Spinal Cord Motor Functions (PDF)

# Spinal Cord Motor Functions: The Cord Reflexes ## PHYSIOLOGY - CNS - Brain and Spinal Cord - CNS intimately connected with the peripheral nervous system - PNS - Sensory and Motor components - Spinal Nerves ### Spinal Cord - Extends from the Foramen Magnum of the Skull, starts from C1 to bet...

# Spinal Cord Motor Functions: The Cord Reflexes ## PHYSIOLOGY - CNS - Brain and Spinal Cord - CNS intimately connected with the peripheral nervous system - PNS - Sensory and Motor components - Spinal Nerves ### Spinal Cord - Extends from the Foramen Magnum of the Skull, starts from C1 to between L1 and L2 - Adults: C1 - L1, ends: L3 ### Spinal Cord Structure | Region | Anatomy | |---|---| | Cervical Region | Cervical enlargement C5-T1 of the spinal cord, more gray matter, supply upper limb skeletal muscles | | Thoracic Region | No enlargement | | Lumbar Region | Lumbar enlargement L2 to S3 of the spinal cord, more gray matter, supply lower limb of skeletal muscles| | Sacral Region | No enlargement | | Coccygeal Region | No enlargement | | Conus Medullaris | Tapered terminal end of the spinal cord, between L1-L2 | | Cauda Equina | Bundle of nerve roots from L2 to Co1, looks like horse tail | ### Spinal Cord Cross-Sectional Anatomy: - White matter - Myelinated axons - As we go down from cervical region to coccygeal region - White matter decreases - Ascending Tracts carrying Sensory Information - Becomes thicker as it ascends - Descending Tracts carrying Motor Information - Thicker at the top - Gray Matter - Contains cell bodies (soma), dendrites, unmyelinated axons - As we go down from cervical region to coccygeal region - Gray matter increases - Higher Muscle Mass in the lower extremities - Needs more motor supply ### Spinal Cord: White and Gray Matter Anatomy White Matter | Anatomy | Overall Function | |---|---| | Anterior (Ventral) Funiculus | Contains bundle of myelinated axons called tract, axons transmits signals between CNS and periphery | | Lateral Funiculus | | | Posterior (Dorsal) Funiculus | | | Anterior Median Fissure | | | Posterior Median Sulcus | | Gray Matter | Anatomy | Overall Function | |---|---| | Posterior Dorsal Horn | Contains cell bodies and dendrites of neurons | | Lateral Hom | | | Anterior (Ventral) Hom | | ### Spinal Cord Nerves - Spinal Nerves Coming out of the Spinal Cord are part of Peripheral Nervous System - There are 31 pairs of spinal nerves: - Cervical: 8 pairs - Thoracic: 12 pairs - Lumbar: 5 pairs - Sacral: 5 pairs - Coccygeal: 1 pair ### Spinal Cord Nerves: Anatomy - There are 7 Cervical Vertebrae, but 8 pairs of spinal nerves come out of the cervical segment - Nerves C1-C7 exit above the corresponding vertebrae - C8 spinal nerve exists below the C7 and above T1 - All other nerves exit below the corresponding vertebrae. ## Spinal Cord Control of Muscle Function - Integration of Information: Occurs at all levels of the nervous system to produce motor responses. - Spinal Cord - Controls simple muscle reflexes. - Brain Stem - Manages more complex responses. - Cerebrum - Regulates the most advanced motor skills. ## Types of Neurons in the Spinal Cord ### Anterior Motor Neurons - Located in the anterior horns controlling skeletal muscles - Alpha Motor Neurons - Give rise to large Aa fibers (14 micrometers) - Innervate extrafusal skeletal muscle fibers forming the motor unit. - Gamma Motor Neurons - Transmit impulses through Ay fibers (5 micrometers) - Control intrafusal fibers within the muscle spindle, regulating muscle tone. ### Interneurons - Present in all areas of gray matter. - More numerous than motor neurons. - Involved in integrative functions like reflexes through various circuits (e.g., diverging, converging, repetitive discharge). - Excitability: Can fire rapidly (up to 1500 times/second). - Interconnected with motor neurons for reflex actions. ## Neuronal Circuits - **Interneurons:** Play a critical role in processing incoming signals before relaying them to motor neurons. - **Renshaw Cells:** Provide lateral inhibition