5. motor functions + spinal reflexes

Voluntary movement requires:

complex interaction of the corticospinal (pyramidal) tracts, basal ganglia, and cerebellum to
ensure smooth, purposeful movement without extraneous muscular contractions

Skeletal motor axis

Projection from motor cortex to motor neuron

Receives extensive input from thalamus, brainstem and cerebellum

Motor Cortex

Divided into 3 sub areas

1. Primary motor cortex

2. Premotor area

3. Supplemental motor area
   

Motor Cortex: Primary motor cortex

Unequal topographic representation (some areas receive more cortical space and attention than others)

Fine motor movement elicited by stimulation

Motor Cortex: Premotor area

Stimulation results in movement of muscle groups (patterns) to perform a specific task

Works in concert with other motor areas

Motor Cortex: Supplemental motor area

Functions in concert with premotor area to provide attitudinal, fixation, or positional movement for the body

Provides background for fine motor control of the arms and hands by premotor and primary motor cortex

Transmission of Cortical Motor Signals: Direct pathway

Corticospinal tract (pyramidal)

Directs discrete detailed movement (fine-skilled motor behavior)

Transmission of Cortical Motor Signals: Indirect pathway (extrpyramidal)

Modulate signal from corticospinal tract to motor neuron

Integrate sensory and posture information

Signals through basal ganglia, cerebellum, and brainstem nuclei

Trunk musculature

Upright posture

Locomotion

Orienting responses

Giant pyramidal cells (Betz cells)

originate in primary motor cortex and give rise to large fibers with fast transmission rates

Cross in pyramid of medulla and descend spinal cord in corticospinal tracts

Synapse with interneurons or directly with motor neurons

antigravity muscles

muscles of the spinal column and the extensor muscles of the legs support the body against gravity

These muscles are under the influence of brainstem nuclei

Pontine reticular nuclei VS Medullary reticular nuclei

Pontine reticular nuclei excite the antigravity muscles

Medullary reticular nuclei inhibit the antigravity muscles

Cerebellum (little brain)

Responsible for coordinating and sequencing muscle activity

Compares actual motor movements with intended movements

Makes corrective changes

Cerebellum: Afferent signals from motor and cortex:

transmits information about intended motion

Cerebellum: Afferent signals from muscle spindles and Golgi tendon organs (proprioception)

Transmits information about status of muscle contraction/tension and limb position

Cerebellum: Efferent signals

Pathways involved in equilibrium control and coordinating opposing influences on motor neurons

Basal Ganglia

Four nuclei involved in control of complex patterns of motor activity

Receives input from primary, premotor, supplemental and prefrontal cortex

Inhibitory output to cortex

Suppression of conflicting motor activity

functions of basal ganglia

Postural stability: with damage, the head falls over chin and the person walks bent over at the waist

Motor programs: converts an overall goal into programs which determine the specific movement

Parkinson disease – inability to initiate movements

“Extrapyramidal symptoms”

“Extrapyramidal symptoms”

involuntary movements

Lack of smoothness in their movements

Tremor

Difficulty in the initiation of a movement

Difficulty maintaining an upright posture

Many of the symptoms of Parkinson’s disease are classic extrapyramidal symptoms

summary of motor control

Cortical level

Issues commands to set into motion the patterns available in the spinal cord
Control the intensity and modifies the timing

Brainstem level

Maintains equilibrium by adjusting axial tone

Cerebellum

Function with all levels of control to adjust cord motor activity, equilibrium, and planning of motor activity

Basal ganglia

Functions to assist cortex in executing subconscious but learned patterns of movement, and to plan sequential patterns to accomplish a purposeful task

Spinal cord level

Preprogramming of patterns of movement of all muscles (i.e., withdrawal reflex, walking
movements, etc.)

Neuronal circuits

for walking and various reflexes

contained within the spinal cord

Higher brain centers activate and command these circuits

walking + maintaining equilibrium

Motor neuron cell body receives inhibitory and excitatory inputs from:

1. Afferent neurons
2. Spinal cord interneurons
3. Cortex via descending tracts

Motor neuron output at synapse is _________ to the muscle fiber

ALWAYS excitatory

Sensory fibers

enter cord and are transmitted to higher centers, or they synapse locally to elicit motor reflexes

