Motor System (Part 2)

Proprioception

sixth sense

ability to sense one’s body position in space

feedback/feedforward loop bw the muscle spindles, golgi tendon organs, joint receptors, and the cerebellum

visual system

humans are visual cues to negotiate the environment

vestibular system

The labyrinthine system in the inner ear—the semicircular canals—contains continuously moving liquid that is constantly monitored by this system to provide feedback regarding the head’s position in space.

also has neural connections to the cerebellum to provide feedback about the head’s position in space.

muscle spindle

proprioceptors located in skeletal muscle.
sensory receptors that provide a constant flow of information regarding length, tension, and load on the muscles.
detect when a muscle has been stretched and initiate a reflex that resists the stretch

extrafusal muscle fibers

bulk of the muscle

intrafusal muscle fibers

the MSs that sit within the bulk of the muscle

attached to the extrafusal muscle fibers

Density of Muscle Spindles in an Extrafusal Muscle Fiber

more MSs in an extrafusal muscle fiber = greater the muscle’s precision control

muscles w the highest density of MSs are small muscles designed for fine motor control

the nuclear chain and bag are structures within the MS

Nuclear bag

is responsive to changes in muscle length

when the length of the muscle changes, it fires

detects the velocity of the muscular stretch, or how quickly the muscle streched

nuclear chain

only fires in response to a new muscle length; it does not respond to velocity, like the nuclear bag

Properties of the Equatorial Regions of the Nuclear Bag and Chain

nuclear bag: elastic and phasic = quick responding

chain: elastic and tonic = slow responding

properties of the Polar Regions of the Nuclear Bag and Chain

bag: contractile and tonic: slow responding

chain: contractile and phasic: quick responding

Ia (Primary Ending)

large and heavily myelinated.
They are fast conducting

fibers wrap around the equatorial region of both the bag and chain.
fibers respond to the rate of muscle stretch (velocity) and to changes in muscle length.

are fast adapting (phasic).

II (Secondary Ending)

medium-size fibers

terminate on the equatorial region of the chain only.
are predominantly located on the chain.
respond to changes in the length of the MS (the rate of the stretch is not involved).
are slow adapting (tonic).

MUSCLE SPINDLE SEQUENCE OF EVENTS

The extrafusal muscle fiber stretches.
This causes the MS (the intrafusal muscle fibers) to stretch.
The equatorial region of the bag stretches right away because the equatorial region of the bag is elastic and responds more to stretch than does the polar region. (If the stretch is a sustained stretch, the equatorial region of the chain will also stretch.)
Stretching of the MS causes the Ia fibers to fire.
The Ia fibers will fire in response to a quick or phasic response because the Ia fibers from the bag are phasic.
The Ia fibers will also fire in response to a tonic or sustained and slow response because the Ia fibers from the chain are tonic.
The secondary fibers, the II fibers, then fire.
The II fibers are attached only to the chain, and the chain only detects length and position changes.
The II fibers are tonic and respond to a slow, sustained stretch

Golgi tendon organs (GTOs)

embedded in the tendons, close to the skeletal muscle insertions

function

The GTOs are proprioceptors that detect tension in the tendon of a contracting muscle

Golgi Tendon Organs Use Ib Afferent Neurons

The GTOs use Ib afferent (or sensory) neurons to send proprioceptive information to the dorsal horn.
The Ib fibers synapse on interneurons in the ventral horn.
In the ventral horn, the interneurons synapse on alpha MNs

Autogenic Inhibition

Activation of the GTOs causes a contracting muscle to be inhibited; in other words, it relaxes.
This is a protective function. If the GTOs did not become activated in response to a muscle’s stretch, an individual could easily tear his or her muscles.

