Moving Neurophysiology 5-6 2024 PDF

Neurophysiology 5-6: Somatic Motor Pathways Somatic motor pathways are the chains of neurons that send signals from the brain to the skeletal muscles to control voluntary movement. Those chains are composed of two neurons: one that is completely within the central nervous system (upper motor neuron)...

Neurophysiology 5-6: Somatic Motor Pathways Somatic motor pathways are the chains of neurons that send signals from the brain to the skeletal muscles to control voluntary movement. Those chains are composed of two neurons: one that is completely within the central nervous system (upper motor neuron) and one neuron (lower motor neuron) that relays the signal from the central nervous system to the skeletal muscle. Both the upper and motor neuron also receive a variety of modulatory inputs from interneurons that act to fine tune and coordinate skeletal muscle activity. When an animal has a deficit in voluntary motor function, it is often possible to determine whether the problem is with upper motor neurons, lower motor neurons, or modulatory inputs to these neurons. The cell bodies of upper motor neurons are located in the grey matter of the cerebral cortex (part of the telencephalon) in a region called the primary motor cortex. Rostral to the primary motor cortex are the prefrontal cortex and premotor cortex, which contain cell bodies of interneurons that synapse on the primary motor neurons and regulate the activity of those cells. The motor cortex has a somatotopic organization, meaning that specific parts of the cortex contain the cell bodies that send signals to specific parts of the body. Damage to part of the motor cortex (by a tumor or a stroke) could cause inability to move one part of the body. Motor neurons in the left primary motor cortex control skeletal muscle on the right side of the body, and motor neurons in the right cortex control movement on the left side of the body. Nuclei (singular nucleus) are groups of neuronal cell bodies within the central nervous system, and ganglia Singular ganglion) are groups of neuronal cell bodies outside of the central nervous system. Tracts are groups of axons in the white matter of the central nervous system. Tracts in the central nervous system are named according to their origin and destination: The corticospinal tract contains axons of upper motor neurons whose cells are located in the motor cortex and the axons terminate in synapses on lower motor neurons in the spinal cord. Axons of those lower motor neurons are found in spinal nerves. The corticobulbar tracts contain axons of upper motor neurons whose cell bodies are in the motor cortex that terminate in synapses on lower motor neurons in brainstem nuclei. Axons of those lower motor neurons are found in cranial nerves. The corticospinal tract is also called the pyramidal tract because as the axons pass through the medulla, they form two bulges (called pyramids) parallel to midline on the ventral surface of the brainstem. Most axons in the corticospinal tract cross to the opposite side of the brainstem in the medulla and form the lateral corticospinal tract. In the spinal cord, those axons are located in the lateral funiculus contralateral to the location of their cell bodies. Some of the corticospinal axons do not cross midline in the medulla, and continue along the spinal cord in the ventral funiculus as the ventral corticospinal tract. These axons do eventually cross midline just before they synapse with the lower motor neuron. The extrapyramidal system is an array of neuronal pathways that synapse on, and therefore influence the activity of, the upper and lower motor neurons. These include inputs from the reticular activating system, basal nuclei, the reticular formation, the red nucleus, and the vestibular nuclei. Upper motor neurons are also influenced by information from sensory receptors via the thalamus. The basal nuclei are groups of neurons in the brain (telencephalon, diencephalon, mesencephalon, and metencephalon) that function in the initiation of movement and in motor learning. These nuclei include the nucleus accumbens, caudate nucleus, putamen, globus pallidus, claustrum, and substantia nigra. Many of the neurons in these nuclei release dopamine as their neurotransmitter. The reticular formation or reticular activating system is another group of nuclei in the brain that can regulate motor activity. These nuclei are located in the brainstem and midbrain and determine the animal’s level of alertness and consciousness. The transmitters released by these neurons include norepinephrine, acetylcholine and serotonin. The red nucleus is located in the midbrain and is important in the regulation and coordination of gait. Neurons in the red nucleus primarily release glutamate as their neurotransmitter. The vestibular nuclei are also located in the brainstem and receive input about head position and movement/acceleration from the inner ear (more about this in Sensing and Seeing). This information is used to fine tune movement. Neurons in the vestibular nuclei also use glutamate as their neurotransmitter. The major function of the cerebellum is to coordinate movement. The cerebellum consists of 2 hemispheres separated by the vermis (“worm”) which forms a ridge along the midline of the structure. Like the cerebrum, the cerebellum has an outer layer of grey matter and an inner core of white matter (which contains some clusters of cell bodies or nuclei). The surface of the cerebellum forms folds called folia (similar to the gyri in the cerebrum). The grey