Chapter 10

voluntary movement

(1) The movement is accompanied by a conscious awareness of what we are doing and why we are doing it, and (2) our attention is directed toward the action or its purpose.

hypertonia

Abnormally high muscle tone, called hypertonia, accompanies a number of diseases and is seen very clearly when a joint is moved passively at high speeds. The increased resistance is due to an increased level of alpha motor neuron activity, which keeps a muscle contracted despite the attempt to relax it. Hypertonia usually occurs with disorders of the descending pathways that normally inhibit the motor neurons.

proprioception

Afferent information about the position of the body and its parts in space is called proprioception.

motor neuron pool

As described in Chapter 9, the building blocks for these movements—as for all movements—are motor units, each comprising one motor neuron together with all the skeletal muscle fibers innervated by that neuron. All the motor neurons that supply a given muscle make up the motor neuron pool for the muscle. The cell bodies of the pool for a given muscle are close to each other either in the ventral horn of the spinal cord or in the brainstem.

upper motor neuron disorders

Clinically, the descending pathways and neurons of the motor cortex are often referred to as the upper motor neurons (a confusing misnomer because they are not really motor neurons). Abnormalities due to their dysfunction are classified, therefore, as upper motor neuron disorders.

somatosensory cortex

Directly behind primary motor cortex

lower motor neurons

In this clinical classification, the alpha motor neurons—the true motor neurons—are termed lower motor neurons.

MPTP

One chemical clearly linked to destruction of the substantia nigra is MPTP (1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine).

synergistic muscles

Path C in Figure 10.6 activates motor neurons of synergistic muscles—that is, muscles whose contraction assists the intended motion. In the example of the knee-jerk reflex, this would include other muscles that extend the leg.

rigidity

Rigidity is a form of hypertonia in which the increased muscle contraction is continual and the resistance to passive stretch is constant (as occurs in the disease tetanus,

somatotopic map

Show what regions of the primary motor cortex control what parts of the body.

parietal-lobe association cortex

Small area behind somatosensory cortex, which lies behind primary motor cortex.

polysnaptic

Synapses on interneurons

reciprocal innervation

The activation of neurons to one muscle with the simultaneous inhibition of neurons to its antagonistic muscle is called reciprocal innervation.

descending pathways

The information determined by the motor program is transmitted via descending pathways to the local level of the motor control hierarchy. There, the axons of the motor neurons projecting to the muscles exit the brainstem or spinal cord.

intrafusal fibers

The modified muscle fibers within the spindle are known as intrafusal fibers.

levodopa (L-dopa)

The most widely prescribed drug is Levodopa (L-dopa), which falls into the third category. L-dopa enters the bloodstream, crosses the blood-brain barrier, and is converted in neurons to dopamine. The newly formed dopamine activates receptors in the basal nuclei and improves the symptoms of the disease.

extrafusal fibers

The skeletal muscle fibers that form the bulk of the muscle and generate its force and movement are the extrafusal fibers.

cramps

Two other forms of hypertonia that can occur suddenly in individual or multiple muscles sometimes originate as problems in muscle and not nervous tissue: Muscle spasms are brief, involuntary contractions that may or may not be painful, and muscle cramps are prolonged, involuntary, and painful contractions

beyond sensorimotor cortex

We have described the various areas of sensorimotor cortex as giving rise, either directly or indirectly, to pathways descending to the motor neurons. However, additional brain areas are involved in the initiation of intentional movements, such as the areas involved in memory, emotion, and motivation. Association areas of the cerebral cortex also have other functions in motor control. For example, neurons of the parietal-lobe association cortex are important in the visual control of reaching and grasping. These neurons contribute to matching motor signals concerning the pattern of hand action with signals from the visual system concerning the three-dimensional features of the objects to be grasped.

