Immediately following insult or injury of the pyramidal system, during a period of neurogenic shock, there may be substantially diminished muscle tone and sluggish or absent deep tendon reflexes.14 As inflammation from the initial insult subsides, severely damaged neurons degenerate and are resorbed, while minimally damaged neurons may repair themselves and resume function.^15,16 The more neurons that are destroyed, the greater the likelihood that hypertonicity will develop over time due to the altered balance of descending input of pyramidal and extrapyramidal systems. As the recovery period continues, individuals may begin to move in abnormal synergy patterns whenever volitional movement is attempted.^17,18 When the damage to the system is less extensive, individuals may eventually recover some or all volitional motor control; the more extensive the damage to the system, the more likely there will be residual motor impairment.^15,18

The extrapyramidal system is made up of several subcortical subsystems that influence muscle tone, organize patterns of movement from among the many possible movement strategies, and make both feedforward adjustments (in anticipation of movement) and refining feedback adjustments (in response to sensations generated as movement occurs) during performance of functional tasks.19 The motor planning subsystem is a series of neural loops interconnecting the premotor and accessory motor cortices in the frontal lobes, the nuclei of the functional basal ganglia (caudate, putamen, globus pallidus, substantia nigra, subthalamus), and several nuclei of the thalamus. Damage to the premotor and accessory motor cortex leads to apraxia, the inability to effectively sequence components of a functional task and to understand the nature of a task and the way to use a tool in performance of the task.²⁰ If there is damage to the caudate and putamen (also called the corpus striatum), underlying muscle tone may fluctuate unpredictably (athetosis) and involuntary dancelike movements (chorea) are likely to occur.²¹ Damage to the subthalamic nuclei can lead to forceful, often disruptive, involuntary movement of the extremities (ballism) that interrupts purposeful activity.²² Damage to the substantia nigra characteristically leads to resting tremor, rigidity of axial and appendicular musculature (hypertonicity in all directions), and bradykinesia (difficulty initiating movement, slow movement with limited excursion during functional tasks), which are most commonly seen in persons with Parkinson’s disease.²³ Motor impairments resulting from damage to the ventral anterior nucleus and related thalamic nuclei are less well understood but may contribute to less efficient motor planning, especially when the individual is learning or performing novel tasks.²⁴

Extrapyramidal structures influence muscle tone and readiness to move via a network of interconnections among motor centers in the brainstem. The reticulospinal tracts, originating in the lower pons and medulla, are thought to influence tone by acting on γ motor neurons and their associated muscle spindles. They play a major role in balancing the stiffness required for antigravity position and the flexibility necessary for movement of the limbs through space during functional activity and are likely the effectors for tonic hindbrain reflexes.^19,25 The vestibulospinal tracts, also originating from the pons and medulla, influence anticipatory postural adjustment in preparation for movement and reactionary postural adjustments as movement occurs. These tracts are thought to be the “effectors” for postural control and balance.^19,26 The tectospinal tracts, originating in the collicular nuclei of the dorsal midbrain, influence linkages between the head and extremities (especially arms and hands) so that the visual and auditory systems can be used effectively to orient the head and body during tasks that require visual (eye-hand) and auditory (ear-hand) guidance.^16,27

The error control, or coordination subsystem, has several interactive components.28 Feedforward information (how movement is likely to occur) from the forebrain’s motor cortex is relayed through the thalamus to the deep nuclei of the cerebellum via the middle cerebellar peduncle (brachium pontis). Feedback information generated by movement travels from the muscle spindle and anterior horn of the spinal cord via the inferior cerebellar peduncle (restiform body), as does sensory information from static and dynamic vestibular receptors (head position and movement in space) and the vestibular nuclei in the brainstem. Through interaction of Purkinje cells in the cerebellar cortex and neurons in the deep cerebellar nuclei, the cerebellum judges how “in sync” these various types of information are (essentially asking the questions, “Did the movement occur as planned? Was the outcome of the movement as intended?”) and suggests refinements for more precise and coordinated movement.²⁹ These adjustments are relayed to the red nucleus in the midbrain via the superior cerebellar peduncle (brachium conjunctiva) and are forwarded back to the thalamus and the motor cortex, as well as to the spinal cord via the rubrospinal tract. The rubrospinal tract is thought to be essential for refinement and correction of direction and control of movement as it occurs.²⁹

