Reflexes

• The typical pattern of a neural “circuit” is an arc.

sensory afferents integration in CNS motor efferents effector tissue

• Reflexes are the simplest form of integration by CNS.– spinal cord and brain stem only– monosynaptic– polysynaptic

• Somatic reflexes– effector tissue = skeletal muscle

Fig. 13.21

Skeletal Muscle Proprioceptors

• Muscle Spindles– detect muscle length

• inhibit muscle stretch

• Golgi Tendon Organs– detect muscle tension

• inhibit over-contraction

Golgi Tendon Organ

Fig. 13.23

Type Ib fibers

Muscle Spindlesfus- = spindle intrafusal fibers – specialized muscle fibers of the spindleextrafusal fibers – regular skeletal muscle fibers

A

AType II fiberType II fiber

Type Ia fiber

Fig. 13.20

• The (Ia) sensory nerve ending is a stretch receptor, wrapped around the center of an intrafusal fiber.

• The intrafusal fibers have sarcomeres only on their ends.

• A stimulation of intrafusal fibers will cause contraction of the ends and stretch of the middle.

• There is always some tonic discharge by the A motor neurons.

• Therefore, the stretch receptors are always slightly stimulated – warmed up and ready to respond to any stretch of the muscle.

Fig. 6-3 Ganong

Muscle Spindles

Stretch Reflex

• a monosynaptic reflexa monosynaptic reflex

• e.g., the knee-jerk reflex

• negative feedback• ipsilateral stimulation

of extension (simultaneous inhibition

of flexion – polysynaptic)

A

Fig. 13.21

Polysynaptic Reflexes

• via interneurons– sensory neuron from skin

[interneuron(s)]

A motor neuron

• examples– withdrawal (flexor) reflex– crossed extensor reflex

Withdrawal Reflex + = stimulatory = stimulatory

- = inhibitory= inhibitory

Fig. 13.22

• a flexor reflex– ipsilateral

• stimulation of flexion

• inhibition of extension

Crossed Extensor

Reflex

Fig. 13.22

• an extensor reflex– contralateral

• stimulation of extension

• inhibition of flexion

Somatic Motor Pathways(from brain)

• organizing principles– phylogenetic

• lateral and medial motor systems– medial tracts are more primitive (see ventral/anterior columns)

» control proximal muscles: posture and gross movements– lateral tracts are more advanced

» control distal muscles: fine, skilled movements

– clinical• pyramidal and extrapyramidal systems

– pyramidal system (direct pathway)– extrapyramidal systems (indirect pathways)

Somatic Motor System

• The frontal lobe is a major control center for somatic motor functions.– precentral gyrus

• 30-60% of cortical motor fibers originate here.

Fig. 14.23

Pyramidal System

• direct pathway– The axon of one

neuron travels from the cerebral cortex to lower motor neurons (such as the A motor neurons).

– two paths• lateral corticospinal

tracts• ventral corticospinal

tracts

Fig. 13.6

• lateral corticospinal tracts– lateral motor system– major importance only in primates– precise control of voluntary

movements in distal muscles– In humans, 80-90% of cortical

motor fibers travel in these tracts.– cross over in the pyramids of the

medulla• giving the “pyramidal” system its

name

• ventral corticospinal tracts– medial motor system– cross over (via interneurons) in

the spinal cord, at the level of the lower motor neuron

Fig. 12-2Ganong

Medullary Pyramids

Fig. 14.27

Fig. 14.9