Spinal Cord Gray Matter & Motor Neurons

Spinal Reflex

• Spinal reflex study notes written by Prof. Dr. Özgür Kasımay.

Spinal Cord Organization for Motor Functions

• Sensory Neurons
• Anterior Motor Neurons
• Alpha motor neurons
• Gamma motor neurons
• Interneurons
• Renshaw Cell Inhibitory System

Multisegmental Spinal Cord Connections

• Multisegmental connections exist from one spinal cord level to another.

Muscle Sensory Receptors and Reflexes

• Muscle Spindles
• Golgi Tendon Organs
• Muscle Stretch Reflex
• Clinical Applications of the Stretch Reflex

Cord Gray Matter Integration

• The cord gray matter serves as the key integration area for cord reflexes.
• Sensory signals enter via posterior roots and travel to two destinations.
• One branch ends in the gray matter, triggering local segmental reflexes.
• The other sends signals to higher nervous system levels like the brain stem or cerebral cortex.

Spinal Cord Neurons

• Spinal cord segments contain millions of gray matter neurons, mainly sensory neurons, anterior motor neurons, and interneurons.
• Anterior motor neurons
• Are located in the anterior horns of the gray matter.
• Are several thousand neurons that are larger than most others in the area
• They send nerve fibers via anterior roots to innervate skeletal muscle fibers directly.
• There are two types: alpha and gamma motor neurons.

Alpha Motor Neurons

• Alpha motor neurons give rise to large type A alpha (Aa) motor nerve fibers that branch to innervate large skeletal muscle fibers.
• Single alpha nerve fiber stimulation excites hundreds of skeletal muscle fibers, collectively called a motor unit.
• The transmission of nerve impulses into skeletal muscles causes contraction of muscle motor units.

Gamma Motor Neurons

• Gamma motor neurons, smaller than alpha motor neurons, are located in the spinal cord anterior horns.
• They transmit impulses through A gamma (Ay) fibers to special skeletal muscle fibers called intrafusal fibers.
• These fibers form the muscle spindle center and control basic muscle "tone."

Interneurons

• Interneurons, located throughout the cord gray matter, are small and highly excitable, often spontaneously active.
• They interconnect extensively and synapse directly with anterior motor neurons.
• Interconnections mediate spinal cord integrative functions.
• The interneuron pool contains various neuronal circuits, including diverging, converging, and repetitive-discharge circuits.
• Most incoming sensory signals from spinal nerves or the brain are first transmitted via interneurons before reaching anterior motor neurons.

Renshaw Cell Inhibitory System

• Renshaw cells are located in the anterior horns of the spinal cord, closely associated with motor neurons.
• Collateral branches from anterior motor neuron axons connect with adjacent Renshaw cells.
• Renshaw cells are inhibitory and transmit signals to surrounding motor neurons.
• Stimulation of a motor neuron inhibits adjacent motor neurons via lateral inhibition.
• The motor system utilizes lateral inhibition to focus or sharpen signals.

Multisegmental Connections

• More than half of all the nerve fibers ascending and descending in its spinal cord are propriospinal fibers.
• These fibers connect one spinal cord segment with another.
• Sensory fibers bifurcate as they enter from the posterior cord roots, with some branches transmitting signals locally and others to multiple segments.
• Ascending and descending propriospinal fibers provide pathways for multisegmental reflexes.

Muscular Sensory Receptors

• Muscles and tendons contain two types of sensory receptors: muscle spindles and Golgi tendon organs.
• These receptors provide info about muscle length, instantaneous tension, and rate of change.
• Muscle spindles are distributed throughout the muscle belly.
• Golgi tendon organs exist in the tendons.

Muscle Spindles

• Muscle spindles, 3-10 mm long, contain 3-12 small intrafusal muscle fibers pointed at their ends and attached to the glycocalyx of surrounding extrafusal muscle fibers.
• Intrafusal fibers are small skeletal muscle fibers with a central region that has few or no actin and myosin filaments.
• The central region of the intrafusal muscle fibers does not contract when contracted ends

Gamma Motor Nerve Fibers

• The end portions that contract are excited by gamma motor nerve fibers coming from gamma motor neurons of type A.
• There are efferent alpha fibers that innervate extrafusal skeletal muscle.

Sensory Innervation of Muscle Spindles

• The receptor portion of the spindles exists in the central portion, intrafusal fibers
• Don't have actin and myosin.
• Muscle spindle receptors are stimulated by stretching the spindle midportion.
• The muscle spindle receptor can be excited by lengthening the muscle and by contraction of these end portions.

Sensory Endings Around Receptor Area

• There are two types: primary and secondary endings.
• Primary Endings: A sensory fiber encircles the central portion of each intrafusal fiber, forming the primary/annulospiral ending, and the nerve fiber is type Ia.
• Secondary Endings: one or two smaller sensory nerve fibers (II) innervate the receptor region on one or both sides, encircling the intrafusal fibers or spread like branches.

