The Nervous System Part I: - PowerPoint Presentation

Slide1

The Nervous System

Part I:

Organization, Spinal Cord, and Peripheral Nerves

Slide2

Introduction

Most of the rapid changes in a vertebrate’s internal or external environments are mediated by the nervous system.

Activation of the correct combination of effectors so that the appropriate responses are made requires a processing of signals within the nervous system that is called coordination or integration.

The nervous system is constantly inundated with nervous signals

many are filtered as they pass to the higher centers so that some are suppressed and others enhanced.

Slide3

Sensory inputs from different types of receptors are combined to provide a more complete picture of the changes taking place.

And these inputs are combined with memories of past events before a response is determined

Often, information about the outcome of the response is fed back into the nervous system reinforcing, or altering the past memories.

Although much of vertebrate behavior is based on inherited neuronal connections, vertebrates learn from their experiences.

Slide4

Cellular components of the Nervous System

There are two basic cellular components to the nervous system:

Neurons, which transmit signals to other neurons, glands, muscle cells, or receptor cells, and

Glial

cells, which provide for structural and functional needs of the neuronal cells.

Slide5

Neurons and Synapses

The primary functional units of the nervous system are the neurons, or nerve cells.

Neurons are arranged into two groups

Central Nervous System (CNS)

T

he brain and spinal cord, and

Peripheral Nervous System (PNS)

C

ontaining the nerves between the CNS and the receptors and effectors.

Slide6

Many morphological types of neurons are known, but all contain the same 4 essential parts:A cell body, or trophic segmentDendrites, or the receptive segmentAn axon, or conductive segmentA terminal arborization, or transmissive segment

Slide7

The cell body contains the nucleus and metabolic machinery of the neuron, and is the site of protein and neurotransmitter synthesis.

The cell body varies in its position relative to the dendrite;

in

multipolar

motor neurons the cell body lies close to the dendrite and a long axon extends outward from the body;

in a bipolar cell the body lies in the middle of the axon;

or it may be set off to the side, as in the

pseudounipolar

sensory neuron (13-1).

Slide8

Bipolar

Multipolar

Pseudounipolar

Slide9

Dendrites receive stimuli and initiate the nerve impulse;

Axons are long, slender, cytoplasmic processes often called nerve fibers.

Conduct sensory

input from the dendrites of receptors to the CNS,

impulses within the CNS,

impulses with the effectors, and

motor impulses from the CNS to the effectors.

Slide10

The branching terminal

aborizations

, called

telodendria

make contact with the receptive segments of other neurons at special junctions, called synapses or at synapse

like

myoneural

junctions or

axoglandular

junctions.

A nerve terminal contains many minute synaptic vesicles, in which neurotransmitter is stored.

Many types of neurotransmitter are known

As a given rule, only one type of neurotransmitter is found in each neuron.

When a nerve impulse reached the synapse,

some of the neurotransmitter is released from the synaptic vesicle into the extracellular space,

it crosses the narrow synaptic cleft to bind with the receptor sites of the target cell;

And either excites or inhibits this cell.

Slide11

The polarity of communication of the nervous system is determined by the synapse locations.

Nerve impulses only travel in one direction, from

presynaptic

neuron to postsynaptic receptors.

Neurotransmitters can either be excitatory, partially depolarizing a cell membrane, or inhibitory, hyperpolarizing the membrane.

A threshold level of depolarization must be reached before an action potential is generated

Slide12

Schwann Cells and Neuroglia

Glia

is the general term used to describe non-neuronal cells of the nervous system.

In

gnathostomes

, all axons are surrounded during their embryonic development by

glial

components called Schwann cells.

The inner tongue-like process of these cells grow around the axon many times so that multiple layers of the Schwann cell’s plasma membrane are laid down, forming the myelin sheath.

Gaps between successive Schwann cells interrupt the myelin sheath at areas called Nodes of

Ranvier

Slide13

Because Nodes of Ranvier are the only regions of the axon accessible to the aqueous extracellular fluids, the waves of depolarization jump from node to node, increasing the speed of transmission of the nerve impulse.

Slide14

Schwann cells associated with many peripheral axons envelop the axon but do not grow into a sheath.

T

hese cells are referred to an

nonmyelinated

.

