The Motor System
General Overview
Sensory systems transform physical energy into neural information
Motor systems transform neural information into physical energy by issuing commands that are transmitted by the brain stem and spinal cord to skeletal muscles.
Muscles translate this neural information into a contractile force that produces movements.
The proper moment to moment functioning of motor systems depends upon a continuous flow of sensory information.
Vision, hearing and receptors on the body surface inform us about where objects are in space and our own position relative to them.
Proprioceptors (sensory receptors) in the muscles, joints and vestibular apparatus inform the motor systems about the length and tension of muscles, the angles of the joints and the position of the body in space.
This info is essential for planning movements and refining those that are in progress.
Components of Motor Systems
Muscles, spinal cord, brain stem, motor cortex, premotor and supplemental areas of the cortex.
Non cortical areas - cerebellum and basal ganglia play important role in motor function.
Serial (hierarchical) organization ù the highest motor levels influence the levels below them e.g. premotor area provides input to the motor cortex which provides input to the brain stem and so on.
Parallel organization - Info from the motor cortex and brain stem can simultaneously and independently affect the spinal cord.
Targets of all outgoing info are the motor neurons in the ventral horn of the spinal cord.
Muscles:
(i) cardiac/ heart muscle
(ii) smooth muscles surround blood vessels and a variety of other organ systems. Primarily under the control of the autonomic nervous system.
(iii) Skeletal (striated ) muscles responsible for locomotion, fine movement, balance and posture
Skeletal muscle is made up of muscle cells (muscle fibers)
Muscle fibers are multinucleate
Two types of muscle fibers
(i) extrafusal fibers - most common and strongest fibers that form the exterior of the muscle.
(ii) intrafusal fibers - located inside the muscle parallel with the extrafusal fibers
Types and actions of skeletal muscle
Muscle tendons - Strong connective tissue. Where muscles attach to bones.
Two points of attachment : insertion and origin
Flexor muscle - a muscle that by contracting causes the joint to close up
Extensor muscle - a muscle that by contracting causes the joint to stretch out.
Synergistic muscles - a flexor for a joint is synergistic to another flexor for the same joint.
Antagonistic muscles ù A flexor and extensor muscle for the same joint are antagonistic e.g bicep and tricep muscles
Fast skeletal muscle - contracts rapidly.
poorly supplied with blood vessels
operate under anaerobic conditions
fatigue rapidly
suited for physical activity e.g. sprinting
Slow skeletal muscle - contract slowly
Operate under aerobic metabolism
More resistant to fatigue
Suited for maintaining posture
Contraction of skeletal muscles necessary for movement controlled by alpha motor neurons located in the ventral horn of the spinal cord
Large diameter (up to 70 mm) with axons of 12-20 mm diameter
Form synaptic connections with skeletal muscles at neuromuscular junctions (NMJ)
Innervate extrafusal fibers
(ii) Gamma Motor Neurons ù
Fusimotor system
Small cells with axons of 1-8 mm diameter
Innervate intrafusal muscle fibers - see later for info about function
Motor neurons branch close to their targets, the muscle fiber.
Allows a single motor neuron to innervate many muscle fibers at one time.
The motor unit is the basic functional unit of skeletal muscle
It consists of the alpha motor neuron, its motor axon and all the skeletal muscle fibers it supplies.
Each skeletal muscle fiber is supplied with just one alpha motor neuron that can innervate a variable number of skeletal muscle fibers depending upon the degree of control the muscle requires.
small motor unit - a motor neuron innervates few muscle fibers.
controls very fine movements e.g. contraction of a finger
large motor unit - a motor neuron innervating many muscle fibers.
responsible for gross movements, maintenance of posture
The Neuromuscular Junction
At NMJ ù neurotransmitter released by motor axons - acetylcholine
ACh receptor on surface of skeletal muscle membrane: nicotinic receptors
Activation of ACh receptors leads to opening on channels selective for Na and K. Results in depolarization of the membrane resulting ultimately in contraction of the muscle
Motor neurons that innervate fast muscles discharge high frequency action potentials and have fast action potential conduction velocities.
Motor neurons that innervate slow muscles
discharge low frequency action potentials and have slow
conduction velocities.
Frequency code of contractile force ù the frequency of action potentials generated by the motor neuron controls the tension developed by a single muscle fiber.
The more motor neurons and consequently the more motor units that are activated the greater a muscleùs force of contraction - the population code of contractile force
Motor neurons organized in two ways in spinal cord ventral horn:
Motor neurons innervating a single muscle are functionally grouped in motor neuron pools.
Motor neurons in dorsal portion of ventral horn ù responsible for flexor movement
Motor neurons in ventral portion of ventral horn ù responsible for extensor movement.
Motor neurons in dorsolateral region of spinal cord ù innervate muscles in extremities.
Motor neurons in ventromedial portion ù innervate axial muscles of body to maintain posture.
Located in the intermediate zone of the spinal cord
Form synapses with alpha motor neurons and mediate excitatory and inhibitory actions.
