Neuromotor Basis for Motor Control Study Notes

Chapter 4: Neuromotor Basis for Motor Control

Application of Neuromotor Basis for Motor Control

• Practical Example: Picking up a pen to write.

• This concept applies to all movements requiring coordination and control.

Central Nervous System (CNS) - The Basic Components

• Neuron: The fundamental building block (basic component) of the nervous system or nerve cell.

• Dendrites: Extensions from a nerve cell that receive neural impulses (signals) from other neurons.

• Axons: Nerve fibers that send information away from the neuron. Each neuron typically has one axon.

  • Axon Terminals: The ends of axons that provide a pathway for neurotransmitters to be released.

  • Myelin: A protective cover surrounding axons that significantly speeds up neural transmissions.

Types and Functions of Nerves

• Sensory Neurons (Afferent Neurons): Transmit sensory information from the body to the CNS.

• Motor Neurons (Efferent Neurons): Transmit motor commands from the CNS to muscles and glands.

  • Alpha Motor Neurons (Motor Horn Cells): Found in the spinal cord, they directly innervate voluntary muscle fibers. They are crucial for muscle contraction and movement.

  • Gamma Motor Neurons: Supply the intrafusal fibers, which are specialized muscle fibers within muscle spindles, playing a role in regulating muscle tone and proprioception.

• Interneurons: Function entirely within the CNS, originating and terminating within the brain or spinal cord. They connect ascending and descending axons, facilitating communication between sensory and motor neurons.

Brain Structure - Cerebrum

• Cerebrum: The largest part of the brain, consisting of two cerebral hemispheres (left and right).

  • Corpus Callosum: A large bundle of nerve fibers that connects and allows communication between the left and right cerebral hemispheres.

• Cerebral Cortex: The gray-colored surface (gray matter) that covers the cerebral hemispheres.

  • Gray Matter: Characterized by ridges and grooves.

    • Gyrus (Gyri): The ridges or convolutions on the surface of the cerebral cortex.

    • Sulcus (Sulci): The grooves or indentations on the surface of the cerebral cortex.

  • Cortical Neurons: Two main types:

    • Pyramidal Cells: Send signals from the cortex to other parts of the CNS, primarily involved in motor control.

    • Nonpyramidal Cells: Primarily involved in processing information within the cortex.

Lobes of the Cerebral Cortex and Their Functions

• The cerebral cortex is divided into four main lobes:

  • Frontal Lobe: Critically involved in the control of voluntary movement, decision-making, problem-solving, and personality.

  • Parietal Lobe: Responsible for the perception and integration of sensory information (e.g., touch, temperature, pain, pressure, spatial awareness).

  • Occipital Lobe: Primarily dedicated to the processing of visual information.

  • Temporal Lobe: Important for memory, abstract thought, judgment, and auditory processing.

Specialized Cortical Areas for Motor and Sensory Control

• Sensory Cortex: Located posterior to the central sulcus.

  • Receives and processes sensory information from various parts of the body, including vision, taste, speech comprehension, body sensation (pain, temperature, pressure).

• Primary Motor Cortex (PMC): Located in the frontal lobe, just anterior to the central sulcus.

  • Crucial for the initiation of voluntary movements.

  • Involved in the coordination of movements, particularly for fine motor skills and postural coordination.

• Premotor Area: Located anterior to the PMC.

  • Plays a significant role in the organization of movements before they are initiated.

  • Involved in the rhythmic coordination of movements, such as playing the piano.

• Supplementary Motor Area (SMA): Located on the medial surface of the frontal lobe, next to the PMC.

  • Important for the control of sequential movements.

  • Involved in the preparation and organization of movements.

Subcortical Structures - Basal Ganglia

• Basal Ganglia: A group of four large subcortical nuclei vital for motor control.

  • Components: Caudate nucleus, Putamen, Substantia Nigra, and Globus Pallidus.

  • Functions: Integral for planning and initiation of movement, controlling antagonist muscles during movement, and regulating the force of movements.

  • Parkinson's Disease: Associated with the degeneration of dopamine-producing neurons in the substantia nigra, leading to motor control deficits.

Diencephalon

• Located between the cerebrum and the brainstem, containing two primary groups of nuclei:

  • Thalamus: Functions as a relay station for sensory information to the cerebral cortex. Also involved in controlling attention, mood, and the perception of pain.

  • Hypothalamus: Controls the endocrine system and plays a critical role in regulating body homeostasis, including temperature, hunger, thirst, and physiological responses to stress.

Other Key Brain Structures

• Cerebellum: Located at the back of the brain. Essential for controlling smooth and accurate movements, balance, and motor learning.

• Brainstem: Connects the cerebrum and cerebellum to the spinal cord. It comprises:

  • Pons: Involved in chewing, swallowing, salivating, breathing control, and balance.

  • Medulla Oblongata: Regulates vital involuntary functions, including respiration (interacting with the pons) and heart rate.

  • Reticular Formation: A network of neurons extending through the brainstem that acts as an integrator of sensory and motor neural impulses, involved in controlling arousal and sleep-wake cycles.

• Limbic System: An integrated system within the frontal and temporal lobes, diencephalon, and other CNS structures.

  • Plays an important role in the learning of motor skills, the control of emotions, and several visceral behaviors.

Spinal Cord Structure and Neural Pathways

• The spinal cord is composed of gray and white matter:

  • Gray Matter: Located centrally, forming H-shaped horns.

    • Posterior (Dorsal) Horns: Primarily receive and process sensory information.

    • Anterior (Ventral) Horns: Contain the cell bodies of alpha motor neurons, which send motor commands to muscles.

  • White Matter: Surrounds the gray matter and consists of myelinated axons organized into tracts.

    • Ascending Tracts (Sensory Neural Pathways): Carry sensory information up to the brain.

      • Dorsal Column: Transmits proprioception (sense of body position), touch, and pressure.

      • Anterolateral System: Carries pain and temperature sensations.

      • Spinocerebellar Tracts: Convey proprioceptive information specifically to the cerebellum.

    • Descending Tracts (Motor Neural Pathways): Carry motor commands from the brain down to the spinal cord and muscles.

      • Pyramidal Tract (Corticospinal Tract): Primarily responsible for the control of fine motor skills. These pathways typically cross over (decussate) to the opposite side of the body.

      • Extrapyramidal Tracts (Brainstem Pathways): Primarily involved in postural control and the facilitation of muscles involved in flexion and extension of the hands and fingers. These pathways generally do not cross over to the opposite side of the body.

The Motor Unit

• Definition: The fundamental functional unit of motor control for muscle innervation, consisting of an alpha motor neuron and all the muscle fibers it innervates.

• Motor Unit Recruitment: The process by which the nervous system increases the number of active motor units to increase the number of muscle fibers active at any given time, thereby increasing the amount of force the muscle can exert.

• Variability: The number of muscle fibers innervated by a single alpha motor neuron can vary significantly:

  • Fine Movements: Involve motor units with a small number of muscle fibers per alpha motor neuron (e.g., muscles controlling eye movements or finger dexterity).

  • Gross Movements: Involve motor units with a large number of muscle fibers per alpha motor neuron (e.g., large limb muscles like the quadriceps).