotatic reflex
B. The myotatic reflex probably operates to maintain muscle tone and occurs when the muscle is stretched.
1. studied by Charles Sherrington in late 19th century using cats and dogs
2. occurs in both flexor & extensors but especially in anti-gravity muscles
3. muscle spindle 1a afferents excite homonymous muscle and synergists monosynaptically
4. single 1a fiber may synapse on all alpha motor neurons innervating the muscle (300)[GLUTAMATE IS NT]
5. response may involve two types of inhibitory synaptic interactions[GLYCINE IS NT]
-RECIPROCAL INHIBITION through excitation of an interneuron connected to antagonist muscle
-RECURRANT INHIBITION via a Renshaw cell that is excited by the alpha-motor neuron and in turn inhibits it; this has the effect of dampening the intensity and duration of the reflex
Spinal Reflexes - Inverse Myotatic Reflex (clasp-knife reflex)
C. Inverse myotatic reflex (clasp-knife reflex) occurs when 1b fibers from Golgi tendon organs are stimulated.
1. functions to dampen excessive tension by exciting antagonists and inhibiting homonymous muscle
2. action is via glycinergic interneurons
3. elicitation requires more tension than myotactic reflex
4. crossed extensor component
Explain the principle of hierarchical organization of motor function.
The function of reflexes is not always apparent. Spinal reflexes are the first level of a hierarchy of motor response systems. This concept of hierarchial organization was first elaborated by the 19th century British neurologist, Hughling Jackson.
Discuss the differences between reciprocal and recurrent inhibition?
RECIPROCAL INHIBITION acts by exiting an interneuron connected to antagonist muscle

RECURRANT INHIBITION acts via a Renshaw cell that is excited by the alpha-motor neuron and in turn inhibits it; this has the effect of dampening the intensity and duration of the reflex
Explain local sign and how it applies to the flexion withdrawal reflex?
D. Flexor Withdrawal reflexes are mediated by skin receptors and pain receptors.
1. ipsilateral flexion and contralateral extension
2. functions to remove limb from potentially harmful stimulus and maintain balance
3. non-linear input-output relationship; full blown response requires certain threshold; hence, resembles fixed action patterns
4. light touch to foot pads may have opposite effect causing reflex extension (positive supporting reaction)
5. divergence occurs within the cord
6. final limb position is a function of site of stimulation (local sign)
7. withdrawal reflexes are prepotent, i.e., they preempt the spinal pathways from any other reflex activity taking place
Explain what sensory organs are involved in the myotatic and inverse myotatic reflex?
Myostatic involves innervation of muscle

Inverse myotatic involves innervation of golgi tendon organs
1. Understand that circuits are a fundamental element in the nervous system and understand the principles that apply to them.
By connecting cells into specific circuits information can be transmitted in a highly specific fashion. A knowledge of particular circuits can be useful in diagnosing pathology in the nervous system.

A. Circuits transmit information through action potentials, i.e., nerve impulses.

B. Several simple circuits may be hooked up to form more complex circuits.

C. Most circuits are produced from genetic instructions during embryonic and fetal development.

D. Unused circuits are often dismantled unless maintained by activity dependent processes, e.g., neurotrophic factors.

E. Reflexes consist of simple circuits and are the elementary units of function of the nervous system. They involve a sensory neuron, a motor neuron and often one or more interneurons. Generally speaking, the more interneurons involved, the less "reflexive" a reflex is. Reflexes, like other more sophisticated behavior programs such as fixed action patterns, are indicators of the underlying neural function. Abnormal reflexes indicate pathology along the circuit. The location of the neurons in the circuits of many reflexes are known and the clinician can test for pathology in many brain regions by artificially eliciting reflexes.

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