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Description : A 55-year-old African man presented with a chief complaint of neck pain since 5 months and weakness of his right foot in the last 2 weeks. History revealed that the neck pain was aggravated with any neck bending activities, and associated with pain and numbness radiating to right arm up to the index and middle finger. Two weeks prior to this presentation, the patient had developed right lower limb radicular pain, numbness, and weakness of the right foot, with no symptoms of back pain.

Tags : Physiology,Spinal Cord Reflexes,spinal,spinal cord

Published on : Mar 27, 2023 | Views : 723 | Downloads : 1

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Slide 1-

Dr. Ramadan Saad Spinal Cord Reflexes (part I)
Slide 2-

Case scenario At the end of this lecture the student should be able to : Describe the pathway of stretch reflex. Describe the structure , innervations and function of the muscle spindle. Explain what is meant by static and dynamic stretch reflex. Explain what is muscle tone and its mechanism and aim. Describe the spinal and supraspinal regulation of the stretch reflex.
Slide 3-

Case scenario A 55-year-old African man presented with a chief complaint of neck pain since 5 months and weakness of his right foot in the last 2 weeks. History revealed that the neck pain was aggravated with any neck bending activities, and associated with pain and numbness radiating to right arm up to the index and middle finger. Two weeks prior to this presentation, the patient had developed right lower limb radicular pain, numbness, and weakness of the right foot, with no symptoms of back pain. Neuro-examination of the lower limbs showed that the patient had a right high steppage gait and normal tone bilaterally. On the right side, the power of right dorsiflexion was 2/5, extensor hallucis longus was 2/ 5, and plantar flexion was 5/5. The power of the other muscle groups of both lower limbs was normal. The ankle reflex was normal bilaterally, and right knee reflex was 3+. Babinski's test was positive on the right side but negative on the left side.
Slide 4-

WHAT IS STRETCH REFLEX? Stretch stimulates the muscle spindle, which activates Ia fibers that excite the motor When a skeletal muscle with an intact nerve supply is stretched, it contracts. This response is called the stretch reflex or myotatic reflex.
Slide 5-

Stretch (Myotatic) Reflex Pathway; Stimulus:→ passive stretch Receptors: → ms spindles Afferents: → fast-conducting Aα (Ia) and Aβ (II) nerve fibers Centers: → alpha motor neurons in AHCs. Efferents: → axons of alpha motor neurons. Effector organ: → Extrafusal ms fibers Response: → ms contraction.
Slide 6-

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 fibres (4-12 fibers) called intrafusal fibres. Types of intrafusal ms fibres: 1) Nuclear bag fibres; Have a dilated central area filled with nuclei. 2) Nuclear chain fibres; Have nuclei which are arranged as a chain in the receptor area. Each ms spindle 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
Slide 7-

Functions of muscle spindle:- 1- keep CNS informed about muscle length & rate or velocity of change in muscle length. 2- muscle spindle act to maintain muscle length against rupture:- if muscle is stretched>> muscle spindle discharge>> reflex shortening of muscle by contraction to keep its length
Slide 8-

B) Efferent (motor) innervations Innervation of Muscle Spindle A) Afferent (sensory) innervations
Slide 9-

Alpha motor neuron 70% Gamma motor neuron 30 % Gamma motor Neurons
Slide 10-

The adequate stimulus that excites the ms spindles is stretching of their central part→ depolarizes it→ initiates an AP in 1ry and 2ry endings. This can occur in 2 ways: 1) Passive stretch of the whole ms: It causes stretch of the ms spindle which lies parallel to ms fibers. Stimulation of Muscle Spindle 2) Activation of the γ-MNs: By supraspinal centers or reflexely It causes contraction of the peripheral part the intrafusal fibres→ stretch of receptor area
Slide 11-

Types of Responses of Ms spindle to Stretch (Types of Stretch Reflex)
Slide 12-

ppt slide no 12 content not found
Slide 13-

Dynamic stretch Reflex Sudden stretch Nuclear bag 1 ry endings AHCs Alpha MNs
Slide 14-

Static stretch reflex maintained stretch e.g. gravity Nuclear chain 1ry and 2 ry endings AHCs Alpha MNs
Slide 15-

Functions of stretch reflex or Ms Spindle
Slide 16-

Def. It is a state of continuous (partial or subtetanic) contraction of skeletal ms during rest. Distribution: It is present in all skeletal ms but specially in the antigravity ms Skeletal Ms Tone
Slide 17-

Muscle Tone maintained stretch e.g. gravity Nuclear chain 1ry and 2 ry endings AHCs Alpha MNs Dynamic state of skeletal ms tone: The ms tone is not static but dynamic in upright standing the magnitude of the ms tone is greater in the antigravity ms especially extensors of lower limb and trunk If the trunk is tilted backward the ms tone ↑es in ms of anterior abdominal wall and ↓es in extensors. This by modulation of ɤ-MNs activity by the supraspinal centers which may facilitate or inhibit the muscle tone
Slide 18-

How Skeletal Ms Tone is affected by higher centers
Slide 19-

Regulation of skeletal ms tone: By supraspinal centers (facilitatory or inhibitory) Skeletal Ms Tone Facilitatory areas Inhibitory areas Area 4 Neocerebellum Pontine (Excitatory) RF Vestibular N. Area 6 (premotor area) Paleocerebllum Medullary (Inhibitory) RF Red N. Basal ganglia. + -
Slide 20-

Regulation of skeletal ms tone: By supraspinal centers (facilitatory or inhibitory) Skeletal Ms Tone Facilitatory areas Inhibitory areas Area 4 Neocerebellum Excitatory RF Vestibular N. Area 6 Paleocerebllum Inhibitory RF Red N. + -
Slide 21-

Inhibitory supra spinal centers Facilitatory supra spinal centers
Slide 22-

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. Skeletal Ms Tone
Slide 23-

Stretch reflex prevents oscillations or jerkiness of body movements Motor signals from the motor areas are transmitted to the ms in an unsmooth form (↑ for few Sec and ↓ for another Sec) This causes irregularities or oscillations of movements The signals discharged from the ms spindles cause partial activity of αMNs of the ms So, the motor signals find αMNs in state of partial activity, so they cause continuous activation of them → cause smooth ms contraction. Smoothing or Damping Function
Slide 24-

Def., It is a reflex responsible for keeping the hand or foot in position when a moderate loads are applied E.g. when a person holding up a cup while someone is filling it with tea, as the load gets bigger and bigger, the force required to keep the hand in position must be continually increased. Mechanism (Coactivation of α-MNs and γ-MNs) The higher motor centers activate α-MNs to initiate contraction of a ms and also activate γ-MNs discharge →co-activation of α-MNs and γ-MNs. This increases the sensitivity of the ms spindles to any slight degree of stretch produced by increasing the load consequently 3. Load Reflex
Slide 25-

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