**4.1 Overview of brain areas and pathways involved in control of posture and movement**

The normal control of movement (voluntary and involuntary, gross and fine), maintenance of a stable posture and balance, muscle tone and coordination of motor activity involve intricate interactions between the cerebral motor cortex, basal nuclei/ subcortical grey mater (putamen, globus pallidus, subthalamic nucleus, substantial nigra, thalamus), brainstem nuclei (vestibular nucleus, superior colliculus, red nucleus) and cerebellum with the cells of the ventral or anterior horn of the spinal cord (alpha and gamma motor neurons) [65]. On emerging from the anterior horn cells, the motor neurons in peripheral nerves (lower motor neurons [LMN]) innervate the muscles of the body to effect movement/muscle contraction [65]. Each alpha motor neuron and all muscle fibres that it innervates constitute a motor unit—the functional unit of the motor system [65]. Functionally, the descending motor pathways from the brain to spinal cord (upper motor neurons [UMN]) are subdivided into pyramidal and extrapyramidal pathways [65]. The pyramidal pathway arise from the cerebral cortex and send motor signals to the spinal cord (corticospinal tract) and to brainstem nuclei (corticobulbar tract) for voluntary control of muscles of the body and face respectively [65]. The extrapyramidal tracts (vestibulospinal, reticulospinal, rubrospinal, tectospinal tracts) take their origin from different brainstem nuclei and project to the spinal cord for control of involuntary/automatic muscle activity like control of muscle tone through the stretch reflex, posture and movement [65]. The stretch reflex arc for maintenance of muscle tone is controlled by the inhibitory influence of corticospinal and dorsal reticulospinal tracts and facilitatory influence by medial reticulospinal and vestibulospinal tracts [65, 66]. The basal nuclei function to facilitate or fine tune voluntary movement while inhibiting undesired movements and they receive projections from the motor cortex and project back to the motor cortex through the thalamus [65]. Thus, the occurrence of unwanted involuntary movements in basal ganglia injury. The cerebellum is also deeply involved in maintenance of balance, posture and

coordination of movement and damage to it produces ataxia [65, 66]. The pyramidal/ corticospinal tract is increasingly vulnerable to damage at different points along their long course to the spinal cord and commonly include their site of origin at the cerebral cortex, corona radiata and the white matter (internal capsule) between the thalamus and basal nuclei [65]. This may contribute to the high prevalence of spastic CP and its combination with other forms of CP in the so-called "mixed CP" subtypes such as spastic dystonic CP and combinations of spasticity and choreoathetosis.

#### **4.2 Neuromotor impairments and musculoskeletal deficits in CP**

In spastic CP, the brain lesions in the various predominant locations disrupt the descending pyramidal pathways resulting in an UMN syndrome whose primary manifestations are categorized into positive and negative features that act in concert to cause secondary progressive musculoskeletal pathology/impairments [66–68]. The positive features of UMN syndrome are spasticity, hyperreflexia, clonus, co-contraction while the negative features include weakness, loss of selective motor control (SMC), poor balance, fatigability and sensory deficits [66–68]. The positive features result from brain lesions disrupting the facilitatory corticobulbar fibres (from the premotor cortex), thus leading to inhibition of the dorsal reticulospinal tract (from the brainstem ventromedial reticular formation) which exerts inhibitory control over the stretch reflex [66–68]. Spasticity refers to a velocity-dependent increase in muscle tone with exaggerated tendon jerks due to hyperexcitable or increased tonic spinal stretch reflex [65–68]. This implies that the loss of inhibition of the spinal stretch reflex by descending pathways result in overactivity of the spinal stretch reflex and underlies the findings of spasticity, hyperreflexia and clonus in pyramidal CP [66–68]. The voluntary output from the motor cortex activates motor neurons targeting the agonist muscles while simultaneously inhibiting the antagonist muscles through the Ia interneurons (reciprocal inhibition) [66]. It is the loss of this reciprocal inhibition of antagonist muscles during voluntary command that underlies cocontraction and it makes generation of force or movement difficult [66].

