Spinal Reflexes Overview

Spinal Reflexes

• Definition: A reflex is an automatic, involuntary response to a specific stimulus, rapid and predictable.
• Function: Involuntary control of skeletal muscles, protecting against injury, emptying pelvic organs, movement and balance, and facilitating skilled movements.
• Components: Sensory receptors, integration in the CNS, and effectors
• Sensory Receptors:
  • Exteroceptors: Respond to external stimuli like temperature, pressure, and information.
  • Proprioceptors: Located in muscles, tendons, and joints, provide information about position.
  • Interoceptors: Found in internal organs, send information about pain and stretching of organs.
• Integration in the CNS: Takes place in the brain or spinal cord. Sensory information is processed, integrated, and commands are sent to effectors. Interneurons distribute information and coordinate commands.
• Effectors: Execute the response. In the somatic system, this is skeletal muscle. In the autonomic system, it's cardiac muscle, smooth muscle, and gland cells.

Muscle Tone

• Muscle spindle: A sensory receptor in muscle that detects changes in muscle length. Responsible for regulating muscle tone.
• Golgi tendon organ: A sensory receptor in muscle tendon that detects muscle tension. Inhibits motor neurons preventing excessive tension.

Testing Stretch Reflexes

• Testing: Used in neurological examinations to assess the integrity of neural pathways.
• Grading: Evaluated on a scale from 0 (absent) to 4 (very brisk with clonus).
• Hyporeflexia: A diminished reflex response, can indicate damage to the reflex pathway. Causes include poliomyelitis, muscular dystrophy, and lower motor neuron lesions.
• Areflexia: An absent reflex, can be due to lesions in the reflex arc or lack of clinical experience.
• Hyperreflexia: An exaggerated reflex, can indicate damage to the upper motor neurons (UMNL). Caused by stroke, brain tumor, or damage to the motor cortex.

Reciprocal Innervation

• Definition: When a muscle contracts (protagonist), its opposing muscle (antagonist) relaxes to allow movement.
• Mechanism: Neuronal connections inhibit antagonist muscle stretch reflexes.

Polysynaptic Reflexes

• Definition: Involve multiple synapses between afferent and efferent nerves, producing complex responses.
• Example: The withdrawal reflex, moving a body part away from a painful stimulus.

Types of Polysynaptic Reflexes

• Flexor Withdrawal Reflex: A protective reflex that withdraws a body part from a painful stimulus.
• Crossed Extensor Reflex: A reflex that extends the opposite limb to maintain balance during a withdrawal reflex.

Conditioned Reflexes

• Definition: Learned reflexes developed through practice.
• Example: Certain responses in music or sports.

Reflexes

• A reflex is an automatic response to a stimulus, rapid, and predictable
• Reflexes can be influenced by higher brain centers
• Reflexes can be monosynaptic (one synapse) or polysynaptic (more than one)

Components of a Reflex

• Sensory receptors, sensory nerve
• Area for integration in the CNS
• Efferent nerve and effectors
• The pathway connecting these components is called a reflex arc

Types of Sensory Receptors

• Exteroceptors: Respond to external stimuli like temperature and pressure
• Proprioceptors: Respond to stimuli within the body, especially muscles, tendons, and joints
• Interoceptors: Respond to stimuli within internal organs

Area for Integration in the CNS

• This area receives sensory information and integrates it
• Interneurons in the CNS are responsible for processing and distributing information
• Interneurons can be excitatory or inhibitory
• The CNS sends commands to the effectors based on information integration

The Effectors

• Effectors are the muscles or glands that produce the reflex response
• In the somatic nervous system, effectors are skeletal muscles
• In the autonomic nervous system, effectors are cardiac muscle, smooth muscle, and glands

Spinal Cord Reflexes

• Spinal cord reflexes provide involuntary control of skeletal muscles
• Two types of reflexes in the somatic nervous system: monosynaptic and polysynaptic

Monosynaptic Reflexes

• Only one synapse between afferent and efferent neurons in the CNS
• Only monosynaptic reflexes in the body are stretch (myotactic) reflexes
• Function is to counteract sudden changes in muscle length
• Components are:
  • Muscle spindles as sensory receptor
  • Spinal cord as integration area
  • Stretched skeletal muscle as effector