to adjacent motor neurons. - Inhibit nearby neurons to sharpen motor signals, much like sensory lateral inhibition sharpens sensory inputs. ## Multisegmental Connections - **Propriospinal Fibers:** Facilitate connections between different segments of the spinal cord. - These fibers allow multi segmental reflexes, coordinating simultaneous limb movements. - Sensory fibers bifurcate upon entry into the spinal cord, spreading signals to multiple segments, enabling coordinated motor actions. ## Muscle Sensory Receptors and Their Roles in Muscle Control - **Key Sensory Receptors:** - **Muscle Spindles:** Located in the muscle belly, providing information about muscle length and rate of change of length. - **Golgi Tendon Organs:** Found in tendons, transmitting data on tendon tension and rate of change of tension. - **Sensory Feedback:** - Operates mostly at a subconscious level. - Provides essential input to the spinal cord, cerebellum, and cerebral cortex for precise muscle control. ### Muscle Spindle Function - **Structure:** - Contains 3-12 intrafusal fibers surrounded by larger extrafusal muscle fibers. - Central region acts as a sensory receptor (non-contractile), while end portions contract through gamma motor nerve fibers. - **Sensory Innervation:** - **Primary Afferent Ending:** (Type la fibers) - Encircles intrafusal fibers. - Transmits signals at 70-120 m/s, among the fastest in the body. - **Secondary Afferent Ending:** (Type II fibers) - Located beside primary endings. - Transmits sensory signals at 8 micrometers diameter, often spreading like branches. - **Intrafusal Fiber Types:** - **Nuclear Bag Fibers:** - Several nuclei form a "bag" in the central receptor area. - Primary endings respond to both nuclear bag and nuclear chain fibers. - **Nuclear Chain Fibers:** - Smaller and thinner, with aligned nuclei. - Secondary endings mainly respond to these fibers. - **Responses of Muscle Spindle:** - **Static Response:** - Slow stretch of the spindle increases impulse transmission from both primary and secondary endings. - Continues signaling for several minutes. - **Dynamic Response:** - Sudden stretch triggers strong signals from the primary endings only, corresponding to rapid changes in length. - Important for detecting quick changes in muscle position. - **Control of Responses via Gamma Motor Nerves:** - **Gamma-Dynamic (y-d) Nerves:** Enhance dynamic response by exciting nuclear bag fibers. - **Gamma-Static (y-s) Nerves:** Enhance static response by exciting nuclear chain fibers. ### Muscle Stretch Reflex - **Basic Circuit:** A monosynaptic pathway involving Type la fibers from muscle spindle to anterior motor neurons, resulting in reflex contraction. - **Types of Stretch Reflex:** - **Dynamic Stretch Reflex:** Reacts to sudden stretch, causing strong contraction to resist changes in muscle length. - **Static Stretch Reflex:** Maintains steady muscle contraction after the initial response, keeping muscle length stable. - **Damping Function of Stretch Reflex** - **Prevents Jerkiness:** Both dynamic and static stretch reflexes smooth out contractions, ensuring fluid movements. - **Absence of this damping mechanism:** Leads to jerky, unsmooth muscle movements, as demonstrated experimentally. - **Role of the Muscle Spindle in Voluntary Motor Activity:** - **Gamma Efferent System:** - 31% of motor fibers to muscles are type A gamma efferent fibers. - **Coactivation:** Alpha and gamma motor neurons are activated simultaneously, causing both extrafusal and intrafusal fibers to contract together. - **Functions of Coactivation:** - Prevents changes in the length of the muscle spindle receptor during contraction. - Maintains the damping function of the spindle for smooth muscle control. - **Control of the Gamma Motor System:** - **Excitatory Inputs to the gamma system come from:** - Bulboreticular facilitatory region of the brain stem. - Cerebellum. - Basal ganglia. - Cerebral cortex. - **Importance in Antigravity Muscles:** Gamma system helps stabilize posture during walking and running