the role of the nervous system in locomotion

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Control

The Role of the Nervous System in Locomotion

Dr Dan Dudek

North Cross School [email protected]

http://sites.google.com/a/northcross.org/dr-dan-dudek/

Outline

•  Nerves•  Structure of the Nervous System•  Role of Higher Centers•  Controlling Locomotion

» Reflexes» Central Pattern Generation (CPG’s)

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Neural Control

Sensory Receptors

Central Nervous System

Muscles

Peripheral Nervous System

Afferent

Efferent Motor Output

Sensory Input Higher Centers (Brain)

Nerve Cord

Central Pattern

Generator (CPG)

Neural Clock

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Neurobiology

What is the Functional Unit of

the Nervous System?

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Structure of Neuron

Axon

SPIKE INITIATION

TRANSMITTER SECRETION

Dendrite

Neuron is the

Functional Unit of the Nervous System

SIGNAL CONDUCTION

INTEGRATION Receive

signals from many

neurons

Send signals to other neurons

Soma

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Real Neuron

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Diversity Enables Discovery

“For a large number of problems there will be some animal of choice, on which it can be most conveniently studied.” ___ August Krogh

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Swimming Squid?

Foundations of Modern

Neurobiology Discovered using Squid

Axon

Tentacle Strike Video

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Swimming Squid = Nobel Prize!

The Nobel Prize in Physiology or Medicine – 1963 John C Eccles, Alan L Hodgkin, Andrew F Huxley

"for their discoveries concerning the ionic mechanisms involved in excitation and inhibition in the peripheral and central portions of the nerve cell membrane".

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Excitable Cells

Axon

SPIKE INITIATION

TRANSMITTER SECRETION

Dendrite Sum all continuous signals

Send discrete signals or spikes

Action Potentials

Soma

Monitor

Amplifier

0 mV _

_ +

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Signal transmission

Graded Passive Potential

Analog

All or none Action Potenital

Digital

All or none Action Potenital

Digital


Analog


Analog

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Neuromorphic Chip Computer Chips are typically digital in operation. 0 or 1

Dendrites of neurons are analog

(continuous)

New neuromorphic chips are part analog

(continuous)

Greater information transfer with less power!

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Functional Characterization

1. Afferent (to carry) - Sensory (skin, sense organs) to brain/spinal cord

2. Efferent (to carry away) - Motor (brain/spinal cord) to muscles, glands

3. Interneuron - conduction among neurons (in C.N.S.), integrate and store information from other neurons

Types of Sensory Receptors •  Electromagnetic and thermal energy

»  light»  infrared radiation»  thermal - heat and cold»  electro-magnetic

•  Mechanical energy»  sound and sonar»  touch and vibration»  pressure»  gravity»  inertia

•  Chemical energy»  taste»  smell»  humidity

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Energy Transduction

Withers, 1992

Deformation Temperature Taste Olfaction Light Voltage

Olfacto- receptor

Gustato- receptor

Thermo- receptor

Mechano- receptor

Photo- receptor

Electro- receptor

Stimulus

Stretch-mediated Na channel

Temperature Dependent Na/K channel enzymes

Phosphorylation-mediated Na/K channel

cAMP- mediated Na/K channel

GMP- mediated Na channel

Voltage dependent Na channel

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Receptor Potentials

Stimulus

Receptor Membrane

Chemical Synapse

Receptor Potential

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Frequency vs Intensity St

retc

h (D

ispl

acem

ent)

A

ctio

n Po

tent

ials

Increasing Stimulation

Frequency

Intensity

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Saturation vs Sensitivity

Stimulus IntensityResp

onse

Fre

quen

cy(A

ctio

n Po

tent

ials

/ sec

) Saturation

Slope = Sensitivity

High Sensitivity

Low Sensitivity

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Response vs Intensity

Stimulus Intensity

Resp

onse

Fre

quen

cy

Log Stimulus Intensity

Weber-Fechner Relationship

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Tonic Receptors

Log DisplacementResp

onse

Fre

quen

cy

Displacement Detector

Displacement

Action Potentials

TONIC

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Tonic vs Phasic Receptors

Log VelocityResp

onse

Fre

quen

cy

Velocity Detector

Log DisplacementResp

onse

Fre

quen

cy

Displacement

Action Potentials

Displacement Detector

TONIC PHASIC

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Range Fractionation

Stimulus IntensityResp

onse

Fre

quen

cy(A

ctio

n Po

tent

ials

/ sec

)Individual receptor cells sensitive over narrow range.

Group of receptors sensitive over broad range

Outline



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The Brain

Eckert

Motor Cortex Stimulate & Move

Sensory Cortex Stimulate & Feel

Brain sends commands for initiation and

navigation

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Structure of Nervous System

Nerve Net

Diffuse

Ganglia - cluster of nerve cell bodies

Directional Repeated in all segments

Condensation Specialization

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Nerve Cords

Invertebrate Ventral

Vertebrate Dorsal

Segmentation Condensation Specialization

Remarkable Similarities

X-section

Outline



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Neural Control

Sensory Receptors

Central Nervous System

Muscles

Peripheral Nervous System

Afferent


Sensory Input Higher Centers (Brain)

Nerve Cord

Central Pattern

Generator (CPG)

Neural Clock

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Motion Restoration

Many strategies are available depending on what is functional and

where in the path malfunction is present.

