21 - Peripheral Nervous System

7/27/2019 21 - Peripheral Nervous System

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Lecture 21 Peripheral Nervous SystemKardong Chapter 16, Hildebrand Chapter17


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Basic Layout of the Nervous SystemThe nervous system stimulates the action of muscles in light of information

from sense organs.

sense organs central nervous

system

afferent or

sensory neurons

muscles

efferent or motorneurons

sensory and motor neurons = peripheral nervous system

association

neurons

Nerve cells (neurons) are the basic units of the nervous

system. Bundles of neurons are called nerves in the

peripheral nervous system or tracts in the central nervous

system.


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Cell Body or perikaryonwhere the nucleus resides.

Axonthe cellular extension

that transmits information.

Axon cylinder = axon plus

Schwann cells which wrap theaxon in myelin.

Dendrites - the connection to

adjacent nerves, muscles, or

sense organs.

Ganglion - a group of cellbodies outside the CNS.

The morphology of a motor neuron

KK 16.2, H&G (Fig. 17.2)


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Other Types of Neurons

While motor neurons are typicallymonopolar(cell body at one end), sensory

neurons are typicallybipolar(cell body in

the middle of the axon).

To complete a circuit between sense organ

and muscle, motor neurons and sensoryneurons are linked in the CNS by association

neurons, which are typically multipolar.

KK 16.3, H&G 17.2


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Synapses and Neurotransmitters

Dendrites of neurons do not

touch; transmission is across a

synapse via diffusion of a

neurotransmitter produced by

presynaptic vesicles.

At least 3 neurons are

involved in going from

sensory information

(stimulus) to muscleresponse. Such a circuit from

sensor to effector is called a

reflect arc.

KK 16.5, H&G 17.1, 17.8


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Sensory and motor peripheral nerves

Sensory nerves enter the CNS via

dorsal roots. Their cell bodies are indorsal root ganglia outside the CNS.

Motor nerves leave the CNS via

ventral roots. In spinal nerves, these

roots come together just outside the

cord and separate again into dorsaland ventral rami.

KK 16.7, H&G 17.8


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Embryological OriginsCNSThe brain and spinal

cord are derived from the

neural tube (neurectoderm).

PNSMotor nerves have

their cell bodies in the CNS,

and their axons grow out of

the CNS to targeted musclesand organs.

The dorsal root ganglia

containing the cell bodies of

sensory nerves are derived

from the neural crests.Bipolar axons grow towards

the CNS and towards their

target sense organs.

KK 16.8, H&G 17.7, 17.8


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Relationship of peripheral nerves to

embryonic mesoderm

The dorsal ramus serves

the epaxial musclesand

dermis, whereas the ventral

ramus serves the hypaxial,appendicular, and visceral

muscles.

KK 16.9


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1) Central Nervous System: brain and spinal cord

2) Peripheral Nervous System: spinal and cranial nerves

A) Somatic Nerves- sensory - information from the integument, skeletal muscles

- motor - information to integument and muscles

B) Visceral Nerves- sensory - information from receptors in the viscera

- motor - information to visceral muscles (gut, heart)

Divisions of the Nervous System

Visceral motor nerves comprise autonomic system. They are of two sets that actin opposition to each other. One type comprises the sympathetic nervous system

and the other the parasympathetic nervous system.


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Again :

Sensory neurons travel via

dorsal root into the spinalcord. Their cell bodies are in

the dorsal root ganglia.

Motor neurons travel via

ventral root out of the spinal

cord. Their cell bodies are inthe grey matter of the cord.

Somatic neurons (sensory

or motor) use somatic or

dorsal ramus.

Visceral neurons use

visceral or ventral ramus.


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A complication:

The previous diagram

illustrates the pattern for

spinal nerves in amniotes,but in lampreys only

somatic motor fibres use

the ventral root.

Non-amniotes may havevisceral motor neurons using

either root, as below left.

Knowing this helps us

understand the cranial

nerves (which are not so

neatly arranged as spinal

nerves!)

Fig. 17-12


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Cranial Nerves - peripheral nerves emerging from the brain

The cranial nerves are numbered with Roman numerals I through up

to XII, and some recognize an anterior nerve 0 (terminalis).

Special Sensory Nerves. Three cranial nerves are associated with the

special sense organs. They are not serially homologous with the rest

of the peripheral nervous system. These are the olfactory nerve (I),

the optic nerve (II) and the auditory (statoacoustic) nerve (VIII).

Dorsal Root Cranial Nerves. These cranial nerves are sensory and

visceral motor in function, and serially homologous with dorsal roots

of spinal nerves.

Ventral Root Cranial Nerves. These are somatic motor in function,

and serially homologous with the ventral roots of spinal nerves.


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Ventral view of shark brain showing the cranial nerves.

Dorsal Root (sensory or

visceral motor)

Special

Visceral Root (somatic motor)

KK 16.14, H&G 17.14


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Cranial nerves help us

understand the

segmentation of the

vertebrate head, which has

been highly modifed and no

longer appears segmented.

The diagram illustrates the

hypothesized ancestralsituation of the dorsal roots,

and the dorsal root cranial

nerves of a fish. The

vertebrate head has at least

7 segments (see Table 16.2in Kardong) but if the upper

diagram is correct it would

be 8 or more.

KK 16.16, H&G 10.7


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21 - Peripheral Nervous System

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