Chapter 10 - Movement Control PDF

Chapter 10 Control of Movement Game 6 1986 World Series Super Bowl xiii Pittsburgh 35- Dallas 31...

Chapter 10 Control of Movement Game 6 1986 World Series Super Bowl xiii Pittsburgh 35- Dallas 31 Two out, down 2, Bottom of 10th https://www.youtube.com/watch?v=VbO2wlxYeWw https://www.youtube.com/watch?v=18caPNisP2U  Objectives Understand: Components of proprioception The neural structures involved in movement Main movement pathways The muscle structures involved in movement Some common reflexes Toxin/Diseases that impair movement Figure 10.3 Converging Inputs to Local Interneurons That Control Motor Neuron Activity 3  Muscle Sensory Organs Proprioception (kinesthesia) is the body's ability to sense movement, action, and location. 1.Muscle spindle 2.Golgi Tendon Organs 3.Joint receptors Length and Tension Monitoring Systems Figure 10.4 Muscle Spindle and Golgi Tendon Organ GTO 5 Golgi Tendon Organs Musculo-Tendinous Junction GTO  From an GTO Negative Feedback to SKM Earlier lecture!! AP to Muscle Muscle Contracts GTO fires SENSORY- AFFERENT InterNeuron inhibits a Motor Neuron Ib Afferent Ceases to fire Neuron Tension drops Ib Inhibitory “INTEGTRATIVE” - neuron Firing Golgi Tendon Organ a Motor Neuron Tension AP Time Time Function- EFFERENT There can be Descending pathway involvement  Golgi Tendon Organs Respond to Muscle Tension Golgi tendon organs (GTO) – found in junction of tendons and muscle fibers Golgi tendon organs: respond to tension a muscle puts on a tendon Composed of free nerve endings that wind between collagen fibers inside connective tissue capsule When a muscle contracts, its tendons act as a series elastic element during isometric phase Sends sensory information to the CNS -constantly monitor tension in tendons -sensory neuron stimulates interneuron in spinal cord. -interneuron inhibits motor neuron. -tension in tendon is reduced. Activation of Golgi Tendon Organs 10  GTO Resting Tension Contraction  Muscle Spindles 1. Muscles are made of thin muscle cells (MS) called intrafusal fibers and regular muscle fibers called extrafusal fibers 2. Muscle spindle apparatus: respond to muscle length (change) a. Muscles that require more control have more spindles. b. Stretching a muscle causes spindles to stretch. Relaxation Contraction Sends impulses to CNS Spindle intrafusal fiber  Muscle Sensory Organs Muscle Spindle Apparatus, c. Muscle spindle apparatus contains thin muscle cells called intrafusal fibers d. Two types of intrafusal fibers 1) Nuclear bag fibers – nuclei in loose central aggregates 2) Nuclear chain fibers – nuclei in rows e. Two types of sensory cells wrap around the fibers: 1) Primary (annulospiral) – most stimulated at the beginning of the stretch 2) Secondary (flower-spray) – respond more during sustained stretch Muscle Spindles Muscle Spindle Apparatus  Muscle Spindles Phasic receptors adapt rapidly and inform, about Contracted the rate of change of a stimulus Relaxed - adapt rapidly Tonic receptors adapt slowly and inform about the presence and strength of a stimulus -adapt slowly The could be applied to cold receptors in the skin– a plunge vs wading in  Muscle Spindles Respond to Muscle Stretch Muscle spindles are stretch receptors encode signals about muscle length and changes in muscle length Capsule encloses a group of small muscle fibers known as intrafusal fibers Innervation from gamma motor neurons Stretch reflex is a contraction response to the muscle stretch Alpha-gamma coactivation – Simultaneous activation of the alpha and gamma motor neurons Alpha motor neurons fire, muscle shortens, tension released Gamma motor neurons fire, intrafusal fibers contract, maintains stretch – Result: Spindle remains active  GTO vs MS Golgi Tendon Organ Ib afferents contraction Muscle Spindle Ia afferents load is now placed on the biceps, Ib afferents fire Golgi Tendon Organ contraction Muscle Spindle Ia afferent (same as above) is not affected until the arm moves Alpha (a) and Gamma (g) Motoneurons 1. Alpha: innervate extrafusal (contracting) muscle fibers 2. Gamma: innervate intrafusal (active stretch) muscle fibers a. Contraction of these fibers (g) does not shorten the muscle, but does increase sensitivity to stretch. b. Provides enough tension during relaxation to maintain muscle tone 3. Both types are stimulated by upper motor neurons at the same time - coactivation Alpha-gamma coactivation Figure 10.5 Alpha- Gamma (a) Muscle stretch Coactivation of Muscle Cells Maintains Muscle Spindle Sensitivity (b) Extrafusal fiber contraction to Muscle Length (c) Alpha-gamma coactivation 21  The Stretch Reflex REFLEX -an action performed as a response to a stimulus and without conscious thought  Figure 10.6 Neural Pathways Involved in the Knee-Jerk Reflex Patellar Ligament MONO vs POLY Synaptic G-P G-R-P Only stretch reflexes are monosynaptic, in that they have no interneuron. All other reflexes are polysynaptic and have at least one interneuron. 