matter of the cerebellum has three distinct cellular layers: molecular layer (outer), Purkinje cell layer (middle), and granule cell layer (inner). The tracts of axons that connect the cerebellum with the rest of the brain are called cerebellar peduncles. The neurons in the cerebellum receive inputs from the vestibular system, the peripheral sense organs that detect limb position (spinocerebellar tracts), the motor cortex, and the red nucleus. These neurons process information about body position and movement, and send out axons that synapse on the upper motor neurons in the motor cortex and on the neurons in cranial nerve nuclei. The transmitters include GABA (Purkinje cells) and glutamate. If upper motor neurons are sufficiently depolarized to generate an action potential, that electrical signal travels along the axon to the synapse with the lower motor neuron. For spinal nerves, the lower motor neuron cell body and dendrites are found in the ventral horn of the spinal cord. For cranial nerves, the lower motor neurons cell body and dendrites are found in nuclei in the brain. The upper motor neuron axon terminus releases glutamate, which binds to ionotropic glutamate receptors on the lower motor neuron and generates an excitatory postsynaptic potential. The lower motor neuron is also receiving signals from the extrapyramidal system and from excitatory and inhibitory interneurons. If the sum of all these signals depolarizes the membrane at the axon hillock to the action potential threshold, and action potential will result. That action potential will result in the release of neurotransmitter at the synapse with the target organ (muscle or gland). The axons of lower motor neurons are found in spinal or cranial nerves. For all spinal and some cranial nerves, axons of sensory neurons are also found in those nerves, mixed with the motor axons. For spinal nerves in the somatic motor system (we will talk about cranial nerves and the autonomic motor system later), the lower motor neuron is called an alpha motor neuron and is located in the ventral horn of the spinal cord. Each alpha motor neuron can innervate up to 1000 muscle fibers. A motor unit is a single alpha motor neuron and all of the nerve fibers it innervates. Motor units are small in muscles that require fine control (like the extraocular muscles that control eye movement), but are much larger in muscles that require greater strength (like the quadriceps). Each muscle fiber is innervated by just one motor neuron. A motor pool is the group of alpha motor neurons that innervate all of the muscle fibers in a muscle (like the biceps brachii). These alpha motor neurons are grouped together in the spinal cord, but may extend over several spinal cord segments. The axons from each segment of the spinal cord (for example, T1 or L2) exit the spinal canal as a ventral root. The ventral roots then combine to form named nerves that innervate the skeletal muscle. For example, the femoral nerve is composed of axons from the 4 , 5 , and 6 lumbar segments of the spinal cord. That nerve innervates the quadriceps, sartorius, and iliopsoas muscles. Spinal nerves are composed of axons, which may be myelinated or unmyelinated, wrapped in connective tissue, along with blood vessels and Schwann cells. The connective tissue around a single axon is called the endoneurium. The connective tissue around bundles of axons is called the perineurium. The connective tissue around the entire nerve is called the epineurium. The axons in a peripheral nerve synapse on skeletal muscle fibers at the neuromuscular junction. The transmitter released into the synapse is acetylcholine and the receptors are nicotinic cholinergic. Binding of the ACh to the nicotinic receptor causes voltage gated sodium channels to open, which depolarizes the muscle cell and leads to release of calcium from the sarcoplasmic reticulum. Calcium binds to a regulatory protein (troponin) which allows myosin to interact with actin, and the muscle fiber contracts. Damage to upper and lower motor neurons will cause different types of skeletal muscle dysfunction. When lower motor neurons are damaged, the innervated skeletal muscle cannot contract and the result is flaccid paralysis with muscle atrophy. Reflexes, like the patellar reflex, will be diminished or absent. Upper motor neuron lesions also result in the inability to voluntarily contract skeletal muscle, but local circuits, like tendon reflexes, will be unaffected. In fact, reflexes may be exaggerated because of loss of normal control of the alpha motor neurons by the upper motor neurons. Muscle atrophy and th th th flaccid paralysis are not seen with upper motor neuron lesions. The cranial nerves that innervate skeletal muscle are III, IV and IV (extraocular muscles), V (chewing muscles), VII (muscles of facial expression), IX (pharyngeal muscles), XI (neck muscles), and XII (tongue muscles). Each of these nerves has a nucleus in the midbrain, pons, or medulla that contains the alpha motor neuron cell bodies. The axons of those neurons send electrical signals to neuromuscular junction in the head and neck. Just like in the spinal nerves, the neurotransmitter is acetylcholine and the receptors are nicotinic. Upper motor neurons from the primary motor cortex synapse on the lower motor neurons in the cranial nerve motor nuclei (the axons travel through the corticobulbar tracts; bulb = brainstem), and neurons from other nuclei (vestibular, basal nuclei, etc.) also synapse on the upper and lower motor neurons that control skeletal muscle activity in the head and neck.