Walking

We initiate walking by allowing the body to fall forward to an unstable position and then moving one leg forward to provide support. When the extensor muscles are activated on the supported side of the body to bear the body's weight, the contralateral extensors are inhibited by reciprocal innervation to allow the nonsupporting limb to flex and swing forward. The cyclical, alternating movements of walking are brought about largely by networks of interneurons in the spinal cord at the local level. afferent inputs and local spinal cord neural networks contribute substantially to the coordination of locomotion.

stretch reflex

When the afferent fibers from the muscle spindle enter the central nervous system, they divide into branches that take different paths. In Figure 10.6, path A makes excitatory synapses directly onto motor neurons that return to the muscle that was stretched, thereby completing a reflex arc known as the stretch reflex. Path D of Figure 10.6 is not explicitly part of the stretch reflex; it demonstrates that information about changes in muscle length ascends to higher centers. The axon of the afferent neuron continues to the brainstem and synapses there with interneurons that form the next link in the pathway that conveys information about the muscle length to areas of the brain dealing with motor control.

muscle-spindle stretch receptors

Within a given spindle are two kinds of stretch receptors. One, the nuclear chain fiber, responds best to how much a muscle is stretched; whereas the other, the nuclear bag fiber, responds to both the magnitude of a stretch and the speed with which it occurs. Although the two kinds of stretch receptors are separate entities, we will refer to them collectively as the muscle-spindle stretch receptors. an external force stretching the muscle also pulls on the intrafusal fibers, stretching them and activating their receptor endings (Figure 10.5a). The more or the faster the muscle is stretched, the greater the rate of receptor firing.

deep brain stimulation

accomplished by surgically implanting electrodes in regions of the basal nuclei; the electrodes are connected to an electrical pulse generator similar to a cardiac artificial pacemaker

brainstem pathway

and a second group we will refer to as the brainstem pathways, which originate in the brainstem.

Parkinson's disease drugs

. They fall into three main categories: (1) agonists (stimulators) of dopamine receptors, (2) inhibitors of the enzymes that metabolize dopamine at synapses, and (3) precursors of dopamine itself.

premotor area

A large number of neurons that give rise to descending pathways for motor control come from two areas of sensorimotor cortex on the posterior part of the frontal lobe: the primary motor cortex (sometimes called simply the motor cortex) and the premotor area (Figure 10.10).

primary motor cortex

A large number of neurons that give rise to descending pathways for motor control come from two areas of sensorimotor cortex on the posterior part of the frontal lobe: the primary motor cortex (sometimes called simply the motor cortex) and the premotor area (Figure 10.10).

flaccid

Although hypotonia may develop after cerebellar disease, it more frequently accompanies disorders of the alpha motor neurons (lower motor neurons), neuromuscular junctions, or the muscles themselves. The term flaccid, which means "weak" or "soft," is often used to describe hypotonic muscles.

substantia nigra

Although the symptoms of Parkinson's disease reflect inadequate functioning of the basal nuclei, a major part of the initial defect arises in neurons of the substantia nigra These neurons normally project to the basal nuclei, where they release dopamine from their axon terminals. The substantia nigra neurons degenerate in Parkinson's disease and the amount of dopamine they deliver to the basal nuclei is decreased. This decreases the subsequent activation of the sensorimotor cortex.

tension-monitoring systems

Any given set of inputs to a given set of motor neurons can lead to various degrees of tension in the muscles they innervate. The tension depends on muscle length, the load on the muscles, and the degree of muscle fatigue. Therefore, feedback is necessary to inform the motor control systems of the tension actually achieved. An additional receptor type specifically monitors how much tension the contracting motor units are exerting

local afferent input - afferent fibers

As just noted, afferent fibers sometimes impinge on the local interneurons. (In one case that will be discussed shortly, they synapse directly on motor neurons.) The afferent fibers carry information from sensory receptors located in three places: (1) in the skeletal muscles controlled by the motor neurons; (2) in other nearby muscles, such as those with antagonistic actions; and (3) in the tendons, joints, and skin of body parts affected by the action of the muscle.

corticospinal versus brainstem pathways

As stated previously, the corticospinal neurons generally have their greatest influence over motor neurons that control muscles involved in fine, isolated movements, particularly those of the fingers and hands. The brainstem descending pathways, in contrast, are involved more with coordination of the large muscle groups used in the maintenance of upright posture, in locomotion, and in head and body movements when turning toward a specific stimulus. There is, however, much interaction between the descending pathways. For example, some fibers of the corticospinal pathway end on interneurons that have important functions in posture, whereas fibers of the brainstem descending pathways sometimes end directly on the alpha motor neurons to control discrete muscle movements. Because of this redundancy, one system may compensate for loss of function resulting from damage to the other system, although the compensation is generally not complete.

