are alpha motor neurons myelinated

Like other neurons, α-MNs transmit signals as action potentials, rapid changes in electrical activity that propagate from the cell body to the end of the axon. To increase the speed at which action potentials travel, α-MN axons have large diameters and are heavily myelinated by both oligodendrocytes and Schwann cells

.

Full Answer

What are the α-motor neurons?

The α-motor neurons are the largest neurons in the spinal cord, with myelinated axons that exit the spinal cord through the ventral roots and travel in peripheral nerves to innervate muscles.

What influences the function of the alpha motor neurons?

The alpha motor neurons are influenced not only by upper motor neurons in the motor cortex and motor control neurons in the brainstem but also by sensory inputs from the periphery.

What is the difference between alpha and gamma motor neurons?

Like the alpha motor neurons, the gamma motor neurons lie in the ventral horn of the spinal cord interspersed among the alpha motor neurons innervating the same muscle.

What is a myelinated motor neuron?

When the axon is covered with a myelin sheath then the nerve fibre is known as a myelinated motor neuron. The myelin sheath is present at specific gaps and these gaps are known as nodes of Ranvier. The presence of a myelin sheath makes the neuron thicker and helps in faster nerve impulse conduction.

What is the difference between alpha and gamma motor neurons?

Motor neurons are divided into two groups. Alpha motor neurons innervate extrafusal fibers, the highly contracting fibers that supply the muscle with its power. Gamma motor neurons innervate intrafusal fibers, which contract only slightly.

What type of neurons are alpha motor neurons?

Alpha (α) motor neurons (also called alpha motoneurons), are large, multipolar lower motor neurons of the brainstem and spinal cord. They innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their contraction.

Which motor neurons are myelinated?

Motor neurons are myelinated by either oligodendrocytes or Schwann cells. Oligodendrocytes and Schwann cells are glia. However, they are very different!

Do all motor neurons have myelination?

For example, the motor neurons of the peripheral nervous system are myelinated but the neurons which form the autonomic nervous system (sympathetic and parasympathetic nervous system) are unmyelinated motor neurons i.e. the axons of these motor neurons don't have a myelin sheath covering around their axons.

What type of peripheral nerve fibers are Unmyelinated?

Group C nerve fibers are one of three classes of nerve fiber in the central nervous system (CNS) and peripheral nervous system (PNS). The C group fibers are unmyelinated and have a small diameter and low conduction velocity, whereas Groups A and B are myelinated.

What are alpha motor neurons quizlet?

Alpha motor neuron. A neuron whose axon forms synapses with extrafusal muscle fibers of a skeletal muscle; activation contracts the muscle fibers.

Which neurons are Unmyelinated?

Unmyelinated Axons are present in the autonomic nervous system. Both the parasympathetic nervous system and sympathetic nervous system are a part of autonomic nervous system. These systems are formed by motor neurons whose axons are unmyelinated.

Where are myelinated and unmyelinated neurons found?

Note: The Myelinated fibre are present in the white part of the brain and also in the spinal cord, while the non myelinated nerve fibre is present in the autonomic nervous system.

Are motor nerves Unmyelinated?

It is necessary for motor neurons to be myelinated for the proper function of neurons such as rapid conduction velocity, protection from environmental toxin and metabolic support of axons. Spinal motor neuron leaves its cell body in spinal cord and extends own axon to PNS for the innervation of muscle fiber.

Why are some neurons not myelinated?

Myelinated neurons are neurons whose axons are surrounded by myelin; the myelin has has an insulating effect and allows the axons to conduct neural impulses faster - but at some metabolic cost, so neurons are not myelinated unless there is a significant advantage to they're being able to conduct faster.

Which neurons have myelin sheath?

Myelin sheath is a substance which is found on neurons within the central nervous system (CNS) and the peripheral nervous system (PNS).

What part of a neuron is sometimes myelinated?

Axon. The axon is the elongated fiber that extends from the cell body to the terminal endings and transmits the neural signal. The larger the diameter of the axon, the faster it transmits information. Some axons are covered with a fatty substance called myelin that acts as an insulator.

What are the types of motor neurons?

There are in fact two types of motor neurons: those that travel from spinal cord to muscle are called lower motor neurons, whereas those that travel between the brain and spinal cord are called upper motor neurons.

Are alpha motor neurons cholinergic?