Sequence of Golgi Tendon Organs Events

The agonist muscle contracts.
This activates the GTOs (which are embedded in the contracting muscle).
The GTOs send proprioceptive information along the Ib sensory fibers to the dorsal horn.
In the dorsal horn, the Ib fibers synapse with an interneuron.
The interneuron connects with an alpha MN in the ventral horn.
Information from the GTOs travels to 3 places:
1. An alpha MN to inhibit the contracting agonist muscle. Referred to as autogenic inhibition
2. An alpha MN to facilitate the antagonist of the contracting muscle
3. The cerebellum for further proprioceptive feedback

joint receptors

Joint receptors are located in the connective tissue of a joint capsule.

joint receptor function

respond to mechanical deformation occurring in the joint capsule and
ligaments.
send this proprioceptive information to the cerebellum and to the ventral horn

What Stimulates the Joint Receptors?

ruffini endings

paciniform corpuscles

ligament receptors

free nerve endings

ruffini endings

ocated in the joint capsule.
These sensory receptors are active both during rest and joint movement.

signals static joint position, dynamic joint movement, and the
direction and velocity of joint movement

Paciniform Corpuscles

These sensory nerve endings only respond to dynamic movement; that is, when the joint is
moving.
signal dynamic joint movement onset and termination, and joint movement velocity

Ligament Receptors

similar in function to GTOs.
These sensory nerve endings are located in the ligaments of a joint capsule and become active at the end of joint range.
respond to tension in the joint capsule

free nerve endings

located in the joint capsule.
These sensory nerve endings are most often stimulated by inflammation and irritation.
When these sensory nerve endings fire, they signal the detection of pain in a joint

Joint Receptor Pathway

The joint receptors use sensory fibers to send proprioceptive information to the dorsal horn.
Here, the messages synapse on an interneuron.
The interneuron connects with a MN in the ventral horn and sends proprioceptive messages back to the muscles surrounding the joint.
An alpha MN in the ventral horn also sends joint receptor information to the cerebellum for constant feedback about joint position and movement

Joint Receptors and Proprioception

While MSs, GTOs, and joint receptors are critical for normal proprioception, joint receptors alone may not be essential for proprioception.
This is suggested because patients with joint replacements still retain good proprioception in joint midrange

plasticity

describing the ability to show modification

learning

acquisition of knowledge or ability

memory

the outcome of learning, including the retention and storage of that knowledge or ability

short-term memory

working memory, which has a limited capacity for information and lasts for only for only a few moments

reflects a momentary attention to something, such as when we remember a phone number only long enough to dial it and then forget it

long-term memory

intimately related to the process of learning.

seen as a continuum

reflect functional changes in the efficiency of synapses

later stages reflect structural changes in synaptic connections.

less subject to disruption

Supplementary Motor Area (SMA)

movements that are initiated internally are controlled primarily by this motor area

also contributes to activating the motor programs involved in learned sequences

movements that are activated by external stimuli (a visual cue; a traffic light changing from red to green) are controlled premotor area (dorsal and ventral premotor cortex). this area controls how these stimuli are to be used to direct the action, specifically associating a given sensory event with a movement to be made.

inputs from the putamen of the basal ganglia complex, while the premotor area receives inputs from the cerebellum.

premotor lesions

cause impairment of retrieval of movements in accordance with visual cues, while SMA lesions disrupt retrieval of self-initiated movements

Parkinson’s disease and SMA

there is a massive depletion of dopamine in the putamen, and patients with Parkinson’s disease have difficulty with self-initiating movements such as walking

movement arises from

the interaction of both perception and action systems, with cognition affecting both systems at many different levels

somatosensory cortex

major processing area for all the somatosensory modalities, and marks the beginning of conscious awareness of somatosensation

divided into: primary (Brodmann’s area) and secondary

association cortices

we see the transition from perception to action in here.

interplay between cognitive and perceptual processing

inputs to motor areas come from…

the basal ganglia, the cerebellum, and sensory areas, including the periphery, SI, and sensory association areas in the parietal lobe

M1 neurons receive sensory inputs from

own muscles and also from the skin above the muscles

outputs from the primary motor cortex contribute to the

corticospinal tract

membrane potential

The membranes of nerve cells are structured so that a difference in electrical potential exists between the inside (negative) and the outside (positive). This results in a resting potential across the cell membrane, which is normally about −70 mV (70 one-thousandths of a volt).

affected by the permeability to each ion.

closer to the equilibrium potential for an ion is cause

permeability to a certain ion increases (more openings of pores or channels to that ion)

farther to the equilibrium potential for an ion is cause

permeability to a certain ion decreases (more closings of pores or channels to that ion)

action potential

Neurons communicate by producing all-or-none electrical impulses called nerve impulses

are self-regenerative electrical signals that tend to propagate throughout a neuron and along its axon.

all or none!!

