6 Innervation of muscle

6.1 Motor neurons

• A neuron is a nerve cell.
• A motor neuron is a nerve cell innervating myofibres.
• The cell bodies of  motor neurons innervating meat carcass myofibres are located in the ventral horn of the spinal cord.
• The body of the neuron is called the perikaryon.
• The perikaryon has root-like projections called dendrites.
• A motor neuron has one long axon leading to the myofibres innervated.
  

A transverse section through the spinal cord.

•  Basic dyes for light microscopy show the perikaryon is packed with Nissl granules.
•  Electron microscopy reveals each granule is an aggregate of cisternae (membranous structures) belonging to the rough endoplasmic reticulum (involved in protein synthesis).
•  Colloidal silver (as in a photographic emulsion) stains neurofilaments (diameter 10 nm) which are bundled together to form neurofibrils.
•  Neurofilaments form a skeleton supporting the complex structure of a neuron.
•  Silver stains show the Nissl granules occur in spaces surround by a tightly woven meshwork of neurofibrils.
  
•  Interspersed among the neurofibrils are a few 20 nm diameter neurotubules (microtubules).
•  Small mitochondria are found throughout the perikaryon and dendrites.
•  The single long axon linking each motor neuron to its group of myofibres (motor unit), usually departs from the perikaryon at a small axon hillock devoid of Nissl granules.
• The axoplasm within the axon contains tubules of endoplasmic reticulum, slender mitochondria, neurotubules and neurofilaments.
•  The nucleus of the motor neuron has a conspicuous nucleolus (composed largely of RNA and showing the nucleus is no longer capable of division by mitosis) .

6.2 Schwann cells & myelin sheaths

• Motor axons in ventral nerve roots and peripheral nerves are insulated by a myelin sheath formed by Schwann cells.
  
• The Schwann cell is tightly rolled around the axon many times and concentric layers of Schwann cell membranes form a thick layer of insulation - myelin.
  
• In the depth of the insulating layer of myelin there are major dense lines (3-nm thick) formed by the apposition of the cytoplasmic faces of unit membranes, while faint intraperiod lines are formed by apposition of outer faces.
  
• Major dense lines are 12 nm apart.
• Schwann cells migrate along axons and become evenly distributed.
• Schwann cells are separated by nodes of Ranvier where the myelin leaves a narrow bare ring of axon.
• The positions of the nodes of Ranvier are determined by the axon..
• Action potentials (nerve impulses) jump from one node to another to increase the velocity of neural conduction.

6.3 Nerves

• A nerve is composed of bundles of bundles of axons surrounded by connective tissue. Not a typo - I do mean bundles of bundles!
  
• Axons in peripheral nerves are bound together by an endoneurium of fine collagen fibres, fibroblasts (the cells that form the collagen fibres) and fixed macrophages (defensive cells).
• Bundles of axons are delineated by a more substantial perineurium to form fascicles.
•  Fascicles are bound together by epineurium to form a complete nerve with a blood supply and often with protective adipose cells.
• The smallest diameter axons (0.3 to 0.4 micrometres) in peripheral nerves are not myelinated, and are mainly postganglionic sympathetic fibres and afferent fibres of the peripheral nervous system entering the dorsal roots of the spinal cord.
•  The smallest myelinated axons (up to 3 micrometres diameter) are efferent preganglionic fibres of the autonomic nervous system.
•  Myelinated sensory and motor axons range in diameter from 1 to 22 micrometres.
•  In nonmyelinated axons, conduction velocity is proportional to the square root of axon diameter whereas, in myelinated axons, conduction velocity is proportional to diameter.

6.4  Motor unit

• One motor neuron innervates a group of myofibres - the group is called a motor unit.
• All the myoibres of a motor unit are within one muscle - but they are scattered through the muscle.
• All the myofibres in a motor unit contract at the same time when activated by their motor neuron.
• The contraction of  motor unit is all or nothing - the myoibres are either relaxed, or contracting as hard as they can.
•  Thus, fine control of muscle activity is regulated by the number of motor units contracted.
• Muscles capable of precise contraction have many small motor units (really only the extraocular muscles controlling the eyes of our meat animals).
• When one motor unit is fatigued, another may replace it.
• Only terrified animals can contract all the motor units in a muscle at once - this can produced an astonishing amount of force. Even strong muscle movements do not normally activate all motor units.
• When an axon (inside a nerve) enters a muscle, it branches to innervate all the scattered myofibres of its motor unit.
•  In small muscles, the zone of myofibre innervation may be in an equatorial plane.
•  In most muscles of meat animals the situation is more complex and, in pennate muscles (with an angular arrangement of muscle fibres), even short fasciculi in which all myofibres run from one end to the other may have two innervation bands.
• This is advance warning  - the arrangement of myofibres in meat animals is complex and  has some important consequences when we study muscle growth in meat animals.

•  The terminal axon is the final axonal branch innervating a myofibre.
•  The functional terminal innervation ratio (FTIR) is the mean number of myofibres innervated by the axonal branches radiating from an intramuscular nerve.
•  The absolute terminal innervation ratio (ATIR) gives the mean number of neuromuscular junctions or motor end plates innervated by the axonal branches radiating from an intramuscular nerve.
• Both FTIR and ATIR are increased when our meat animals have extra myoibres (as in the case of double-muscled cattle).

6.5 Neuromuscular junctions

• The neuromuscular junction is where a terminal axon innervates its myofibre.
• Neuromuscular junctions in farm mammals are compact in structure and are called motor end plates.
• At the neuromuscular junction, the terminal axon forms a terminal ramification of small branches, often with bulbous ends.
•  The terminal ramification sits on a mound on the myofibre called the Doyère hillock.
•  The terminal ramification and the Doyère hillock constitute a motor end plate.
•  The motor end plate is capped by perineural epithelial cells (something like special Schwann cells).
•  At the point of contact between the terminal ramification and the myofibre surface, the myoibre (covered by its basement membrane) is corrugated to form synaptic clefts.
  
• When an action potential arrives at a motor end plate, synaptic vesicles in the axoplasm fuse with the axonal membrane, and each vesicle releases a quantal amount of acetylcholine into the synaptic cleft.
• Acetylcholine diffuses across the synapse, from the axonal membrane to the myofibre membrane, and causes a change in the ionic permeability of the postsynaptic area of the myofibre membrane.
•  This causes the end plate potential to change.
•  If the change is of sufficient magnitude, it initiates an action potential on the surface of the myofibre, which then contracts.
•  Acetylcholine is rapidly destroyed by acetylcholinesterase enabling the myofibre to relax.
  
• In the brief instant of one second, this whole sequence of events at the neuromuscular junction may occur several hundred times.
• The growth of motor end plates is correlated with the growth of myofibres until motor end plates reach a certain size, after which, myoibre growth is no longer accompanied by motor end plate growth.
 

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