corticobulbar pathway

As the corticospinal fibers descend through the brain from the cerebral cortex, they are accompanied by fibers of the corticobulbar pathway (bulbar means "pertaining to the brainstem"), a pathway that begins in the sensorimotor cortex and ends in the brainstem. The corticobulbar fibers control, directly or indirectly via interneurons, the motor neurons that innervate muscles of the eye, face, tongue, and throat. These fibers provide the main source of control for voluntary movement of the muscles of the head and neck, whereas the corticospinal fibers serve this function for the muscles of the rest of the body.

extrapyramidal system

Axons from neurons in the brainstem also form pathways that descend into the spinal cord to influence motor neurons. These pathways are sometimes referred to as the extrapyramidal system, or indirect pathways, to distinguish them from the corticospinal (pyramidal) pathways. Axons of most of the brainstem pathways remain uncrossed and affect muscles on the same side of the body (see Figure 10.12), although a few do cross over to influence contralateral muscles. In the spinal cord, the fibers of the brainstem pathways descend as distinct clusters, named according to their sites of origin. For example, the vestibulospinal pathway descends to the spinal cord from the vestibular nuclei in the brainstem, whereas the reticulospinal pathway descends from neurons in the brainstem reticular formation.

monosynaptic reflex

Because the afferent nerve fibers in the stretched muscle synapse directly on the motor neurons to that muscle without any interneurons, this type of reflex is called a monosynaptic reflex. Afferent fibers synapsing directly on motor neurons without any interneurons.

tension-monitoring systems

Branches of the afferent neuron from the Golgi tendon organ cause widespread inhibition of the contracting muscle and its synergists via interneurons They also stimulate the motor neurons of the antagonistic muscles (path B in Figure 10.8). Note that this reciprocal innervation is the opposite of that produced by the muscle-spindle afferents. This difference reflects the different functional roles of the two systems: The muscle spindle provides local homeostatic control of muscle length, and the Golgi tendon organ provides local homeostatic control of muscle tension. During very intense contractions that have the potential to cause injury, Golgi tendon organs are strongly activated. The resulting high-frequency action potentials arriving in the spinal cord stimulate interneurons that inhibit motor neurons to the muscle associated with that tendon, thus reducing the force and protecting the muscle.

akinesia

Clinically, Parkinson's disease is characterized by a reduced amount of movement (akinesia), slow movements (bradykinesia), muscular rigidity, and a tremor at rest.

bradykinesia

Clinically, Parkinson's disease is characterized by a reduced amount of movement (akinesia), slow movements (bradykinesia), muscular rigidity, and a tremor at rest.

upper motor neurons

Clinically, the descending pathways and neurons of the motor cortex are often referred to as the upper motor neurons (a confusing misnomer because they are not really motor neurons).

convergence and divergence

Convergence and divergence are hallmarks of the corticospinal pathway. For example, a great number of different neuronal sources converge on neurons of the sensorimotor cortex, which is not surprising when you consider the many factors that can affect motor behavior. As for the descending pathways, neurons from wide areas of the sensorimotor cortex converge onto single motor neurons at the local level so that multiple brain areas usually control single muscles. Also, axons of single corticospinal neurons diverge markedly to synapse with a number of different motor neuron populations at various levels of the spinal cord, thereby ensuring that the motor cortex can coordinate many different components of a movement.

subcortical and brainstem nuclei

During activation of the cortical areas involved in motor control, subcortical mechanisms also become active. We now turn to these areas of the motor control system. Numerous highly interconnected structures lie in the brainstem and within the cerebrum beneath the cortex, where they interact with the cortex to control movements. Their influence is transmitted indirectly to the motor neurons both by pathways that ascend to the cerebral cortex and by pathways that descend from some of the brainstem nuclei.

