Neuromotor Basis for Motor Control Chapter 4 Neuromotor Basis for Motor Control Concept: The neuromotor system forms the foundation for the control of movement

Introduction The Neuromotor System Components of the central nervous system (CNS) and peripheral nervous system (PNS) involved in the control of coordinated movement Focus of current chapter is CNS structure and function Chapter 6 will include PNS related structure and function for tactile, visual, and proprioceptive sensory systems

The Neuron General Structure [see Fig. 4.1] Neuron = Nerve cell Cell body Contains nucleus Dendrites Extensions from cell body – range from 1 to thousands per neuron Receive information from other cells Axon (also known as a “nerve fiber”) Extension from cell body – one per neuron with branches (known as collaterals) Sends information from neuron Neuron = Nerve cell Basic component of the nervous system Range in size from 4 to 100 microns

Types and Functions of Neurons Three Types of Neurons 1. Sensory Neurons [see Fig. 4.2] Also known as “afferent” neurons Send information to CNS from sensory receptors Unipolar – 1 axon; no dendrites Cell body and most of the axon located in PNS; only axon central process enters CNS

Types and Functions of Neurons, cont’d 2. Motor Neurons [see Fig. 4.2] Also known as “efferent” neurons Two types influence voluntary movement: 1. Alpha motor neurons Predominantly in spinal cord – axons synapse on skeletal muscles 2. Gamma motor neurons In intrafusal fibers of skeletal muscles

Types and Functions of Neurons, cont’d 3. Interneurons [see Fig. 4.2] Specialized neurons that originate and terminate in the brain or spinal cord Function as connections between: Axons from the brain and synapse on motor neurons Axons from sensory nerves and the spinal nerves ascending to the brain

Central Nervous System (CNS) Two components: Brain and spinal cord The Brain 4 structural components most directly involved in the control of voluntary movement: 1. Cerebrum 2. Diencephalon 3. Cerebellum 4. Brainstem

Brain Components: 1. Cerebrum One of two components of forebrain Two halves Right cerebral hemisphere Left cerebral hemisphere Covered by cerebral cortex Gray tissue; 2- to 5-mm thick Undulating covering of Ridges – each is called a gyrus Grooves – each is called a sulcus Cortex motor neurons Pyramidal cells Nonpyramidal cells Connected by the corpus callosum

Cerebral Cortex Four lobes Sensory cortex Frontal Parietal Occipital Temporal Sensory cortex Posterior to central sulcus Receives neuron axons specific to type of sensory information Named according to nearest skull bone

Cerebral Cortex, cont’d Association areas [see Fig. 4.4] Location Adjacent to specific sensory areas of sensory cortex Function To “associate” information from the several different sensory cortex areas Allow the interaction between perceptual and higher-order cognitive functions e.g., selection of the correct response in a choice-RT situation Possible locations for transition between perception and action

Cerebral Cortex, cont’d Primary motor cortex Location & Structure Frontal lobe just anterior to central sulcus Contains motor neurons that send axons to skeletal muscles Function Involved in control of: Initiation and coordination of movements for fine motor skills Postural coordination

Cerebral Cortex, cont’d Premotor area Location: Anterior to the primary motor cortex Functions include Organization of movements before they are initiated Rhythmic coordination during movement -- enables transitions between sequential movements of a serial motor skill (e.g. keyboard typing, piano playing) Control of movement based on observation of another person’s performing a skill

Cerebral Cortex, cont’d Supplementary motor area (SMA) Location: Medial surface of frontal lobe adjacent to portions of the primary motor cortex Functions include involvement in the control of Sequential movements Preparation and organization of movement

Cerebral Cortex, cont’d Parietal lobe Location One of the 4 lobes of the cerebral cortex Function Involved in the integration of movement preparation and execution Interacts with the premotor cortex, primary motor cortex, and SMA before and during movement

Subcortical Brain Area Important in Motor Control Basal Ganglia Buried within cerebral hemispheres Consist of 4 large nuclei Caudate nucleus Putamen Substantia nigra Globus pallidus Function involves control of Movement initiation Antagonist muscles during movement Force - Receive info from cerebral cortex and brainstem - Send info to brainstem

Basal Ganglia, cont’d Parkinson’s Disease Common disease associated with basal ganglia dysfunction Lack of dopamine production by substantia nigra Motor control problems [BART] Bradykinesia (slow movement) Akinesia (reduced amount of movement) Rigidity of muscles Tremor

