Spinal Reflexes 2024 PDF - RCSI

Spinal reflexes Course Year 2 CNS module Lecturer Prof David Henshall; Presented by Dr Ebrahim Rajab Date 3rd November, 2024 Learning outcomes Discuss the role of involuntary control of skeletal muscles Define a reflex and describe its basic components Explain the function o...

Spinal reflexes Course Year 2 CNS module Lecturer Prof David Henshall; Presented by Dr Ebrahim Rajab Date 3rd November, 2024 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 by definition is “Any response that occurs automatically without conscious effort. Reflexes are rapid, automatic and stereotyped responses to a specific stimulus” A rapid, automatic and stereotyped* response to a specific stimulus *Stereotyped: This means that the response does not vary and is therefore predictable Example of a reflex, 100 people are asked to touch a metal plate with their index finger. Unknown to them the plate is hot. Immediately the hand is withdrawn. The response is predictable, similar in all, automatic and immediate. Skeletal muscle and spinal cord reflexes (overview) Skeletal muscle is under voluntary control from brain but, Skeletal muscles can contract without conscious control in a reflex manner in response to certain stimuli There are varying degrees of complexity of these spinal reflexes monosynaptic reflexes through to polysynaptic reflexes involving muscle control on both sides of body Functions of the Spine Ventral side Dorsal side 1) Link for transmission of information between brain and remainder of body 2) Integrate reflex activity between afferent input and efferent output without involving the brain From Physiology by Sherwood Information “root” of spinal nerves Afferent sensory fibres enter spinal cord through dorsal root (their cell bodies are in the dorsal root ganglion; DRG) Cell bodies of efferent [motor] neurons originate in grey matter, axons exit through ventral route -motor neurons (A axons innervate skeletal muscle)  motor neurons (A axons innervate intrafusal fibres) Dorsal horn: cell bodies of interneurons on which afferents Dorsal & ventral roots form spinal nerve terminate Ventral horn: cell bodies of efferent motor neurons [supply skeletal muscles] (of which there are 31 in total) From Physiology by Sherwood Spinal cord reflexes: why are they important? Coordinate the rapid withdrawal responses to painful stimuli Protect against over-stretching of muscles Execute emptying pelvic organs e.g. bladder contraction Carry out purposeful muscle movement and contribute to proper balance & movement (e.g. during walking) Spinal reflexes coordinate skilled movements of the trunk and limbs and the often-taken-for- granted activities of standing erect, walking, and running Learned reflexes (e.g. somersaults in sports) https://youtu.be/l59Hu4CiV1Q Reflexes continued A reflex response is the most basic form of integrated neural activity Reflex responses occur in: 1. The somatic nervous system 2. The autonomic nervous system  What are the Components of a reflex? Reflex activity involves three components and their connecting neurons The three components are: 1. Peripheral sensory receptors [& sensory nerve] 2. Area for integration in the CNS 3. [Efferent nerve] & effectors “Reflex arc” is the name for the neural pathway that accomplishes reflex activity.  The sensory receptors These receptors respond to changes in the periphery 1-Exteroceptors: (respond to external stimuli) Information and temperature, pressure 2- Proprioceptors: (receptors in muscles, tendons, joints) send information about position 3- Interoceptors: (internal organs) - send information about pain, stretching internal organs Area for integration in the CNS This area is the brain or spinal cord Receives, processes and integrates incoming sensory information sent from the sensory receptors Based on integration of incoming information, the CNS integration area issues appropriate commands to effectors. Integration in the CNS depends on the activity of neurons known as interneurons which are responsible for the distribution of sensory information and co-ordination of commands within the CNS Interneurons can have excitatory or inhibitory effects The effectors i.e. executing the response. In the somatic nervous system, the effectors are skeletal muscles. In the autonomic nervous system, the effectors are cardiac muscle, smooth muscle and gland cells. Spinal reflexes of the somatic nervous system These reflexes provide