23  Crossed-Extensor Reflex Figure 10.8 In Response to Pain Detected By Nociceptors, the Ipsilateral Flexor Muscle’s Motor Neuron Is Stimulated (Withdrawal Reflex) and the Opposite Limb Is Extended (Crossed-Extensor Reflex) to Support the Body’s Weight Reciprocal Inhibition 24 Back to the Brain  Movement Overview Corticospinal Tract Motor Areas Motor Areas (pyramidal) Cerebellum Basal Ganglia Upper Motor Neuron to Cranial Nerve Motor nuclei and Spinal Cord Decussation Spinal Cord Lower Motor Neuron to Voluntary Muscles Figure 10.3 Converging Inputs to Local Interneurons That Control Motor Neuron Activity 27 Figure 10.1 Simplified Hierarchical Organization of the Neural Systems Controlling Body Movement 28  Figure 10.9 Major Motor Areas of the Cerebral Cortex (a) Major motor areas of the (b) Some components of cortex the sensorimotor cortex 29  WHERE CAN MOVMENT START? Primary Supplemental Primary Motor Motor Somatosensory Cortex Area Cortex Premotor Cortex Basal Prefrontal Nuclei Cortex 52 Brodmann Cerebellum Areas Brodmann Area 4  ~1mm Primary Supplemental Motor Primary Motor Cortex Somatosensory Area Premotor Cortex Cortex 1. Molecular Layer 2. External Granular Layer 3. External Pyramidal Layer 4. Internal Granular Layer 5. Internal Pyramidal Layer 6. Multiform Layer Layer 5 – Cells of Betz 6 layers  Layer 5 – Cells of Betz Cells of Betz –(aka pyramidal cells of Betz) - giant pyramidal cells (neurons) - the largest in the CNS, reaching 100 μm in diameter - located within the fifth layer of the grey matter in the primary motor cortex. - axons go down the spinal cord via the corticospinal tract, - synapse directly with anterior horn cells, which in turn synapse directly with their target muscles. (neurons) located within the fifth also known as pyramidal cells of sometimes reaching 100 μm in Betz) are giant pyramidal cells layer of the grey matter in the neurons are the largest in the primary motor cortex. These central nervous system, diameter. Figure 10.11 The Corticospinal and Brainstem Pathways Corticospinal Tract Cerebral Cortex Upper Motor Internal Neuron Capsule Brainstem Pyramids Spinal Cord Anterior Gray Horn Primary Motor pathway for voluntary movement (UMN,LMN) Lower Motor Neuron 33  Corona Radiata Knee Ankle Toes Eyelids Nares Lips Tongue Larynx Face INTERNAL CAPSULE  Corticonuclear Fibres Descending Tracts Corticospinal Subcortical Tracts UMN Lat. CST Ant. CST Brain - sagittal 3rd, 4th, 11th, 12th cranial nerves Nuclei III IV XI XII Cortico-bulbar/nuclear fibres LMN Figure 10.1 Simplified Hierarchical Organization of the Neural Systems Controlling Body Movement 36  Primary Motor Area BASAL (GANGLIA) Nuclei Start Movement ? Stop Movement Modulate Movement Reverberating Circuit MC to BG and/or Cerebellum  Basal (Ganglia) Nucleus 3 main pathways (Direct, Indirect, Nigrostriatal) Caudate Striatum Subthalamic Nucleus Putamen Substantia MC Globus Pallidus (not visible) Nigra BG/N -a group of subcortical nuclei primarily involved in motor control (start, stop and Modulate) -has other roles such as motor learning, executive functions and behaviors, and emotions. -Disruption of the BG network result in several movement disorders (PD). To LMN  VA/VL complex is the main motor thalamic relay to the cortex. Basal ganglia consist of several masses of grey matter located deep within the cerebral white matter Reverb Primary functions: (efferent copy) MC – inhibiting muscle tone throughout the body – selecting, maintaining purposeful motor activity – suppressing unwanted patterns of movement BG – coordinates slow, sustained contractions Starts movement Stops movement Modulates movement To LMN Intended vs actual movements  Basal (Ganglia) Nucleus Motor Cortex Caudate Thalamus Striatum Subthalamic Nucleus Putamen Substantia Located Globus Nigra Pallidus (2 parts) in the Externus Globus midbrain Pallidus pars compacta (SNpc) -dopaminergic neurons Internus pars reticulata (SNpr) GABA neurons.  