Transcript Neurons - Images

Chapter 9
Nervous System
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Organization of
Nervous System
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Organization of
Nervous System
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II. A & B:
There are 2 types of cells in the nervous
system:
• Neurons are the
structural & functional
units of the nervous
system
• Glial cells (aka
neuroglial cells)
provide support for
the neurons.
• In fact, neurons could
not exists without the
glial cells performing
their various
functions.
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II. C: Organs of the Nervous System
The organs of the nervous system can be
divided into 2 groups:
1. The central nervous system (CNS)
consists of the brain and the spinal cord.
2. The peripheral nervous system (PNS)
consists of nerves (peripheral nerves)
that connect the CNS to other body
parts.
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II. D: Functions of the Nervous
System
Together the CNS and PNS allow us to
think, remember, move, to be aware of,
and to respond, to conditions in the world
in which we live.
This is possible because the nervous
system provides sensory, integrative, and
motor functions to the body.
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III. A: Sensory Functions
• Sensory receptors at the end of peripheral
nerves gather information by detecting
changes to the body’s environment (both
external & internal changes)
• then convert the information into nerve
impulses that travel to the CNS.
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III. A: Integrative functions
• To integrate means to combine parts into
a whole, so …
• In the brain, sensory nerve impulses are
integrated into perceptions ( a sensation
that is an interpretation of the combined &
coordinated impulses)
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III. A: Motor functions
• Once a decision (conscious or
unconscious) is made,
 motor impulses travel to effector organs and
a response to the sensory input is stimulated.
• Motor responses are carried out by
effectors (a muscle or gland that effects
change in the body).
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III. B: PNS & Motor Functions
The motor functions of the PNS can be divided into
2 categories:
• the somatic system is under conscious control;
the somatic motor pathways lead to the skin and
to skeletal muscles (aka voluntary muscles)
• the autonomic system controls effectors that
are involuntary – the heart, smooth muscles in
blood vessels and in viscera, and glands
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IV. Neuroglial Cells
• Neuroglial cells perform functions that are
vital to neurons by filling spaces, support
neurons by providing nutrition and
structural frameworks, producing myelin,
and performing phagocytosis.
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IV. CNS Glial Cells
There are 4 types of glial cells in the CNS:
• microglia cells are small cells that are
scattered throughout the CNS an function
to phagocytize bacterial cells and cellular
debris
• oligodendrocytes occur in rows along
nerve fibers and provide insulating layers
of myelin in the brain and spinal cord
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IV. CNS Glial Cells
(glial cells in the CNS continued)
• astrocytes, found between blood vessels
& nerves, support structures, aid in
metabolism, and respond to brain injury by
filling in spaces (also, form scar tissue
following injuries to the CNS)
• ependymal cells form epithelial-like
membranes found in specialized parts of
the brain and spinal cord; they help
regulate the production of spinal fluid
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IV. Glial Cells in the PNS
Schwann cells are found in the PNS and form a
covering called the myelin sheath around neural
axons (Iike a band-aide around a finger)
The outer, living layer of the Schwann cells
(neurilemma) plays an important role in the
regeneration of damaged peripheral nerves (the
axons can regenerate)
 In contrast, when neurons of the CNS (myelinated by
oligodendrocytes) are damaged the axons do not
generally regenerate.
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Head lines!! p. 206
• Excess neuroglial cells cause harm…
• Why are babies so uncoordinated?!...
• Neuron cell body the size of a tennis
ball?!...
• Grey matter vs white matter…
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V. Neurons
• Neurons vary on size and shape but ALL
neurons have 3 parts:
 a cell body
dendrites
an axon
• Mature neuron cells do not divide BUT
some parts of the nervous system contain
neural stem cells that can differentiate
into neurons or neuroglia.
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Neuron Cell Body
• The cell body of a neuron consists of:
granular cytoplasm, cell membrane, various
organelles including a nucleus (with a
prominent nucleolus), mitochondria, & Nissl
bodies (similar to the rough endoplasmic
reticulum) with attached ribosomes (Do you
remember the function of ribosomes?)
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Dendrites
• The receptive part of the neuron consists
of short, highly branched structures called
dendrites.
• Dendrites are receive nerve impulses from
the axons of other neuron cells and
transmit the impulse to the cell body.
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Axon
• A single axon carries the nerve impulse away
from the cell body.
• Large axons of the PNS are enclosed in myelin
sheaths composed of many Schwann cells.
• The portions of the Schwann cell that contains
most of the cytoplasm & the nucleus remain
outside the sheath and form the neurilemma,
which assists in the repair of damaged axons.
• Gaps in the myelin sheath are called nodes of
Ranvier; these “gaps” facilitate rapid conduction
of nerve impulses
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Myelinated vs Unmyelinated
• Myelinated axons
have myelin sheaths
are appear white.
• Masses of myelinated
axons and their cell
bodies form the white
matter within the
CNS
• Unmyelinated axons
lack myelin sheaths
are appear gray.
• Masses of
unmyelinated axons
and their cell bodies
form the gray matter
within the CNS
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Neurons Classified by Structure
• Bipolar neurons have only 2 processes
arising from each end of their cell body;
one is a dendrite and one is an axon.
both processes are structurally similar
Neurons within specialized parts of the eyes,
nose, & ears are bipolar
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Neurons Classified by Structure
• Unipolar neurons have a single process
extending from their cell body; a short distance
from the cell body this process divides into 2
branches. The cell bodies of these neurons
aggregate in groups called ganglia which are
located outside the CNS;
 both branches really function as 1 axon
 the peripheral branch is associated with dendrites
near a peripheral body part and has a “trigger zone”
– the initial portion of the axon
 the other branch enters the brain or spinal cord
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Neurons Classified by Structure
• Multipolar neurons have many process
arising from their cell body – many
dendrites and one axon;
most of the neurons whose cell bodies lie in
the brain and spinal column are multipolar.