The somatosensory system is composed of a set of ascending pathways, each carrying a specific sensory modality from the spinal cord and brainstem to the thalamus and postcentral gyrus of the cerebral cortex, reticular formation, or cerebellum. The anterolateral (spinothalamic) system carries exteroceptive information from mechanoreceptors that monitor protective senses (e.g., pain, temperature, irritation to skin and soft tissue).30 This tract originates in the dorsal horn (substantia gelatinosa) of the spinal cord and the spinal trigeminal nucleus, crosses the midline of the neuraxis to ascend in the lateral funiculus of the spinal cord to the contralateral ventral posterior thalamus, and then continues to the postcentral gyrus. The dorsal column/medial lemniscus carries information from encapsulated receptors that serve as internal monitors of body condition and motion.³¹ This tract ascends from the spinal cord to reach the nuclei gracilis and cuneatus in the medulla of the brainstem, then crosses midline to ascend to the contralateral ventral posterior thalamus and on to the posterior central gyrus. The postcentral gyrus (somatosensory cortex) is organized as a homunculus, with each region of the body represented in a specific area.³² Sensation from the lower extremities (lumbosacral spinal cord) is located at the top of the gyrus near the sagittal fissure. Moving downward toward the lateral fissure, the next area represented is the trunk (thoracic spinal cord), followed by upper extremities and head (cervical spinal cord), and finally face, mouth, and esophagus (trigeminal nuclei) just above the lateral fissure. A lesion such as an MS plaque in one of the ascending pathways may result in a discrete area of loss of exteroception or of conscious proprioception in one area of the body; a lesion on the somatosensory cortex can lead to a more profound, multimodality impairment on the opposite side of the body.

Although sensory information is logged in at the postcentral gyrus, the location of the somatosensory cortex, interpretation and integration of this information occurs in the somatoperceptual system in the parietal association areas, with specialization in the right hemisphere.³² These association areas give meaning to the sensations that are generated as people move and function in their environments. This is where people understand the relationships among their various extremities and trunk (body schema), as well as their relationship to and position within our physical environment. Damage to the parietal lobes leads to problems ranging from left-right confusion to the inability to recognize and monitor the condition of a body part (neglect, agnosia), depending on how much of the association area is involved.^33,34

The visual system begins with processing of information gathered by the rods and cones in the multiple layers of specialized neurons in the retina, located in the posterior chamber of the eye. Axons from retinal ganglion cells are gathered into the optic nerve, which carries information from that eye toward the brain. At the optic chiasm there is reorganization of visual information, such that all information from the left visual field (from both eyes) continues in the right optic tract and that from the right field continues in the left tract. This information is relayed, through the lateral geniculate body of the thalamus, via the optic radiations, to the primary visual cortex on either side of the calcarine fissure of the midsagittal occipital lobe.35 Damage to the retina or optic nerve results in loss of vision from that eye. Damage to the optic chiasm typically leads to a narrowing of the peripheral visual field (bitemporal hemianopsia); a lesion of one of the optic tracts or radiations leads to loss of part or all of the opposite visual field (homonymous hemianopsia). Damage to the visual cortex can result in cortical blindness, in which visual reflexes may be intact but vision is impaired.³⁶

Visual information is interpreted in the visual association areas in the remainder of the occipital lobe.³⁷ The visual association areas in the left hemisphere are particularly important to interpretation of symbolic and communication information, while spatial relationships are of more interest in the right hemisphere. Specific details about the environment, especially about speed and direction of moving objects with respect to the self and of the individual with respect to a relatively stationary environment, are important contributors to functional movement and to the development of skilled abilities.³⁸ Interconnections between the parietal and occipital association areas serve to integrate visual and somatic/kinesthetic perception and provide important input to motor planning and motor learning systems.³⁹

Effective visual information processing is founded on three interactive dimensions: visual spatial orientation, visual analysis skills, and visual motor skills.⁴⁰ Developmentally, visual spatial orientation includes spatial concepts used to understand the environment, the body, and the interaction between the body and environment that are part of functional activity (e.g., determining location or direction with respect to self, as well as respect to other objects or persons encountered as people act in their environment). Visual analysis skills allow people to discriminate and analyze visually presented information, identify and focus on key characteristics or features of what people see, use mental imagery and visual recall, and respond or perceive a whole when presented with representative parts. The visual motor system links what is seen to how the eyes, head, and body move, allowing one to use visual information processing skills during skilled, purposeful activities. This also provides the foundation for fine manipulative skills requiring eye-hand coordination.³⁷

