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 dogs 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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Last updated 16 November 2004