muscle tone

Even when a skeletal muscle is relaxed, there is a slight and uniform resistance when it is stretched by an external force. This resistance is known as muscle tone, Muscle tone is due both to the passive elastic properties of the muscles and joints and to the degree of ongoing alpha motor neuron activity. When a person is very relaxed, the alpha motor neuron activity does not make a significant contribution to the resistance to stretch. As the person becomes increasingly alert, however, more activation of the alpha motor neurons occurs and muscle tone increases.

alpha motor neurons

Extrafusal fibers of a muscle are activated by large-diameter motor neurons called alpha motor neurons. If action potentials along alpha motor neurons cause contraction of the extrafusal fibers, the resultant shortening of the muscle removes tension on the spindle and slows the rate of firing in the stretch receptor

middle level for voluntary movements

Function: converts plans received from higher centers to a number of smaller motor programs that determine the pattern of neural activation required to perform the movement. These programs are broken down into subprograms that determine the movements of individual joints. The programs and subprograms are transmitted through descending pathways to the local control level. Structures: sensorimotor cortex, cerebellum, parts of basal nuclei, some brainstem nuclei.

higher centers for voluntary movements

Function: forms complex plans according to individual's intention and communicates with the middle level via command neurons. Structures: areas involved with memory, emotions and motivation, and sensorimotor cortex. All these structures receive and correlate input from many other brain structures.

local level for voluntary movements

Function: specifies tension of particular muscles and angle of specific joints at specific times necessary to carry out the programs and subprograms transmitted from the middle control levels. Structures: brainstem or spinal cord interneurons, afferent neurons, motor neurons.

hypotonia

Hypotonia is a condition of abnormally low muscle tone accompanied by weakness, atrophy (a decrease in muscle bulk), and decreased or absent reflex responses

alpha-gamma coactivation

If muscles were always activated as shown in Figure 10.5b, however, slackening of muscle spindles would reduce the available sensory information about muscle length during rapid shortening contractions. A mechanism called alpha-gamma coactivation prevents this loss of information.

descending pathways affecting afferent systems

Importantly, some of the descending fibers affect afferent systems. They do this via (1) presynaptic synapses on the terminals of afferent neurons as these fibers enter the central nervous system, or (2) synapses on interneurons in the ascending pathways. The overall effect of this descending input to afferent systems is to regulate their influence on either the local or brain motor control areas, thereby altering the importance of a particular bit of afferent information or sharpening its focus. For example, when performing an exceptionally delicate or complicated task, like a doctor performing surgery,

Parkinson's disease

In Parkinson's disease, the input to the basal nuclei is diminished, the interplay of the facilitatory and inhibitory circuits is unbalanced, and activation of the motor cortex (via the basal nuclei-thalamus limb of the circuit just mentioned) is reduced. Symptoms of Parkinson's disease reflect inadequate functioning of basal nuclei.

postural reflexes

Many of the reflex pathways previously introduced (for example, the stretch and crossed-extensor reflexes) are active in posture control. people can operate under conditions of unstable equilibrium because complex interacting postural reflexes maintain their balance. The afferent pathways of the postural reflexes come from three sources: the eyes, the vestibular apparatus, and the receptors involved in proprioception (joint, muscle, and touch receptors, for example). The efferent pathways are the alpha motor neurons to the skeletal muscles, and the integrating centers are neuron networks in the brainstem and spinal cord. In addition to these integrating centers, there are centers in the brain that form an internal representation of the body's geometry, its support conditions, and its orientation with respect to vertical. This internal representation serves two purposes: (1) It provides a reference framework for the perception of the body's position and orientation in space and for planning actions, and (2) it contributes to stability via the motor controls involved in maintaining upright posture.

intention tremor

They typically cannot perform limb or eye movements smoothly but move with a tremor—a so-called intention tremor that increases as a movement nears its final destination. This differs from patients with Parkinson's disease, who have a tremor while at rest.