The ability to problem solve, consider alternatives, plan and organize, understand conceptual relationships, multitask, set priorities, and delay gratification, as well as the initial components of learning are functions of the frontal association areas of the forebrain.41^,42 These dimensions of cognitive function are often described by the phrase higher executive function. Quantitative and other analytical skills are thought to be primarily housed in the frontal association areas of the left hemisphere, while intuitive understanding and creativity may be more concentrated in the right hemisphere. Most people tap the resources available in both hemispheres (via interconnections through the corpus callosum) during daily life, although some may fall toward one end or another of the analytical-intuitive continuum. Individuals with acquired brain injury involving frontal lobes often exhibit subtle deficits that have a significant impact on their ability to function in complex environments, as well as under conditions of high task demand; difficulty in these areas certainly compromises functional efficiency and quality of life.^43,44

The neuroanatomical structures that contribute to the motivational system include the nuclei and tracts of the limbic system, prefrontal cortex, and temporal lobes; all play major roles in managing emotions, concentration, learning, and memory.⁴⁵ The motivational system not only has an important impact on emotional aspects of behavior but also influences autonomic/physiological function, efficacy of learning, interpretation of sensations, and preparation for movement (Figure 10-1).⁴⁶ Dimensions of limbic function that influence motivation and the ability to manage challenges and frustration include body image, self-concept, and self-worth as related to social roles and expectations, as well as the perceived relevance or importance (based on reward or on threat) of an activity or situation.^47,48 Central set is a phrase used to describe the limbic system’s role as a motivator and repository of memory on readiness to move or act.⁴⁶ Central set helps people predict movement needs relevant to a given situation or circumstance (considering both the physical and affective dimensions of the environment in which they are acting) from past experience.⁴⁹

The ability to be alert and oriented when functioning in a complex environment is the purview of the consciousness system and is a function of interaction of the brainstem’s reticular formation, the filtering system of thalamic nuclei, and the thought and problem solving that occur in the association areas of the telencephalon, especially in the frontal lobes.50 The reticular activating system, found in the inferior mesencephalon and upper pons of the brainstem, is the locus of sleep-wake and level of alertness.⁵¹ The thalamus and the reticular formation help people habituate to repetitive sensory stimuli while they focus on the type of sensory information that is most germane to the task at hand.⁵² The frontal association areas add content to one’s consciousness: the ability to reason and to adapt to challenges encountered as one moves through daily life.⁵³ Alteration in quality and level of consciousness and behavior are indicators of evolving problems within the CNS.⁵⁴ Increasing intracranial pressure, the result of an expanding mass or inflammatory response following trauma or ischemia, may be initially manifest by confusion or agitation; progression into a state of lethargy, stupor, or unresponsiveness (coma) indicates deteriorating compromise of the CNS structures.⁵⁵

Homeostasis and the ability to respond to physiological stressors are functions of the components of the autonomic nervous system.⁵⁶ The nuclei of the hypothalamus serve as the command center for parasympathetic and sympathetic nervous system activity via projections to parasympathetic cranial nerve nuclei in the brainstem, the sympathetic centers of the intermediate horn in the thoracic spinal cord, and the parasympathetic centers in the lumbosacral spinal cord. The hypothalamus has extensive interconnections with the limbic system, bridging physiological and emotional/psychological aspects of behavior and activity.⁵⁷ The hypothalamus also integrates neural-endocrine function through interconnections with the pituitary gland.⁵⁸ Clearly, this relatively small area of forebrain plays a substantial integrative role in physiological function of the human body. Damage or dysfunction to this area therefore has significant impact on physiological stability and stress response.

