Reflexes and Muscles

• A receptor is a specialized part of a neuron that receives a signal
• an afferent neuron is a neuron that transmits the signal to the spinal cord
• an interneuron is a neuron in the spinal cord that connects input to output
• an efferent neuron is a neuron that transmits the signal to the effectors
• the effector is the tissue that turns the signal into an action, usually a muscle
• the types of reflexes depend on where the reflex is integrated: Urination and defecation originates in the spine, whereas blinking and swallowing originate in the brain
• Autonomic reflexes are involuntary responses to stimuli that are not consciously controlled and they include blood pressure, heart rate, breathing, eating, and coughing. They are also called visceral reflexes
• All autonomic pathways consist of a pre-ganglionic neuron originating in the CNS and a post-ganglionic neuron
• Excitation of a somatic alpha motor neuron always causes a skeletal muscle to contract
• Flexor-extensor pairs are called antagonistic muscle groups because they exert opposite effects
• A skeletal muscle is a collection of muscle cells, or muscle fibers, just as a nerve is a collection of neurons
• Satellite cells become active and differentiate into muscle when needed for muscle growth and repair
• If a response requires a less contracted muscle, sensory input activates inhibitory interneurons called Renshaw cells in the ventral horn of the spinal cord. The RC dampens the alpha motor neuron signal which contracts that muscle - known as recurrent inhibition
• Relaxation is the absence of excitatory input by somatic neurons
• Proprioceptors are receptors located in our musculoskeletal system and they provide information about where our body is and how our body is moving through space
• The three types of proprioceptors are joint receptors, Golgi tendons, and muscle spindles
• Muscle spindles are small sensory organs enclosed in a capsule, buried among and in parallel with extrafusal fibers (regular muscle fibers); the central region of spinal intrafusal fibers is wrapped by sensory nerve endings which project to the spinal cord and synapse with the alpha motor neuron, innervating the muscle
• Joint receptors are a group of 4 sense organs located in the capsules of synovial joints. They are sensitive to pressure and may be involved in kinaesthetics, providing information to the central nervous system about the position of a joint
• Golgi Tendons are sensory nerve endings that monitor muscle tension. They have a protective reflex, meaning they have a higher threshold to fire
• Extrafusal fibers are the actual contractile fibers the alpha neurons excite in order to contract a muscle
• Infrafusal muscle fibers are within the extrafusal muscle fibers and the ends are stimulated by axons of gamma motor neurons to maintain spindle sensitivity, regardless of muscle length
• The Golgi tendon reflex is polysynaptic. It is a reflex initiated by Golgi tendon organs in response to tension that sends a signal to the Group Ib afferent neuron which excites inhibitory interneurons. Simultaneously, it sends a signal to the antagonistic muscle telling it to contract
• Muscle tone is the basal state of muscle contraction that results from the tonic activity of the muscle spindles usually by the cerebellum to maintain posture and balance. The extrafusal fibers maintain a certain level of tension even at rest
• Excitation of gamma motor neurons and alpha motor neurons at the same time is a process known as alpha-gamma coactivation.
• Voluntary movements originate in the primary motor cortex of the left cerebral hemisphere, go to the midbrain, the medulla, and to the contralateral somatic motor neurons to skeletal muscles
• Tendons are connective tissue that attaches skeletal muscle to the bone and they are made of collagen
• Committed stem cells called satellite cells lie just outside the muscle fiber membrane. Satellite cells become active and differentiate into muscle when needed for muscle growth and repair
• Muscle fibers are bundled together into units called fascicles
• The cell membrane of a muscle fiber is called the sarcolemma. The main intracellular structures in striated muscles are myofibrils, highly organized bundles of contractile and elastic proteins that carry out the work of contraction.
• Skeletal muscle fibers also contain extensive sarcoplasmic reticulum (SR), a form of modified endoplasmic reticulum that wraps around each myofibril like a piece of lace
• The SR consists of longitudinal tubules with enlarged end regions called the terminal cisternae. These are adjacent to T (transverse) tubules. One t-tubule and its two flanking terminal cisternae are called a triad.
• The membranes of t-tubules are a continuation of the muscle fiber membrane, which makes the volume of t-tubules continuous with the extracellular fluid. They are extensions of the cell membrane that associate with the ends of the SR. They allow action potentials to move rapidly from the cell surface into the interior of the fiber so that they reach the terminal cisternae nearly simultaneously.
• Each myofibril is composed of several types of proteins organized into repeating contractile structures called sarcomeres.
• Myofibril proteins include the motor protein called myosin, which forms thick filaments; the microfilament actin, which creates thin filaments; the regulatory proteins tropomyosin and troponin; and two giant accessory proteins, titin and nebulin
• myosin is composed of heavy light chain helical filament and globular head, activated by myosin ATPases
• Actin makes up the thin filaments of the muscle fiber. In skeletal muscle, two F-actin polymers twist together like a double strand of beads, creating the thin filaments of the myofibril
• the parallel thick and thin filaments of the myofibril are connected by myosin cross-bridges that span the space between the filaments
• Z lines are zigzag protein structures that serve as the attachment site for thin filaments
• The M line band represents proteins that form the attachment site for thick filaments, equivalent to the Z disk for the thin filaments
• Titin is an elastic giant protein that maintains spatial structure and provides elasticity to myofibrils