Receive inputs from sensory nerves and other motor centers
Can act (i) locally within the same spinal segment or
(ii) can send axons up and down the cord - information transmitted to other spinal levels.
Excitatory interneurons are associated with the activation of synergistic muscles
Inhibitory interneurons are associated with suppressing the activation of
antagonistic muscles
Reflex Pathways
Components of Reflex Arcs
(i) Sensory receptor
Sensory neuron associated with that receptor send info to the spinal cord
(ii) The integration center (a) synapse between sensory and motor neuron
(monosynaptic)
(b) synapse between sensory neuron and interneurons (polysynaptic)
(iii) The motor neuron
(iv) The effector organ (muscle or gland). Receives input from the motor neuron
Spinal cord reflexes are the most basic of motor responses.
Reflexes are carried out entirely within spinal cord
May be modified by descending inputs from higher centers.
(i) Muscle Spindles Length detectors. (Fig 13-3
and 13-4 )
Sensory fibers innervating muscle fibers transmit info regarding the state of contraction and the tension in muscle fibers to the CNS. Thus the CNS can monitor the muscle contraction and it can revise its instructions as necessary.
Muscle spindle is a sensory receptor that is activated by stretch.
Muscle spindles are formed by sensory fibers that entwine intrafusal muscle fibers.
The muscle spindle has a fusiform shape and is arranged in parallel with extrafusal fibers (the skeletal muscle fibers that mediate contraction of the muscle).
The sensory spindle fibers entwined around the intrafusal fibres are tonically active i.e. fire action potentials when the muscle is at its resting length.
These sensory fibers synapse with alpha motor neurons in the spinal cord producing tonic excitation and contraction of the muscle extrafusal fibers ù gives rise to muscle tone whereby a muscle maintains a certain level of tension even at rest. (Fig. 13-4a)
Spindle fibers activity (i.e. rate of action potential production) is linked to the length of the intrafusal fibres of the spindle.
When intrafusal fibers stretch as a result of stretching the muscle, the sensory fibers increase their firing rate.
How?
Stretching of spindle lengthens the central region of the intrafusal fiber around which the sensory fibers are entwined.
Leads to activation of stretch sensitive channels that depolarize the membrane and generates action potentials.
This rapid firing of the muscle spindle fibers initiates reflex muscle contraction , relieving stretch on the muscle and preventing damage from overstretching the muscle (Fig. 13-4b)
The Gamma Motor neurons control the sensitivity of Muscle Spindles During
Gamma motor neurons innervate contractile elements at the poles of the muscle spindles.
Activation of these contractile elements causes the central region of the muscle spindle to stretch like a rubber band.
The parallel arrangement of muscle spindles relative to the extrafusal fibers raises a problem:
Because the intrafusal fibers slacken when a muscle shortens, the spindle discharge will cease when the muscle shortens.
But if this happens the muscle spindle would fail to transmit info about changes in muscle length to the CNS at the very time when the info is most critical i.e. when the contracting muscle is shortening.
Activation of gamma motor neurons ensures that the muscle spindle continues to fire action potentials during muscle contraction.
How? (Fig. 13-5b)
Activation of gamma motor neurons (that innervate contractile elements at the poles of the muscle spindles) causes the central region of the muscle spindle to stretch
Thus activation of the gamma motor neuron prevents the muscle spindle from slackening during shortening of the extrafusal fibers i.e. it maintains the tension of the intrafusal fibers of the muscle spindle.
In this way, the sensory fibers fire
continuously in response to muscle shortening
Information from the muscle spindles is transmitted to the spinal cord where it is
(i) used for reflex actions
(ii) forwarded to the somatic sensory cortex
Located in series with the extrafusal muscle fiber
As the muscle contracts and a greater tension develops in the muscle the Golgi tendon organ firing rate increases.
Activation of Golgi tendon organs inhibits alpha motor neurons and decreases muscle contraction
Under normal circumstances, the Golgi tendon organ functions to slow muscle contraction as the force of contraction increases.
Also provides a protective function ù see later.
Sensory input from Golgi tendon organs about the tension produced by muscles is useful for a variety of motor acts such as maintaining a steady grip on an object.
(i) Stretch Reflex also called the knee jerk reflex. (Fig 13-7)
Sensory receptor involved: muscle spindles.
An example:
Tap the patellar tendon below the knee.
Muscle spindles are stretched and activated.
Action potentials conducted along the sensory afferent fiber to the spinal cord.
On entering the spinal cord through the dorsal root the sensory fiber branches
(i) one branch enters the dorsal column pathway
(ii) the other branch synapses with motor neurons in the ventral horn (monosynaptic reflex)
Motor neurons in spinal cord activate muscle fibers in the leg causing leg to extend quickly out.
Two things have to happen for leg extension
(i) leg extensor fibers must be activated
(ii) leg flexor muscles must be inhibited.
To produce activation of extensor fibers, sensory fibers synapse with motor neurons innervating the appropriate muscle.
To produce inhibition of antagonistic flexor muscle sensory fiber synapses with inhibitory interneuron which then synapses onto motor neuron innervating flexor muscle and inhibits activity in the muscle.