Neurons can generate these because they contain specialized molecules, called sodium channels, that respond to depolarization by opening (activating).

Myelination

present around some axons within the peripheral nervous system (PNS) (where it is produced by Schwann cells) and within the central nervous system (CNS) (where it is produced by oligodendrocytes).

profound effects on the conduction of action potentials along the axon.

the energy requirement for impulse conduction is lower in these fibers; therefore, the metabolic cost of conduction is lower. Second, it results in an increased conduction velocity.

reduces the time it takes for impulses to travel from one region to another, thus reducing the time needed for reflexes and motor activities and permitting the brain to operate as a high-speed computer.

A fibers

large and myelinated, conduct rapidly, and carry various motor or sensory impulses.

B fibers

are smaller myelinated axons that conduct less rapidly than A fibers. These fibers serve autonomic functions.

C fibers

are the smallest and are nonmyelinated; they conduct impulses the slowest and serve pain conduction and autonomic functions.

synapses

the junctions between neurons that permit them to communicate with each other. Some are excitatory (increasing the probability that the postsynaptic neuron will fire), whereas others are inhibitory (decreasing the probability that the postsynaptic neuron will fire).

Electrical (or electrotonic) synapses

haracterized by gap junctions, which are specialized structures in which the presynaptic and postsynaptic membranes come into close apposition.

chemical synapse.

the overwhelming majority of synapses in the mammalian brain and spinal cord

communicate via diffusion of neurotransmitter molecules; some common transmitters that consist of relatively small molecules are listed with their main areas of concentration in the nervous system

simple reflex arc

includes a receptor (eg, a special sense organ, cutaneous end-organ, or muscle spindle, whose stimulation initiates an impulse); the afferent neuron, which transmits the impulse through a peripheral nerve to the central nervous system, where the nerve synapses with an LMN or an intercalated neuron; one or more intercalated neurons (interneurons), which for some reflexes relay the impulse to the efferent neuron; the efferent neuron (usually an LMN), which passes outward in the nerve and delivers the impulse to an effector; and an effector (eg, the muscle or gland that produces the response)

stretch reflexes

(also called tendon reflexes or deep tendon reflexes) are routinely assessed as part of the neurological examination.

are functionally important since they provide a feedback mechanism for maintaining appropriate muscle tone (see Fig 5–18). The stretch reflex depends on specialized sensory receptors (muscle spindles), afferent nerve fibers (primarily Ia fibers) extending from these receptors via the dorsal roots to the spinal cord, two types of LMNs (alpha and gamma motor neurons) that project back to muscle, and specialized inhibitory interneurons (Renshaw cells).

muscle spindles and reflexes

mechanoreceptors are located within muscles and provide information about the length and rate of changes in length of the muscle.

alpha motor neurons

when fired, action potentials propagate, via axons in the ventral roots and peripheral nerves, to the motor end-plate, where they have an excitatory effect and produce muscle contraction.

gamma motor neurons

small, specialized motor neurons whose cell bodies are located in the ventral horn

when fired, excites the intrafusal muscle fibers so that they contract.

increase tension on the muscle spindle, which increases its sensitivity to overall muscle stretch. Thus, this neuron/intrafusal muscle fiber system sets the “gain” on the muscle spindle.

GTO and reflexes

present within muscle tendons.

These stretch receptors are arranged in series with extrafusal muscle fibers and are activated by either stretching or contracting the muscle. Group Ib afferent fibers run from the tendon organs via the dorsal roots to the spinal gray matter. Here, they end on interneurons that inhibit the alpha motor neuron innervating the agonist muscle, thus mediating the inverse stretch reflex. This feedback arrangement prevents overactivity of alpha motor neurons.