Stretch Reflex PDF

Lecture 4 Stretch Reflex  OBJECTIVES By the end of this lecture the student should be able to: Describe stretch reflex Differentiate between two types of stretch reflex Explain the neurophysiological base of muscle tone and tendon jerk Identify inverse stretch reflex. ...

Lecture 4 Stretch Reflex  OBJECTIVES By the end of this lecture the student should be able to: Describe stretch reflex Differentiate between two types of stretch reflex Explain the neurophysiological base of muscle tone and tendon jerk Identify inverse stretch reflex.  MUSCLE SPINDLE ▪ Muscle spindles: are small, spindle-shaped sensory receptors located in skeletal muscle tissue , Muscle spindle cells are located within the belly of a muscle and they run parallel to the main muscle fibers (extrafusal fibers). ▪ A muscle spindle is a type of muscle proprioceptor that is located within a muscle and is sensitive to a stretch (i.e., lengthening of the muscle).  MUSCLE SPINDLE ▪❍ A muscle spindle is sensitive to the stretch (i.e., the lengthening) of the muscle within which it is located. More specifically, it is sensitive to two aspects of the stretch: ▪ 1. The phasic stretch (the rate (speed) at which a muscle stretches) ▪ 2. The tonic stretch (the extent to which the muscle is stretched). ▪ ❍ When the muscle is stretched sufficiently, the muscle spindle is also stretched and becomes stimulated, creating an impulse in a sensory neuron that enters the spinal cord to alert the CNS that the muscle has just been stretched. ▪ ❍ Because a stretched muscle may be overly stretched and torn, this impulse in the spinal cord causes a reflex contraction of the muscle. ▪ By contracting and shortening, the muscle stops any excessive stretching that might tear the muscle. ▪ This reflex is called the muscle spindle reflex or the stretch reflex. ▪ ❍ The muscle spindle reflex is also known as the myotatic reflex. ▪ ❍ Therefore a muscle spindle and its stretch reflex are. They prevent a muscle from being overly stretched and torn.  STRETCH (MYOTATIC) REFLEX Def ▪ It is a reflex contraction of a muscle when it is passively stretched ▪ Pathway 1. Stimulus:→ passive stretch 2. Receptors: → muscle spindles 3. Afferents: → fast-conducting Aα (Ia) and Aβ (II)nerve fibers 4. Centers: → alpha motor neurons in AHCs. 5. Efferent: → axons of alpha motor neurons. 6. Effector organ: → Extrafusal muscles fibers 7. Response: → muscle contraction. STRETCH REFLEX  1ry and 2ry endings Ms spindle AHCs Extrafusal Alpha motor ms fibers neurons  MUSCLE SPINDLE Site: Fleshy parts of skeletal ms parallel to their fibres (extrafusal ms fibers). Shape: It is capsulated fusiform stretch receptor. Structure: Each spindle consists of several small ms fibers (4-12 fibers) called intrafusal fibers. Muscle Spindle  MUSCLE SPINDLE Types of intrafusal ms fibers: There are 2 types; 1)Nuclear bag fibers; Have a dilated central area filled with nuclei. Are 2 of these fibers per spindle. 2)Nuclear chain fibers; Have nuclei which are arranged as a chain in the receptor area. Are 5-8 of these fibers per spindle.  MUSCLE SPINDLE Each ms fiber consists of 2 parts; a)Central part: It is non-contractile part constitutes the receptor areas of the spindles It receives sensory innervation b)Peripheral part: It is a contractile part when contracts, it causes stretch of the central receptor area. It receives motor innervation Muscle Spindle 1ry endings 2ry endings  STATIC STRETCH REFLEX IS THE NEUROPHYSIOLOGICAL BASE OF MUSCLE TONE MUSCLE TONE Spontaneous local axon stretch reflex The length of any skeletal muscle is shorter than the distance between its origin & insertion. This puts the muscle in a state of constant slight stretch. This stretch stimulates reflex some muscle spindles which send excitatory impulses through the afferent sensory nerve & the dorsal root to the A.H.C. The excited A.H.C. send motor impulses through the anterior root & the efferent motor nerve to the muscle. This results in reflex continuous subtetanic contraction of the muscle; this constitutes the muscle tone which is important for nourishment of the muscle & the posture of the body. The muscle tone receives higher control; facilitatory and inhibitory. Therefore: UMNL(∆ lesion): results in loss of ∆ inhibition of the intact reflex arc leading to increased muscle tone (spasticity) below the level of the lesion. LMNL: results in interruption of the reflex arc leading to decrease muscle tone (flaccidity) at the level of the lesion.  