Hypertonus is a term used to describe muscles that are influenced to be too stiff or are excessively biased toward supporting antigravity function. Spasticity is a type of hypertonus that typically occurs when there is damage to one or more CNS structures of the pyramidal motor system and is encountered as a component of many neuromuscular pathologies.75–79 Decrements in underlying tone and muscle performance, in bipedal humans with impairment of the pyramidal system, most often occurs in a decorticate pattern: The upper extremity is typically biased toward flexion, such that the limb can be easily moved into flexion but not into extension (decreased compliance of flexors). The lower extremity is biased toward extension, such that the impaired compliance of extensors makes movement into flexion difficult (Figure 10-3).⁸⁰ In persons with severe acquired brain injury the entire body may be biased toward extension, a condition or posture described as decerebrate pattern spasticity (Sherrington originally described this phenomenon as decerebrate rigidity).⁸⁰ Both decorticate and decerebrate conditions are unidirectional in nature; there is an increase of muscle stiffness and resistance to passive elongation (impaired compliance) in one group of muscles (agonists) with relatively normal functioning of opposing muscle groups (antagonists).

Spasticity is a velocity-dependent phenomenon. Under conditions of rapid passive elongation, spastic muscle groups “fight back” with increased stiffness, a result of a hypersensitive deep tendon reflex loop (Figure 10-4). This has been described as clasp-knife spasticity, in which the spastic limb “gives” after an initial period of resisting passive movement in the way that a pocket knife initially resists opening when near its initially closed position but then becomes more compliant once moved past a threshold position as it is opened. Growing evidence indicates that the stiffness encountered during passive movement has both neurological (spasticity) and musculoskeletal (changes in muscle and associated soft tissue) components that combine to increase the risk of contracture development.^81,82

Given the unidirectional nature of severe hypertonus, it is common for persons with severe hypertonus to develop chronic abnormal postures. The limb or body segment assumes an end-range position that the limb would not normally be able to assume (e.g., extreme equinovarus with marked supination in an individual with severe acquired brain injury, equinovalgus with marked pronation in a child with CP).80^,81 If persistent, these fixed postures are associated with a significant likelihood of secondary contracture development.⁸³

Hypertonicity is also associated with deficits in muscle performance, most notably diminished strength, diminished ability to produce power (generate force quickly), diminished ability to effectively isolate limb and body segments, diminished excursions of movements within joints (i.e., moving within a limited range of motion [ROM]), and inefficiency with altering force production or timing of contractions to meet changing (fluid) demands of tasks.^84–86 Muscle performance deficits can contribute to an imbalance of forces around a joint that leads to habitual abnormal patterns of movement. These habitual patterns are often biomechanically inefficient and, over time, contribute to the development of secondary musculoskeletal impairments such as adaptive shortening or lengthening of muscles and malalignment of joints.⁸³ Strengthening exercises can have a positive impact on function, even in the presence of hypertonicity.^85–87

Individuals with Parkinson’s disease and related neurological disorders often demonstrate varying levels of rigidity: a bidirectional, co-contracting hypertonicity in which there is resistance to passive movement of both agonistic and antagonistic muscle groups.88 Co-contraction of flexor and extensor muscles of the limbs and trunk creates a bidirectional stiffness that interferes with functional movement. Rigidity is often accompanied by slowness in initiating movement (bradykinesia), decreased excursion of active ROM, and altered resting postures of the limbs and trunk (Figure 10-5). The rigidity of Parkinson’s disease can be overridden under certain environmental conditions: Persons with moderate to severe disease can suddenly run reciprocally if they perceive danger to themselves or a loved one; once this initial limbic response has dissipated, they will resume a stooped and rigid posture, with difficulty initiating voluntary movement, limited active ROM, and bradykinesia. Because rigidity creates a situation of excessive stability of the trunk and limbs, orthoses are not typically a component in the plan of care for persons with Parkinson’s disease.⁸⁹

Hypotonus (low muscle tone) describes a reduced stiffness of muscle that does not effectively support upright posture against gravity or to generate force during contraction; as a result, hypotonic muscles are more compliant than they are stiff.90 In children, hypotonia can arise from abnormal function within the CNS (approximately 75%) or from problems with peripheral structures (peripheral nerve and motor units, neuromuscular junction, the muscle itself, or unknown etiology).⁹¹ Hypotonia can be congenital (seen as “floppy” infants), transient (e.g., in preterm infants), part of the clinical presentation of CP, Down syndrome and other genetic disorders, as well as Autism spectrum disorders.^91–96

Hypotonic muscles are considerably more compliant on rapid passive elongation (i.e., less resistant to passive stretch) than muscles with typical tone, as well as those with hypertonus/spasticity. Because their postural muscles are less stiff, individuals with hypotonicity often have difficulty when assuming and sustaining antigravity positions.⁹⁷ To compensate for their reduced postural tone, persons with hypotonicity may maintain postural alignment by relying on ligaments and connective tissue within joint capsule and muscle to sustain upright posture (Figure 10-6). With overreliance on ligaments in extreme ends of range, further degradation to joint structures often occurs.

