The pathway taken by nerve impulses in a reflex action is called a reflex arc. It is an automatic response to a stimulus. When a stimulus is given to a receptor, the receptors produce animpulse in the sensory fibre. The sensory fibre carries this message in the form of an electrical stimulus across the synapse. From the synapse, the impulse travels in the motor fibre and stimulates the effector organ to produce the desired response. The correct order for a simple reflex arc is: (b) →(d) → (a) → (c). Show
If you're seeing this message, it means we're having trouble loading external resources on our website. If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked. Introduction[edit | edit source]Motor neurones are cells in the brain and spinal cord that allow us to move, speak, swallow and breathe by sending commands from the brain to the muscles that carry out these functions. Their nerve fibers are the longest in the body, a single axon can stretch from the base of the spinal cord all the way to the toes. Image: Corticospinal tract demonstrating distinction between upper motor neuron and lower motor neuron. Motor neurons divided into either upper or lower motor neurones[1], forming various tightly controlled, complex circuits throughout the body. This controls both voluntary and involuntary movements through the innervation of effector muscles and glands. The upper and lower motor neurons form a two-neuron circuit.
Upper and lower motor neurons utilize different neurotransmitters to relay their signals.
Development of Motor Neurones[edit | edit source]Motor neurons to develop early in embryonic development. Cells in the neural tube either take the rostral-caudal axis or ventral-dorsal axis. The axons of motor neurons begin to appear in the fourth week of development from the ventral region of the ventral-dorsal axis (the basal plate) also known as the motor neural progenitor domain (pMN). Transcription factors include Pax6, OLIG2, Nkx-6.1, and Nkx-6.2, which are regulated by sonic hedgehog (Shh). The OLIG2 gene being the most important due to its role in promoting Ngn2 expression, a gene that causes cell cycle exiting as well as promoting further transcription factors associated with motor neuron development. [3] Classes of Motor Neurones[edit | edit source]Location:
When complex motor operations are required, the lower motor neurons consult the upper motor neurons and both work in union to provide a meaningful response.
Image: Relex arc Upper Motor Neurone[edit | edit source]An upper motor neurone exists in the brain and sends higher-level motor information to the medulla, located in the brain, or to the correct spinal cord level outside of the brain. From the medulla or the spinal cord, lower motor neurons carry motor information to muscle fibers, making them much more directly responsible for movement than upper motor neurons. Upper motor neurones
Image: Rubrospinal tract. There are many different pathways down which an upper motor neuron can send electrical impulses that eventually translate to movement. Generally
Lower Motor Neurone[edit | edit source]The lower motor neuron is responsible for transmitting the signal from the upper motor neuron to the effector muscle to perform a movement. There are three broad types of lower motor neurons: somatic motor neurons, special visceral efferent (branchial) motor neurons, and general visceral motor neurons. Somatic motor neurons are in the brainstem and further divide into three categories: alpha, beta, and gamma.
Branchial motor neurons (or special visceral neurones) innervate the muscles of the head and neck that derive from the branchial arches (typically, six arches in vertebrates; in lower vertebrates, they bear gills; they are pharyngeal arches in human embryos).
General visceral motoneurons contribute to both the sympathetic and parasympathetic functions of the autonomic nervous system.
Neuromuscular Junctions[edit | edit source]The junction between a motor neuron and muscle fiber is a specialized synapse called the neuromuscular junction. The motor neuron releases a flood of acetylcholine (Ach) neurotransmitters upon stimulation from the axon terminals from synaptic vesicles that bind with the post- synaptic receptors at the plasma membrane. This response is contractile causing muscle contraction and inhibition does not require a neurotransmitter release. [5] Lesions[edit | edit source]Snap your fingers or wiggle your toes. Those quick movements result from a sequence of electrical signals that begin in the brain. Motor neurons carry instructions from the brain along axons that stretch from the spinal cord to the muscles in the hands and feet. In diseases eg spinal muscular atrophy, motor neuron axons become damaged and degenerate, which means signals from the brain never reach the muscles and movement becomes impaired. Upper and lower motor neuron lesions cause very different clinical findings.
References[edit | edit source]
Which of these is the correct order of the structure of a motor neuron?Thus, the correct answer is 'Cell body-Axon-Nerve terminal'.
What is the structure of the motor neuron?The structure of a motor neuron can be categorized into three components: the soma, the axon, and the dendrites. The soma is the cell body where the nucleus lies, and which controls the cells and is also where proteins are produced to maintain the functioning of the neuron.
What are the 4 structures of a neuron?Following are the different parts of a neuron:. Dendrites. These are branch-like structures that receive messages from other neurons and allow the transmission of messages to the cell body.. Cell Body. ... . Axon. ... . Synapse.. What is the order of the 3 neurons?For neurons in the brain, at least, this isn't an easy question to answer. For the spinal cord though, we can say that there are three types of neurons: sensory, motor, and interneurons.
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