Transcript Chapter 4. Neuromotor Basis for Motor Control

Chapter 4
Neuromotor Basis for Motor Control
Concept: The neuromotor system forms the
foundation for the control of movement
Introduction
The Neuromotor System
– Components of the central nervous system (CNS)
and peripheral nervous system (PNS) involved in the
control of coordinated movement
Focus of current chapter is CNS structure and
function
– Chapter 6 will include PNS related structure and
function for tactile, visual, and proprioceptive sensory
systems
The Neuron
Neuron = Nerve cell
– Basic component of
the nervous system
– Range in size from 4
to 100 microns
General Structure [see Fig.
4.1]
– Cell body
Contains nucleus
– Dendrites
Extensions from cell body –
range from 1 to thousands
per neuron
Receive information from
other cells
– Axon (also known as a
“nerve fiber”)
Extension from cell body –
one per neuron with
branches (known as
collaterals)
Sends information from
neuron
Types and Functions of
Neurons
Three Types of Neurons
1. Sensory Neurons [see Fig. 4.2]
– Also known as “afferent”
neurons
– Send information to CNS from
sensory receptors
– Unipolar – 1 axon; no
dendrites
– Cell body and most of the axon
located in PNS; only axon central
process enters CNS
Types and Functions of Neurons,
cont’d
2. Motor Neurons [see Fig. 4.2]
–
–
Also known as “efferent” neurons
Two types influence voluntary movement:
1. Alpha motor neurons
Predominantly in spinal cord – axons synapse on
skeletal muscles
2. Gamma motor neurons
In intrafusal fibers of skeletal muscles
Types and Functions of Neurons,
cont’d
3. Interneurons [see Fig. 4.2]
– Specialized neurons that originate and
terminate in the brain or spinal cord
– Function as connections between:
Axons from the brain and synapse on motor
neurons
Axons from sensory nerves and the spinal nerves
ascending to the brain
Central Nervous System
(CNS)
Two components: Brain and spinal cord
The Brain
4 structural components most directly involved in the
control of voluntary movement:
1. Cerebrum
2. Diencephalon
3. Cerebellum
4. Brainstem
Brain Components: 1.
Cerebrum
One of two components of forebrain
Two halves
– Right cerebral hemisphere
– Left cerebral hemisphere
Covered by cerebral cortex
– Gray tissue; 2- to 5-mm thick
– Undulating covering of
Ridges – each is called a gyrus
Grooves – each is called a sulcus
– Cortex motor neurons
Pyramidal cells
Nonpyramidal cells
Connected by the
corpus callosum
Cerebral Cortex
Four lobes
–
–
–
–
Frontal
Parietal
Occipital
Temporal
Named according to
nearest skull bone
Sensory cortex
– Posterior to central sulcus
– Receives neuron axons
specific to type of sensory
information
Cerebral Cortex, cont’d
Association areas [see Fig. 4.4]
Location
– Adjacent to specific sensory areas of sensory cortex
Function
– To “associate” information from the several different sensory cortex areas
– Allow the interaction between perceptual and higher-order cognitive
functions
e.g., selection of the correct response in a choice-RT situation
– Possible locations for transition between perception and action
Cerebral Cortex, cont’d
 Primary motor cortex
Location & Structure
– Frontal lobe just anterior to central
sulcus
– Contains motor neurons that send
axons to skeletal muscles
Function
Involved in control of:
– Initiation and coordination of
movements for fine motor skills
– Postural coordination
Cerebral Cortex, cont’d
Premotor area
– Location: Anterior to the primary motor cortex
– Functions include
Organization of movements before they are initiated
Rhythmic coordination during movement
-- enables transitions between sequential
movements of a serial motor skill (e.g. keyboard
typing, piano playing)
Control of movement based on observation of
another person’s performing a skill
Cerebral Cortex,
cont’d
Supplementary motor area (SMA)
– Location: Medial surface of frontal lobe
adjacent to portions of the primary motor
cortex
– Functions include involvement in the
control of
Sequential movements
Preparation and organization of
movement
Cerebral Cortex,
cont’d
Parietal lobe
Location
– One of the 4 lobes of the cerebral cortex
Function
– Involved in the integration of movement
preparation and execution
Interacts with the premotor cortex, primary
motor cortex, and SMA before and during
movement
Subcortical Brain Area
Important in Motor Control
Basal Ganglia
– Buried within cerebral hemispheres
– Consist of 4 large nuclei
- Receive info from
Caudate nucleus
cerebral cortex and
Putamen
brainstem
Substantia nigra
- Send info to brainstem
Globus pallidus
– Function involves control of
Movement initiation
Antagonist muscles
during movement
Force
Basal Ganglia, cont’d
Parkinson’s Disease
– Common disease associated with basal
ganglia dysfunction
Lack of dopamine production by substantia nigra
– Motor control problems [BART]
Bradykinesia (slow movement)
Akinesia (reduced amount of movement)
Rigidity of muscles
Tremor
Brain Components:
2. Diencephalon
2nd component of forebrain
Contains two groups of nuclei
– Thalamus
Functions:
– A type of relay station - receives and integrates sensory info from
spinal cord and brainstem; sends info to cerebral cortex
– Important role in control of attention, mood, and perception of pain
– Hypothalamus
Critical center for the control of the endocrine system and
body homeostasis
Brain Components: 3.