Ex

axons

of autonomic nerves,

Many other types of glial cells, called neuroglia, occur in the CNS

T

hey are 30-50 times more numerous than neurons.

Slide15

Oligodendrocytes

send out processes that wrap around and

myelinate

the axons within the CNS.

Regions of the CNS containing

myelinated

axons appear whitish when stained, and therefore, are called white matter, and contain bundles axons extending between different parts of the CNS.

Regions of non-

myelinated

axons, dendrites, and cell bodies are termed gray matter.

Dense aggregations of cell bodies within the gray matter are called nuclei, and act as land marks within the CNS because they indicate areas of processing of sensory inputs.

Star–shaped

glail

cells known as

astrocytes

are interposed among neuronal cell bodies.

They function to support adjacent cells and also to maintain the ionic composition of the extracellular fluid.

Slide16

Organization of the Nervous System

The basic structure and plan of the nervous system is similar in all vertebrates.

Because of the way they develop; the spinal cord and brain of a vertebrate are hollow and lined by a non-nervous and partially ciliated ependymal epithelium.

The adult spinal cord has a small central canal, which enlarges in parts of the brain to form a series of interconnected ventricles.

Cerebrospinal fluid slowly circulates within these spaces.

Slide17

Bilateral features of the nervous system are often referred to by the contrasting terms

ipsilateral

and

contralateral

.

These terms are especially important in describing the targets of the particular tracts of nerve cells,

Typical spinal nerves attach to the cord at segmental intervals by dorsal and ventral roots.

In most vertebrates, the dorsal and ventral roots of a body segment unite slightly peripheral to the spinal cord to form a spinal nerve

Dorsal, ventral, and communicating rami extend to different parts of the body from these nerves.

Most of the cranial nerves appear to be serially homologous to either a ventral or dorsal root of a typical spinal nerve.

Slide18

The Three Connectional Categories of Neurons and their Distribution

Regardless of the differences in their morphologies, all neurons belong to one of three categories:

Primary sensory,

or

afferent

neurons that carry impulses from free nerve endings or specialized receptor cells into the CNS;

Motor

, or

efferent

neurons that carry signals from the CNS to the effectors, such as muscles or glands.

Interneurons

which are confined to the CNS

They receive input from afferent neurons, integrate this information, and send it to the periphery through efferent neurons.

Slide19

Basic Neuronal Circuitry

Activities of the nervous system are controlled by many types of circuit cells among sensory neurons,

interneurons

, and motor neurons.

There are three basic neuronal pathways common to all vertebrates;

reflexes,

ascending pathways,

descending pathways.

Much activity is controlled by reflexes, mediated by cells located in the spinal cord.

Slide20

Touching a hot stove initiates a reflex termed a 3-neuron reflex arc because it includes three types of neurons: A sensory neuron, an interneuron, and a motor neuron.

In contrast, the knee-jerk reflex requires only two types of neurons because the sensory neurons synapse directly with motor neurons.

Reflexes like the 2- and 3-neuron reflex arc involve neurons that are only on one side of the body and one body segment.

Other reflexes include neurons that cross the midline of the body and affect many body segments.

Axons

that cross the midline are called

commisures

, or

decussations

.

Slide21

Reflexes also may involve neurons that ascend or descend along the body axis and affect many body segments; these are described as

intersegmental

reflexes.

For example, the coordinated movements of swimming and walking are integrated reflexively by pools of

intersegmental

and decussating neurons.

Many reflexes in the brain and spinal cord are products of long evolutionary history, they are innate reflexes.

Others called, conditioned reflexes, develop during thee lifetime of the animal’s repetitive experiences.

Conditioned reflexes are not inherited directly, although

the capacity

for them to develop is.

Slide22

Many sensory impulses ascend from the spinal cord or brainstem to higher centers in the brain.

Groups of axons with similar functions terminate in the same part of the brain and ascend together in common tracts that are described by their points of origin and termination.

Most sensory impulses decussate on their way to higher centers so that impulses originating on the left side of the body terminate on the right side of the brain,

The

decussation

is not always complete.

Slide23

Spinal Cord

The spinal cord lies between the brain and spinal nerves, but the cord is far more than a simple passage way for nerve impulses between receptors and effectors and the brain.

