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Bio 168 Reflexes and Central Motor Mechanisms Study Guide
Fall Semester 2004

Learning Objectives, Topics & Keywords



Recommended Reading: Martini (6th edition) 448 ‚ 455; 523 ‚ 527.

1. Understand the elements of a spinal motor reflex arc

 

Overview: much motor behavior is based on spinal reflexes

 

reflex arc: sensory input --> neural integration --> motor output

 

may be simple or complex pattern of output

 

2. Know the kinds of sensory input that drive reflexes.

sensory axons enter at the dorsal root

all sensory afferents are excitatory

cutaneous receptors

mechanoreceptors (e.g., light touch, itch, vibration)

for example, scratch reflex (tickle dog’s tummy, it scratches)

 

nociceptors (stimuli that may damage tissue --> pain)

for example, withdrawal reflex (touch something hot --> reflexly pull back hand)

 

proprioceptors ("sense of self")

provide information about mechanical state of muscles and body position in space

muscle spindles (sense muscle length)

specialized muscle fibers

send signals on fastest (largest diameter) axons in the body

 

Golgi tendon organs (sense muscle tension)

 

joint mechanoreceptors (sense joint angle and load)

 

3. Understand the organization of several spinal reflexes

stretch reflex (also, deep tendon reflex, "knee jerk reflex")

input is stretch of spindles (i.e., increase in muscle length)

integration is monosynaptic EPSPs in ipsilateral motor neurons of same muscle

output is twitch (if motor neurons reach threshold)

reflex opposes increase in muscle length by external load

 

Golgi tendon organ reflex

input is tension in muscle tendon which excites the Golgi tendon organ

integration is disynaptic (via an interneuron)

--> IPSPs in ipsilateral motor neurons of same muscle

output is inhibition of motor neuron leading to less muscle tension

reflex protects muscle and tendon from excessive tension

 

withdrawal/crossed extensor reflex

input is nociceptor (painful)

integration is multisynaptic, bilateral

output is both excitatory and inhibitory

interneurons produce "reciprocal inhibition"

reflex withdrawal plus weight bearing (crossed extensor)

 

 

4. Understand the basics of motor control by higher centers

Overview: upper motor neurons --> descending pathways --> lower motor neuron --> muscle

motor cortex (precentral gyrus of the frontal lobe)

primary motor area

upper motor neuron

 

motor homunculus (means "little human")

skeletal muscles mapped to cortex on precentral gyrus

fineness of motor control depends on neuron number

 

direct pathway (also called pyramidal pathway or corticospinal pathway)

 

from upper motor neuron in motor cortex to lower motor neuron in  ventral horn

synapse on or near lower motor neuron

control fine movement

 

anterior and lateral corticospinal tracts (how are tracts named?)

made of axons of upper motor neuron

tracts cross (decussate) in the medulla

anatomical basis of left body control by right brain

 

indirect (extrapyramidal) pathway

everything else

upper motor neuron in basal ganglia

basal ganglia (groups of neurons in base of cerebrum and in brainstem)

control automatic movements (e.g., walking)

control muscle tone

inhibitory (e.g., need to stop 'standing' before starting 'walking')

 

 

 

cerebellum

detects and corrects errors in movement

inputs:

"intentions" (from collaterals of corticospinal tract fibers)

actual movement (as reported by proprioceptors)

visual input

vestibular input (sense of balance)

compares intended movement to actual movement

outputs:

generates an "error" signal if intended movement is not equal to actual movement

sends corrective information to motor cortex and basal ganglia

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Durham Technical Community College
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Last updated 16 November 2004