Basal (Ganglia) Nucleus Direct Pathway Indirect Pathway Promotes Voluntary Movement Inhibits Movement In muscles In non contributing muscles Coordinated, smooth movement. Selected Recruitment – excess force is removed (inhibited)  DISinhibition VA/VL Upper Motor Globus Complex* Neurons in Cortex Striatum Pallidus Thereby So there is B is tonically active inhibiting C no excitation of D Nil Stimulation B is transiently And C is So D is inhibited DIS-inhibited excited *VA/VL complex is the main motor thalamic relay to the cortex.  Glutamate GABA an amino acid gamma-aminobutyric acid Most prevalent excitatory NT An INHIBITORY NT- stops Facilitates memory and learning cells from firing Excess glutamate is associated ~40% of synapses work with with GABA Panic attacks, anxiety, impulsivity, Anti-anxiety, anti convulsant OCD, -Depression  Parkinson’s Disease 3 Main Symptoms: 30% Bradykinesia (slowed movement) NO Tremors 30% More Severe Muscle Rigidity at the onset of the disease Symptoms Resting Tremors Tremors at the Less Severe onset of the disease Symptoms Leads to problems with speech, writing and balance Risk Factors: Age, Male (50% higher risk), Environmental Chemicals 100 Possible Causes: % Dopaminergic Neurons Remaining Protein Misfolding, aggregation and toxicity N Defective Proteolysis Mitochondrial Dysfunction 50 Oxidative Stress PD 0 AGE With PD - the input to the basal nuclei is diminished, the interplay of the facilitatory and inhibitory circuits is unbalanced, and activation of the motor cortex is reduced.  Dopamine as a Neurotransmitter Motor Cortex Nigrostriatal Dopamine System a. Neurons from the substantia nigra (part of the basal nuclei) of the brain send dopaminergic neurons to the corpus striatum (major input site of Substantia Nigra the basal nuclei) (2 parts) b. Important step in the control and pars compacta (SNpc) - initiation of movements dopaminergic neurons pars reticulata (SNpr) GABA c. Parkinson disease is caused by neurons. degeneration of these neurons. 1) Patients are treated with L-dopa and MAOIs (monoamine oxidase inhibitors*).  Cerebellum Balance Co-ordinated Movement Motor learning “Little Brain” -75% of the Cerebrum surface area- contains over 50% of the brain neurons Cerebellum 3 lobes (Anterior, Posterior, Flocculonodular) Proprioceptors Inner Ear Dense with folds- arbor vitae Coordinates Movements (smoothens, balance and posture, tone) Cerebellar Peduncles Maintains posture and equilibrium Superior - Midbrain ~10% of brain mass - ~ 50% of neurons S Middle - Pons Regions:outer cortex (gray), Arbour Vitae (white), Deep M I Inferior - Medulla Cerebellar nuclei Receives input: Equilibrium, Proprioceptive information (where limbs are in space),  Cerebellum Spinocerebellum Cerebrocerebellum Vestibulorcerebellum Spinocerebellum Vestibulorcerebellum Cerebrocerebellum  Cerebellum Three functionally distinct parts Vestibulocerebellum: balance and eye movement Spinocerebellum: enhances muscle tone and coordinates skilled movements Cerebrocerebellum: planning and initiating voluntary activity and stores procedural memories  Cerebellum Vermis Cerebrocerebellum Vestibulocerebellum Spinocerebellum (Flocculonodular Lobe) Cerebellar Hemispheres  Cerebellum Intermediate Vermis Intermediate Primary Zone Zone Fissure Anterior Spinocerebellum Lobe Cerebrocerebellum Horizontal Fissure Posterior Lobe Posterior Fissure Vestibulocerebellum Flocculus Flocculus (Flocculonodular Lobe) Nodulus Hemisphere Hemisphere  Cerebellum Arbor Vitae -tree-like appearance - dense with folds -cerebellar white matter -present in both cerebellar hemispheres Purkinje cells brings sensory and motor information to 5 types of neurons exist the cerebellar cortex and from the 4 are inhibitory (Purkinje, basket, stellate and cerebellum. Golgi) 2 are excitatory (granule cells, unipolar brush) A 6th type is found primarily in the floccular lobe and the vermis (the unipolar brush cell) https://nba.uth.tmc.edu/neuroscience/m/s3/chapter05.html  Cerebellum The cerebellum influences posture and movement through its 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. receives information from the sensorimotor cortex and also from the vestibular system, eyes, skin, muscles, joints, and tendons. 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 and is involved in “muscle memory.” 