Some

interneurons

of the spinal cord span from one segment of the trunk or tail to one or more other segments and thus are known as

intersegmental

neurons.

Their presence makes possible considerable sensory-motor integration within the spinal cord.

The rhythmic and coordinated movements of swimming and walking are controlled and integrated reflexively by pools of

intersegmental

and decussating spinal nerves, these are central pattern generators.

Slide24

The complexity of the spinal cord, and the degree to which the brain exerts control over spinal activity, increases during the course of vertebrate evolution.

Hagfishes and lamprey have relative simple spinal cords;

none of their axons are myelinated and there is no distinction between gray and white matter

.

In

gnathostomes

, the spinal cord is larger, well vascularized, and more rounded.

The evolution of a highly vascular nervous systems was an important part of vertebrate evolution allowing for more complex nervous systems to evolve.

Slide25

As more ascending and descending fiber tracts evolved, the white matter increased and bulged outward except near the midline, where the dorsomedian sulcus and ventromedian fissure remain.The gray matter in the spinal cord of amniotes has a characteristic butterfly shape when seen in transverse sections.Synapses among neurons of amniotes are confined to the gray matter, white matter contains only ascending and descending fiber tracts.

Slide26

The brains of birds and mammals exert more control over body activities than do the brains of other vertebrates

tracts to and from the brain are correspondingly more numerous.

The degree of

vascularization

of the spinal cord also increases, and the number of

neuroglial

cells that service the neurons also is greater in birds and mammals.

Slide27

The spinal cord in most vertebrates is nearly as long as the vertebral column, although in mammals, frogs, and a few

teleosts

it is shorter because it grows more slowly than the rest of the body (13-13a).

As a consequence, the more caudal spinal nerves form a bundle, known as the cauda

equina

,

Slide28

Vertebrates

with well-developed limbs have a large number of neurons supplying them, and the diameter of the spinal cord is correspondingly greater in the cervical and lumbar

regions

Where

most nerves to the limbs originate.

The spinal cord of mammals is protected

by:

Thin

pia mater, which overlies the neural tissue;

T

he

highly vascularized arachnoid layer outside of it; and

The

tough

dura

matter, which

ensheaths

the entire cord and roots of the spinal nerves.

Slide29

Spinal Nerves

The spinal nerves of vertebrates usually attach by roots to the spinal cord,

However,

in lampreys, the homologues of the dorsal and ventral roots form distinct dorsal and ventral spinal nerves.

The ventral nerves of lampreys are

segmentally

spaced and lead directly into the

myomeres

, they only carry somatic motor neurons.

The dorsal nerves of lamprey are

intersegmental

and pass between the

myomeres

to the body surface and gut regions, and contain both somatic and visceral sensory neurons

.

Most visceral motor neurons reach the viscera of lampreys through a cranial nerve

.

Slide30

The

presence of a distinct dorsal and ventral spinal

nerve,

each with its own type of neurons, is believed to be a

plesiomorphic

character of vertebrates.

In all other craniates, including hagfishes, the distinct dorsal and ventral spinal nerves of a trunk segment unite to form a distinct spinal nerve.

The originally distinct dorsal and ventral nerves are now referred to as the dorsal and ventral roots of the spinal nerve.

All sensory nerves continue to enter through the dorsal root, and somatic motor fibers continue to leave through the ventral root.

Slide31

Cranial Nerves of

Gnathostomes

as Represented by

Chondrichthyans

Slide32

Sensory Nerves Unique to the Head

These include four sensory nerves and two general categories of nerves found only in the

head.

Terminal Nerve

(0): The special somatic sensory terminal nerve may be a part of the chemosensory system regulating certain aspects if reproduction in response to olfactory

pheromones.

Slide33

Olfactory Nerve

(I): A special

somatosensory

nerve that returns chemosensory fibers from the nasal sac.

Optic Nerve

(II):

They are interneurons and carry impulses from the retina to the brain.

Because

the retina develops as a part of the brain the optic nerve is technically a brain tract and not a nerve.

Somatoacoustic

,

Vestibulochochlear

, or

Octaval

Nerve

(VIII): The different names are recognized in different groups.

Nerve

VIII can be characterized as a special somatic sensory nerve that returns fibers from all parts of the inner ear.

