Reflex And Spinal Control Of Movement

Front 1

Reflexes Definition

Back 1

stereotyped efferent response to a afferent input

 -consistent in static (resting, passive conditions)
 -Can they be altered???

Front 2

Proprioceptors

Back 2

Muscle receptors
– Group Ia (Aalpha) – dynaminc spindles
– Group II (Abeta)– static spindles
– Group Ib (Aalpha) – GTOs

Joint receptors

Front 3

Exteroceptors

Back 3

-cutaneous (mechanical, thermal, noxious) – group II, III, IV (A, A, C)

-vestibular and visual included, not in SC reflexes

Front 4

Muscle Spindle Pathways

Back 4

-Homonymous motoneuron

-Heteronymous (synergist) motoneuron

-Ia inhibitory interneuron

Front 5

Sensory Receptors: muscle spindle

Back 5

-The muscle spindle has two sets of
afferent axons: group Ia and group
II.

- The afferent axons contact the
intrafusal fibers in the center.

Front 6

Group Ia

Back 6

afferents detects changes in muscle length and the
rate with which this occurs

- velocity sensitive”
- monosynpatic to MNs

Front 7

Group II

Back 7

afferent detects changes in muscle length

- “position sensitive”
- Polysynaptic to MNs

Front 8

muscle spindle Study

Back 8

- Recordings of discharge rate along a Ia afferent coming from the
extensor digitorum muscle of a human.

-The muscle was moved passively (left) and performed a voluntary
contraction (right). Increasing joint angle = flexion = stretch.

How is the sensitivity of the spindle adjusted?

Front 9

Intrafusal muscle fibers

Back 9

-Concentric contraction causes shortening … but still get spindle discharge

- Use of the gamma (and beta) MNs
- Intrafusal muscle fibers
- “Gain” (input/output ratio) altered by changing neural drive

Front 10

efferent axons

Back 10

1. In addition to afferent axons, the intrafusal
fibers are innervated by efferent axons.

2. The efferent innervation is provided by gamma
motor neurons.

3. The two types of gamma motor neurons (dynamic
and static) cause the ends of the intrafusal fibers to contract, which stretches the central part and thereby alters the sensitivity of the fibers.

Front 11

Sensitivity

Back 11

the response relative to the
stimulus (stretch)

Front 12

length detector

Back 12

1. The muscle spindle, therefore, is
not a simple length detector.

2.When a muscle spindle is stretched, it generates action potentials at a frequency that depends on the amount and rate of the stretch.

3. Selective stimulation of two types of gamma MNs has different effects on the firing of the primary sensory endings in the spindle (the Ia fibers)

4.The response of the muscle spindle to a given stretch (bottom panel) can vary.

Front 13

Mostly...

Back 13

alpha-gamma co-activation, but can
dissociate

Front 14

Sensory Receptors: modulation of pathways

Back 14

Incoming Ia input can be reduced
by presynaptic inhibition.

Front 15

Mechanisms:

Back 15

1. Increased conductance of chloride causing a hyperpolarization in the afferent synaptic terminal

2. Or, metabotropic receptors can close Ca2+ channels, decreasing the influx of Ca into the Ia afferent.

Front 16

hyperpolarization in the
afferent synaptic terminal

Back 16

 This results in antidromic collision and a refractory period in which Na channels will not open.

Result: Less Ca influx, less depolarization and less NT release from Ia afferent.

Result: depressed potential in the post synaptic cell

Front 17

H-reflex – analog of stretch reflex

Back 17

1. ‘electrical analogue of the
stretch reflex?’

2. Direct motor response
precedes reflex response

3. Reflex response decreases
with increasing intensity

Front 18

The H-reflex amplitude changes

Back 18

1. between tasks

2. possibly due to presynaptic inhibition

3. and within tasks

Front 19

golgi tendon organ

Back 19

is a sensor of muscle force arranged in series with the muscle fibers

1. The GTO is encapsulated and innervated by a single group Ib axon.

2. The axon branches into many fine endings that
intertwine among the braided collagen fascicles.

3. Contracting muscle fibers stretch the collagen
fascicles and squeeze the Ib afferent.

Front 20

rate

Back 20

The rate at which the tendon organ generates action potentials is proportional to the force in muscle (passive or active).

Each line indicates the discharge rate recorded in a single Ib afferent at different muscle forces.

Front 21

Recordings of discharge
rate along a Ib afferent
coming from the extensor
digitorum muscle of a
human.

Back 21

The muscle contracted against a zero load in a and
a light load in b. Increasing joint angle = flexion.

Note the association between EMG (muscle
force) and Ib discharge rate.

What does this indicate?