Taken together, these findings indicate that α-MNs are uniquely vulnerable among cholinergic neuron populations in the SMA mouse spinal cord, with γ-MNs and other cholinergic neuronal populations being largely spared.

Where is alpha motor neuron found?

the spinal cordalpha motor neurons are lower motor neurons whose cell bodies are found in the anterior horn of the spinal cord and whose axons travel down to the body to innervate skeletal muscle to cause muscle contraction.

Is the alpha motor neuron in the final common pathway?

Abstract. Alpha motor neurons (also known as lower or skeletal motor neurons) have been studied extensively for over 100 years. Motor neurons control the contraction of skeletal muscles and thus are the final common pathway in the nervous system responsible for motor behavior.

What is the alpha motor neuron?

An alpha motor neuron and the muscle fibers it innervates is a motor unit. A motor neuron pool contains the cell bodies of all the alpha motor neurons involved in contracting a single muscle.

Where do alpha motor neurons originate?

Alpha motor neurons originate in the basal plate, the ventral portion of the neural tube in the developing embryo. Sonic hedgehog (Shh) is secreted by the nearby notochord and other ventral structures (e.g., the floor plate ), establishing a gradient of highly concentrated Shh in the basal plate and less concentrated Shh in the alar plate. Under the influence of Shh and other factors, some neurons of the basal plate differentiate into α-MNs.

How does loss of -MNs affect muscle?

Because α-MNs provide the only innervation to extrafusal muscle fibers, losing α-MNs effectively severs the connection between the brainstem and spinal cord and the muscles they innervate. Without this connection, voluntary and involuntary (reflex) muscle control is impossible. Voluntary muscle control is lost because α-MNs relay voluntary signals from upper motor neurons to muscle fibers. Loss of involuntary control results from interruption of reflex circuits such as the tonic stretch reflex. A consequence of reflex interruption is that muscle tone is reduced, resulting in flaccid paresis. Another consequence is the depression of deep tendon reflexes, causing hyporeflexia .

What is the primary output of a lower motor neuron?

The primary output of α-MNs is to extrafusal muscle fibers. This afferent and efferent connectivity is required to achieve coordinated muscle activity.

Where are motor neurons located?

While their cell bodies are found in the central nervous system (CNS), α motor neurons are also considered part of the somatic nervous system —a branch of the peripheral nervous system (PNS)—because their axons extend into the periphery to innervate skeletal muscles .

Which type of neuron is responsible for initiating contraction?

They innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their contraction. Alpha motor neurons are distinct from gamma motor neurons, which innervate intrafusal muscle fibers of muscle spindles .

Which neuron sends fibers that mainly synapse on extrafusal muscle fibers?

Alpha motor neurons send fibers that mainly synapse on extrafusal muscle fibers. Other fibers from α-MNs synapse on Renshaw cells, i.e. inhibitory interneurons that synapse on the α-MN and limit its activity in order to prevent muscle damage.

Where is the motor neuron located?

A single α motor neuron (located within the ventral horn of the spinal cord; see Fig. 1 ), with the muscle fibers that it innervates, is defined as the motor unit. As soon as the large myelinated axon of an α motor neuron enters a skeletal muscle, it branches many times. Each small branch terminates on muscle fibers at the motor end plate or neuromuscular junction (Fig. 11). The plasma membrane of the axon terminal (axolemma) is separated by the synaptic cleft (gap of 20–50 nm) from the plasma membrane of the muscle fiber (sarcolemma). The surface area of sarcolemma (postsynaptic membrane) at a motor end plate is thrown into numerous folds. These serve to increase the contact area of muscle to the naked axon (presynaptic membrane). The neurotransmitter, acetylcholine, is released from synaptic vesicles into the synaptic cleft when a nerve impulse reaches the motor end plate. Once the acetylcholine is released, it will stimulate receptor sites on the postsynaptic muscular membrane, causing the contraction of skeletal muscle fibers. The acetylcholine remains in contact with the postsynaptic membrane for a very short period of time (about 1 msec), and it is rapidly inactivated by the enzyme acetylcholinesterase (AChE). Acetylcholinesterase hydrolyzes acetylcholine into acetic acid and choline. The choline is taken up by the presynaptic terminals for the synthesis of new molecules of acetylcholine.

What are the functions of motor neurons?