Slide34

Lateral Line Nerves

(Not Numbered): The vast majority of amniotes have lateral line and

electroreceptive

organs innervated by a series of 6 lateral line nerves that develop from distinct lateral line

placodes

on the surface of the head.

Gustatory Nerves

(Not Numbered): These are specialized visceral sensory nerves that carry taste sensations. The gustatory fibers become associated with cranial nerves VII, IX, and X.

Slide35

Ventral Cranial Nerves

Those cranial nerves designated as ventral cranial nerves resemble the spinal nerves of the lamprey because they contain primarily somatic motor neurons going to many somatic muscles of the head.

This group consists of those cranial nerves that supply the

extrensic

ocular muscles and the

epibranchial

and

hypobranchial

muscles.

Some also carry somatic sensory

proprioreceptive

neurons, which return information about the degree of contraction of the

extrensic

ocular muscles.

Slide36

Oculomotor

Nerve

(III): The

oculomotor

nerve supplies the 4

extrensic

ocular muscles: dorsal rectus, medial rectus, ventral rectus, and inferior oblique.

Trochlear

Nerve

(IV): The

trochlear

nerve supplies the superior oblique muscle.

Abducens

Nerve

(VI): The

abducens

nerve supplies the lateral rectus muscle.

Occipital Nerve

:

Epibranchial

and

hypobranchial

muscles of fishes, which are located in the caudal region of the head and beneath the pharynx, are innervated by 3-4 occipital nerves and by a variable number of anterior spinal nerves.

Slide37

Dorsal Cranial Nerves

The next 4 cranial nerves (V, VII, IX, X) appear to be comparable with the dorsal spinal nerves of lampreys.

They are mixed nerves that contain some combination of sensory and motor neurons.

All supply somatic motor neurons to the

branchiomeric

muscles of the visceral arches, and they are sometimes referred to as

branchiomeric

nerves.

Slide38

Glossopharyngeal

Nerve

(IX): These nerves carry somatic motor neurons to the muscles of the third visceral arch, and returns visceral sensory neurons from the adjacent part of the gill pouch. In mammals, the free nerve endings and specialized receptors that are the source of the sensory information lie on the back of the tongue and pharynx.

Vagal

Nerve

(V): The

vagal

nerve has 4 branches that supply the

branchiomeric

muscles of the last 4 visceral arches and return visceral sensory neurons from the last 4 gill pouches.

Slide39

Facial Nerve

(VII) and Trigeminal Nerve (V): These nerves carry somatic motor neurons to the

branchiomeric

muscles of the hyoid and

mandibular

arches, respectively. The

hyomandibular

nerve also carries visceral sensory neurons, and the

mandibular

ramus

carries general somatic sensory neurons from the skin over the lower jaw.

Slide40

Changes in the Cranial Nerves of Amniotes

The pattern of the cranial nerves we have seen in

chondrichthyans

persists with predictable modifications in amniotes.

A few evolutionary changes have occurred in the nerves returning from the special sensory organs.

Slide41

These changes include:

Most terrestrial

verts

have a

vomeronasal

organ, and they acquire with it a

vomeronasal

component to the olfactory nerve.

In most non-mammalian vertebrates, most of the fibers of the optic nerve decussate in the optic chiasm, located at the point of attachment of the optic nerves to the brain.

In

many mammals, only half of the fibers decussate. This change may correlate with overlapping visual fields.

A cochlea is well developed in some

sauropsids

, especially birds, as well as in mammals, so nerve VIII in amniotes is often termed the

vestibulocochlear

nerve.

The lateral line system is entirely lost in amniotes, so the six lateral line nerves are absent.

Slide42

The Brain

Craniates differ from early choanates in having a markedly enlarged brain;

The brain is not a completely new craniate feature.

We find a well-developed brain in many protostomes, including the octopus and arthropods

.

Slide43

Embryonic Development and Regions of the Brain

The brain develops by an enlargement of the cranial end of the neural tube.

First, three brain expansions can be recognized:

a forebrain, or prosencephalon,

a midbrain, or mesencephalon,

and the hindbrain, or

rhombencephalon

.

Slide44

The

prosencephalon

first gives rise to the optic

vessicles

, which will form the retina of the

eye.

The

telencephalon, develops soon after as a pair of rostral extensions from the prosencephalon.