In addition to postural control dysfunction, individuals with hypotonia often have difficulty with coordination of movements. This may be due to the decreased efficacy of afferent information collected by a lax muscle spindle during movement execution.98 Children with hypotonia often have impaired control of movements at midrange of muscle length, suggesting that kinesthetic information is not being used efficiently to guide movement or that the ability to regulate force production throughout movements is compromised. In either case, muscle performance is notably less efficient, especially in activities that require eccentric control (e.g., controlled lowering of the body from a standing position to sitting on the floor).⁹⁷ Individuals with hypotonia have difficulty regulating force production and collaboration between agonists and antagonists is not well coordinated.⁹⁷

Immediately after an acute CNS insult or injury there is often a period of neurogenic shock in which the motor system appears to shut down temporarily, with apparent loss of voluntary movement (paralysis) and markedly diminished or absent deep tendon reflex responses.^98–100 This phenomenon is observed following cervical or thoracic spinal cord injury and early on following significant stroke. During this period, individuals with extreme levels of hypotonus are sometimes (erroneously) described as having flaccid paralysis. Most individuals with acute CNS dysfunction will, within days to weeks, begin to show some evidence of returning muscle tone; over time, many develop hyperactive responses to deep tendon reflex testing and other signs of hypertonicity.⁹⁹ If they continue to have difficulty activating muscles voluntarily for efficient functional movement, these individuals are described as demonstrating spastic paralysis.

The term flaccidity is best used to describe muscles that cannot be activated because of interruption of transmission or connection between lower motor neurons and the muscles they innervate.101 True flaccidity is accompanied by significant atrophy of muscle tissue, well beyond the loss of muscle mass associated with inactivity; this is the result of loss of tonic influence of lower motor neurons on which muscle health is based.¹⁰²

The flaccid paralysis seen in persons with myelomeningocele (spina bifida) occurs because incomplete closure of the neural tube during the early embryonic period (soon after conception) prevents interconnection between the primitive spinal cord and neighboring somites that will eventually develop into muscles of the extremities.¹⁰³ The flaccid paralysis observed in persons with cauda equina level spinal cord injury is the result of damage to axons of α and γ motor neurons as they travel together as ventral roots to their respective spinal foramina to exit the spinal column as a spinal nerve.¹⁰⁴ After acute poliomyelitis, the loss of a portion of a muscle’s lower motor neurons will lead to marked weakness; the loss of the majority of a muscle’s lower motor neurons will lead to flaccid paralysis.⁶⁵ In Guillain-Barré syndrome, the increasing weakness and flaccid paralysis seen in the early stages of the disease are the result of demyelination of the neuron’s axons as they travel toward the muscle in a peripheral nerve.¹⁰⁵ After injection with botulinum toxin, muscle tone and strength are compromised because of the toxin’s interference with transmitter release from the presynaptic component of the neuromuscular junction.¹⁰⁶

Athetosis is the descriptor used when an individual’s underlying muscle tone fluctuates unpredictably.107 Athetosis is characterized by random changes in postural tone, with variations from hypertonus to hypotonus. Athetosis, although less common than spastic CP, can have as significant (and often more pronounced) an impact on daily function.¹⁰⁸ Although etiology of athetosis is not well understood, it is most frequently described as a type of basal ganglia or thalamic dysfunction associated with bilirubin toxicity or significant perinatal anoxia.¹⁰⁷ Persons with athetosis typically demonstrate truncal hypotonia, with fluctuating levels of hypertonus in antigravity musculature and the extremities. In some individuals, movements appear to have a writhing dancelike quality (choreoathetosis) in addition to the tonal influences. Others may compensate for postural instability by assuming end-range positions, relying on mechanical stability of joints during functional activity. Although persons with athetosis are less likely to develop joint contracture than those with long-standing hypertonicity, they are more likely to develop secondary musculoskeletal complications that compromise stability of joints, a result of extreme posturing, imbalance of forces across joint structures, and the need to stabilize in habitual postural alignment patterns for function.¹⁰⁹