Download Structure of the Nervous System Functional Classes of Neurons

Structure of the Nervous System
Afferent neurons
Interneurons
Efferent neurons
Functional Classes of Neurons
Characteristics of the Functional Classes of Neurons
Central Nervous System: Brain
Fig. 6-38
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Central Nervous System: Spinal Cord
Fig. 6-41
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Peripheral Nervous System
• Neurons in the peripheral nervous system transmit signals between the central nervous system and receptors and effectors in all other parts of the body. • The peripheral nervous system has 43 pairs of nerves: 12 pairs of cranial nerves and 31 pairs that connect with the spinal cord as the spinal nerves. • The 31 pairs of spinal nerves are designated by the vertebral levels from which they exit: cervical (8), thoracic (12), lumbar
(5), sacral (5), and coccygeal (1).
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Peripheral Nervous System
• The eight pairs of cervical nerves control the muscles and glands and receive sensory input from the neck, shoulders, arms, and hands. • The 12 pairs of thoracic nerves are associated with the chest and upper abdomen. • The five pairs of lumbar nerves are associated with the lower abdomen, hips, and legs.
• The five pairs of sacral nerves are associated with the genitals
and lower digestive tract. (A single pair of coccygeal nerves associated with the tailbone brings the total to 31 pairs.)
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Peripheral Nervous System
• These peripheral nerves can contain nerve fibers that are the axons of efferent neurons, afferent neurons, or both. • All the spinal nerves contain both afferent and efferent fibers,
whereas some of the cranial nerves contain only afferent fibers or only efferent fibers.
• Efferent neurons carry signals out from the central nervous system to muscles or glands. The efferent division of the peripheral nervous system is more complicated than the afferent, being subdivided into a somatic nervous system and an autonomic nervous system. 8
Spinal Nerves
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The brachial plexus is a collection of nerves formed in the ventral horns of spinal cord of the lower four cervical and first thoracic vertebrae. It proceeds through the neck, the axilla (armpit region), and into the arm. The brachial plexus is responsible for cutaneous and muscular innervation of the entire upper limb The brachial plexus is divided into Roots, Trunks, Divisions, Cords, and Branches. There are five "terminal" branches and numerous other "pre‐
terminal" or "collateral" branches that leave the plexus at various points along its length. The five roots are the five anterior rami of the spinal nerves, after they have given off their segmental supply to the muscles of the neck. Each trunk then splits in two, to form six divisions. These six divisions will regroup to become the three cords. The cords are named by their position with respect to the axillary
artery. These cords branch the branch to travel to specific muscles. Each branch is a named motor nerve
Radial nerve
Structure of the radial nerve
The radial nerve consists of connective tissue wrapped around axons of many Somatic Efferent Motor Neurons to convey Action Potentials to skeletal muscle contractile cells in the triceps brachii, supinator, anconeus, the extensor muscles of the forearm, and brachioradialis
Radial Nerve
Nerve Fascicle
Somatic Efferent Motor Neuron
Nerve Fascicle
The Radial Nerve consists of many NERVE FASCICLES, which are bundles of axons surrounded by connective tissue. Each Somatic Efferent Motor Neuron is myelinated by a myelin sheath, which allows it to conduct action potentials at high velocity. Spinal cord (section)
Axon of somatic efferent
motor neuron of named nerve
Axon terminals
Axon
terminals
Muscle
Terminal buttons
Neuromuscular
junction
Muscle fibers
Muscle Terminal Neuromuscular
fibers button
junction
Fig. 7-4, p. 189
Somatic efferent pathway
Motor neurons originate ventral horn

Innervate skeletal muscle cells
The Somatic Nervous System
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Cerebral cortex can exert voluntary control to drive somatic
efferent neuron’s axon hillock to threshold, so that voluntary or
conciously controlled muscle contraction occurs
Lower brain and spinal level reflex activity also effects somatic
efferent neuron’s axon hillock ability to reach threshold for
reflex control of skeletal muscle contraction also occurs
Regardless of how they reach threshold, the somatic efferent
motor neuron are the only neurons that can activate contractile
activity in skeletal muscle. FINAL COMMON PATHWAY
From “higher” voluntary centers
From “lower” or reflex centers
Motor Unit: A somatic efferent motor neuron and all the muscle
fibers (cells) it innervates

A named motor nerve
innervates skeletal muscle

Each motor neuron within
named nerve innervates a
distinct motor unit that is
made up of that neuron and
multiple muscle (cells)

Any specific muscle cell is
innervated by only one
motor neuron.