interneurons

Most of the synaptic input to motor neurons from the descending pathways and afferent neurons does not go directly to motor neurons but, rather, goes to interneurons that synapse with the motor neurons Interneurons comprise 90% of spinal cord neurons, and they are of several types. Some are near the motor neuron they synapse upon and thus are called local interneurons. Others have processes that extend up or down short distances in the spinal cord and brainstem, or even throughout much of the length of the central nervous system. The interneurons with longer processes are important for integrating complex movements such as stepping forward with your left foot as you throw a baseball with your right arm.

motor program

Neurons of the middle level of the hierarchy integrate all of this afferent information with the signals from the command neurons to create a motor program—defined as the pattern of neural activity required to properly perform the desired movement. If a complex movement is repeated often, learning takes place and the movement becomes skilled. Then, the initial information from the middle hierarchical level is more accurate and fewer corrections need to be made. Movements performed at high speed without concern for fine control are made solely according to the initial motor program.

supplementary motor cortex

Other areas of sensorimotor cortex shown in Figure 10.10 include the supplementary motor cortex, which lies mostly on the surface on the frontal lobe where the cortex folds down between the two hemispheres

reflex circuits

Reflex circuits acting entirely at the local level are also important in refining ongoing movements. Thus, some proprioceptive inputs are processed and influence ongoing movements without ever reaching the level of conscious perception.

spasticity

Spasticity is a form of hypertonia in which the muscles do not develop increased tone until they are stretched a bit; after a brief increase in tone, the contraction subsides for a short time.

clasp-knife phenomenon

Spasticity is a form of hypertonia in which the muscles do not develop increased tone until they are stretched a bit; after a brief increase in tone, the contraction subsides for a short time. The period of "give" occurring after a time of resistance is called the clasp-knife phenomenon.

muscle spindle

Stretch receptors embedded within muscles monitor muscle length and the rate of change in muscle length. These receptors consist of peripheral endings of afferent nerve fibers wrapped around modified muscle fibers, several of which are enclosed in a connective-tissue capsule. The entire apparatus is collectively called a muscle spindle

cerebellar disease

The importance of the cerebellum in programming movements can best be appreciated when observing its absence in individuals with cerebellar disease. They typically cannot perform limb or eye movements smoothly but move with a tremor—a so-called intention tremor that increases as a movement nears its final destination. This differs from patients with Parkinson's disease, who have a tremor while at rest. People with cerebellar disease also cannot combine the movements of several joints into a single, smooth, coordinated motion. Unstable posture and awkward gait are two other symptoms characteristic of cerebellar disease. For example, people with cerebellar damage walk with their feet wide apart, and they have such difficulty maintaining balance that their gait is similar to that seen in people who are intoxicated by ethanol.

cerebellum

The cerebellum is located dorsally to the brainstem It influences posture and movement indirectly by means of input to brainstem nuclei and (by way of the thalamus) to regions of the sensorimotor cortex that give rise to pathways that descend to the motor neurons. The cerebellum receives information from the sensorimotor cortex and also from the vestibular system, eyes, skin, muscles, joints, and tendons—that is, from some of the very receptors that movement affects. One role of the cerebellum in motor functioning is to provide timing signals to the cerebral cortex and spinal cord for precise execution of the different phases of a motor program, in particular, the timing of the agonist/antagonist components of a movement. It also helps coordinate movements that involve several joints and stores the memories of these movements so they are easily achieved the next time they are tried. The cerebellum also participates in planning movements—integrating information about the nature of an intended movement with information about the surrounding space. The cerebellum then provides this as a feedforward (see Chapter 1) signal to the brain areas responsible for refining the motor program Moreover, during the course of the movement, the cerebellum compares information about what the muscles should be doing with information about what they actually are doing. If a discrepancy develops between the intended movement and the actual one, the cerebellum sends an error signal to the motor cortex and subcortical centers to correct the ongoing program.

cerebral cortex

The cerebral cortex has a critical function in both the planning and ongoing control of voluntary movements, functioning in both the highest and middle levels of the motor control hierarchy Although these areas of the cortex are anatomically and functionally distinct, they are heavily interconnected, and individual muscles or movements are represented at multiple sites. Thus, the cortical neurons that control movement form a neural network, meaning that many neurons participate in each individual movement. The neural networks can be distributed across multiple sites in parietal and frontal cortex, including the sites named in the preceding two paragraphs. The interactions of the neurons within the networks are flexible so that the neurons are capable of responding differently under different circumstances.

