Spinal Reflexes Overview

Spinal Reflexes

• Spinal reflexes are rapid, automatic, and stereotyped responses to stimuli.
• They occur without conscious effort.
• Skeletal muscles are voluntarily controlled by the brain, but can also contract in a reflex manner in response to stimuli.
• Spinal reflexes vary in complexity, ranging from monosynaptic to polysynaptic reflexes. These polysynaptic reflexes involve muscle control on both sides of the body.
• Reflexes are crucial for involuntary control of skeletal muscles.
• Reflexes are integrated within the spinal cord.

Learning Outcomes

• Discuss the role of involuntary control of skeletal muscles.
• Define a reflex and describe its basic components.
• Explain the function of muscle sensory receptors in spinal reflexes.
• Describe a stretch reflex.
• Discuss the testing of stretch reflexes in neurological testing.
• Interpret an atypical knee jerk response.
• Explain the flexor withdrawal and cross extensor reflexes.

Reflex

• A reflex is a response that occurs automatically without conscious effort.
• Reflexes are rapid, automatic, and stereotyped responses to a specific stimulus.
• They are predictable.
• A reflex response example is immediately withdrawing your hand when touching a hot plate.

Skeletal Muscle and Spinal Cord Reflexes (Overview)

• Skeletal muscle is under voluntary control from the brain, but skeletal muscles can contract without conscious control in a reflex manner in response to certain stimuli.
• Spinal reflexes display varying degrees of complexity.

Functions of the Spine

• The spinal cord links the brain with the rest of the body (transmission of information).
• It integrates afferent input and efferent output.
• It carries out reflexive activity without brain involvement.

Information "Root" of Spinal Nerves

• Sensory fibres, enter the spinal cord through the dorsal root ganglion (DRG), which contains their cell bodies.
• Efferent (motor) neurons' cell bodies are situated in the spinal cord's gray matter. Their axons leave the spinal cord through the ventral root.
• Alpha motor neurons innervate skeletal muscle, while gamma motor neurons innervate intrafusal fibres.
• Dorsal and ventral roots form the spinal nerve. There are 31 spinal nerves in total.

Spinal Cord Reflexes: Why They Are Important

• Spinal reflexes coordinate rapid withdrawal responses to painful stimuli.
• They protect muscles from overstretching.
• They execute emptying of pelvic organs, such as bladder contraction.
• They orchestrate purposeful muscle movements, contributing to proper balance and movement, including activities like walking.
• They are involved in learned reflexes (e.g., somersaults).

Reflexes Continued

• Reflex responses occur in the somatic nervous system and the autonomic nervous system.
• A reflex response is the most basic form of integrated neural activity.

Components of a Reflex

• Peripheral sensory receptors and sensory nerves
• Area for integration in the central nervous system (CNS)
• Efferent nerve and effectors
• The neural pathway that accomplishes reflex activity is called a reflex arc.

The Sensory Receptors

• Exteroceptors respond to external stimuli (e.g., touch, temperature, pressure).
• Proprioceptors provide information regarding the position of muscles, tendons, and joints.
• Interoceptors detect changes within the body (e.g., pain, organ stretching).

Area for Integration in the CNS

• The brain and spinal cord integrate incoming sensory information.
• Integration in the CNS relies on interneurons' activity to distribute sensory information and coordinate commands within the CNS.
• Interneurons can either excite or inhibit.

The Effectors

• Effectors are the body parts that respond to a stimulus.
• In the somatic nervous system, effectors are skeletal muscles.
• In the autonomic nervous system, effectors are cardiac muscle, smooth muscle, and glands.

Spinal Reflexes of the Somatic Nervous System

• Spinal reflexes provide involuntary control over skeletal muscle.
• The area for integration is the spinal cord.
• Spinal reflexes are influenced and modified by higher centers in the CNS.
• Two main types: Monosynaptic and Polysynaptic.

Monosynaptic Reflexes

• Monosynaptic reflexes involve only one synapse between afferent and efferent nerves.
• The only monosynaptic reflexes in the body are stretch reflexes (also known as myotactic reflexes).
• A stretch reflex is elicited in a skeletal muscle when it is briefly stretched, resulting in a brief contraction of that muscle.
• The components of a stretch reflex include a sensory receptor (muscle spindle), a processing area (spinal cord), and an effector (the skeletal muscle being stretched).

Muscle Sensory Receptors

• Muscles and tendons have sensory receptors that provide continuous feedback about muscle function (length, tension, rate of change).
• Muscle spindles monitor muscle length and rate of change in length.
• Golgi tendon organs detect muscle tension and rate of change in tension. Both are proprioceptors

Muscle Spindle

• Muscle spindles consist of fibrous capsule, intrafusal muscle fibers, and sensory axons
• Sensory axons (Ia and II) report on muscle length and rate of change

Neuronal Connections in Stretch Reflex

• All skeletal muscles respond to brief stretching with brief contractions.
• This stretch reflex involves one synapse between afferent and efferent nerves.