Motor neurons

located in the anterior portion of the cord

50 - 100% bigger than other neurons

Interneurons

30x more abundant than anterior motor neurons

Small and highly excitable

Most signals from brain terminate on interneurons

Comprise neural circuitry for motor reflexes

Propriospinal fibers

Travel up and down cord for 1 - 2 segments

Provide pathways for multi-segmental reflexes

Important for proprioception

Motor unit

single motor neuron and all associated muscle fibers

Alpha motor neurons

Large type Aα fibers (~14 microns)

Stimulation can excite 3 to >100 extrafusal muscle fibers (Motor unit)

Gamma motor neurons

Smaller type Aγ fibers (~5 microns)

Stimulation excites intrafusal fibers in muscle spindle

Maintains tone

how are α-motor neurons and γ-motor neurons activated during a muscle contraction?

co-activated

Extrafusal fibers

Make up bulk of muscle

Stimulated by alpha motor neurons

Provide force for muscle contraction

Intrafusal fibers

Smaller than extrafusal fibers

Encapsulated in sheaths to form muscle spindles

Run in parallel with extrafusal fibers, but much shorter (~10 mm)

Sensory Receptors of Muscle

Muscle Spindle + Golgi Tendon Organ

Signals from muscle sensory receptors are (mainly) for intrinsic muscle control.
mainly occur subconsciously.

transmit information to cerebrum and cerebellum (as well as to spinal cord).

Sensory Receptors of Muscle: Muscle Spindle

Located in muscle belly

Senses muscle length and rate of change in length

Detects both static and dynamic changes in muscle length

Consist of intrafusal muscle fibers (3 to 12 per spindle) in parallel with extrafusal muscle
fibers

Intrafusal fibers (3 to 12 per spindle)

Do not contribute to muscle tension (Functions as sensory receptor only)

Innervated by gamma motor neurons (type Aγ)

Two types of intrafusal fibers: Bag + Chain

Stretch Reflex (myotatic reflex)

Sudden stretch of muscle stimulates receptors in muscle spindle

Afferent neuron synapses with α-motor neuron and inhibitory interneuron

Causes contraction of stretched muscle and inhibition of opposing muscle

Opposes further stretch of muscle

MONOSYNAPTIC REFLEX ARCH

Myotatic (stretch) reflex in the jaw

Helps determine resting position of mandible

Increases tone of elevator muscles to counteract effects of gravity

Sensory Receptors of Muscle: Golgi Tendon Organ

Low-threshold mechanoreceptors located in tendon

Senses tendon tension and rate of change in tension

Increased muscle tension compresses nerve endings, opens stretch-sensitive ion channels

Function is to equalize force among muscle fibers

Only affects an individual muscle (adjacent muscles are not affected)

Autogenic inhibition reflex

a sudden relaxation of muscle at very high muscle tensions (protects against muscle tear)

However, Golgi tendon organs signal muscle force through the entire physiological range,
not only at high levels of tension

Inverse stretch reflex (Golgi tendon reflex)

Protective: prevent muscle damage

DISYNAPTIC reflex arc

Golgi tendon organ: tension receptor (NOT length)

In series with the extrafusal muscle fibers

Stimulus: excessive contraction of the muscle

Sensory afferent synapses with an inhibitory interneuron

Hyperpolarizes the α motor neuron to the extensor muscle and causes muscle to relax

Transmission of muscle length and tension information to brain

Signal from Muscle Spindle and Golgi tendon are also transmitted to higher centers (not just the spinal cord)

This informs brain of instantaneous changes in muscle tension and length

Information is transmitted at 120 m/sec

Important for feedback control of motor activity

Flexor Withdrawal and Crossed Extensor Reflex

Withdraws a limb from a painful stimulus

Activated by Nociceptors (pain afferent)

Polysynaptic reflex arch

Reciprocal innervation: to obtain movement around a joint, need to contract flexors and inhibit extensors

Painful stimulus causes combined reflex:

Withdrawal

• Ipsilateral (same side)

• Contract flexors

• Inhibit extensors

Crossed extensor

• Contralateral (opposite side)

• Inhibit flexors

• Contract extensors

Withdrawal reflex in the jaw

Withdrawal reflex in the jaw

can be initiated by any painful stimulus in the oral cavity

Also called jaw-opening reflex or nociceptive reflex

Noxious stimulus

e.g.: biting down on popcorn kernel

causes inhibition of elevator muscles and stimulates jaw-opening muscle

Other Reflexes for Posture and Locomotion

Pressure on bottom of feet causes extensor reflex

more complex than flexor-crossed extensor reflex

Basic walking reflexes reside in spinal cord