52  Cerebellum -participates in planning movements: integrating information about the nature of an intended movement with information about the surrounding space. During movement, -compares information about what the muscles should be doing with information about what they actually are doing and can send correction signals if needed. Individuals with cerebellar disease have uncoordinated movements, cannot start or stop movements quickly or easily, and cannot combine the movements of several joints into a single smooth, coordinated motion (have trouble walking). 53  Lesions Upper Motor Upper Motor Neurons Lower Motor Neurons Neurons Tonicity elastic Tonicity [ Hyper Reflexia [ Hypo Reflexia tension Spasticity stiff or tight + Atrophy +++ Atrophy Single No Fasciculations* Fasciculations fibres contract + Babinski Sign - Babinski Sign Ç Lower √ Motor Neurons a Motor Neuron * involuntary rapid muscle twitches that are too weak to move a limb but are easily felt by patients and seen or palpated by clinicians.  Babinski’s reflex. (a) The stimulation pattern on the sole of the foot. (b) A negative Babinski test. (c) A positive Babinski test. Babinski Test UMN LMN - Mass Strength 15-20% Spastic Paralysis 80% Flaccid Paralysis Tone Hypertonia Hypotonia DTR’s Hyper-reflexia Hyporeflexia Fasciculations Absent Present + Fibrillations Absent Present + Babinski Present + Absent Pronator Drift Present + Absent Hoffman Sign* Present + Absent + Fasciculation - a visible, involuntary muscle twitching. DTR = Deep Tendon Reflex *The reflexive pathway causes the thumb to flex and adduct quickly. A positive Hoffman sign indicates an upper motor neuron lesion and corticospinal pathway dysfunction likely due to cervical cord compression.  Summary MOVEMENT DESCENDING SYSTEMS (Upper Motor Neurons) BASAL GANGLIA Motor Cortex Gating proper initiation of Movement Planning, initiating and directing voluntary movements CEREBELLUM Brain Stem Centers Sensory motor Basic Movements and Postural Control Coordination of ongoing Movement Local circuit neurons SPINAL CORD Lower motor neuron AND Motor neuron pools integration BRAINSTEM CIRCUITS (Lower motor neurons) Sensory inputs Skeletal Muscles  Clinical Case Study A 55-year-old woman was brought into an urgent-care clinic with muscle pain and trouble speaking. Her husband explained that over the previous 3 days, her back and jaw muscles had grown gradually stiffer and more painful. At this point, she could barely open her mouth. At the time of examination, her blood pressure and temperature were normal, findings from a head and neck exam were unremarkable (other than the stiff jaw), her lung sounds were clear, and her heart sounds were normal. While evaluating her extremities, the physician noticed that her right leg was bandaged. A little over a week prior to this visit, she had been working in her garden and had stumbled and fallen onto a rake. She had washed and bandaged the wound herself. Removal of the bandage revealed a raised 5-centimeter-wide erythematous region surrounding a 0.5 centimeter puncture wound that had scabbed over. When asked when she had received her latest tetanus booster shot, she guessed it had been more than 20 years. This information, along with her leg wound and symptoms, led the physician to conclude that the woman had tetanus. She was immediately hospitalized, her leg wound was thoroughly cleaned, and she was treated aggressively with tetanus immune globulin (TIG) and antibiotics. What are the effects of the tetanus toxin on the nervous system Why are the jaw muscle affected early on (leading to the common name, “lockjaw”)? 57  Terms Muscle tone is the resistance to stretch exhibited by a relaxed muscle. - due both to the passive elastic properties of the muscles and joints and to the degree of ongoing alpha motor neuron activity. Hypertonia- Abnormally high muscle tone -due to a greater-than- normal level of alpha motor neuron activity. – Hypertonia is accompanied by either spasticity, in which the excess tone diminishes as the muscles are stretched, or rigidity, in which the excess tone is constant. Hypotonia- Abnormally low muscle tone - due to disorders of alpha motor neurons, neuromuscular junctions, or the muscles themselves. – Hypotonia is accompanied by weakness and atrophy. 