Chapter 15 Nervous System In Kinesiology

The nervous system is a highly dynamic and complex network that is constantly adapting and changing, a phenomenon referred to as plasticity. Plasticity encompasses not only the ability to learn and remember but also to recover from injuries and adapt to new experiences. This adaptability stems from various mechanisms within the nervous system, including structural and functional changes at the synapses, the strengthening or weakening of neural connections, and the formation of new neurons (neurogenesis).

The nervous system is structurally divided into two major parts:

  1. Central Nervous System (CNS): Consisting of the brain and the spinal cord, the CNS acts as the command center, processing information and orchestrating body functions. It is responsible for higher-order functions such as cognition, emotion, and coordination of movements. Within the brain, there are various regions that specialize in different functions, including areas responsible for sensory processing, motor control, and integration of information.

  2. Peripheral Nervous System (PNS): Comprised of all the nerves that branch out from the CNS, the PNS carries sensory information from the body to the CNS (afferent pathways) and transmits commands from the CNS to the muscles (efferent pathways). These pathways enable the body to respond to stimuli and maintain homeostasis.

Neurons and Synapses: Neurons, or nerve cells, are the fundamental building blocks of the nervous system. They are specialized for receiving and transmitting information. The neuron consists of several key components:

  • Cell Body: Contains the nucleus and organelles; it regulates the cell's metabolic activities.

  • Dendrites: Tree-like structures that receive signals from other neurons and sensory receptors, serving as communication pathways.

  • Axon: A long extension that transmits signals away from the cell body to other neurons or muscles. Synapses are the junctions where neurons communicate, allowing for effective two-way transmission of signals. The synaptic cleft is the gap between neurons where neurotransmitters are released, enabling chemical communication that influences the postsynaptic neuron's activity.

In Kinesiology, understanding different types of neurons is crucial:

  • Regular Sensory Neurons: Typically lack dendrites but possess a couple of axons with terminal branches that carry sensory information.

  • Alpha Motor Neurons: Located in the spinal cord, these neurons have numerous dendritic branches and a long axon, primarily innervating skeletal muscles for movement.

  • Gamma Motor Neurons: These neurons innervate intrafusal fibers located in muscle spindles, helping regulate muscle tone and proprioception.

  • Pyramidal Cells: Found in the motor cortex, they facilitate long-distance transmission of motor signals.

  • Purkinje Cells: Located in the cerebellum, known for their elaborate dendritic trees that provide extensive input for motor coordination.

  • Interneurons: Feature complex dendritic structures enabling them to connect with multiple neurons, playing a critical role in reflexes and local processing.

Glial Cells: Supporting cell types that outnumber neurons, providing metabolic, structural, and immunological support. They include astrocytes, oligodendrocytes, microglia, and Schwann cells, each fulfilling essential functions for maintaining neural health and function.

Myelin: A fatty substance that insulates axons, increasing the speed of signal transmission through saltatory conduction. It is produced by oligodendrocytes in the CNS and Schwann cells in the PNS, contributing to the efficiency of neural communication.

Sensory Receptor Systems:

  • Proprioception: Provides critical information about body position and movement, gathered from kinesthetic receptors in muscles, tendons, and joints, as well as from vestibular receptors in the inner ear that assist with balance.

Visual System: The first step in visual processing occurs as light hits the retina, activating photoreceptors (rods and cones). Rods are sensitive to low light levels, facilitating night vision, while cones are responsible for color vision and functioning best in bright light. Both types of photoreceptors communicate visual information through nerve impulses to various brain regions for processing.

Visual Pathways:

  • Approximately 70% of optic nerve fibers connect to the lateral geniculate nucleus (LGN) in the midbrain, contributing to focal vision important for recognizing objects and fine motor movements.

  • The remaining fibers terminate in the superior colliculus, facilitating ambient vision, which aids in tracking moving objects and providing an overall awareness of the visual field.

Kinesthetic Muscle System:

  • Muscle Spindles: Sensitive to changes in muscle length, muscle spindles provide vital sensory feedback during movement. Composed of intrafusal fibers, they signal muscle stretch to the central nervous system via type Ia and II afferent neurons. Type Ia afferent neurons are large-diameter, myelinated fibers that convey information about the rate and degree of stretch in the muscle. They are rapidly adapting and sensitive to changes in muscle length, allowing for precise detection of muscle dynamics. Type II afferent neurons, also myelinated, transmit information regarding the static length of the muscle and are slower to adapt compared to type Ia fibers.

  • Golgi Tendon Organs: Located in tendons near the musculotendinous junction, these receptors sense tension generated by muscle contractions. They play a protective role by inhibiting excessive muscle contraction (autogenic inhibition) to prevent injury and provide sensory feedback for precise control of movement.

  • Skin Receptors: Meissner’s corpuscles, Merkel’s discs, Ruffini corpuscles, and free nerve endings provide sensory information about light touch or low-frequency vibration. Pacinian corpuscles located deeper in the skin and respond more to deep compression and high-frequency vibration.