by damping movements through muscle spindles. - **Stabilization of Body Position During Motor Activity:** - **Muscle Spindle System:** - Helps stabilize joint position during intense or delicate motor actions. - Activation of muscle spindles on both sides of a joint increases reflexive muscle contraction, stabilizing the joint. - **Application:** Stabilizes body during intricate motor tasks, such as fine movements of the fingers during detailed voluntary actions. - **Clinical Applications of the Stretch Reflex:** - **Purpose:** Used to assess the background muscle tone and level of brain excitation sent to the spinal cord. - **Knee Jerk Test:** - Striking the patellar tendon elicits a dynamic stretch reflex, causing the quadriceps muscle to contract and the leg to jerk forward. - Muscle jerks can be elicited from various muscles by striking their tendons or bellies. - **Assessment of Spinal Cord Sensitivity:** - **Exaggerated Jerks:** Indicate heightened facilitation from the brain. - **Weakened or Absent Jerks:** Suggest reduced facilitation or damage to the motor areas. - **Clonus: Oscillation of Muscle Jerks:** - **Definition:** Repeated rhythmic contractions of muscles due to oscillation of the stretch reflex. - **Example:** Ankle clonus occurs when stretching the gastrocnemius muscle triggers a stretch reflex, lifting the body, followed by muscle relaxation, leading to repeated oscillations. - **Clinical Significance:** Clonus indicates a highly sensitized stretch reflex, often due to brain lesions or conditions like decerebration. ### Golgi Tendon Reflex: Key Functions - **Golgi Tendon Organ:** - Sensory receptor located in muscle tendons, connected to 10-15 muscle fibers. - Detects muscle tension, unlike muscle spindles which detect muscle length. - **Responses:** - **Dynamic Response:** Reacts to sudden increases in muscle tension. - **Static Response:** Provides steady-state firing proportional to ongoing muscle tension. - **Transmission of Impulses:** - **Type Ib nerve fibers:** Conduct signals rapidly to the spinal cord, cerebellum, and cerebral cortex. - **Inhibitory Reflex:** - Local inhibitory interneurons in the spinal cord inhibit anterior motor neurons. - Prevents excessive muscle tension, protecting muscle from damage. - **Protective Mechanism: Lengthening Reaction** - **Function:** Prevents muscle tearing or tendon avulsion. - When muscle tension becomes extreme, the reflex causes instantaneous muscle relaxation. - A key negative feedback mechanism. - **Equalization of Contractile Force** - **Role:** Ensures even distribution of muscle load. - Fibers with excess tension are inhibited, while fibers with less tension are stimulated. - Prevents overloading and damage to specific areas of the muscle. - **Higher-Level Motor Control:** - **Instantaneous Feedback:** - Dorsal spinocerebellar tracts transmit rapid signals (up to 120 m/sec) to the cerebellum and brain stem. - Critical for feedback control in motor areas of the cerebral cortex. - **Clinical Relevance:** Provides feedback for fine-tuning motor signals originating from higher brain centers (cerebellum, brainstem, cerebral cortex). ## Flexor Reflex and Withdrawal Reflexes - **Flexor Reflex:** - Triggered by cutaneous sensory stimuli (e.g., pain, pinprick). - Causes contraction of flexor muscles to withdraw the limb from a stimulus (also called nociceptive reflex). - **Neuronal Mechanism:** - Pathway involves interneurons before reaching motor neurons. - Includes diverging circuits (activating necessary muscles), reciprocal inhibition circuits (inhibiting antagonist muscles), and afterdischarge (sustained reflex response). - **Crossed Extensor Reflex:** - **Delayed Reflex:** (0.2-0.5 seconds after flexor reflex) - Opposite limb extends to stabilize the body or push away from the painful stimulus. - Involves signals crossing the spinal cord to activate extensor muscles on the opposite side. - **Afterdischarge:** Prolonged reflex keeps the body away from the painful stimulus. - **Reciprocal Inhibition and Innervation:** - **Reciprocal