58  Cerebral Cortex Primary motor cortex – Located in frontal lobe – Confers voluntary control over movement produced by skeletal muscles – Primarily controls muscles on the opposite side of the body – Motor homunculus Depicts location and relative amount of motor cortex devoted to output to muscles of each body part Other Brain Regions and Motor Control Supplementary motor area: preparatory role in programming of complex sequences Premotor cortex: assists primary motor cortex Primary Somatosensory Cortex : guides premotor cortex  Basis for Control of Body Movement Body movement is controlled through Motor Units: Motor neurons + all the skeletal muscle fibers they innervate Motor neurons are considered the final common pathway out of the CNS, because all neural input for skeletal muscles merges into motor neurons. Motor Neuron Pool for a skeletal muscle consists of all of the motor neurons that affect the muscle Coordinated movement requires a balance of inputs between both excitatory and inhibitory messages Almost all body movements, including very simple ones, include the timed activation of many motor units in several muscles. Many skeletal muscle contractions, like maintenance of posture, are isometric, and do not involve movement. 60  Motor Programs Motor Program: Sequence of neural activity required to carry out a desired movement Created by middle level neurons, as they coordinate information about the current position of body parts and the space in the immediate vicinity of the body Information from the motor program is relayed to the lowest level neurons in the appropriate local area, via Descending Pathways. Motor programs require constant adjustments during the execution of a movement Middle level neurons continue to receive updated information about the movements in progress 61  Table 10.1 Conceptual Motor Control Hierarchy for Voluntary Movements Higher centers 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. The middle level 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 and the thalamus The local level 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. 62  Voluntary and Involuntary Actions Voluntary movements are accompanied by a conscious awareness of what we are doing and why we are doing it, and our attention is directed toward the action or its purpose. Involuntary movements are often characterized as unconscious, automatic, or reflex. Almost all motor behavior involves both voluntary and involuntary components. 63  Local Control of Motor Neurons Local control systems receive instructions from higher brain centers and make adjustments based on information received from sensory receptors in the muscles, tendons, joints, and skin of the body parts to be moved. Local control systems help adjust motor unit activity to unexpected obstacles to movement and painful stimuli in the environment. To make adjustments to movement, local control systems use information carried by afferent fibers from sensory receptors in muscles, tendons, joints and skin 64  Muscle Spindles & Golgi Tendon Organs Muscle Spindles monitor muscle length, while Golgi Tendon Organs monitor muscle tension. They are types of stretch receptors. By relaying impulses to the brain, muscle spindles and golgi tendon organs provide information about muscle position and stretch in order to finely regulate the speed and intensity of muscle contraction. Their action also prevents excessive stretching of the muscles and tendons, to prevent possible injury. 65  Stretch Reflexes and Muscle Spindles Stretch Reflex: A monosynaptic reflex, in which stretching of a muscle leads to contraction of the same muscle Stretching of intrafusal fibers of the muscle by an external force results in increased action potentials and a rapid response of muscle contraction Contraction of extrafusal fibers of the muscle and shortening of a muscle release the tension on the spindle, decrease action potentials, and decrease neuronal messages sent to the CNS Example: Knee Jerk Reflex – A reflex in which stretching of the patellar tendon & ligament results in contraction of the Quadriceps femoris muscles and extension of the knee – Stretch receptors for this reflex are within the Quadriceps muscles – This is part of the walking mechanism; important for balance & posture Only stretch reflexes are monosynaptic, in that they have no interneuron. All other reflexes are polysynaptic and have at least one interneuron. 66  The 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 known as a withdrawal reflex. The same stimulus causes 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. 