Structure of the Nervous System

Afferent neurons

Efferent neurons

Interneurons

Functional Classes of Neurons

Characteristics of the Functional Classes of Neurons

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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 genitalsand 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. 

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Spinal Nerves

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 axillaryartery. 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

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. 

Nerve Fascicle

Nerve Fascicle

Somatic Efferent Motor Neuron

Axon terminals

Neuromuscularjunction

Spinal cord (section)

Muscle

Axonterminals

Axon of somatic efferentmotor neuron of named nerve

Musclefibers

Terminalbutton

Terminal buttons

Neuromuscularjunction

Muscle fibers

Fig. 7-4, p. 189

Motor neurons originate ventral horn

Innervate skeletal muscle cells

Somatic efferent pathway

The Somatic Nervous System

1. 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

2. 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

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

A somatic efferent motor neuron and all the muscle fibers (cells) it innervates

Motor Unit:Motor Unit:

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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.

ALL SKELETAL ALL SKELETAL MUSCLE FIBERS IN MUSCLE FIBERS IN ONE MOTOR UNITONE MOTOR UNIT

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 ALL SKELETAL MUSCLE FIBERS IN MUSCLE FIBERS IN ONE MOTOR UNITONE 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

1. An action potential in a somatic efferent motor neuron is propogated to the axon terminal (terminal button

2. The local action potential triggers opening of voltage regulated Ca2+ channels and subsequent entry of Ca2+ into terminal button

3. Ca2+ triggers release of ACH by exocytosis of a portion of the vesicles4. 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

5. This opens ion channels for Na+ into muscle cell compared to smaller movement of K+ outward

6. The result is an end-plate potential. Local current flow occurs between the end plate and the adjacent muscle cell membrane

7. Local current flow opens Na+ channels in the adjacent membrane8. 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

9. Ach is destroyed by cholinesterase, an enzyme located on the motor end plate, terminating the muscle cell’s response

10. 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

Acetylcholine-gatedreceptor-channel (fornonspecific cation traffic)

Action potentialpropagationin muscle fiber

Voltage-gatedCa2+ channel

Action potentialpropagationin motor neuron

Na+

Na+

Na+

Plasma membraneof muscle fiber

Voltage-gatedNa+ channel

Terminal button

Motor end plate

Vesicle ofacetylcholine

Ca2+

Myelin sheath

Axon terminal ofmotor neuron

K+Acetylcholinesterase

Fig. 7-5, p. 190

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2

34

6

7

88

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6

959

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Animation: Neuromuscular junction