Brain Components: 2. Diencephalon 2nd component of forebrain Contains two groups of nuclei Thalamus Functions: A type of relay station - receives and integrates sensory info from spinal cord and brainstem; sends info to cerebral cortex Important role in control of attention, mood, and perception of pain Hypothalamus Critical center for the control of the endocrine system and body homeostasis

Brain Components: 3. Cerebellum Location: Behind cerebrum and attached to brainstem [See Fig. 4.3] Structure includes Cortex covering Two hemispheres White matter under the cortex contains Red nucleus – Where cerebellum’s motor neural pathways connect to spinal cord Oculomotor nucleus

Brain Components: 3. Cerebellum cont’d Functions Involved in control of smooth and accurate movements Clumsy movement results from dysfunction Involved in control of eye-hand coordination, movement timing, posture Serves as a type of movement error detection and correction system Receives copy of motor neural signals sent from motor cortex to muscles (efference copy) Involved in learning motor skills

Brain Components: 4. Brainstem Location Beneath cerebrum; connected to spinal cord 3 components involved in motor control Pons Medulla Reticular formation Functions Pons Involved in control of various body functions (e.g. chewing) and balance Medulla Regulatory center for internal physiologic processes (e.g. breathing) Reticular formation Integrator of sensory and motor info Inhibits / Activates neural signals to skeletal muscles

Spinal Cord A complex neural system vitally involved in motor control Structure [See Fig. 4.5] Gray matter – H-shaped central portion Consists of cell bodies and axons of neurons Two pairs of “horns” Dorsal (posterior) horns – Cells transmit sensory info Ventral (anterior) horns – Contains alpha motor neurons with axons terminating on skeletal muscle Interneurons (Renshaw cells) – In ventral horn

Sensory Neural Pathways Several neural tracts (called ascending tracts) Pass through spinal cord and brainstem Connect to sensory areas of cerebral cortex and cerebellum 2 tracts to sensory cortex especially important for motor control Dorsal column Anterolateral system Tract to cerebellum important for motor control Spinocerebellar tract – Primary pathway for proprioceptive info

Motor Neural Pathways Descending tracts Travel from brain through spinal cord Pyramidal tracts (corticospinal tracts) 60% from motor cortex Most fibers cross to other side body (decussation) in medulla of brainstem Involved in control of fine motor skill performance Nonpyramidal tracts (brainstem pathways) Fibers do not cross to other side of body Involved in postural control and control of hand and finger flexion – extension

The Motor Unit An alpha motor neuron and all the skeletal muscle fibers it innervates [See Figure 4.6] When a motor neuron activates (fires) all its connected muscle fibers contract The ultimate end of the motor neural information ~ 200,000 alpha motor neurons in spinal cord Number of muscle fibers served by a motor unit depends on type of movement associated with the muscle Fine movements e.g. eye muscles = 1 fiber / motor unit Gross movements e.g. posture control = many fibers (up to ~ 700) / motor unit

Motor Unit Recruitment Amount of force generated by muscle contraction depends on number of muscle fibers activated To increase force, need more motor units Process of increasing number of motor units involved = recruitment Recruitment follows “size principle” Size = motor neuron cell body diameter Size principle = recruit smallest motor units first (i.e., weakest force produced) then systematically increase size recruited until achieve desired force

From Intent to Action: The Neural Control of Voluntary Movement Think about the entire process of deciding to perform a skill and actually performing it The neural activity involved in this process typically follows a hierarchical organization pattern From higher to lower levels of the neuromuscular system This process is described conceptually in Figure 4.7 and Table 4.1

Neural Control of Voluntary Movement 1. Higher centers Function - Form complex plans according to intent, communicates with the middle level via command neurons. Structures – areas involved with memory and emotions, SMA, associations cortex 2. Middle level - Function – converts plans to a number of smaller motor programs which determine the pattern of neural activation required. Structures – sensorimotor cortex, cerebellum, basal nuclei, brainstem nuclei Lowest level Function – specifies tension of particular muscles and angle of joints at specific times necessary to carry out programs from middle control level Structures – brainstem or spinal cord

From Intent to Action: Brain Structures Associated with Movement Research by Carson and Kelso (2004) Demonstrated there is more involved in understanding how we control voluntary coordinated movement than knowing which brain structures involved in which type of movements Cognitive intention is a critical component Experiment Participants performed finger-flexion movement to a metronome On the beat (synchronize) Between beats (syncopate) Task involved exactly the same movement but two different cognitive intentions fMRI results showed Different brain regions active for the two movement intentions