67  Cerebral Cortex 1 A network of connected neurons in the frontal and parietal lobes of 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. 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 (also called the motor cortex) and the premotor area. The neurons of the motor cortex that control muscle groups in various parts of the body are arranged anatomically into a somatotopic map similar to that seen in the somatosensory cortex. 68  Cerebral Cortex 2 Other areas of sensorimotor cortex include the supplementary motor cortex, which lies mostly on the surface of the frontal lobe where the cortex folds down between the two hemispheres, the somatosensory cortex, and parts of the parietal-lobe association cortex. Although these areas are anatomically and functionally distinct, they are heavily interconnected, and individual muscles or movements are represented at multiple sites. The cortical neurons that control movement form a neural network, meaning that many neurons participate in each individual movement. 69  Cerebral Cortex 3 The interactions of the neurons within the networks are flexible so that the neurons are capable of responding differently under different circumstances. This adaptability enhances the possibility of integrating incoming neural signals from diverse sources and the final coordination of many parts into a smooth, purposeful movement. It also accounts for the remarkable variety of ways in which we approach a goal. For example, you can comb your hair with the right hand or the left, starting at the back of your head or the front. This same adaptability also accounts for some of the learning that occurs in all aspects of motor behavior. 70  Cerebral Cortex 4 Additional brain areas are involved in the initiation of intentional movements, such as the basal nuclei, cerebellum, and 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. 71  Subcortical and Brainstem Nuclei 1 Numerous highly interconnected structures lie in the brainstem and in 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. These structures may play a minor role in motivation and initiating movements. Their role is to establish the programs that determine the specific sequence of movements needed to accomplish a desired action. 72  Subcortical and Brainstem Nuclei 2 Subcortical and brainstem nuclei are also important in learning skilled movements. Prominent among the subcortical nuclei are the paired basal nuclei, which consist of a closely related group of separate nuclei. This explains why brain damage to subcortical nuclei following a stroke or trauma can result in either hypercontracted muscles or flaccid paralysis—it depends on which specific circuits are damaged. 73  Parkinson’s Disease 1 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 is reduced. Clinically, Parkinson’s disease is characterized by a reduced amount of movement (akinesia), slow movements (bradykinesia), muscular rigidity, and a tremor at rest. Other motor and nonmotor abnormalities may also be present, such as a change in facial expression resulting in a mask-like, unemotional appearance, a shuffling gait with loss of arm swing, and a stooped and unstable posture. 74  Parkinson’s Disease 2 The initial defect in Parkinson’s disease 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 activation of the sensorimotor cortex. 75  Parkinson’s Disease 3 It is not known what causes the degeneration of neurons of the substantia nigra and the development of Parkinson’s disease. It may have a genetic cause, and exposure to environmental toxins such as manganese, carbon monoxide, and some pesticides may also be a contributing factor. The drugs used to treat Parkinson’s disease are all designed to restore dopamine activity in the basal nuclei. There are three categories: – agonists (stimulators) of dopamine receptors, – inhibitors of the enzymes that metabolize dopamine at synapses, and – precursors of dopamine itself (example - Levodopa, or L-dopa) 76  Cerebellum 1 The cerebellum influences posture and movement through its 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. 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 and is involved in “muscle memory.” 