Control of Body Movements: Reflex Physiology

Neural Reflexes and Pathways

• Neural reflexes and their integration pathways.

• Differentiation between autonomic and skeletal muscle reflexes.

• Integrated control of body movements by the brain and subconscious level.

• Brief overview of control of movements in visceral muscles.

Definition of Reflexes

• Reflexes are rapid, predictable, and involuntary responses to stimuli.

  • Rapid: Quick responses due to nearby integration centers (spinal cord or brainstem).

  • Predictable: Simple movements due to less sophisticated integration centers.

  • Involuntary: Not under voluntary control, even if skeletal muscles are involved.

Classification of Neural Reflexes

• Based on the efferent division controlling the muscle:

  • Somatic Reflexes: Controlled by somatic motor neurons (voluntary control).

    • Example: Patellar knee-jerk reflex.

  • Autonomic Reflexes: Controlled by the autonomic nervous system (involuntary control).

    • Example: Pupillary light reflex.

• Based on the integration center:

  • Spinal Cord: Most skeletal muscle reflexes are integrated here.

    • Example: Patellar knee-jerk reflex.

  • Brainstem: Reflexes involving cranial nerves are integrated here.

    • Example: Pupillary light reflex.

• Based on the time of development:

  • Inborn Reflexes: Present at birth.

    • Example: Urination reflex in newborns.

  • Learned (Conditioned) Reflexes: Acquired through experience.

    • Example: Withdrawing hand from a hot object.

• Based on the number of neurons in the reflex pathway:

  • Monosynaptic: One synapse (junction) in the pathway, involving at least two neurons.

  • Polysynaptic: Multiple synapses (junctions) in the pathway, involving more than two neurons.

  • Autonomic reflexes are always polysynaptic.

    • Example: Cardiac reflexes.

  • Somatic reflexes can be either monosynaptic or polysynaptic.

Reflex Arc Components

• Five components of a reflex arc:

  1. Receptors: Detect the stimulus.

  2. Sensory Neuron (Afferent): Carries the signal from the receptor to the integration center.

  3. Integration Center: Spinal cord or brainstem.

  4. Motor Neuron (Efferent): Transmits the signal from the integration center to the effector. Can be somatic or autonomic.

  5. Effector: Muscle (skeletal, smooth, or cardiac) or gland that carries out the response.

Monosynaptic vs. Polysynaptic Reflexes

• Monosynaptic Reflex: Involves only two neurons and one synapse.

• Polysynaptic Reflex: Involves multiple neurons and synapses, including interneurons.

  • Interneurons: Neurons located between the sensory and motor neurons, creating additional synapses.

Autonomic Reflexes

• All autonomic reflexes are polysynaptic.

• Involve at least one synapse in the CNS and another in the autonomic ganglion.

• Motor neurons are autonomic (sympathetic or parasympathetic).

• Target cells are smooth muscle or cardiac muscle.

Skeletal Muscle Reflexes

• Involve proprioceptors, sensory neurons, the spinal cord (integration center), somatic motor neurons, and skeletal muscles (effectors).

Proprioceptors

• Located in skeletal muscles, joint capsules, and ligaments.

• Types of proprioceptors:

  • Muscle Spindles: Detect muscle stretch.

  • Golgi Tendon Organs: Detect muscle tension.

  • Capsule Receptors: Located in joint capsules.

Muscle Spindles

• Located within the muscle.

• Contain intrafusal fibers (inside the spindle) and extrafusal fibers (outside the spindle).

• Intrafusal fibers are innervated by gamma motor neurons.

• Extrafusal fibers are innervated by alpha motor neurons and are responsible for muscle contraction.

Golgi Tendon Organs

• Located at the junction of the muscle and tendon.

• Detect muscle tension and protect the muscle from overstretch by causing relaxation.

Example: Biceps Muscle

• Spindle detects stretch when a box is placed on the arm, leading to increased muscle contraction.

• If the load becomes too heavy, the Golgi tendon organ causes the muscle to relax to prevent injury.

Patellar Tendon Reflex (Knee-Jerk Reflex)

• Receptor: Spindle stretched by tapping the patellar tendon.

• Sensory neuron: Carries the impulse to the spinal cord.

• Integration center: Spinal cord.

• Motor neuron: Somatic motor neuron to the quadriceps muscle.

• Effector: Quadriceps muscle contracts, causing the leg to extend.

• Inhibition of hamstring muscles (antagonists) is necessary for the reflex to occur.

Cross Extensor Reflex

• Postural reflex that is learned.

• When stepping on something painful, the affected limb flexes.

• The opposite limb extends to maintain balance.

• Involves integration in the central nervous system and coordination of multiple inputs.

