HOW IS VOLUNTARY MOVEMENT PERFORMED? Dr.M.Tamilkodi M.D. Professor of Physiology, Tirunelveli Medical College.

NERVOUS SYSTEM – communications and control network that facilitates the organism to interact in appropriate ways with its environment. GENERAL FUNCTIONS Sensory detection Information processing- including learning and memory Expression of behavior – that includes cognition, motor act ( movement or autonomic response) - because of intercellular communications in the neural circuits.

Voluntary movements requires the interaction of multiple levels the nervous system--- Ultimately contractions of skeletal muscles are responsible for the movements of the body α-motor neurons in the spinal cord form the final common path for motor control Peripheral sensory input and spinal cord tracts descending from the brainstem and cerebral cortex influence the motor neurons Cerebellum and basal ganglia contribute to the motor control by modifying brainstem and cortical activity

Proprioceptors - - - Muscle spindle – inform muscle length and velocity of stretch Golgi tendon organs – inform the force generated Alpha motor neurons – larger cells for high threshold fast twitch (innervate extrafusal fibers) Gamma motor neurons – smaller cells for low threshold slow twitch; fatigue resistant.(innervate intrafusal fibers)

Smaller motor neurons are activated first - muscles concerned with continuous postural support ( anti gravity muscles) are supplied by them. Forceful movement is carried by fast- twitch fibers which are less sustainable and easily fatigued. Spinocerebellar tracts – carry information to cerebellum regarding the status of muscle length and tension.

SPINAL CORD Muscles interact extensively in the maintenance of posture and production of coordinated movement. Much of these interaction is controlled by the circuitry in the spinal cord Axial muscles – innervated by motor neurons that are more medially situated Limb muscles – innervated by more laterally placed motor neurons Neurons innervating flexors and extensors are also segregated

SPINAL REFLEXES SEGMENTAL ORGANISATION – refers to the basic circuits and connections that take place at each level of the spinal cord Basic spinal reflexes are a. Myotatic or stretch reflex b. Inverse stretch reflex c. Flexor withdrawal reflex

2.PROPRIOSPINAL SYSTEM– the series of neurons whose axons run up and down to connect different levels of the spinal cord. This allows co-ordination of activities at different spinal levels which is important for behavior such as locomotion 3.DESCENDING MOTOR PATHWAYS – that interact with the spinal circuits they carry the signals related to voluntary movement; they are also important for the more automatically (non consciously) controlled aspects of motor functions such as setting of muscle tone (the resting resistance of the muscles to changes in length)

Spinal cord can produce basic locomotor actions Normal strategy for generating basic locomotion engages central pattern generators (neural circuitry in the spinal cord, that can produce contraction of the limb flexors and the extensors needed for walking) and uses both sensory feedback and efferent impulses from higher motor control centers for the refinement of control

BRAINSTEM Contains neural circuitry for initiating locomotion and for controlling posture. Maintenance of posture requires the coordinated activity of both axial and limb muscle in response to input from proprioceptors and spatial position sensors such as inner ear Cerebral cortical input through corticospinal system is necessary for the control of fine individual movement of distal limbs and digits.

Brainstem is the origin of three descending tract that influence movements Rubrospinal tract – active during locomotion e.g. stepping (in humans, corticospinal tract super cede) Vestibulospinal tract- maintains posture in response to changes in position of head Reticulospinal tract – inhibitory to interneurons modulating extensor motor neurons

THEREFORE Voluntary movements depend upon interactions among motors areas of cerebral cortex, cerebellum and the basal ganglia

MOTOR CORTICAL AREAS Primary motor cortex or excitomotor cortex or Brodmann’s areas 4 Premotor cortex – area 6 Supplementary motor cortex (Penfield) mapped out on the medial surface Second motor cortex (Sugar) – on the upper bank of sylvian fissure Area 4S – suppressor area for area 4 Area 8 – Frontal eye field

OUTPUT – Corticospinal or Pyramidal system Extrapyramidal system FUNCTION – Initiation of voluntary and skilled movements of the body Control of both voluntary and involuntary movements of the body

EXTRAPYRAMIDAL SYSTEM COMPONENTS: Cortical- COEEPS & COIEPS Subcortical – Caudate nucleus, Putamen, External Pallidum & Internal Pallidum Thalamus- Nucleus Ventro Anterior& Nucleus Ventro Lateral of lateral thalamic mass Subthalamic nucleus Brainstem – Red nucleus, Substantia Nigra, Reticular nuclei Cerebellum

CONNECTIONS: Cortico-thalamo cortical circuit Dentato-rubro-thalamo-cortical circuit Neo-cortico-neo-cerebellar tracts Cortico-nigral fibers Cortico-strio-nigral fibers Domestic connections among basal ganglia – described by HESSLER & JUNG Reticulo-cerebellar fibers Cortico-ponto-cerebellar fibers OUTPUT: Rubrospinal tract Reticulospinal tract – (inhibitory) Tectospinal tract Vestibulospinal tract – (facilitatory)

FUNCTIONS OF CEREBELLUM Maintenance of muscle tone and posture – spinocerebellum Coordination of movements – compares the intention of the motor cortex & the momentary status of the muscles concerned and feeds back motor cortex Influences the rate, range, force & direction of movements Influences the ocular movements Maintenance of posture and equilibrium

NEUROTRANSMITTAORS FOUND IN BASAL GANGLIA

Examples for high-skilled movements- Finger movements of a neurosurgeon manipulating micro-surgical instruments while repairing a cerebral aneurysm Eye-hand-body coordination of a professional basket ball player making a shot

MOTOR CONTROL SYSTEM WORKING AT A SEEMINGLY AUTOMATIC LEVEL: Coordinated contraction of hip flexors and ankle extensors while clearing a pavement irregularity encountered during walking. EXAMPLE FOR PERTURBED MOTOR CONTROL 1) Stiff- legged stride of stroke patient 2) Swaying walk of an intoxicated person

Studies have been done to understand the above said facts – 1)Spinal animal 2)Midbrain animal 3)Hypothalamic lesion 4)Thalamic animal 5)Decerebrate animal 6)Decorticate rigidity

SUMMARY Initiation of voluntary movement –Pyramidal or Corticospinal system Preparedness, go smooth and finish movement – Extra pyramidal system Affect and mood involved at the moment decided by – Limbic system Attention and alertness provided by – Reticular formation Reflex activity carried by – spinal cord Control of reflex activities – Brain stem