To Restore Function: 1.  Express cognitive control

over relevant motor functions in residual anatomy

2.  Device to pick up and decipher the cognition

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Pre-Motor Commands Plan motor events in the premotor cortex

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Direct Connections

A direct electronic interface is proving

useful in brain-computer interfaces

and opens the possibility of neural

prosthetics

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By-passing Spine and Limbs

Human Brain Machine

Interface is allowing “thought” control of prosthetic

limbs

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Brain-Machine Interface

Implant microwires into owl monkey

Measure signals as monkey performs reaching tasks

Analyze data to interpret signals

Wessberg et al., 2000 Nature

Real-time Decoding of Brain Signals

Signals are decoded using simple linear models and artificial

neural networks (ANN), predictions made and signals

sent to a robot arm locally and over the

internet

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Thought Control of Cursor

Monkey moves arm to track target. Brain

signals recorded and correlated with hand position. Brain signals alone used to track cursor - no

arm movement needed.

Serruya et al. 2002 Nature

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Thought Control of Arm

Velliste et al. 2008 Nature

Use motor cortical activity to control a mechanized arm in 3-D movement with proportion grasp

Outline



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Neurobiology

Two ways to control Biomotion:

1. Chain of reflexes 2. Motor programs (CPGs)

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Chain of Reflexes

Sensory Receptors

Central NS

Muscles

Afferent


Sensory Input

Chain of Reflexes

Neural Reflexes

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Golgi Tendon Organ

Ib Afferent Nerve

Muscle Spindle Organ

Ia Afferent Nerve

Extrafusal Muscle Fiber

Intrafusal Muscle Fiber

Efferent α-Motor Nerve

Length and Force Sensors

Muscle Spindle Fibers

Stretch activated sensors to detect

length AND velocity of muscle

Have there ownγ-motor neurons to

adjust length

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Spindle Fibers

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γ-motor neurons maintain sensitivity as length changes

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Neural Reflex Pathway

Length Sensors

Sense stretch and feedback signal to contract muscle to

shorten. Locomotion can provide rhythmic

signal.

Spinal Cord Cross-section

Interneuron

Length Sensor (muscle spindle)

Afferent Sensory Neuron

Efferent Motor Neuron

Withers

Pathways

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Interneurons relay sensory information to other levels (and

limbs) within the spinal cord, as well

as to opposing muscles within the

same limb

Pathways

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Spindle afferents synapse directly back onto motor

neurons of the same muscle, forming a

monosynaptic pathway which

facilitates a rapid motor response to

stretch of the muscle

Pathways

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Interneurons mediate reciprocal

inhibition (or activation) of

opposing sets of muscles within and

between limbs.

Examples - Pain Flexion and Crossed-

Extension Reflex

Noxious stimulus causes activation of flexor and

inhibition of extensor on “injured” limb

Opposite limb activates extensors and inhibits

flexors to maintain stance

Ouch! 50

Examples - Stretch

Tendon Stretch Reflex

Unexpected stretch causes activation of flexor and inhibition of extensor

Maintains original joint angle

Tap! 51

Golgi Tendon Organs

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http://www.ualberta.ca/~aprochaz/research_spindle_intro.html

Stretch activated sensors to detect force of muscle

Tends to inhibit muscle contraction,

protects from developing too high

force

GTO Firing Rate and Force

Linear increase in firing rate is unusual

for neurons

Good for negative feedback control of

fine forces

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Force (N)

Ib A

ffere

nt F

iring

Rat

e (H

z)

GTO’s Are Team Players

The Ib inhibitory interneurons receive

convergent input from GTO’s, muscle

spindles (not shown), joint and cutaneous

receptors, and descending pathways

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GTO’s Don’t Just Inhibit At rest, stimulation of Ib afferent fibers from

the ankle extensor muscle inhibits ankle

extensor motor neurons

During locomotion the same stimulus excites the motor neurons (via

polysynaptic excitatory pathways)

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The Motor Servo

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Descending Commands α a. p. Motor

Unit force Load Kinematics

-position -length

-velocity

-

GTO

Spindles

Adjusting Stiffness

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-position -length

-velocity

- GTO

Spindles

Hypothesis Motor Servo acts to

adjust muscle stiffness

Reciprocal Inhibition

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-position -length

-velocity

- GTO

Spindles

Motor Servo reciprocally

inhibits opposing

muscles and limbs

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Neurobiology

Two ways to control Biomotion:

1. Chain of reflexes 2. Motor programs (CPGs)

the role of the nervous system in locomotion

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