  • Joint Receptors: Modified Ruiffini and Pacinian corpuscles located in the joint capsule. Golgi organs are located in the ligaments that bind the joint together. Role is to protect the joint from injury by signaling the central nervous system when the full range of motion of a joint has been reached

Vestibular System:

  • Vestibular apparatus - two types

    • Semicircular canals - consist of the superior, horizontal, and posterior canals; these respond to angular acceleration

    • Otolith organs - consist of the utricle and saccule; these respond to linear acceleration

Intersensory Integration and Sensory Dominance

  • Vision is usually the dominant sensory modality

Effector Systems for Movement

  • Motor Unit - consists of a singe alpha motor neuron plus all the skeletal muscle fibers it innervates

    • The fewer the number of fibers, the more precise the control that is possible

Motor Control Functions of the Spinal Cord

  • Brain is more responsible for higher-order creative and executive mental and motor functions

  • Spinal cord is responsible for routine, repetitive control functions

  • Structure of the Spinal Cord

    • Functions:

      • Dual-transmission pathway that carries both afferent and efferent information

      • Supports reflexes at the local spinal level to provide rapid, essentially automatic responding to noxious stimuli and to ensure the successful execution of movements is already underway

    • Is protected in the vertebral column

    • Spinal Nerves: a total of 31 pairs attached to the spinal cord; attaches to two roots, ventral and dorsal

      • Ventral: a ventral root carries the efferent information

      • Dorsal: a dorsal root carries afferent information

  • Spinal Reflex: the simplest functional unit of integrated nervous-system behavior

    • Reflex Arc Requirements

      • A sensory receptor

      • An afferent neuron

      • An efferent neuron

      • An effector

    • Monosynaptic reflex - only requires two neurons and has one synapse

      • Mytotaic reflex (muscle-stretch reflex)

    • Polysynaptic reflex - requires more than two neurons and has more than one synapse

      • Flexion reflex - causes the withdrawal of the limbs in order to avoid potentially harmful stimuli

      • Reciprocal inhibition - the neural control phenomena that ensures that agonist and antagonist muscles do not typically cocontract in opposition to each other

      • Crossed Extensor reflex - works with flexion reflex to maintain postural stability

      • Extensor Thrust reflex - aids in supporting the body’s weight against gravity

      • Gait Control Spinal Reflexes - characterized by continuous patterns of limb flexion and extension

Motor Control Functions of the Brain

  • Brain serves higher-order functions, of which some are directly related to motor control

  • Main Areas of the Brain for Motor Control

    • Motor Cortex - located immediately forward of the central sulcus in the frontal lobe or the cerebrum

      • Divided into two hemispheres by the corpus callosum

      • Involved in the production and control of skilled movement

      • Two Principal Means of relaying commands

        • Pyramidal tract - most direct route; allows neurons from the motor cortex to synapse directly with the alpha motor neurons the the spinal level; carries impulses that are excitatory in nature

        • Extrapyramidal tracts - allow nerve impulses front the motor cortex to reach the spinal level through a range of pathways via the cerebellum, basal ganglia, thalamus and brain stem; carries impulses that are inhibitory in nature

      • Each hemisphere further divided into three parts

        • Motor Cortex - lies immediately forward of the central sulcus

        • Pre motor Cortex - lies forward of the motor cortex

          • In front of the pre motor cortex

            • frontal eye fields for voluntary eye movement

            • Broca’s area - located in the left hemisphere only

        • Supplementary Motor Area - lies on the medial wall of the cerebral hemispheres and forward of the motor cortex

    • Cerebellum - located off the brain stem and bellow the occipital lobe of the cerebrum

      • Two types of afferent fibers: climbing and mossy fibers

      • Deals with regulation of muscle tone, the coordinated smoothing of movement, timing, and learning

    • Basal Ganglia - located in the inner layers of the cerebrum

      • Consists of five pairs of interconnected nuclei (globes pallidus, caudate nucleus, putamen, subthalamic nucleus, and the substantial nigra)

      • Receive input from two major sources (motor areas of the cerebral cortex and the brain stem)

      • Send output to two locations (thalamus and brain stem)

    • Brain Stem - located forward of the cerebellum and continuous with the spinal cord and cerebrum

      • Principal function is to act as a relay center, especially for the transmission of information or and from the cerebral cortex

      • Also functions in the control of muscle tone and posture

        • Righting reflexes - maintain the orientation of the body in respect to gravity

        • Tonic reflexes - concerned with the position of one body part in relation to the other body parts

      • Consists of three major areas: the pons, medulla, and the reticular formation

        • Pons and medulla: main structures of brain stem, receive input from the cerebral cortex, basal ganglia, and cerebellum.

        • Reticular formation - network of neurons that extend through the brain stem

        • Has a major role in regulating the ability of the cerebral cortex