Muscle Spindles and Their Role in Muscle Tone

• Muscles maintain a degree of tension known as resting muscle tone.
• Muscle spindles are essential for maintaining this muscle tone. Appropriate muscle tone avoids overstretching or insufficient contraction.

Muscle Tone

• Muscle tone is regulated by impulses from the brain, mediated by y-motoneurons to maintain balance between muscle relaxation and contraction.

Muscle Spindles Keep the Brain Continuously Informed

• Muscle spindles continuously inform the brain and cerebellum of any muscle tone changes.

Interruption of the Reflex Arc Controlling Muscle Tone

• Loss of muscle tone due to a disrupted reflex arc.
• Different kinds of muscle tone and how they manifest

Golgi Tendon Organ

• Golgi tendon organs are encapsulated sensory receptors found within tendons.
• Golgi tendon organs detect muscle tension.
• They transmit signals to the spinal cord and brain to inhibit the motor neuron.
• Golgi tendon organs control the force exerted by a muscle and the stiffness of particular joints.

Golgi Tendon Organ Pathway

• Golgi tendon organs trigger an inhibitory response through spinal cord interneurons.
• Recent studies suggest they may not directly initiate reflexes.

The Knee Jerk Response

• The knee jerk response is a stretch reflex that's used to assess the integrity of the nervous system.
• Briefly stretching the quadriceps muscle triggers a rapid contraction of the same muscle (allowing the knee to extend).

Testing and Grading Stretch Reflex Responses

• Assessing the quality of a stretch reflex is a part of standard neurological testing (e.g., examining tendon reflexes).
• Grades (0-4) are used to describe the strength of the response.

Diminished Stretch Reflex Response (Hyporeflexia)

• Hyporeflexia (diminished stretch reflexes) occurs when any part of the reflex pathway from the spindle to the muscle is damaged.
• Examples of conditions which cause hyporeflexia include: Poliomyelitis, muscular dystrophy, lower motor neuron lesions.

Absent Stretch Reflex (Areflexia)

• Areflexia is the absence of tendon reflexes, possibly due to a clinical experience issue.
• Some potential reasons for areflexia include a lesion of the reflex arc, a root lesion, or peripheral neuropathy.

Exaggerated Stretch Reflex Responses (Hyperreflexia)

• Hyperreflexia (exaggerated stretch reflexes) follows damage to motor pathways from the brain to the spinal cord.
• Common causes of hyperreflexia include a stroke or brain tumour.
• This phenomenon often results from a loss of the inhibitory input from higher brain areas to some motor neurons.

Reciprocal Innervation

• Somatic reflexes orchestrate limb movements like the knee-jerk reflex, by simultaneous contraction of agonist and relaxation of antagonist muscles.
• Reciprocal innervation facilitates this agonist-antagonist interaction.

Polysynaptic Reflexes

• Polysynaptic reflexes involve multiple synapses between afferent and efferent nerves in the spinal cord.
• Polysynaptic reflexes produce more complex responses than monosynaptic reflexes.
• Interneurons may control multiple muscle groups

Polysynaptic Reflexes: Withdrawal Reflexes

• Withdrawal reflexes are polysynaptic reflexes that cause a part of the body to move away from painful stimuli. This is a protective mechanism.
• Hand withdrawal from a hot plate.

Flexor Withdrawal Reflex

• The flexor withdrawal reflex is triggered when pain receptors (nociceptors) are activated.
• This reflex contracts flexor muscles and withdraws the stimulated limb to prevent further damage.

Crossed Extensor Reflex

• The crossed-extensor reflex is a polysynaptic reflex that occurs in response to a stimulus, such as stepping on a sharp object.
• The reflex is on the opposite side of the body. It is on the opposite side to the withdrawal of the foot, enabling the other foot to take the weight of the body.

Controlling Your Reflexes

• Higher brain centers can override spinal reflexes, at least temporarily.
• For example, voluntarily overriding a reflex during a medical procedure.

Acquired/Conditioned Reflexes

• Some reflexes develop through practice and learning, for instance certain sporting techniques and musical performances.
• Repeated skill performance modifies neural pathways.

Further Reading

• Further information on spinal reflexes can be found in human physiology and Clinical Neurology texts (e.g. Human Physiology by Sherwood, Clinical Neurology by Greenberg et al).

This quiz explores the concept of spinal reflexes, including their automatic nature and how they function without conscious effort. It covers the basic components of reflexes, their classification, and their importance in involuntary muscle control. Students will also learn about specific reflex tests and what atypical responses may indicate.