Somatic nervous system
Encyclopedia
The somatic nervous system (SoNS) is the part of the peripheral nervous system
associated with the voluntary control of body movements via skeletal muscles. The SoNS consists of efferent nerve
s responsible for stimulating muscle contraction
, including all the non sensory neuron
s connected with skeletal muscle
s and skin
.
s.
The basic route of nerve signals within the efferent
somatic nervous system involves a sequence that begins in the upper cell bodies
of motor neuron
s (upper motor neuron
s) within the precentral gyrus
(which approximates the primary motor cortex
). Stimuli from the precentral gyrus are transmitted from upper motor neurons and down the corticospinal tract
, via axon
s to control skeletal (voluntary) muscles. These stimuli are conveyed from upper motor neurons through the ventral horn of the spinal cord
, and across synapse
s to be received by the sensory receptor
s of alpha motor neurons (large lower motor neuron
s) of the brainstem and spinal cord
.
Upper motor neurons release a neurotransmitter
, acetylcholine
, from their axon terminal knobs, which are received by nicotinic receptors
of the alpha motor neurons. In turn, alpha motor neurons relay the stimuli
received down their axons via the ventral root
of the spinal cord. These signals then proceed to the neuromuscular junction
s of skeletal muscles.
From there, acetylcholine is released from the axon terminal knobs of alpha motor neurons and received by postsynaptic receptor
s (Nicotinic acetylcholine receptor
s) of muscles, thereby relaying the stimulus to contract muscle fibers.
s, however, the response of a muscle fiber to a neurotransmitter (always acetylcholine
(ACh)) can only be excitatory.
is a neural circuit that creates a more or less automatic link between a sensory input and a specific motor output. Reflex circuits vary in complexity—the simplest spinal reflexes are mediated by a three-element chain, beginning with sensory neurons which activate interneuron
s in the spinal cord, which then activate motor neurons. Some reflex responses, such as withdrawing the hand after touching a hot surface, are protective, but others, such as the patellar reflex
"knee jerk" activated by tapping the patellar tendon, contribute to ordinary behavior.
Peripheral nervous system
The peripheral nervous system consists of the nerves and ganglia outside of the brain and spinal cord. The main function of the PNS is to connect the central nervous system to the limbs and organs. Unlike the CNS, the PNS is not protected by the bone of spine and skull, or by the blood–brain...
associated with the voluntary control of body movements via skeletal muscles. The SoNS consists of efferent nerve
Efferent nerve
In the nervous system, efferent nerves, otherwise known as motor or effector neurons, carry nerve impulses away from the central nervous system to effectors such as muscles or glands...
s responsible for stimulating muscle contraction
Muscle contraction
Muscle fiber generates tension through the action of actin and myosin cross-bridge cycling. While under tension, the muscle may lengthen, shorten, or remain the same...
, including all the non sensory neuron
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...
s connected with skeletal muscle
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
s and skin
Skin
-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...
.
Nerve signal transmission
The somatic nervous system controls all voluntary muscular systems within the body, with the exception of reflex arcReflex arc
A reflex arc is a neural pathway that controls an action reflex. In higher animals, most sensory neurons do not pass directly into the brain, but synapse in the spinal cord...
s.
The basic route of nerve signals within the efferent
Efferent
Efferent is an anatomical term with the following meanings:*Conveying away from a center, for example the efferent arterioles conveying blood away from the Bowman's capsule in the kidney. Opposite to afferent....
somatic nervous system involves a sequence that begins in the upper cell bodies
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
of motor neuron
Motor neuron
In vertebrates, the term motor neuron classically applies to neurons located in the central nervous system that project their axons outside the CNS and directly or indirectly control muscles...
s (upper motor neuron
Upper motor neuron
Upper motor neurons are motor neurons that originate in the motor region of the cerebral cortex or the brain stem and carry motor information down to the final common pathway, that is, any motor neurons that are not directly responsible for stimulating the target muscle...
s) within the precentral gyrus
Brodmann area 4
Brodmann area 4 comprises the primary motor cortex of the human brain. It is located in the posterior portion of the frontal lobe.Brodmann area 4 is about the same as the precentral gyrus...
(which approximates the primary motor cortex
Primary motor cortex
The primary motor cortex is a brain region that in humans is located in the posterior portion of the frontal lobe. Itworks in association with pre-motor areas to plan and execute movements. M1 contains large neurons known as Betz cells, which send long axons down the spinal cord to synapse onto...
). Stimuli from the precentral gyrus are transmitted from upper motor neurons and down the corticospinal tract
Corticospinal tract
The corticospinal or pyramidal tract is a collection of axons that travel between the cerebral cortex of the brain and the spinal cord....