(ii) The Inverse Myotatic reflex (Fig13-6)
A reflex that involves the Golgi tendon sensory organ.
A protective mechanism seen when a person tries to carry too much weight e.g. weight lifters lifting bar bells
Bar bell is lifted by contracting the biceps muscle.
If weight is too great for biceps to lift, the biceps relaxes and the bar bells are dropped.
How?
The Golgi tendon organs detect the excessive force in the muscle and discharge action potentials.
Sensory info from Golgi tendon organ enters spinal cord.
Sensory fiber branches.
(i) one branch enters dorsal column pathway
(ii) other branch synapses with inhibitory interneurons in the spinal cord which inhibit the motor neuron activating the biceps muscle.
Also
Interneurons (i) activate motor neurons that innervate extensor muscles that are antagonistic to the biceps
(ii) inhibit motor neurons that innervate flexor muscles that are synergistic with the biceps muscle
(iii) Net Result ù the arm extends and drops the weight
A reflex that involves cutaneous sensory receptors
Example:
When you step on a tack, pain receptors on the bottom of the foot transmit info to the spinal cord.
Sensory fibers activate motor neurons innervating flexor muscles while inhibiting motor neurons innervating extensor muscles. Result you lift your foot up away from the tack.
But body still has to be supported so the flexor muscles in the opposite leg are inhibited while the extensor muscles are activated.
Motor cortex located rostral to the somatic sensory cortex
Cells in the motor cortex are organized in a somatotopic manner similar to that of
the somatic sensory cortex
Also, parts of body involved in fine movement e.g. fingers , occupy more space than parts of body involved in gross bodily movement e.g. torso
Cells of motor cortex are arranged in functional columns ù cortical efferent zones.
All cells in one efferent zone are involved in the contraction of a given muscle.
Also organized horizontally into 6 layers.
Info from motor cortex to spinal cord transmitted via corticospinal pathway
Axons descending from the motor cortex form
the Pyramidal tract.
Massive bundle of axons contains approx. 1 million axons
At the level of the brain stem, the MAJORITY of the fibers that continue to the spinal cord cross the midline.
These fibers enter the lateral corticospinal tract and innervate motor neuron pools in the spinal cord that control distal limb movement.
The minority of fibers that do NOT cross the mid line at the level of the brain stem descend in the ventral corticospinal tract.
They innervate motor neurons in the medial region of the ventral horn associated with axial muscles of the body (posture)
Three basic functions:
(i) planning of a movement
(ii) control of posture and equilibrium
(iii) control of smooth limb movement
Involved in planning of movement.
Interconnections between basal ganglia and substantia nigra play prominent role in diseases of the motor system e.g. parkinsionism
Parkinsonùs disease is associated with loss of the neurotransmitter dopamine in neurons of the basal ganglia
The Brain in the Motor Response System
(Fig 13-10)
Three levels involved in control of movement: spinal cord, brain stem and motor areas of the cerebral cortex
(i) Simplest movements involve spinal reflexes e.g. stretch reflex etc.
Spinal reflexes may or may not be modulated by descending inputs from higher regions of the CNS.
(ii) Voluntary movements require participation of the cerebral cortex, cerebellum and the basal ganglia (Fig 13-11)
Voluntary movement divided into three phases:
Planning
Involves exchange and coordination between cortical association areas, the basal ganglia and the cerebellum
Initiation
Motor cortex is responsible for the initiation of movement
Execution
Carried out by descending pathways that impinge on somatic (alpha) motor neurons
Different parts of the motor system have distinctive role therefore lesions of different areas result in characteristic movement disorders
Damage to descending pyramidal fibers.
Characterized by paralysis on side opposite lesion (majority of fibers cross over mid line)
Also increase in muscle tone, hyperactive reflexes, extension of the big toe and fanning of other toes in response to stroking the bottom of the foot (The Babinski sign).
Muscle atrophy not prominent.
Occur within spinal cord, disrupt inputs to motor neurons
Effects are ipsilateral
Characterized by loss of strength and movement in muscle groups
Loss of strength in voluntary muscle contraction
The Babinski sign
Occur with spinal cord injury that affects motor neurons directly not inputs to motor neurons.
Characterized by ipsilateral hypoactive reflexes, paralysis limited to specific groups of muscles
Flaccid muscles with prominent atrophy.
Amyotrophic Lateral Sclerosis (ALS or Lou
Gehrig disease)
Devastating, progressive degenerative disease affecting the motor system.
Cause remains unknown
Neuropathologic findings include degeneration of motor neurons in the cortex, lower brain stem and spinal cord.
Weakness and wasting of the upper extremities usually occurs followed by impaired speech, swallowing and respiration.
Mean survival time after diagnosis about 3 years.
Lead to disturbances in motor coordination.
Produce ipsilateral disturbances.
Characterized by inaccurate range and direction of movement.
Inability to rapidly move a limb and to stop sharply
Tremor with movements.
Careful chronic consumption of alcohol affects the cerebellum!