Somatic Motor System Flashcards
What are lower motor neurons?
alpha motor neurons
The only neuron that innervate skeletal muscle - final common pathway of nervous system for interacting with external environemtn
What do LMNs use as their neurotransmitter?
ACh & skeletal muscle have nicotinic Ach receptors
Describe a Nicotinic Ach Receptor
o nAChR are ligand-gated ion channels; when 2 ACh molecules bind to the nAChR, the gate opens, and cations (Na+, K+ and Ca++) can flow through.
• So from resting membrane potential you get binding of ACh to nAChR which trigger the opening of Na+ channels (sodium & calcium in, K+ out) -> membrane depolarizes, AP occurs, channels close & you get repolarization
o Voltage change from AP causes Ca++ channels to open & Ca++ released from SR -> muscle contraction
• The net effect is to depolarize the skeletal muscle membrane (excitatory post-synaptic potential EPSP) enough to reach the threshold potential, so that an AP is generated & then propagated along the full length of the skeletal muscle membrane via voltage gated sodium & potassium channels.
What is the enzyme which breaks down Ach at the synaptic clefts?
- Acetylcholinesterase (AChE) breaks down ACh in the synaptic cleft basal lamina, and the breakdown products can be recycled by the presynaptic terminal
- A single AP in a LMN causes all of its muscle fibers to fire an AP, hence muscle twitch occurs
What is a motor neuron pool?
• A motor neuron pool is all of the LMNs innervating one muscle
o Typically thousands of LMNs supply a single muscle
What is a motor unit
• A motor unit is one LMN and all the muscle cells (myofibers) that it innervates
o A motor unit in an extraocular muscle has about 10 cells, while a motor unit in the quadriceps femoris or gluteus maximus has about 1000 muscle cells.
o After an initial development period of multiple innervation, an individual muscle cell is only innervated by a single LMN throughout life
How do you increase the strength of contraction of a muscle
To increase the strength of contraction of a muscle, a single LMN can fire action potentials more frequently, leading to the summation of forces generated by individual twitches, up to a maximal response for muscle fibers, called (fused) tetanus. Additionally, more & more LMNs & their muscle fibers (motor units) can be recruited to fire APs, and because of the properties of LMNs, this tends to occur in the order S, FFR, FF motor units as more & more force is required. S fibers can contract for long periods of time (hours), FFR for many minutes, and FF for only about 1 minute.
*S- slow, red: aerobic
F- Fast fatiguable, white- anaerobic
FFR- fast fatigue resistant: intermediate properties or red and white fibres
What happens to a muscle when a LMN dies
• If an LMN dies, all its muscle fibers are denervated
o Because only 1 LMN supplies each muscle fiber
• But surviving LMNs nearby can sprout axon collaterals & take over innervation of the surrounding muscle fibers
o May induce a switch (if necessary) in the muscle fiber type
• Leads to ‘type grouping’ (myofibres switch fiber type to new LMN’s type) and larger motor units
o Groups/clusters of different fiber types
• If a muscle loses its motor neuron pool (all LMNs innervating it), this causes complete paralysis of the muscle & severe muscle atrophy over time
Where are LMNs in the spinal cord?
Anterior horn
innervate muscles on the IPSILATERAL side
Where are LMNs in the brainstem
• LMNs in cranial nerve motor nuclei in the brainstem associated with CN III- XII, except VIII
• Almost all LMNs innervate ipsilateral muscles
o EXCEPTIONs (contralateral)
• Trochlear nucleus – superior oblique (cross before they leave midbrain – so R trochlear nucleus supplies L superior oblique)
• Part of oculomotor nucleus – superior rectus
Names the heirarchical control of LMNs
• Direct sensory input
• Interneurons
o integrate a lot of other input – sensory input & input from CPG, UMN systems & indirect influences from other types of cerebral cortex
• Indirect sensory input
• Central pattern generators (CPG)
• Upper motor neurons (UMN)
• Cerebral cortex, cerebellum, basal ganglia
• Input to LMNs may be excitatory or inhibitory
o From interneurons – depending on the neurotransmitter released
Explain the muscle stretch reflex
- The muscle stretch reflex is the simplest reflex, involving only 2 neurons & one synapse (monosynaptic) between the sensory & motor neuron in the CNS (of course, stretching a muscle activates many sensory neurons, which synapse on many motor neurons; to say it is a two neuron reflex & monosynaptic is to discuss only the essence of the organization of the reflex)
- Muscle spindles detect a stretch of the muscle, which activates Ia afferent fibers (the cell bodies are in the DRG) which synapse (excitatory) directly on LMNs in the ventral horn of the spinal cord. The LMNs innervate the same muscle that was stretched (the homonymous muscle – note, that’s another synapse, so the reflex isn’t really monosynaptic) & cause it to contract, to return it towards its length prior to the stretch. You should revise the main muscle stretch reflexes that are tested clinically, and the related spinal segments, eg S1-2, L3-4, C5-6, C7-8
Explain the flexor reflex (withdrawl reflex)- painful stimuli
• It is initiated by cutaneous receptors & involves multiple muscle groups in the affected limb to move the limb away from the noxious stimulus, as well as the contralateral limb & postural muscles to maintain balance
o Particularly when the stimulus is applied to a lower limb, this typically results in a ‘crossed extensor’ response – eg the opposite lower limb is extended to take the weight of the body
what are central pattern generators
CPG more complicated than reflex. Reflex just respond to senses.
• CPGs are located in the brainstem & spinal cord
• CPGs are networks of neurons that control LMNs to drive a stereotyped action involving several muscles or groups of muscles (eg breathing, coughing, walking)
o Walking – you are not thinking about which muscle groups are contracting or relaxing.
• Activity of CPG can be modified by descending input & sensory input to adapt to changing conditions
o i.e. not walking on a smooth surface – need to adjust the sequence of contractions, and may involve some reflexes as well
Name some upper neuron systems which influence spinal LMNS
- Corticospinal tract (cerebral cortex)
- Rubrospinal tract (red nucleus in midbrain)
- Tectospinal tracts (tectum)
- Reticulospinal tracts (pontine & medullary reticular formation)
- Vestibulospinal tracts (medial & lateral vestibular nuclei)