sensorimotor cortex

The descending pathways to the local level arise only in the sensorimotor cortex and brainstem. The term sensorimotor cortex is used to include all those parts of the cerebral cortex that act together to control muscle movement Other brain areas, notably the basal nuclei (also referred to as the basal ganglia), thalamus, and cerebellum, exert their effects on the local level only indirectly via the descending pathways from the cerebral cortex and brainstem.

descending pathways more

The influence exerted by the various brain regions on posture and movement occurs via descending pathways to the motor neurons and the interneurons that affect them. The pathways are of two types: the corticospinal pathways, which, as their name implies, originate in the cerebral cortex; and a second group we will refer to as the brainstem pathways, which originate in the brainstem. Neurons from both types of descending pathways end at synapses on alpha and gamma motor neurons or on interneurons that affect them. The ultimate effect of the descending pathways on the alpha motor neurons may be excitatory or inhibitory.

interneuron function

The interneurons are important elements of the local level of the motor control hierarchy, integrating inputs not only from higher centers and peripheral receptors but from other interneurons as well Moreover, interneurons can act as "switches" that enable a movement to be turned on or off under the command of higher motor centers. For example, if you pick up a hot plate, a local reflex arc will be initiated by pain receptors in the skin of your hands, normally causing you to drop the plate. If it contains your dinner, however, descending commands can inhibit the local activity and you can hold onto the plate until you can put it down safely. The integration of various inputs by local interneurons

pyramidal tracts

The nerve fibers of the corticospinal pathways have their cell bodies in the sensorimotor cortex and terminate in the spinal cord. The corticospinal pathways are also called the pyramidal tracts or pyramidal system because of their triangular shape as they pass along the ventral surface of the medulla oblongata. These fibers provide the main source of control for voluntary movement of the muscles of the head and neck, whereas the corticospinal fibers serve this function for the muscles of the rest of the body. Axons from neurons in the brainstem also form pathways that descend into the spinal cord to influence motor neurons. These pathways are sometimes referred to as the extrapyramidal system, or indirect pathways, to distinguish them from the corticospinal (pyramidal) pathways.

Golgi tendon organs

The receptors employed in this tension-monitoring system are the Golgi tendon organs, which are endings of afferent nerve fibers that wrap around collagen bundles in the tendons near their junction with the muscle (see Figure 10.4). These collagen bundles are slightly bowed in the resting state. When the muscle is stretched or the attached extrafusal muscle fibers contract, tension is exerted on the tendon. This tension straightens the collagen bundles and distorts the receptor endings, activating them. The Golgi tendon organs discharge in response to the tension generated by the contracting muscle and initiate action potentials that are transmitted to the central nervous system.

crossed-extensor reflex

The same stimulus causes just the opposite response in the contralateral leg (on the opposite side of the body from the stimulus); motor neurons to the extensors are activated while the flexor muscle motor neurons are inhibited. This crossed-extensor reflex enables the contralateral leg to support the body's weight as the injured foot is lifted by flexion

gamma motor neurons

The two ends of intrafusal muscle fibers are activated by smaller- diameter neurons called gamma motor neurons The cell bodies of alpha and gamma motor neurons to a given muscle lie close together in the spinal cord or brainstem. Both types are activated by interneurons in their immediate vicinity and sometimes directly by neurons of the descending pathways. The contractile ends of intrafusal fibers are not large or strong enough to contribute to force or shortening of the whole muscle. However, they can maintain tension and stretch in the central receptor region of the intrafusal fibers. Activating gamma motor neurons alone therefore increases the sensitivity of a muscle to stretch Coactivating gamma motor neurons and alpha motor neurons prevents the central region of the muscle spindle from going slack during a shortening contraction (see Figure 10.5c). This ensures that information about muscle length will be continuously available to provide for adjustment during ongoing actions and to plan and program future movements.