In spastic CP, there is significant weakness that contributes to abnormal posture and movement. The weakness is consequent on a number of factors such as reduced muscle size/volume, reduced muscle activation, lower frequency motor unit firing rates and increased Type 1 muscle fibres due to an altered neural input to muscle (reduced neuromuscular activation) caused by the damage to the descending corticospinal tracts [68]. This is also accompanied by decreased muscle endurance and loss of selective motor control (SMC) [67, 68]. Loss of selective motor control is the impaired ability to single out the activation of specific muscles in response to demands of a voluntary posture or movement [67, 68]. For example, the co-activation of quadriceps femoris (knee extension) and gastrocnemius (ankle planter flexion) in a child with severe spastic CP [68]. The weakness, impaired SMC and poor balance (negative features) are pivotal in determining when or if a child with CP will walk [67, 68]. It is also important to note that some surgical interventions for spasticity such as muscle lengthening, tendon transfer, selective dorsal rhizotomy and intrathecal baclofen all reduce muscle strength while orthoses and serial casting may worsen weakness through immobilization [67, 68]. The reduced descending excitatory signals on muscle growth results in impaired muscle growth (smaller muscles) and a short muscle-tendon unit which contributes to muscle weakness in spastic CP("short muscle disease") [67, 68]. The failure of muscle growth to progress at same speed with bone growth (muscle-to-bone growth rate discrepancy) which is more prominent in bi-articular muscles like rectus femoris, hamstrings and

#### *Aetiology and Pathophysiology of Cerebral Palsy DOI: http://dx.doi.org/10.5772/intechopen.106685*

gastrocnemius underlies the joint contractures and gait abnormalities such as toewalking and flexed-knee gait in spastic CP [67, 68].

In non-spastic or extrapyramidal CP with damage to the basal nuclei, the clinical manifestations are abnormal, involuntary, uncontrolled, recurrent and occasionally stereotyped movements with fluctuating tone and persistence of primitive reflexes [68]. In dystonia, there are involuntary sustained or intermittent muscle contractions of both agonist and antagonist muscles causing twisting and repetitive movements and or abnormal postures with increased tone [65–68]. Choreoathetosis is characterized by a combination of random-appearing sequence of one or more discrete, excessive and rapid movements or fragment of movement of proximal body parts/ trunk (chorea) with slow, continuous, writhing movements of distal body parts that impedes maintenance of a stable posture (athetosis) [65–68]. Both dystonic and choreoathetoid movements impair function [68].

However, the terms spastic (pyramidal) and extrapyramidal CP are strictly incorrect [5, 65]. It is more accurate to refer to these as "predominantly spastic" and "predominantly non-spastic" [65] . Due to the complex interactions of the upper motor neuron system (the pyramidal, extrapyramidal and cerebellar pathways) with anterior horn cells to control posture and movement, lesions causing CP in real life usually involve both pyramidal and extrapyramidal pathways [65]. This explains the clinical combination of motor/movement abnormalities such as spasticity with dystonia, and spasticity with choreoathetosis ("mixed CP"). Thus, the mixed CP subtype should actually be very common but spastic CP remains the commonest type thereby exposing the subjectivity and imprecision in assessment of patients based on the physiologic classification of CP [2, 69].

In the rare ataxic CP with damage to the cerebellum, clinical features are hypotonia, limb incoordination, and poor balance and these result in instability and a compensatory wide base of support with elevated, outstretched arm postures to improve balance during gait (ataxia) [65, 68].

Therefore, the primary neurologic correlates of early brain injury in CP include: [2, 66–68].


#### **4.3 Secondary impairments and accompanying disorders in CP**

The accompanying physical, mental or physiological impairments identified in the current definition of CP include epilepsy, cognitive impairment (intellectual disability), speech, visual and hearing impairments and secondary musculoskeletal pathology [1]. These secondary or accompanying impairments are significant since they may cause more functional limitation than the primary motor dysfunction (the core feature of CP) [1, 2].