Muscle Sensory Receptors

• Continuous feedback of sensory information from the muscle to the spinal cord is required
• Muscle spindles and Golgi tendon organs provide information about muscle length, tension, and rate of change
• Both are proprioceptors

Muscle Spindles

• Consist of:
  • Fibrous capsule
  • Intrafusal muscle fibers wrapped by nerve endings
  • Group Ia sensory axons (primary fibers) for stretch and rate of length change detection
  • Group II sensory axons (secondary fibers) for stretch detection only

Neuronal Connections in Stretch Reflex

• All skeletal muscles respond to a brief stretch with a brief contraction (stretch reflex)
• The muscle spindle conveys sensory information to the brain, especially cerebellum

Muscle Spindles and Muscle Tone

• Muscle spindles contribute to muscle tone (resting muscle tension)
• Prevent muscles from being completely relaxed (over-lengthening)
• Regulate muscle tone and contribute to achieving normal resting muscle length and state of tension
• Muscle tone is ultimately controlled by the brain, but spindles provide feedback

Golgi Tendon Organ

• Sensory receptor located in the muscle tendon
• Detects muscle tension
• Inhibits the motor neuron when tension is high
• Prevents excessive tension on the muscle
• Controls force and stiffness of joints

Stretch Reflex Testing

• Testing stretch reflexes (tendon jerks) is part of routine neurological testing
• Helps assess the integrity of reflexes
• Grading:
  • Grade 4: Very brisk, often with clonus (sustained involuntary contractions)
  • Grade 3: Brisk but normal
  • Grade 2: Normal
  • Grade 1: Minimal
  • Grade 0: Absent

Diminished Stretch Reflex Response (Hyporeflexia)

• Results from damage to any part of the reflex pathway
• Causes:
  • Poliomyelitis (virus damages lower motor neurons)
  • Muscular dystrophy (degeneration of skeletal muscle)
  • Lower motor neuron lesions

Absent Stretch Reflex (Areflexia)

• Loss of tendon reflexes
• Causes: Lesion of the reflex arc

Exaggerated Stretch Reflex Response (Hyperreflexia)

• Results from damage to motor pathways from the brain to the spinal cord
• Causes: Upper motor neuron lesion (UMNL) due to stroke or brain tumor
• Leads to loss of inhibitory inputs from higher brain regions to motor neurons

Reciprocal Innervation

• During somatic reflexes, the antagonist muscle relaxes while the protagonist contracts
• This is achieved by inhibiting the stretch reflexes in the antagonist muscle
• Example: Knee-jerk reflex, where the semitendinosus muscle (antagonist) relaxes

Polysynaptic Reflexes

• Involve more than one synapse between afferent and efferent neurons in the CNS
• More complex responses than monosynaptic reflexes
• Several interneurons may be involved
• Example: Withdrawal reflex

Functions of the Spinal Cord

• Link for transmission of information between the brain and the body
• Integrates reflex activity between afferent input and efferent output without brain involvement

Information Root of Spinal Nerves

• Afferent sensory fibers enter the spinal cord through the dorsal root (cell bodies in the dorsal root ganglion - DRG)
• Efferent (motor) neuron cell bodies are in the grey matter, axons exit through the ventral route
• Alpha motor neurons (Aα axons) innervate skeletal muscle
• Gamma motor neurons (Aγ axons) innervate intrafusal fibers (muscle spindle fibers)
• Dorsal horn: Cell bodies of interneurons
• Ventral horn: Cell bodies of efferent motor neurons

Spinal Reflexes: Importance

• Coordinate rapid withdrawal responses to painful stimuli
• Protect against over-stretching of muscles
• Execute emptying of pelvic organs (e.g. bladder contraction)
• Contribute to purposeful movement and balance (e.g., walking)
• Learned reflexes (e.g., somersaults in sports)

This quiz provides an in-depth look at spinal reflexes, their definitions, functions, and components. You will learn about different types of sensory receptors and how the central nervous system integrates sensory information to execute motor responses. Test your knowledge on the mechanisms that protect our bodies and facilitate movement.