Motor neuron

Micrograph of the hypoglossal nucleus showing motor neurons with their characteristic coarse Nissl substance ("tigroid" cytoplasm). H&E-LFB stain.
Spinal cord tracts
Location of lower motor neurons in spinal cord

Neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly or indirectly control effector organs, mainly muscles and glands.

- Motor neuron

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Neuromuscular junction

Electron micrograph showing a cross section through the neuromuscular junction. T is the axon terminal, M is the muscle fiber. The arrow shows junctional folds with basal lamina. Active zones are visible on the tips between the folds. Scale is 0.3 μm. Source: NIMH
At the neuromuscular junction, the nerve fiber is able to transmit a signal to the muscle fiber by releasing ACh (and other substances), causing muscle contraction.
Muscles will contract or relax when they receive signals from the nervous system. The neuromuscular junction is the site of the signal exchange. The steps of this process in vertebrates occur as follows:(1) The action potential reaches the axon terminal. (2) Voltage-dependent calcium gates open, allowing calcium to enter the axon terminal. (3) Neurotransmitter vesicles fuse with the presynaptic membrane and ACh is released into the synaptic cleft via exocytosis. (4) ACh binds to postsynaptic receptors on the sarcolemma. (5) This binding causes ion channels to open and allows sodium and other cations to flow across the membrane into the muscle cell. (6) The flow of sodium ions across the membrane into and potassium ions out of the muscle cell generates an action potential which travels to the myofibril and results in muscle contraction.Labels:A: Motor Neuron AxonB: Axon TerminalC. Synaptic CleftD. Muscle CellE. Part of a Myofibril
Motor Endplate
When ligands bind to the receptor, the ion channel portion of the receptor opens, allowing ions to pass across the cell membrane.
Botulinum toxin injected in human face

A neuromuscular junction (or myoneural junction) is a chemical synapse between a motor neuron and a muscle fiber.

Lower motor neuron

Micrograph of the hypoglossal nucleus showing motor neurons with their characteristic coarse Nissl substance ("tigroid" cytoplasm). H&E-LFB stain.

Lower motor neurons (LMNs) are motor neurons located in either the anterior grey column, anterior nerve roots (spinal lower motor neurons) or the cranial nerve nuclei of the brainstem and cranial nerves with motor function (cranial nerve lower motor neurons).

Motor cortex

Region of the cerebral cortex involved in the planning, control, and execution of voluntary movements.

Topography of human motor cortex. Supplementary motor area labelled SMA.
Motor cortex controls different muscle group
Some commonly accepted divisions of the cortical motor system of the monkey
Map of the body in the human brain.

In that view, a neuron in the motor cortex sends an axon or projection to the spinal cord and forms a synapse on a motor neuron.

Spinal cord

Long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column.

The spinal cord (in yellow) connects the brain to nerves throughout the body.
Part of human spinal cord. 1 – central canal; 2 – posterior median sulcus; 3 – gray matter; 4 – white matter; 5 – dorsal root + dorsal root ganglion; 6 – ventral root; 7 – fascicles; 8 – anterior spinal artery; 9 – arachnoid mater; 10 – dura mater
Diagram of the spinal cord showing segments
A model of segments of the human spine and spinal cord, nerve roots can be seen extending laterally from the (not visible) spinal cord.
Spinal cord seen in a midsection of a five-week-old embryo
Spinal cord seen in a midsection of a 3 month old fetus
Spinal cord tracts.
Spinal Cord Sectional Anatomy. Animation in the reference.
Diagrams of the spinal cord.
Cross-section through the spinal cord at the mid-thoracic level.
Cross-sections of the spinal cord at varying levels.
Cervical vertebra
A portion of the spinal cord, showing its right lateral surface. The dura is opened and arranged to show the nerve roots.
The spinal cord with dura cut open, showing the exits of the spinal nerves.
The spinal cord showing how the anterior and posterior roots join in the spinal nerves.
The spinal cord showing how the anterior and posterior roots join in the spinal nerves.
A longer view of the spinal cord.
Projections of the spinal cord into the nerves (red motor, blue sensory).
Projections of the spinal cord into the nerves (red motor, blue sensory).
Cross-section of rabbit spinal cord.
Cross section of adult rat spinal cord stained using Cajal method.
An overview of the spinal cord.
Sagittal section of pig vertebrae showing a section of the spinal cord.
The base of the brain and the top of the spinal cord
Spinal cord. Spinal membranes and nerve roots.Deep dissection. Posterior view.
Spinal cord. Spinal membranes and nerve roots.Deep dissection. Posterior view.
Spinal cord. Spinal membranes and nerve roots.Deep dissection. Posterior view.
Spinal cord. Spinal membranes and nerve roots.Deep dissection. Posterior view.
Spinal cord. Spinal membranes and nerve roots.Deep dissection. Posterior view.
Spinal cord. Spinal membranes and nerve roots.Deep dissection. Posterior view.
Spinal cord. Spinal membranes and nerve roots.Deep dissection. Posterior view.
Cerebrum.Inferior view.Deep dissection
Cerebrum.Inferior view.Deep dissection
Spinal cord. Brachial plexus. Cerebrum.Inferior view.Deep dissection.
Spinal cord. Brachial plexus. Cerebrum.Inferior view.Deep dissection.
Spinal cord
Medulla spinalis of 8-week-old human embryo

In cross-section, the peripheral region of the cord contains neuronal white matter tracts containing sensory and motor axons.