STATIC STRETCH REFLEX AHCs 1ry and 2 ry endings Alpha MNs Nuclear chain maintained stretch e.g. gravity Functions of skeletal ms tone: a) Postural control: Is the basic mechanism for control of posture and equilibrium By adjusting the magnitude of ms tone of different groups of ms. b) Help in heat production and maintain of body temperature c) It helps both the venous return & lymph flow: Ms tone has a mild squeezing effect on the walls of veins and lymphatics of skeletal ms→ help venous return to the heart. d) Keeps viscera in position and prevents visceroptosis:  DYNAMIC STRETCH REFLEX IS THE NEUROPHYSIOLOGICAL BASE OF TENDON JERK (DEEP REFLEX) Induced local axon stretch reflex. It is induced by tapping the tendon of the muscle with a hammer. This tap stretches the muscle with synchronous stimulation of muscle spindles → resulting in a brief contraction of the muscle. UMNL result in exaggeration of deep reflexes (hyperreflexia) below the level of the lesion. LMNL: result in diminution of deep reflexes (hyporeflexia) at the level of the lesion.  DYNAMIC STRETCH REFLEX 1 ry endings Nuclear bag Sudden stretch AHCs Alpha MNs Def., Rapid contraction followed by relaxation of a ms due to sudden stretching of that ms by tapping on its tendon using a medical hammer Mechanism: It is a dynamic type of the stretch reflex Jerk Center Limb position Tendon Response Biceps C5,6 The elbow is Tapping on biceps Flexion of 120° tendon the forearm jerk Triceps C6,7 The elbow is Tapping on triceps Extension of 90° tendon directly the forearm jerk Knee L2, 3 & 4 knee is semi Tapping on Extension of flexed by patellar tendon the knee jerk seating with the leg to be tested crossing over other Ankle S1,2 feet slightly Tapping on Plantar dorsiflexed tendoachilles flexion. jerk Reinforcement of the tendon jerks The response of the tendon jerks can be reinforced by facilitating the spinal centers. This can be done by either; a) Jendrassik's maneuver → ask the patient to hook his fingers or to clench his teeth. b) Distracting patient’s attention→ prevents any voluntary inhibition of the reflex. REINFORCEMENT OF THE TENDON JERKS  Types of Responses of Ms spindle to Stretch (Types of Stretch Reflex) Dynamic Response Static Response Stimulus Sudden stretch constant slight stretch Receptors nuclear bag nuclear chain Afferents 1ry endings primary and secondary endings Center Alpha motor neurons Alpha motor neurons Response Rapid contraction Continous subtetanic followed by rapid contraction relaxation Examples e.g. tendon jerk e.g. muscle tone Facilitatory areas Inhibitory areas 1. Area 4 1. Area 6 2. Neocerebellum +- 2. Paleocerebllum 3. Excitatory RF 3. Inhibitory RF 4. Vestibular N. 4. Red N.  It is a reflex relaxation (or lengthening) of a ms in response to excessive stretch or contraction of that ms. Neural pathway: Stimulus: ↑ed ms tension by; 1. Overstretch or 2. Severe contraction Receptors: Golgi tendon organs 1) Site: tendons of skeletal ms in series with ms fibers 2) Structure: Are encapsulated sensory receptor 6-20 elastic fibers 3) Innervations: Type Ib afferent fibres Receptors: GTOs Stimulated by ↑ed ms tension caused by passive overstretch or active contraction of the muscle Afferents: Ib nerve fiber Center : a)inhibitory interneurons→ inhibit the α-MNs supplying the same muscle b)excitatory interneurons→ excite the α-MNs supplying the antagonistic muscle. Response: Relaxation of the same muscle Contraction of antagonistic group of muscle.  Clinical Significance of Golgi Tendon Organ: a)Physiological significance: Protective reaction which prevent tearing of the ms or avulsion of its tendon from its bony attachment when the ms is overstretched. Clinical Significance of Golgi tendon organ: b)Clinical significance: (clasp knife effect) Demonstrated clinically by passive flexion of a spastic limb (e.g. in upper motor neuron lesions) at its main joint. As the limb is flexed, an initial resistance occurs due to contraction of this ms a result of the stretch reflex. With persistent flexion, at a certain point, GTR is excited→ sudden disappearance of the initial resistance → the limb flexes easily, as occurs due closing-of a pocket knife→ clasp knife effect. E.g. Flexion of knee and ankle Def. Alternating regular rhythmic contractions with incomplete relaxations of a ms (its MNs is in a state of facilitation) in response to sudden maintained stretch. Cause: UMNL Types 1) Ankle Clonus: Produced by sudden maintained dorsiflexion of the foot → leads to regular rhythmic planter flexions due to rhythmic contractions of soleus and gastrocnemius muscles. 2) Knee Clonus: Produced by the sudden downward displacement of the patella → rhythmic oscillations of the patella.  CONCLUSION 1- Neurophysiological base of ▪ Muscle tone → Static stretch reflex ▪ Tendon jerk → dynamic stretch reflex ▪ Clasp-knife → inverse stretch reflex ▪ Clonus → stretch reflex- inverse stretch reflex sequence 2- upper motor neuron is associated with hypertonia, hyperreflexia while lower motor neuron is associated with hypotonia and hyporeflexia