basal nuclei

They form a link in some of the looping parallel circuits through which activity in the motor system is transmitted from a specific region of sensorimotor cortex to the basal nuclei, from there to the thalamus, and then back to the cortical area where the circuit started Instead of going to the thalamus, they can send information to the brainstem, which eventually goes down to muscles. Some of these circuits facilitate movements, and others suppress them. This explains why brain damage to subcortical nuclei following a stroke or trauma can result in either hypercontracted muscles or flaccid paralysis

knee-jerk reflex

This reflex is important in maintaining balance and posture. The examiner taps the patellar tendon (see Figure 10.6), which passes over the knee and connects extensor muscles in the thigh to the tibia in the lower leg. A s the tendon is pushed in by tapping, the thigh muscles it is attached to are stretched and all the stretch receptors within these muscles are activated. This stimulates a burst of action potentials in the afferent nerve fibers from the stretch receptors, and these action potentials activate excitatory synapses on the motor neurons that control these same muscles. The motor units are stimulated, the thigh muscles contract, and the patient's lower leg extends to give the knee jerk. In path B of Figure 10.6, the branches of the afferent nerve fibers from stretch receptors end on inhibitory interneurons. When activated, these inhibit the motor neurons controlling antagonistic muscles whose contraction would interfere with the reflex response. In the knee jerk, for example, neurons to muscles that flex the knee are inhibited. The proper performance of the knee jerk tells the physician that the afferent fibers, the balance of synaptic input to the motor neurons, the motor neurons, the neuromuscular junctions, and the muscles themselves are functioning normally.

motor control hierarchy

To begin a consciously planned movement, a general intention such as "pick up sweater" or "write signature" or "answer telephone" is generated at the highest level of the motor control hierarchy. These higher centers include many regions of the brain (described in detail later), including sensorimotor areas and others involved in memory, emotions, and motivation. Information is relayed from these higher-center "command" neurons to parts of the brain that make up the middle level of the motor control hierarchy. The middle-level structures specify the individual postures and movements needed to carry out the intended action. The middle-level hierarchical structures are located in parts of the cerebral cortex as well as in the cerebellum, subcortical nuclei, and brainstem As the neurons in the middle level of the hierarchy receive input from the command neurons, they simultaneously receive afferent information from receptors in the muscles, tendons, joints, and skin, as well as from the vestibular apparatus and eyes. These afferent signals relay information to the middle-level neurons about the starting positions of the body parts that are "commanded" to move. Neurons of the middle level of the hierarchy integrate all of this afferent information with the signals from the command neurons to create a motor program—defined as the pattern of neural activity required to properly perform the desired movement. The local level of the hierarchy includes afferent neurons, motor neurons, and interneurons. Local-level neurons determine exactly which motor neurons will be activated to achieve the desired action and when this will happen. motor neurons always form the final common pathway to the muscles.

spasms

Two other forms of hypertonia that can occur suddenly in individual or multiple muscles sometimes originate as problems in muscle and not nervous tissue: Muscle spasms are brief, involuntary contractions that may or may not be painful, and muscle cramps are prolonged, involuntary, and painful contractions

corticospinal pathway

corticospinal pathways, which, as their name implies, originate in the cerebral cortex; The nerve fibers of the corticospinal pathways have their cell bodies in the sensorimotor cortex and terminate in the spinal cord. The corticospinal pathways are also called the pyramidal tracts or pyramidal system because of their triangular shape as they pass along the ventral surface of the medulla oblongata. In the medulla oblongata near the junction of the spinal cord and brainstem, most of the corticospinal fibers cross (known as decussation) to descend on the opposite side (Figure 10.12). The skeletal muscles on the left side of the body are therefore controlled largely by neurons in the right half of the brain, and vice versa.

withdrawal reflex

painful stimulation of the skin, as occurs from stepping on a tack, activates the flexor muscles and inhibits the extensor muscles of the ipsilateral leg (on the same side of the body). The resulting action moves the affected limb away from the harmful stimulus and is thus known as a withdrawal reflex The same stimulus causes just the opposite response in the contralateral leg (on the opposite side of the body from the stimulus); motor neurons to the extensors are activated while the flexor muscle motor neurons are inhibited. This crossed-extensor reflex enables the contralateral leg to support the body's weight as the injured foot is lifted by flexion