Neural tube

Embryonic precursor to the central nervous system, which is made up of the brain and spinal cord.

Transverse section of half of a chick embryo of forty-five hours' incubation. The dorsal (back) surface of the embryo is toward the top of this page, while the ventral (front) surface is toward the bottom. (Neural tube is in green.)
Stages of neural tube formation.
Shh secreted from the floor plate creates a gradient along the ventral neural tube. Shh functions in a concentration-dependent manner to specify ventral neuronal fates. V0-V3 represent four different classes of ventral interneurons, and MN indicates motor neurons.

Three main ventral cell types are established during early neural tube development: the floor plate cells, which form at the ventral midline during the neural fold stage; as well as the more dorsally located motor neurons and interneurons.

Acetylcholine

Organic chemical that functions in the brain and body of many types of animals (including humans) as a neurotransmitter.

Acetylcholine pathway.
Acetylcholine processing in a synapse. After release acetylcholine is broken down by the enzyme acetylcholinesterase.
Muscles contract when they receive signals from motor neurons. The neuromuscular junction is the site of the signal exchange. The steps of this process in vertebrates occur as follows: (1) The action potential reaches the axon terminal. (2) Calcium ions flow into the axon terminal. (3) Acetylcholine is released into the synaptic cleft. (4) Acetylcholine binds to postsynaptic receptors. (5) This binding causes ion channels to open and allows sodium ions to flow into the muscle cell. (6) The flow of sodium ions across the membrane into the muscle cell generates an action potential which induces muscle contraction. Labels: A: Motor neuron axon B: Axon terminal C: Synaptic cleft D: Muscle cell E: Part of a Myofibril
Components and connections of the parasympathetic nervous system.
Micrograph of the nucleus basalis (of Meynert), which produces acetylcholine in the CNS. LFB-HE stain.

Acetylcholine is the neurotransmitter used at the neuromuscular junction—in other words, it is the chemical that motor neurons of the nervous system release in order to activate muscles.

Neuron

Electrically excitable cell that communicates with other cells via specialized connections called synapses.

Anatomy of a multipolar neuron
Schematic of an anatomically accurate single pyramidal neuron, the primary excitatory neuron of cerebral cortex, with a synaptic connection from an incoming axon onto a dendritic spine.
Diagram of the components of a neuron
Neuron cell body
Diagram of a typical myelinated vertebrate motor neuron
Golgi-stained neurons in human hippocampal tissue
Actin filaments in a mouse cortical neuron in culture
Image of pyramidal neurons in mouse cerebral cortex expressing green fluorescent protein. The red staining indicates GABAergic interneurons.
SMI32-stained pyramidal neurons in cerebral cortex
Different kinds of neurons:
1 Unipolar neuron
2 Bipolar neuron
3 Multipolar neuron
4 Pseudounipolar neuron
Synaptic vesicles containing neurotransmitters
A signal propagating down an axon to the cell body and dendrites of the next cell
Chemical synapse
An annotated diagram of the stages of an action potential propagating down an axon including the role of ion concentration and pump and channel proteins.
As long as the stimulus reaches the threshold, the full response would be given. Larger stimulus does not result in a larger response, vice versa.
Drawing by Camillo Golgi of a hippocampus stained using the silver nitrate method
Drawing of a Purkinje cell in the cerebellar cortex done by Santiago Ramón y Cajal, demonstrating the ability of Golgi's staining method to reveal fine detail
Drawing of neurons in the pigeon cerebellum, by Spanish neuroscientist Santiago Ramón y Cajal in 1899. (A) denotes Purkinje cells and (B) denotes granule cells, both of which are multipolar.
Guillain–Barré syndrome – demyelination

Motor neurons receive signals from the brain and spinal cord to control everything from muscle contractions to glandular output.

Interneuron

Cartoon of a locust interneuron that integrates information about wind in order to control wing motor neurons during flight

Interneurons (also called internuncial neurons, relay neurons, association neurons, connector neurons, intermediate neurons or local circuit neurons) are neurons that connect two brain regions, i.e. not direct motor neurons or sensory neurons.

Sonic hedgehog protein

Encoded for by the SHH gene.

Studies involving ectopic expression of SHH in vitro and in vivo result in floor plate induction and differentiation of motor neuron and ventral interneurons.

Anterior grey column

Front column of grey matter in the spinal cord.

The location of motor neurons in the anterior grey column of the spinal column
Section of the medulla oblongata through the lower part of the decussation of the pyramids

The anterior grey column contains motor neurons that affect the skeletal muscles while the posterior grey column receives information regarding touch and sensation.