These differentiate into the cerebral hemispheres (which together constitute the cerebrum) and into paired olfactory bulbs.

The rest of the

prosencephalon

remains in the midline and differentiates as the diencephalon.

Slide45

The lateral walls of the diencephalon thicken to form the

thalamus.

As they enter the brain the optic nerves partially decussate forming the optic chiasm, just rostral to the hypothalamus.

The mesencephalon remains undivided.

A narrow cerebral aqueduct of

Sylvius

passes through it and connects the third ventricle with the fourth ventricle in the

rhombencephalon

.

Slide46

The dorsal part of the mesencephalon,

known as

the

tectum

enlarges;

A

s

a pair of optic lobes, in

nonmammalian

vertebrates

And,

in

mammals,

is similarly differentiated into the paired superior and inferior

colliculi

.

This part of the

mesencaphalon

is known as the tegmentum.

Slide47

The rostral 1/3 of the roof of the

rhombencephalon

differentiates into the

metencephalon

,

The

most particular part is the cerebellum.

During the embryonic development of birds and mammals,

neuroblasts

migrate from the cerebellum into the ventral part of the rhombencephalon and differentiate into the pontine and other nuclei,

These

transmit messages between the cerebrum and cerebellum

.

This

region is know as the

pons

A

pons does not differentiate in reptiles and

anamniotes

, so all of the

rhombencephalon

, except the cerebellum, forms the 5

th

brain region, the

myencephalon

, which leads to the spinal cord

Slide48

Major Trends in Tetrapod Brain Evolution

The brain of fishes and tetrapods vary enormously in size and complexity.

But the brain regions of tetrapods, the structures they contain, and their basic organizational features are the same as in fish.

The cerebrum, thalamus, mesencephalic tectum, and cerebellum were the regions most affected as vertebrates adapted to a terrestrial life and became more active with a wider range of behavioral responses.

Slide49

Amphibian Brain

The amphibian brain is little different from the brain of fishes.

Locomotor

movements are not as complex as in fishes, and the cerebellum is

therefore smaller.

Most adult amphibians live on land, and have lost the lateral line system and associated regions of the brain.

Slide50

The Brian of Amniotes

The evolution of the cerebrum, dorsal pallium, and tectum are closely related.

The expanded cerebral hemispheres have grown partially caudal in reptiles and birds, and completely caudal in mammals covering the dorsal and lateral surfaces of the

diencephalon.

Slide51

The expansion

is

closely linked to an expansion of the dorsal thalamus, which relays more somatosensory, optic, and auditory impulses

.

This area becomes

increasingly important as a sensory integration center

.

The cerebrum is characterized by containing three neuronal layers: such a pattern is called an

allocortex

, the more advanced mammals have a 6 layered

isocortex

, or neocortex.

Slide52

As mammals increase in size the isocortex becomes highly convoluted, forming surface folds called gyri, which are separated by grooves called sulci.A convoluted surface accommodates the increased number of neuron cell bodies needed to process the increased sensory input and motor output of these generally larger mammals.

Slide53

The Cerebellum

All amniotes have lost the lateral line and electroreceptive systems, so the cerebellum no longer receives input from these systems, but it still has enlarged greatly relative to the cerebellum of amphibians and fishes.

Amniotes have developed an extraordinary wide range of motion of the head, trunk, tail, limbs, and digits as they have adapted to the more difficult problems of support, locomotion, and feeding in the terrestrial environments.

Slide54

The cerebellum is particularly large in birds and mammals, and its surface are is increased by the formation of many tightly packed, leaf-like folds called folia.

The auricles of the cerebellum remain vestibular centers in mammals.

The body

of

the

cerebellum continues

to receive an extensive input from most sensory modalities.

A prominent feature of the avian and mammalian cerebellum is a pair of lateral expansions of the cerebellar body, which form the cerebellar hemispheres that connect with the isocortex.

Slide55

An important overall effect of the cerebellum is to influence the extent, duration, and timing of muscle contractions.

These factors are essential for the smoothness of muscle contractions, for maintaining balance, and for muscle coordination.

Beyond this there is evidence that the cerebellum plays a role in motor learning and the ability to execute precise, carefully timed, and complex motor movements.

Slide56

FIN

ally