Motor neurons control the contraction of skeletal muscles and thus are the final common pathway in the nervous system responsible for motor behavior. Muscles become paralyzed when their innervating motor neurons die because of injury or disease. Motor neuron diseases (MNDs), such as Amyotrophic Lateral Sclerosis, progressively destroy motor neurons until those inflicted succumb to the illness due to respiratory failure. One strategy being explored to study and treat muscle paralysis due to motor neuron loss involves deriving surrogate motor neurons from pluripotent stem cells. Guided by decades of research on the development of the spinal cord, recent advances in neurobiology have shown that functional motor neurons can be derived from mouse and human embryonic stem (ES) cells. Furthermore, ES cell-derived motor neurons restore motor behavior when transplanted into animal models of motor dysfunction. The recent discovery that mouse and human motor neurons can be derived from induced pluripotent stem (iPS) cells (i.e., somatic cells converted to pluripotency) has set the stage for the development of patient-specific therapies designed to treat movement disorders. Indeed, there is now hope within the scientific community that motor neurons derived from pluripotent stem cells will be used to treat MNDs through cell transplantation and/or to screen molecules that will prevent motor neuron death. In this chapter, we review the journey that led to the generation of motor neurons from ES and iPS cells, how stem cell-derived motor neurons have been used to treat/study motor dysfunction, and where the technology will likely lead to in the future.

What is the neuromuscular junction?

Structurally and functionally, neuromuscular junctions resemble synapses (see Fig. 7.3). As a motor axon approaches its target muscle cell it loses its myelin sheath and forms a flattened expansion that is applied to the surface of the muscle membrane. Once applied, the expansion is covered by a Schwann cell sheath which insulates the neuromuscular junction from the external environment. That portion of the muscle cell to which the nerve is apposed is modified by forming a flattened bump on its surface. This is formed by the focal accumulation of cytoplasm (sarcoplasm) and organelles within the muscle cell that raises its membrane underneath the nerve terminal. That part of the muscle cell membrane applied to the nerve cell is known as the postjunctional membrane ( Fig. 7.49). Reciprocally, that part of the nerve cell membrane applied to the muscle cell is known as the prejunctional membrane.

How many muscle fibers are innervated by a single motor neuron?

The number of muscle fibers innervated by a single motoneuron varies depending on the size and function of the muscle, but averages about 150 muscle fibers per motor unit. While each muscle fiber receives input from a single motor neuron, motor units usually overlap whereby 10–15 fibers from one motor unit are adjacent to similar bundles of a second motor unit. This phenomenon permits motor units to work in unison and to contract in support of each other. Figure 5a shows a representation of four muscle fibers receiving innervation from a single motor neuron. Upon reaching the muscle fiber, the axon of the motoneuron divides into multiple terminal nerve twigs ( Figure 5b and d ), all of which form synaptic contacts with the muscle fiber ( Figure 5c and e ). These synaptic contacts are physically supported by both the basement membrane and overlapping Schwann cells ( Figure 5b and c ). The presence of three cell types (neuron, glial and muscle) in the neuromuscular junction led to the term tripartite synapse.

How many muscle fibers are in a motor unit?

Although the number of muscle fibers innervated by a single motoneuron varies depending on the size and function of the muscle, there are about 150 muscle fibers per motor unit. While each muscle fiber receives input from a single motor neuron, motor units usually overlap such that 10–15 fibers from one motor unit are adjacent to similar bundles of a second motor unit. This permits motor units to work in unison and to contract in support of each other. Shown in Fig. 5A is a representation of four muscle fibers receiving innervation from a single motor neuron. Upon reaching the muscle fiber, the axon of the motoneuron divides into multiple terminal nerve twigs ( Fig. 5B and 5D ), all of which form synaptic contacts with the muscle fiber ( Fig. 5C and 5E ). These synaptic contacts are physically supported by both the basement membrane and overlapping Schwann cells ( Fig. 5B and 5C ). The presence of three cell types (neuron, glial and muscle) in the neuromuscular junction led to the term tripartite synapse.

What is the junction between a motor neuron and a muscle cell?

The junction between a motor neuron and a muscle cell involves elaboration of both the neuron and the muscle cell and of certain surrounding tissues , the entire complex being known as a motor endplate.

What is the transmitter substance in the motor neuron?