Front 22

Ib inhibitory interneuron

Back 22

The Ib axon (afferent) from the golgi tendon organ
contacts an interneuron in the spinal cord; the
interneuron is known as the

Front 23

Reflex circuits/
GTO may not be
hardwired

Back 23

1. The Ib inhibitory IN receives convergent input
from tendon organs, muscle spindles (not
shown) joint receptors, and descending pathways

2. Stimulation of Ib afferents may inhibit the motor neuron pool while at rest, whereas it may excite the motor neuron pool while walking or standing

Front 24

Other Proprioceptive Pathways

Back 24

Cutaneous or joint/fascia

Mechanical (group II, III)

noxious (group III-IV)

Also not hard-wired

Stumbling Corrective Responses

After spinal cord injury??

Front 25

Mechanical

Back 25

Group II and III

-plantar skin during stance phase of walking
-tactile inputs on fingertips during grasp

Front 26

Noxious

Back 26

Group III and IV

-flexion withdrawl reflex
-local sign

Front 27

Reflexes contribute to movement

Back 27

 Different receptors provide various
input based on anatomical and
topographical arrangement

Front 28

Reflexes modulate and even reverse action in different conditions

Back 28

Can not be explained by simple
habituation

Network of cells

Front 29

Spinal and Supraspinal Control
of Movement

Back 29

1. Brainstem and spinal cord can generate
basic patterns of motor output

2. Complex inter- and intralimb coordination
(e.g., standing and walking)

3. Modulated by descending and
afferent inputs

Front 30

Means to study locomotion via lesion models

Back 30

1. Decerebrate
-transection at the brainstem (midbrain) (a-a’)

2. Spinal
-transection at lower thoracic level (b-b’)

3. Deafferented
-transection of the dorsal roots

4. Immobilized
-removes movement related feedback by paralyzing the muscles (fictive locomotion)

5. Neonatal (and/or) in vitro preparation

Front 31

Central Pattern Generators (CPG)

Back 31

Definition: circuit with intrinsic ability to generate rhythmic, coordinated motor behaviors

Responsible for automaticity of simple to complex motor behaviors

Generates intra- and inter-limb coordination

Modulated by descending commands and afferent input

Front 32

CPGs and Locomotion
(Half-Center Model)

Back 32

 Graham Brown’s half-center model (1911) comprises one set of neurons that project to motor neurons innervating extensor muscles and another set that projects to motor neurons innervating flexor muscles.

Two centers inhibit each other reciprocally so that when one half-center is active, the other is inhibited

Front 33

Most studied/detailed Central Pattern
Generator

Back 33

Total of 33 neurons control all feeding behaviors

Approximately 50% of all possible connections
are present

Anatomy ≠ Physiology

Front 34

Modulated by

Back 34

1. sensory information (food in gut)

2. descending modulation
(hunger/satiety of
animal)

Front 35

Two major students:

Back 35

- John Eccles (1963 Nobel) –neurophysiology: first
intracellular recording

- Derek Denny-Brown –neurologist (first one to bring neurology to prominence)

Front 36

Charles Sherrington

Back 36

-1932 Nobel prize in Medicine
- reflex activation of the CNS

Front 37

Internally generated
output

Back 37

- Graham Brown (1911)

- Phasic output in decerebrate, spinalized, and deafferented cat

-“Half-center” mode

Front 38

How to trigger CPGs/locomotion

Back 38

 This was later demonstrated by
electrical stimulation of high
threshold cutaneous & muscle
afferents (FRAs) in spinalized
cats treated with L-Dopa and
nialamide

Front 39

Acute spinal paralyzed cat

Back 39

(Dubuc et al. 1980)

record from hindlimb nerves

i.v. L-DOPA, nialamide, and 4-aminopyridine

Front 40

Use of classical neurotransmitter
pathways

Back 40

1. A number of agents can initiate/ facilitate
locomotion in animal preparations

Excitatory (primarily glutamate)
-Specific agonists elicit rhythmic activity in virtually all systems –inconsistent with locomotion (Cowley and Schmidt 1994)

Inhibitory (antagonists)
 -Strychnine (glycine) and biccuculine (GABAA) (Robinson and Goldberger 1986; Deleon et al 1999)

Front 41

NE and
Serotonin (5-HT

Back 41

• released from brainstem nuclei locus ceruleus and raphae nuclei.

• project densely throughout the spinal cord via the
reticulospinal tract, releasing these monoamines.

• Both NE and 5HT increase the excitability of the
MNs and may lead to the spontaneous generation of locomotor activity

Front 42

Neonatal/in vitro preparations

Back 42

5HT and NMDA trigger locomotor patterns similar to
locomotion

Front 43

Epidural stimulation

Back 43

-Epidural electrodes at T11-L1 (Dimitrijevic et al. 1998)

- Variable “pattern” generation with variable intensity stimulation

Front 44

Can’t really identify CPGs in humans

Back 44

-Unable to isolate spinal cord networks from both descending and afferent sources

-Does it really matter – do humans demonstrate automaticity related to stepping behaviors??