Cerebellum
Location: Behind cerebrum and attached to
brainstem [See Fig. 4.3]
Structure includes
– Cortex covering
– Two hemispheres
– White matter under the cortex contains
Red nucleus – Where cerebellum’s motor neural pathways
connect to spinal cord
Oculomotor nucleus
Brain Components: 3. Cerebellum
cont’d
Functions
– Involved in control of smooth and accurate
movements
Clumsy movement results from dysfunction
– Involved in control of eye-hand coordination,
movement timing, posture
– Serves as a type of movement error detection and
correction system
Receives copy of motor neural signals sent from motor cortex
to muscles (efference copy)
– Involved in learning motor skills
Brain Components: 4. Brainstem
Location
Beneath cerebrum;
connected to spinal cord
3 components involved
in motor control
– Pons
– Medulla
– Reticular formation
Functions
Pons
– Involved in control of various body
functions (e.g. chewing) and balance
Medulla
– Regulatory center for internal
physiologic processes (e.g.
breathing)
Reticular formation
– Integrator of sensory and motor
info
– Inhibits / Activates neural signals
to skeletal muscles
Spinal Cord
A complex neural system vitally involved in motor
control
Structure [See Fig. 4.5]
– Gray matter – H-shaped central portion
Consists of cell bodies and axons of neurons
Two pairs of “horns”
– Dorsal (posterior) horns – Cells transmit sensory info
– Ventral (anterior) horns – Contains alpha motor neurons with
axons terminating on skeletal muscle
Interneurons (Renshaw cells) – In ventral horn
Sensory Neural Pathways
Several neural tracts (called ascending tracts)
– Pass through spinal cord and brainstem
– Connect to sensory areas of cerebral cortex and cerebellum
2 tracts to sensory cortex especially important for motor
control
– Dorsal column
– Anterolateral system
Tract to cerebellum important for motor control
– Spinocerebellar tract – Primary pathway for proprioceptive info
Motor Neural Pathways
Descending tracts
– Travel from brain through spinal cord
Pyramidal tracts (corticospinal tracts)
– 60% from motor cortex
– Most fibers cross to other side body (decussation) in medulla of
brainstem
– Involved in control of fine motor skill performance
Nonpyramidal tracts (brainstem pathways)
– Fibers do not cross to other side of body
– Involved in postural control and control of hand and finger
flexion – extension
The Motor Unit
An alpha motor neuron and all the skeletal muscle
fibers it innervates [See Figure 4.6]
– When a motor neuron activates (fires) all its connected muscle
fibers contract
The ultimate end of the motor neural information
~ 200,000 alpha motor neurons in spinal cord
– Number of muscle fibers served by a motor unit depends on
type of movement associated with the muscle
Fine movements
e.g. eye muscles = 1 fiber / motor unit
Gross movements
e.g. posture control = many fibers (up to ~ 700) / motor unit
Motor Unit Recruitment
Amount of force generated by muscle contraction
depends on number of muscle fibers activated
– To increase force, need more motor units
Process of increasing number of motor units involved =
recruitment
Recruitment follows “size principle”
– Size = motor neuron cell body diameter
– Size principle = recruit smallest motor units first (i.e., weakest
force produced) then systematically increase size recruited until
achieve desired force
From Intent to Action: The Neural
Control of Voluntary Movement
Think about the entire process of deciding to
perform a skill and actually performing it
The neural activity involved in this process
typically follows a hierarchical organization
pattern
– From higher to lower levels of the neuromuscular
system
This process is described conceptually in
Figure 4.7 and Table 4.1
Neural Control of Voluntary
Movement
1. Higher centers
– Function - Form complex plans according to intent,
communicates with the middle level via command neurons.
– Structures – areas involved with memory and emotions, SMA,
associations cortex
2. Middle level – Function – converts plans to a number of smaller motor
programs which determine the pattern of neural activation
required.
– Structures – sensorimotor cortex, cerebellum, basal nuclei,
brainstem nuclei
Lowest level
– Function – specifies tension of particular muscles and angle of
joints at specific times necessary to carry out programs from
middle control level
– Structures – brainstem or spinal cord
From Intent to Action: Brain Structures
Associated with Movement
Research by Carson and
Kelso (2004)
Demonstrated there is more
involved in understanding
how we control voluntary
coordinated movement than
knowing which brain
structures involved in which
type of movements
– Cognitive intention is a
critical component
Experiment
Participants performed fingerflexion movement to a
metronome
– On the beat (synchronize)
– Between beats (syncopate)
Task involved exactly the same
movement but two different
cognitive intentions
fMRI results showed
– Different brain regions active for
the two movement intentions