Types of Movements

• Reflexive Movements: Least complex, integrated at the spinal cord or brainstem, predictable, and involuntary.

• Voluntary Movements: Most complex, integrated in the cerebral cortex, require conscious thought and decision-making, and not predictable.

• Rhythmic Movements: Combination of reflexive and voluntary, such as walking. Started and stopped voluntarily but maintained by automated patterns.

Parkinson's Disease

• Progressive neurological disorder characterized by abnormal movements and cognitive changes.

• Caused by a loss of basal ganglia neurons that release dopamine.

• Treated with levodopa to increase dopamine levels in the brain.

Comparison of Movement Types

• Reflexes: Quick, simple, integrated at the spinal cord or brainstem.

• Voluntary Movements: Slow, complex, integrated at the cerebral cortex, learned and acquired.

• Rhythmic Movements: Intermediate speed and complexity, integrated in the spinal cord with input from higher centers, require brainstem involvement.

Neural Reflexes and Pathways

• Neural reflexes and their integration pathways, including the roles of various brain structures and neural circuits.

• Differentiation between autonomic and skeletal muscle reflexes at the receptor, integration center, and effector levels.

• Detailed explanation of the integrated control of body movements by the brain and subconscious level, highlighting the roles of the cerebral cortex, cerebellum, and basal ganglia.

• In-depth overview of control of movements in visceral muscles, including peristalsis and sphincter control.

Definition of Reflexes

• Reflexes are rapid, predictable, and involuntary responses to stimuli.

  • Rapid: Quick responses due to nearby integration centers (spinal cord or brainstem), minimizing synaptic delay.

  • Predictable: Simple movements due to less sophisticated integration centers, resulting in consistent responses.

  • Involuntary: Not under voluntary control, even if skeletal muscles are involved. These responses occur without conscious intent.

Classification of Neural Reflexes

• Based on the efferent division controlling the muscle:

  • Somatic Reflexes: Controlled by somatic motor neurons (voluntary control). These reflexes involve skeletal muscles.

    • Example: Patellar knee-jerk reflex, where tapping the patellar tendon elicits a lower leg extension.

  • Autonomic Reflexes: Controlled by the autonomic nervous system (involuntary control). These reflexes regulate smooth muscles, cardiac muscles, and glands.

    • Example: Pupillary light reflex, where the pupil constricts in response to bright light.

• Based on the integration center:

  • Spinal Cord: Most skeletal muscle reflexes are integrated here, allowing for quick motor responses without involving higher brain centers.

    • Example: Patellar knee-jerk reflex, mediated directly by spinal cord circuits.

  • Brainstem: Reflexes involving cranial nerves are integrated here, influencing functions like heart rate, breathing, and blood pressure.

    • Example: Pupillary light reflex, controlled by brainstem nuclei.

• Based on the time of development:

  • Inborn Reflexes: Present at birth, ensuring immediate survival functions.

    • Example: Urination reflex in newborns, essential for waste elimination.

  • Learned (Conditioned) Reflexes: Acquired through experience, allowing adaptation to environmental stimuli.

• Example: Withdrawing hand from a hot object, learned through previous encounters with heat.

• Based on the number of neurons in the reflex pathway:

  • Monosynaptic: One synapse (junction) in the pathway, involving only two neurons (sensory and motor). These reflexes are the fastest.

  • Polysynaptic: Multiple synapses (junctions) in the pathway, involving more than two neurons, including interneurons. These reflexes allow for more complex integration.

  • Autonomic reflexes are always polysynaptic, utilizing multiple neurons to modulate target organ activity.

    • Example: Cardiac reflexes, involving both sympathetic and parasympathetic pathways to control heart rate and contractility.

  • Somatic reflexes can be either monosynaptic or polysynaptic, depending on the complexity of the required response.

Reflex Arc Components

• Five components of a reflex arc:

  1. Receptors: Detect the stimulus, such as temperature, pressure, or stretch. These sensors are specific to the type of stimulus.

  2. Sensory Neuron (Afferent): Carries the signal from the receptor to the integration center, transmitting information about the stimulus.

  3. Integration Center: Spinal cord or brainstem, where sensory information is processed, and a motor response is initiated. This center contains interneurons that modulate the reflex.

  4. Motor Neuron (Efferent): Transmits the signal from the integration center to the effector, carrying the command for the appropriate response. Can be somatic or autonomic, depending on the target.

  5. Effector: Muscle (skeletal, smooth, or cardiac) or gland that carries out the response, resulting in a specific action.

Monosynaptic vs. Polysynaptic Reflexes

• Monosynaptic Reflex: Involves only two neurons (sensory and motor) and one synapse, resulting in very fast responses.