77  Cerebellum 2 The cerebellum also participates in planning movements: integrating information about the nature of an intended movement with information about the surrounding space. During movement, the cerebellum compares information about what the muscles should be doing with information about what they actually are doing and can send correction signals if needed. Individuals with cerebellar disease have uncoordinated movements, cannot start or stop movements quickly or easily, and cannot combine the movements of several joints into a single smooth, coordinated motion (have trouble walking). 78  Descending Pathways 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. Types of descending pathways: the corticospinal pathways, which originate in the cerebral cortex, and the brainstem pathways, which originate in the brainstem. 79  Corticospinal Pathway 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 to descend on the opposite side (called decussation). 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. Corticospinal pathways control rapid, fine movements of the distal extremities, such as the manipulation of an object with the fingers. 80  Brainstem Pathways Axons from neurons in the brainstem also form pathways that descend into the spinal cord to influence motor neurons. These pathways are called the brainstem pathways (or extrapyramidal system). Axons of most of the brainstem pathways remain uncrossed and affect muscles on the same side of the body, although a few do cross over to influence contralateral muscles. The brainstem descending pathways are involved with coordination of the large muscle groups of the trunk and proximal portions of the limbs used in the maintenance of upright posture, in locomotion, and in head and body movements when turning toward a specific stimulus. 81  Muscle Tone Muscle tone is the resistance to stretch exhibited by a relaxed muscle. Intrinsic muscle tone in smooth muscle is due to a baseline level of calcium in the cytosol that causes low-level activity of tension-generating cross-bridges. By contrast, muscle tone in skeletal muscles is due both to the passive elastic properties of the muscles and joints and to the degree of ongoing alpha motor neuron activity. 82  Abnormal Muscle Tone Abnormally high muscle tone is called hypertonia, and it is due to a greater-than-normal level of alpha motor neuron activity. – Hypertonia is accompanied by either spasticity, in which the excess tone diminishes as the muscles are stretched, or rigidity, in which the excess tone is constant. Abnormally low muscle tone is called hypotonia, and it is due to disorders of alpha motor neurons, neuromuscular junctions, or the muscles themselves. – Hypotonia is accompanied by weakness and atrophy. 83  Amyotrophic Lateral Sclerosis Amyotrophic lateral sclerosis (ALS) is a lower motor neuron condition in which progressive degeneration of alpha motor neurons causes hypotonia and atrophy of skeletal muscles. In most cases the causes are unknown, but causes may include viruses, neurotoxins, heavy metals, immune system abnormalities, or enzyme abnormalities. About 5 to 10% of cases are inherited, with about half of them being caused by a defect in a gene coding for an enzyme that protects neurons from free radicals generated during oxidative stress. There is currently no cure for ALS; treatment consists of medications and respiratory, occupational, and physical therapies that provide relief from symptoms and maintain comfort and independence as long as possible. 84  Maintenance of Upright Posture and Balance The skeleton cannot stand erect against gravity without the support of muscle activity. Muscles that maintain upright posture, supporting body weight against gravity, are controlled by brain and by reflexes connected to neural networks of brainstem and spinal cord. Added to the problem of maintaining upright posture is that of maintaining balance. For stability, the center of gravity must be kept within the base of support the feet provide. Once the center of gravity has moved beyond this base, the body will fall unless one foot is shifted to broaden the base of support. Yet, people can operate under conditions of unstable equilibrium because complex interacting postural reflexes maintain their balance. 85  Walking Walking requires the coordination of many muscles. Walking is initiated by falling forward to an unstable position and then moving one leg forward to provide support. Under normal conditions, neural activation occurs in the cerebral cortex, cerebellum, and brainstem, as well as the spinal cord during locomotion. Damage to even small areas of the sensorimotor cortex can cause marked disturbances in gait. 86