Inhibition:** Excitation of one muscle group inhibits the antagonist muscles. - Common in flexor and extensor muscle reflexes to prevent co-contraction. - **Reciprocal Innervation:** Seen in bilateral muscle reflexes, such as flexor reflexes in opposite limbs. - Prevents simultaneous contraction of opposing muscle groups on both sides of the body. ## Reflexes of Posture and Locomotion - **Positive Supportive Reaction:** - Pressure on the footpad causes the limb to extend. - Helps maintain body weight and posture, especially in animals with spinal cord injury. - **Cord Righting Reflex:** - An animal with a spinal cord injury makes movements to stand up from a lying position. - Some complex postural reflexes are integrated in the spinal cord. - **Stepping and Walking Movements:** - **Rhythmical Stepping Movements:** Alternating flexion and extension of a limb occurs rhythmically, even after sensory nerves are cut. - Likely due to reciprocal inhibition circuits in the spinal cord. - **Stumble Reflex:** If a foot encounters an obstruction, the spinal cord adjusts by lifting the foot higher to clear the object. - **Reciprocal Stepping of Opposite Limbs:** In intact lumbar spinal cords, stepping in one limb causes backward movement of the opposite limb through reciprocal innervation. - **Diagonal Stepping (Mark Time Reflex):** In animals with spinal cord transection, stepping occurs diagonally between forelimbs and hindlimbs due to reciprocal innervation. - **Scratch Reflex:** - **Trigger:** Initiated by itch or tickle sensations. - **Functions:** - **Position Sense:** Locates the exact point of irritation. - **To-and-Fro Movement:** Involves reciprocal innervation circuits similar to stepping movements. ## Spinal Cord Reflexes Causing Muscle Spasm - **Muscle Spasm from Pain:** - **Broken Bone:** Pain from bone injury causes tonic muscle contraction around the fracture. Pain relief through anesthesia resolves the spasm. - **Peritonitis:** Abdominal muscle spasm occurs due to irritation of the peritoneum. Deep anesthesia is needed during surgeries to prevent excessive muscle contraction. - **Muscle Cramps:** - **Causes:** Cold, lack of blood flow, or over exercise. - **Mechanism:** Positive feedback loop - initial irritation triggers contraction, which stimulates more sensory signals, leading to full-blown muscle cramps. ## Autonomic Reflexes in the Spinal Cord - **Types of Reflexes:** - Vascular tone changes due to local skin heat. - Sweating induced by localized heat. - Intestino Intestinal reflexes control gut motor functions. - Peritoneo Intestinal reflexes inhibit gut motility in response to peritoneal irritation. - Evacuation reflexes for bladder and colon emptying. - **Mass Reflex:** - Triggered by strong pain stimulus or visceral distention (e.g., bladder). - **Effects:** - Flexor muscle spasm throughout the body. - Bladder and colon evacuation. - Elevated blood pressure (over 200 mmHg). - Profuse sweating. - Similar mechanism to epileptic seizures, involving reverberating circuits in the spinal cord. ## Spinal Shock and Recovery - **Spinal Shock:** - Occurs after spinal cord transection in the upper neck, causing immediate depression of all cord functions and reflexes. - **Cause:** Loss of tonic excitation from higher centers, such as reticulospinal, vestibulospinal, and corticospinal tracts. - **Recovery:** - Cord neurons regain excitability after hours to weeks. - Sympathetic nervous system and skeletal muscle reflexes return gradually (stretch reflexes return first). - Bladder and colon reflexes are suppressed initially but usually recover over weeks. ## Transections at the Spinal Cord - **Paraplegia:** Loss of voluntary movements below level of the lesion. - Loss of conscious sensation below the level of the lesion. - **Spinal Shock:** - Limbs flaccid, reflexes absent immediately after transection. Partial recovery may occur after sometime (after several hours to a few weeks) - C7 transection: HR and BP decreases - C3 transection: breathing stops - C1 transection: death