• Polysynaptic Reflex: Involves multiple neurons and synapses, including interneurons, allowing for more complex and modulated responses.

  • Interneurons: Neurons located between the sensory and motor neurons, creating additional synapses and enabling integration of multiple inputs.

• Example: Withdrawal reflex, where stepping on a tack involves sensory neurons, interneurons in the spinal cord, and motor neurons to flex leg muscles and shift weight to the other leg.

Autonomic Reflexes

• All autonomic reflexes are polysynaptic, allowing for modulation and integration of various inputs.

• Involve at least one synapse in the CNS and another in the autonomic ganglion, ensuring regulation by both central and peripheral components.

• Motor neurons are autonomic (sympathetic or parasympathetic), targeting smooth muscle, cardiac muscle, or glands.

• Target cells are smooth muscle or cardiac muscle, controlling functions like heart rate, digestion, and blood vessel diameter.

Skeletal Muscle Reflexes

• Involve proprioceptors, sensory neurons, the spinal cord (integration center), somatic motor neurons, and skeletal muscles (effectors).

• Proprioceptors provide constant feedback about muscle length, tension, and joint position.

Proprioceptors

• Located in skeletal muscles, joint capsules, and ligaments, providing continuous information about body position and movement.

• Types of proprioceptors:

  • Muscle Spindles: Detect muscle stretch, initiating a contraction to resist the stretch.

  • Golgi Tendon Organs: Detect muscle tension, causing muscle relaxation to prevent overstretch and injury.

  • Capsule Receptors: Located in joint capsules, detecting joint position and movement.

Muscle Spindles

• Located within the muscle, parallel to extrafusal muscle fibers.

• Contain intrafusal fibers (inside the spindle) and extrafusal fibers (outside the spindle).

• Intrafusal fibers are innervated by gamma motor neurons, adjusting the sensitivity of the muscle spindle.

• Extrafusal fibers are innervated by alpha motor neurons and are responsible for muscle contraction, producing movement.

Golgi Tendon Organs

• Located at the junction of the muscle and tendon, in series with muscle fibers.

• Detect muscle tension and protect the muscle from overstretch by causing relaxation through inhibitory interneurons in the spinal cord.

Example: Biceps Muscle

• Spindle detects stretch when a box is placed on the arm, leading to increased muscle contraction to support the load.

• If the load becomes too heavy, the Golgi tendon organ causes the muscle to relax to prevent injury, reducing the risk of muscle strain.

Patellar Tendon Reflex (Knee-Jerk Reflex)

• Receptor: Spindle stretched by tapping the patellar tendon, initiating the reflex.

• Sensory neuron: Carries the impulse to the spinal cord, transmitting the signal.

• Integration center: Spinal cord, where the sensory neuron synapses directly with a motor neuron.

• Motor neuron: Somatic motor neuron to the quadriceps muscle, activating the muscle.

• Effector: Quadriceps muscle contracts, causing the leg to extend, demonstrating the reflex.

• Inhibition of hamstring muscles (antagonists) is necessary for the reflex to occur, allowing the quadriceps to contract without opposition.

Cross Extensor Reflex

• Postural reflex that is learned, involving coordination between limbs.

• When stepping on something painful, the affected limb flexes to withdraw from the stimulus.

• The opposite limb extends to maintain balance, preventing a fall.

• Involves integration in the central nervous system and coordination of multiple inputs, ensuring a coordinated response.

• Example: Touching something hot causes one arm to withdraw while the other arm extends to stabilize the body.

Types of Movements

• Reflexive Movements: Least complex, integrated at the spinal cord or brainstem, predictable, and involuntary. These movements are rapid and automatic.

• Voluntary Movements: Most complex, integrated in the cerebral cortex, require conscious thought and decision-making, and not predictable. These movements involve planning and execution.

• Rhythmic Movements: Combination of reflexive and voluntary, such as walking, chewing or breathing. Started and stopped voluntarily but maintained by automated patterns controlled by central pattern generators in the spinal cord and brainstem.

Parkinson's Disease

• Progressive neurological disorder characterized by abnormal movements and cognitive changes, affecting motor control and cognitive function.

• Caused by a loss of basal ganglia neurons that release dopamine, leading to impaired motor coordination.

• Treated with levodopa to increase dopamine levels in the brain, alleviating motor symptoms.

Comparison of Movement Types

• Reflexes: Quick, simple, integrated at the spinal cord or brainstem, ensuring rapid responses to stimuli.

• Voluntary Movements: Slow, complex, integrated at the cerebral cortex, learned and acquired, allowing for deliberate and planned actions.

• Rhythmic Movements: Intermediate speed and complexity, integrated in the spinal cord with input from higher centers, require brainstem involvement, combining automaticity with voluntary control.