, via axon
Axon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....
s to control skeletal (voluntary) muscles. These stimuli are conveyed from upper motor neurons through the ventral horn of the spinal cord
Spinal cord
The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...
, and across synapse
Synapse
In the nervous system, a synapse is a structure that permits a neuron to pass an electrical or chemical signal to another cell...
s to be received by the sensory receptor
Sensory receptor
In a sensory system, a sensory receptor is a sensory nerve ending that responds to a stimulus in the internal or external environment of an organism...
s of alpha motor neurons (large lower motor neuron
Lower motor neuron
Lower motor neurons are the motor neurons connecting the brainstem and spinal cord to muscle fibers, bringing the nerve impulses from the upper motor neurons out to the muscles...
s) of the brainstem and spinal cord
Spinal cord
The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...
.
Upper motor neurons release a neurotransmitter
Neurotransmitter
Neurotransmitters are endogenous chemicals that transmit signals from a neuron to a target cell across a synapse. Neurotransmitters are packaged into synaptic vesicles clustered beneath the membrane on the presynaptic side of a synapse, and are released into the synaptic cleft, where they bind to...
, acetylcholine
Acetylcholine
The chemical compound acetylcholine is a neurotransmitter in both the peripheral nervous system and central nervous system in many organisms including humans...
, from their axon terminal knobs, which are received by nicotinic receptors
Nicotinic acetylcholine receptor
Nicotinic acetylcholine receptors, or nAChRs, are cholinergic receptors that form ligand-gated ion channels in the plasma membranes of certain neurons and on the postsynaptic side of the neuromuscular junction...
of the alpha motor neurons. In turn, alpha motor neurons relay the stimuli
Stimulus (physiology)
In physiology, a stimulus is a detectable change in the internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity....
received down their axons via the ventral root
Ventral root
In anatomy and neurology, the ventral root is the efferent motor root of a spinal nerve.At its distal end, the ventral root joins with the dorsal root to form a mixed spinal nerve....
of the spinal cord. These signals then proceed to the neuromuscular junction
Neuromuscular junction
A neuromuscular junction is the synapse or junction of the axon terminal of a motor neuron with the motor end plate, the highly-excitable region of muscle fiber plasma membrane responsible for initiation of action potentials across the muscle's surface, ultimately causing the muscle to contract...
s of skeletal muscles.
From there, acetylcholine is released from the axon terminal knobs of alpha motor neurons and received by postsynaptic receptor
Receptor (biochemistry)
In biochemistry, a receptor is a molecule found on the surface of a cell, which receives specific chemical signals from neighbouring cells or the wider environment within an organism...
s (Nicotinic acetylcholine receptor
Nicotinic acetylcholine receptor
Nicotinic acetylcholine receptors, or nAChRs, are cholinergic receptors that form ligand-gated ion channels in the plasma membranes of certain neurons and on the postsynaptic side of the neuromuscular junction...
s) of muscles, thereby relaying the stimulus to contract muscle fibers.
Vertebrate and invertebrate differences
In invertebrates, depending on the neurotransmitter released and the type of receptor it binds, the response in the muscle fiber could either be excitatory or inhibitory. For vertebrateVertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s, however, the response of a muscle fiber to a neurotransmitter (always acetylcholine
Acetylcholine
The chemical compound acetylcholine is a neurotransmitter in both the peripheral nervous system and central nervous system in many organisms including humans...
(ACh)) can only be excitatory.
Reflex arcs
A reflex arcReflex arc
A reflex arc is a neural pathway that controls an action reflex. In higher animals, most sensory neurons do not pass directly into the brain, but synapse in the spinal cord...
is a neural circuit that creates a more or less automatic link between a sensory input and a specific motor output. Reflex circuits vary in complexity—the simplest spinal reflexes are mediated by a three-element chain, beginning with sensory neurons which activate interneuron
Interneuron
An interneuron is a multipolar neuron which connects afferent neurons and efferent neurons in neural pathways...
s in the spinal cord, which then activate motor neurons. Some reflex responses, such as withdrawing the hand after touching a hot surface, are protective, but others, such as the patellar reflex
Patellar reflex
The patellar reflex or knee-jerk is a deep tendon reflex and is a myotatic reflex.-Mechanism:Striking the patellar tendon with a tendon hammer just below the patella stretches the sensory nerve fiber of the femoral nerve which synapses at the level of L4 in the spinal cord, completely independent...
"knee jerk" activated by tapping the patellar tendon, contribute to ordinary behavior.