Front 45

Mesencephalic Locomotor Region (MLR)

Back 45

Direct projections from cortex, basal ganglia,
thalamus

Front 46

Actions exerted through:

Back 46

medial reticular formation (reticulospinal
pathway)
– chemical and electrical activation
– modulates with ipsilateral swing phase
– posture/muscle tone (thermostat-like behavior)

ventrolateral/ventromedial spinal pathways
– reduction in postural tone/locomotor ability with
ventral spinal hemisection (Brustein and Rossignol
1998)
– Some recovery can still occur

Front 47

Triggering locomotion through neural
structures

Back 47

1. Mesencephalic locomotor region

2. Median reticular formation

3. Decerebrate preparations

Front 48

Descending pathways initiate
locomotion

Back 48

EMG activity recorded from a walking decerebrate cat with hindlimb muscles that were de-afferented. (LG = lateral gastrocnemius, EDB = extensor digitorum brevis, IP = iliopsoas, ST
= semitendinosus)

The pattern is similar to that before deafferentation.

Front 49

Without afferent input

Back 49

 “Controlled locomotion” initiated by Shik and
colleagues (1960s)

Stimulation of MLR

Paralyzed, record from hindlimb nerves

Front 50

Without descending input

Back 50

Chronic spinal cats transected after birth
(Forssberg et al. 1980)

-(Forssberg et al. 1980)Treadmill walking
(motorized)

-EMG recorded from hindlimb muscles

Front 51

why afferent information important

Load through stance-phase limb

Back 51

1. Too little – reduced Ia, Ib, cutaneous input

2. Too much – 1) can’t bear weight and 2) Doesn’t allow swing initiation in late stance

Front 52

Why afferent information important

Hip extension in late stance

Back 52

1. Stretch of sartorius muscle activates whole limb flexion

2. Moving limb back increases stretch earlier . .

3. Moving limb forward reduces swing initiation (Lam and Pearson 2001)

Front 53

Hip flexion in swing

Back 53

triggers limb extension (McVea et al.
2006)

Front 54

Similar mechanisms in humans

Back 54

Behavior of limb dependent on
multiple afferent inputs

1. IF hip flexed and extensor muscle force high . .
THEN prolong stance phase duration

2. IF hip extended and extensor muscle force low . . . THEN initiate swing phase

Front 55

Pang and Yang (2000)

Back 55

use of infant walking to assess effects of limb position and loading

Front 56

Pharmacology + afferent stimulation
(motorized treadmill)

Back 56

1. Complete injury –NE alpha 2
agonists in cats triggers locomotion 8 days
post-SCI

2. Incomplete injury –specific NE alpha 1 and
5HT agonist facilitate weightbearing

Front 57

“synergies”: What

Back 57

Definition: group of muscles activated in a fixed balance

 -Across single or multiple joints
 -Synchronous (simultaneous) vs
 -Temporal (time-varying) synergies

Front 58

Why

Back 58

- Provides variety of movement strategies based on
“task-relevant subspace of control variables”

- Simplified control of particular biomechanical
features of the limb (global limb vs single joint

- Primitive solution to motor coordination

Front 59

How?

Back 59

Selection of interneuronal subgroups

Front 60

supraspinal structures

Back 60

- telencephalon/diencephalon structures

- brainstem pathways

Front 61

spinal networks

Back 61

- neural circuits (spinal interneurons)

- motoneuron pools

Front 62

afferent contributions

Back 62

muscle, cutaneous, joint receptors and pathways

Front 63

Telencephalon

Back 63

Motor cortex stimulation initiates locomotion

80% of corticospinal fibers modulate with locomotor activity (increased to flexors; Drew et al. 2002)

Humans: MEP modulation greater to TA than MG
(Schubert et al. 1997; Capaday et al. 1999)

Contribution of corticospinal tracts

Front 64

Contribution of corticospinal tracts

Back 64

bilateral pyramidotomy – temporary alteration in gross walking pattern (depends on lesion size)

dorsolateral spinal hemisection (Jiang and Drew 1996)
– toe drag; no ladder or beam walking
– inability to modulate stepping responses to first obstacle

Front 65

Telencephalon: Uses

Back 65

1. Stepping over obstaclesupon first appearance

2. Increase in cortical activity

Front 66

Diencephalon

Back 66

Behavioral context for locomotion (Sinnamon
1993)

Direct projections from these centers to brainstem locomotor regions

Front 67

Behavioral context for locomotion

Back 67

Exploratory system
– basal ganglia, hippocampus - inhibitory control

Appetite system
– lateral hypothalamus, perifornix
– “brings in contact with incentive and consummative stimuli”

Defense system
– Medial hypothalamus, periaqueductal gray
– “increase distance between threatening/painful stimuli”