involuntary control of the skeletal muscular system. The area for integration is in the spinal cord. Spinal reflexes can be influenced and modified by higher centres in the CNS. There are two types of reflexes in the somatic nervous system Monosynaptic Polysynaptic  Monosynaptic reflexes Only one synapse between afferent and efferent nerves in CNS The only monosynaptic reflexes which occur in the body are the stretch (myotactic) reflexes. A stretch reflex is elicited in a skeletal muscle by briefly stretching the muscle. The reflex response is a brief contraction of that muscle This reflex functions to oppose sudden changes in length of the muscle The components of the stretch reflex are 1. Sensory receptor = muscle spindles 2. Area for integration = spinal cord 3. Effector = the skeletal muscle that is stretched Muscle sensory receptors Proper control of muscle function requires continuous feedback of sensory information from muscle to spinal cord What is length? What is tension? How rapidly is length/tension changing? Muscles & their tendons have two main sensory receptors Muscle spindles (length/rate of change in length) Golgi tendon organs (tension/rate of change of tension) Both are types of proprioceptor Muscle spindle Muscle spindle consists of: 1a 1. Fibrous capsule 2. Intrafusal muscle fibers wrapped by nerve ending 3. Group Ia sensory axons (Primary fibre/endings) Large diameter Fast conduction Detect stretch and rate of length change 4. Group II sensory axons (secondary fibre/endings) Detect stretch only Muscle spindle (force-generating) Co-activation of See animation of spindles functioning on VLE  Neuronal connections involved in a stretch reflex All skeletal muscles will respond to a brief stretch with a brief contraction i.e. a stretch reflex Note. one synapse only between N.B. Muscle spindle sensory information afferent and efferent nerves. also conveyed to brain (e.g. cerebellum) Muscle spindles and their role in muscle tone Muscle spindles are important for muscle tone (residual muscle tension): Muscles are always at least partially contracted resting muscle tension Why? If muscles relaxed completely (no resting tone), they would over-lengthen, and too much time would be required to take up slack when a contraction was called for. On the other hand, too much tone would not allow for sufficient rest and recovery. Muscle tone Muscle tone is ultimately controlled by impulses from the brain, but muscle spindles are the principal regulator Under normal conditions, activity of -motoneurons is maintained at the level needed to keep the muscle spindles under proper tension while the muscles are relaxed Undue relaxation is prevented by stretch and activation of the spindles thus, tone is regulated by the stretch reflex and is not a characteristic of the muscle itself Mechanism produces normal resting muscle length and state of tension or muscle tone muscle spindles keep the brain and particularly the cerebellum continually informed of even slight changes in muscle tone. Interruption of the reflex arc controlling muscle tone leads to immediate loss of muscle tone https://youtu.be/gLZoYLxdXCQ  Golgi tendon organ Encapsulated sensory receptor inside muscle tendon Detects muscle tension 10-15 muscle fibres connected ↑ muscle tension → Ib sensory nerves transmit signals to spinal cord (and brain) Inhibits motor neuron Prevents too much tension on muscle controls force within muscles and the stiffness of particular joints see animation of Golgi tendon on VLE  Golgi tendon organ pathway N.B. Recent studies suggest GTO may not directly initiate reflexes The knee jerk response: a The response to stretch of the quadriceps (the stretch reflex which is knee jerk) or of the gastrocnemius (the ankle jerk) may be demonstrated. The quadriceps is informative about integrity of briefly stretched by sharply tapping the patellar tendon CNS Sequence of events in the knee jerk Testing and grading stretch reflex responses Testing stretch reflexes, often called tendon jerks, forms part of routine neurological testing. Testing provides information about the integrity of the pathways involved in these reflex responses Grade 4: very brisk, often with clonus* (record number of beats) Grade 3: brisk but normal Grade 2: normal Grade 1: minimal Grade 0: absent *Clonus is involuntary and rhythmic muscle contractions caused by a permanent lesion in descending motor neurons Diminished stretch reflex response (hyporeflexia) Hyporeflexia results if any part of the reflex pathway from the spindle back to the muscle is damaged Examples of conditions associated with hyporeflexia: Poliomyelitis (virus damages lower motor neurons) Muscular dystrophy (degenerative disease of skeletal muscle) Lower motor neuron lesions (e.g. those serving foot)  Absent stretch reflex (Areflexia) Areflexia: Apparent loss of tendon reflexes Can be due to lack of clinical experience! Causes: any lesion of the reflex arc (e.g. root lesion or peripheral neuropathy)  Exaggerated stretch reflex responses (hyperreflexia) Hyperreflexia: results following damage to motor pathways from the brain to the spinal cord. Greatly exaggerated muscle jerk response Causes: Upper motor neuron lesion, UMNL after stroke or brain tumour Damage to motor areas of cerebral cortex Results in loss of inhibitory inputs from higher areas to some motor neurons - Certain stretch reflexes become exaggerated (e.g. elbow)  Reciprocal innervation Somatic reflexes result in limb movement (e.g. in the knee jerk response, the knee extends and the leg kicks forward). When a specific muscle (the protagonist), contracts the opposing muscles (the antagonist) must relax to allow the movement.* Furthermore, stretch reflexes in the antagonist muscle must be inhibited This is achieved by reciprocal innervation. Neuronal connections involved in *this includes the knee-jerk reflex; the antagonist is the stretch reflex (e.g. knee jerk) showing semitendinosus muscle reciprocal innervation Polysynaptic reflexes From 2 to 100s of synapses between afferent and efferent nerves These produce more complex responses than monosynaptic reflexes May be several interneurons acting between the sensory afferent fibres and the -motoneurons Furthermore, these interneurons may control several muscle groups Polysynaptic reflexes The withdrawal reflexes are examples of polysynaptic reflexes. They move a part of the body away from painful stimulation i.e. are protective e.g. withdrawal of the hand from a hot plate  Flexor withdrawal reflex 1. Hand on hot plate 2. Nociceptors (pain receptors) in hand are stimulated. 3. Action potentials conducted along fast, pain communicating A (sensory) fibres 4. Sensory pathways synapse first on interneurons 5. Motor neurons contacted secondarily 6. Integrative centres of the cord cause those muscles to contract that can most effectively remove the pained part of the body from the object The biceps muscle (flexor) contracts and the hand is pulled up and away Contraction of the extensor (antagonist) triceps muscle is inhibited Value of immediate response to noxious stimulation (i.e. withdrawal reflex) Involuntary reflex control over skeletal muscles provides an immediate response. Immediate response is of great importance to prevent injury/damage. Strength of the reflex is related to the intensity of the stimulus Crossed extensor reflex Not all spinal reflex action is limited to motor responses on side of the body to which the stimulus is applied. If you stand on a sharp object, you lift your foot away from it, but you don’t fall over! This is because of the (flexor) reflex is accompanied by another reflex known as the crossed extensor reflex 0.2 to 0.5 s after stimulus elicits flexor reflex in one limb the opposite limb begins to extend Signals from sensory nerves cross to opposite side of cord to excite extensor muscles Opposite leg simultaneously prepares to suddenly bear all the weight to avoid losing balance/fall Controlling your reflexes Spinal reflexes can be voluntarily “over-rided” (at least temporarily) by higher brain centres Pin prick or needle insertion during a blood draw: e.g. bran sends IPSPs via descending pathways to biceps motorneurons and EPSPs to triceps to keep arm from pulling away e.g. over-riding bladder contraction (autonomic reflex) Acquired/conditioned reflexes Some reflexes are a result of practice and learning e.g. certain responses in music (pianist) and sports (e.g. somersault, catching a ball) May involve “over-riding” basic reflexes Person learns to consciously inhibit postural reflexes Repeated skill performance generates new synaptic patterns in CNS which substitute for natural reflex Becomes automatic See https://www.youtube.com/watch?v=2fdp8SVOSF4 for some examples of skill performance  Further reading Medical Sciences by Niash Further reading in Human Physiology by Sherwood Clinical Neurology by Greenberg, Aminoff & Simon (or similar)