The terminal expansion of the motor neuron contains vesicles filled with the transmitter substance acetylcholine. When an action potential arrives at the nerve terminal, the vesicles release the acetylcholine through the presynaptic membrane into the synaptic deft. The transmitter substance then flows across the cleft to react with molecular receptors on the postjunctional membrane which generates an action potential in the muscle membrane. This potential is then propagated into the muscle cell to cause contraction of the muscle fibres.

Location

Alpha motor neurons innervating the head and neck are found in the brainstem; the remaining α-MNs innervate the rest of the body and are found in the spinal cord. Because there are fewer muscles in the head and neck than in the rest of the body, there are more α-MNs in the spinal cord than in the brainstem.

Connectivity

Like other neurons, lower motor neurons have both afferent (incoming) and efferent (outgoing) connections. Alpha motor neurons receive input from a number of sources, including upper motor neurons, sensory neurons, and interneurons. The primary output of α-MNs is to extrafusal muscle fibers.

Signaling

Like other neurons, α-MNs transmit signals as action potentials, rapid changes in electrical activity that propagate from the cell body to the end of the axon. To increase the speed at which action potentials travel, α-MN axons have large diameters and are heavily myelinated by both oligodendrocytes and Schwann cells.

Role in disease

Poliomyelitis, caused by the poliovirus seen here, is associated with the selective loss of cells within the ventral horn of the spinal cord, where α-MNs are located.

Development

Alpha motor neurons originate in the basal plate, the ventral portion of the neural tube in the developing embryo. Sonic hedgehog (Shh) is secreted by the nearby notochord and other ventral structures (eg, the floor plate ), establishing a gradient of highly concentrated Shh in the basal plate and less concentrated Shh in the alar plate.

Nomenclature

Like other motor neurons, α-MNs are named after the properties of their axons. Alpha motor neurons have Aα axons, which are large- caliber, heavily myelinated fibers that conduct action potentials rapidly. By contrast, gamma motor neurons have Aγ axons, which are slender, lightly myelinated fibers that conduct less rapidly.

Which component of the motor neuron is myelinated?

The axon is the component of the motor neuron that is myelinated (contains myelin sheath) due to which it becomes a myelinated motor neuron. The myelin sheath acts as a protection from the external environmental conditions that can possibly interfere with the nerve impulse conduction process.

Which neuron has long axons?

Myelinated motor neuron fibers have long axons so can transmit signals at farther distances.

What is the name of the nerve fiber that is covered by a myelin sheath?

When the axon is covered with a myelin sheath then the nerve fiber is known as a myelinated motor neuron . The myelin sheath is present at specific gaps and these gaps are known as nodes of Ranvier. The presence of a myelin sheath makes the neuron thicker and helps in faster nerve impulse conduction.

How long are axons?

For example, the axons that make up the sciatic nerve are really long and can be over 1 meter in length. Axon terminals are pre-synaptic components of neuron structure. These are the site of intracellular communication, where the signals are transferred to different motor neurons.

How many axons are in a neuron?

A neuron typically contains a single a xon with multiple branches with many terminals for effective communication with several cells. The length of axons varies greatly they can be really long as well as short. For example, the axons that make up the sciatic nerve are really long and can be over 1 meter in length.

Which neuron is responsible for transmitting signals to glands, tissues, and tissues?

Lower motor neurons contain the motor neurons of the peripheral nervous system (the nerve fibers that transmit signals from CNS to glands, tissues, etc.). The myelinated motor neurons of the peripheral nervous system contain a myelin sheath that is formed of Schwann cells.

Which neuron is responsible for the conduction of nerve impulses?

Interneurons. Motor neurons. Motor neurons are present in the peripheral nervous system. Moreover, the sympathetic and parasympathetic nervous systems also use motor neurons for the conduction of nerve impulses. Motor neurons receive the information from sensory neurons and convert it into action in muscles or glands.

Overview

Signaling

Like other neurons, α-MNs transmit signals as action potentials, rapid changes in electrical activity that propagate from the cell body to the end of the axon. To increase the speed at which action potentials travel, α-MN axons have large diameters and are heavily myelinated by both oligodendrocytes and Schwann cells. Oligodendrocytes myelinate the part of the α-MN axon that lies in the central nervous system (CNS), while Schwann cells myelinate the part that lies in the p…

Location

Development

Connectivity

Clinical significance

See also

External links