Types of Muscle Spindles

• There are two types of spindles: nuclear bag and nuclear chain fibers.
• Nuclear bag: one to three in each spindle, nuclei are in "bags" in the receptor's central portion.
• Nuclear chain: three to nine, half the size of bag fibers, with aligned nuclei in a chain.
• Primary sensory nerve ending is excited by the bag and chain fibers; diameter is smaller than the bag.
• The secondary ending is excited only by chain fibers conversely.

Muscle Spindle Response

• Muscle stimulation is related to the length and rate of change
• Static Response: a static response is when the muscle spindle stretches slowly, the # of transmitted impulses from primary and secondary endings increases, and the endings continue to stimuli
• Dynamic Response: length of the spindle receptor increases suddenly at the primary ending the activation is activated through the 1a fibers
• Muscles can be controlled by gamma motor nerves and are gamma dynamic or gamma static nerves
• Gamma dynamic fibers excite the nuclear bag intrafusal fibers
• Gamma static excites nuclear chain intrafusal fibers
• When the dinamic fibers excite then dynamic response of muscle increases significantly Conversely, the muscle and nerve fibers are stimulated.

Muscle Stretch Reflexes

• Simplest manifestation function of the spinal cord
• The reflex triggers a contraction from the muscle fibers, and the synergists around it
• Neuronal circuitry can also initiate this process as a sensory nerve, after being stimulated enters the dorsal horn.

Neuronal Circuitry With Motor Neurons

• Type 1a proprioceptor nerve fiber
• Enters the dorsal root
• A branch goes to the root on the anterior
• A motor synapsis is added and then continues to innervate the fibers
• The activity of the stretch is carried through the nerves through a monosynaptic pathway.
• Protects the nerve

Dynamic Stretch Reflex

• The signals from this area are transmitted from the primary sensory endings and caused by rapid stretches
• Causes flex to occur
• This reflex happens very quickly oppose the muscle
• The signal is over quickly(short period)

Static Stretch Reflex

• Has a dynamic effect, weaker
• This reflex is stimulated by both primary and secondary muscle endings

Muscle Activity

• Whenever the signals are transmitted from the cortex in the brain to alpha, the motor neurons are simulated at the same time.
• The cause is the extrafusal
• Large skeletal muscle

Gamma Motor Control

• The gamma motor efferent system Is excited from the stem and signals and the areas of the stem.
  • Cerebellum
  • Basal ganglia
  • Cerebral cortex
• The muscle spindle stimulates the central region

Clinical Actions - Knee Jerk

• To find an area that can cause a forward muscle "jerk"
• These reflexes can be found when the muscles are damaged in the area of the cord or brain.

Golgi Tendon

• This tendon can control tension and is encapsulated in this area
• Stimulates the muscle bundle and the fibers
• The change or increase in the stimulus is a huge difference from the the spindle and it is more of a change from the spindle
• There is both a dynamic and static response
• As the pressure increase there is the setting, which results in both

Muscle Tensions

• Signals are transported from 1b fibers at a fast pace in the dorsal horn of the spinal cord
• A single inhibitory sends a signal to inhibit muscle (without damaging anything else)
• Tension is used to prevent an over development of the muscle through the spinal motor neurone

Forces of Muscles

• There is both a contraction and more forces

Flexor Reflexes

• This reflex helps remove pain
• Touch receptors can create the same process a limb is stimulates and a stimulus occurs is
  • stimulus is released

Flexor Reflex Mechanism

• Pain causes flexors Inhibited In the circuits :
• 1. Diverging in nature to facilitate the pull
• 2. To inhibit more of a relaxation or push
• 3. Stimulates
• Patterns happen when a pain feeling is applied and contactions start

Flexor and the Withdrawal Reflex Patterns

• A pain stimulus applies on the arm
• Withdraws hand from painful stimuli
• 0.2-0.5 seconds happen from stimulus causing flexion so the opposite limb extrends .
• Causing the person to be pushed away from the harmful stimuli
• This occurs from all sensory nerves crossing the nerve pathways in the body

Reflexes Causing Muscle Spams

• Results from when the bone is broken stimulating the areas to contract .
• From pain stimuli
• Abnormalities
• Abdominal spasm
• Muscle cramps

Autonomic Reflexes

• Tone in the muscle is changed when the muscle is heated
• Bladder and the bowel movements
• When there are changes in the brain and the cord is shut down it results in combination of hyper/areflexia
• If this cord is transacted or suddenly destroyed the body cant move and the functions dont wotk and the

The Pressure Fall

• Pressure falls and is affected and blocked for some time and is then restored

Muscular Reflexes & Sacaral Actions

• Last for a limited time

Explore the spinal cord's gray matter, focusing on the roles of alpha and gamma motor neurons, interneurons, and sensory receptors, and how they contribute to muscle function and motor control.