The motor unit’s single motor
neuron will excite all muscle
cells it the unit with every
action potential it conducts

Excitation or Inhibition of the
motor unit must occur at the
cell body in the ventral horn
of the grey matter
Spinal Level Somatic Efferent (alpha) motor neurons
• Receive synaptic input from higher brain centers and make adjustments in activation of motor units based on information received from sensory receptors in the muscles, tendons, and joints of the body part to be moved.
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50,000 SYNAPSES MAY
CONVERGE ON THE
DENDRITES AND CELL BODY
OF THE SOMATIC EFFERENT
MOTOR NEURON THAT
EXCITES ALL THE SKELETAL
MUSCLE FIBERS OF ONE
MOTOR UNIT
ALL SKELETAL MUSCLE FIBERS IN ONE MOTOR UNIT
ALL SKELETAL MUSCLE FIBERS IN ONE MOTOR UNIT
THE SOMATIC EFFERENT
MOTOR NEURON SERVES
ONLY GROUP OF MUSCLE
FIBERS; EACH MUSCLE FIBER
IN THE UNIT RECEIVES
INNERVATION FROM ONLY ONE
SOMATIC EFFERENT MOTOR
UNIT. AN ACTION POTENTIAL IN
THE SOMATIC EFFERENT IS
THE SOLE WAY TO ACTIVATE
THE MOTOR UNIT
ALL OR NONE ACTIVATION OF MOTOR UNIT
• When Somatic efferent (alpha) motor neuron conducts an action potential it causes an action potential in every muscle fiber in the motor unit
• Every action potential in the somatic efferent (alpha) motor neurons turns on all the contractile machinery in every muscle fiber (cell) in the motor unit – This always results in force development from the motor unit
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An action potential in a somatic efferent motor neuron is propogated to the
axon terminal (terminal button
The local action potential triggers opening of voltage regulated Ca2+ channels
and subsequent entry of Ca2+ into terminal button
Ca2+ triggers release of ACH by exocytosis of a portion of the vesicles
Ach diffuses across the space separating the nerve and muscle cell and binds
with the nicotinic receptors for it on the motor end plate of the muscle cell
membrane
This opens ion channels for Na+ into muscle cell compared to smaller
movement of K+ outward
The result is an end-plate potential. Local current flow occurs between the end
plate and the adjacent muscle cell membrane
Local current flow opens Na+ channels in the adjacent membrane
Resultant entry of Na+ reduces the resting membrane potential to threshold,
initiating an action potential, which is propagated along the entire surface of
the muscle cell
Ach is destroyed by cholinesterase, an enzyme located on the motor end
plate, terminating the muscle cell’s response
There is a one to one relationship between action potentials in the somatic
efferent and the action potential in the muscle cell. Every action potential turns
on the muscle cell’s contractile proteins
Axon terminal of
motor neuron
Myelin sheath
Action potential
propagation
in motor neuron
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Terminal button
Voltage-gated
Ca2+ channel
Voltage-gated Vesicle of
Na+ channel acetylcholine
Plasma membrane
of muscle fiber
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Action potential
propagation
in muscle fiber
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Ca2+
Na+
2
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Acetylcholinesterase
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K+
Na+
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Acetylcholine-gated
receptor-channel (for
nonspecific cation traffic)
Na+
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Motor end plate
Fig. 7-5, p. 190
Animation: Neuromuscular
junction