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Neurons Classified by Function
• Sensory (afferent) neurons carry nerve
impulses from peripheral receptors to the CNS.
 afferent neurons are usually unipolar (some are
bipolar)
 sensory neurons are located in peripheral body parts
and
 have specialized receptor ends at the tips of their
dendrites or their dendrites are closely associated
with receptor cells in the skin or sensory organs
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Neurons Classified by Function
• Interneurons carry nerve impulses from
one part of the CNS to another.
interneurons are multipolar
interneurons lie entirely within the brain &
spinal cord (the CNS)
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Neurons Classified by Function
• Motor (efferent) neurons carry nerve
impulses from the CNS to effectors.
efferent neurons are multipolar
carry nerve impulses to stimulate muscles to
contract and/or glands to release secretions
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VI. Cell Membrane Potential
A. A cell membrane is usually polarized
(electrically charged), with an excess of
negative charges on the inside of the
membrane.
Polarization is important to the conduction of
nerve impulses.
• This separation of
charge (or potential
difference) is called
the resting potential.
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VI. Cell Membrane Potential
B. The + ions involved in enabling the conduction
of a nerve impulse are Na+ and K+ ions.
• There is a high concentration of Na+ ions
outside the cell while there is a high
concentration of K+ ions inside the cell (along
with many negative ions contained in the
cytoplasm).
• K+ ions pass through the cell membrane more
easily than Na+ ions; this make K+ ions a major
contributor to cell polarization, depolarization,
and repolarization
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VI. Cell Membrane Potential
C. Conduction of a nerve impulse
• Resting potential: higher concentration of Na+
ions outside, higher concentration of K+ ions
inside.
• A threshold stimulus causes sodium channels to
open, allowing Na+ ions to diffuse in
 the cell membrane depolarizes
• SOON after, potassium channels open and K+
ions move out
 the cell membrane repolarizes
This rapid change is called an action potential.
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VI. Cell Membrane Potential
C. A nerve impulse is created as depolarization
creates a wave of action potentials (a localized
bioelectric current) that move down the axon.
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VII. Nerve Impulse
A. Nodes of Ravier:
• Unmyelinated fibers conduct impulses over
their entire membrane surface.
• Myelinated fibers conduct impulses from node
of Ranvier to node of Ranvier; a phenomenon
called saltatory conduction.
• Therefore, impulses move faster along
myelinated fibers that along unmyelinated
fibers.
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VII. Nerve Impulse
B. All-or-nothing Response
•
•
•
When the threshold stimulus is applied to an
axon it responds completely by conducting
the impulse (all-or-nothing response).
The result is that all impulses carried on a
particular axon is of the same strength.
Greater intensity of stimulation triggers more
impulses per second, not stronger impulses.
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VII. Nerve Impulse
C. Refractory Period
• There is a brief period, following a nerve
impulse, when a threshold stimulus will not
trigger another impulse on an axon – this is
called the refractory period.
• This limits the frequency of impulses in an
axon.
Frequency as high as 700 impulses per second is
possible but 100 impulses per second is more
common.
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VIII. The Synapse
A. Synaptic Transmission
2. When an impulse (an action potential)
reaches the synaptic knobs of an axon,
synaptic vesicles release neurotransmitter
into the synaptic cleft.
– The neurotransmitter reacts with specific
receptors on the postsynaptic membrane.
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VIII. The Synapse
3. About 50 types of neurotransmitters have
been identified
• acetylcholine
• monoamines such dopamine, serotonin,
epinephrine, norepinephrine
• several amino acids, and a large group of
neuropeptides
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VIII. The Synapse
3. Stimulation of the nerve fiber stops when
the neurotransmitter is degraded by an
enzyme or reabsorbed by the presynaptic
neuron.
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IX. Impulse Processing
Neuronal Pools
Neurons within the CNS are organized into
neuronal pools with varying numbers of
cells.
Each pool receives input from afferent
nerves and processes the information
according to the special characteristics of
the pool.
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IX. Impulse Processing
Facilitation
A particular neuron of a pool may receive
excitatory or inhibitory stimulation; if the
net effect is excitatory but sub-threshold,
the neuron becomes more excitable to
incoming stimulation (a condition called
facilitation).
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IX. Impulse Processing
Convergence
A single neuron within a pool may receive
impulses from two or more fibers
(convergence), which makes it possible for
the neuron to summate impulses from
different sources.
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IX. Impulse Processing
Divergence
Impulses leaving a neuron in a pool may
be passed into several output fibers
(divergence), a pattern that serves to
amplify an impulse.
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X. Nerves
• A nerve is a bundle of nerve fibers held
together by layers of connective tissue.
 nerves that carry impulses to the brain or
spinal cord (CNS) are referred to as sensory
nerves
nerves that carry impulses to the muscles or
glands are referred to as motor nerves
most nerves include both sensory & motor
fibers and are called mixed nerves
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Nerve Pathways
• The routes that nerve impulses follow as
they travel through the nervous system are
referred to as nerve pathways.
• A reflex (the simplest of nerve pathways)
is an automatic subconscious response to
stimuli (changes within or without the
body).
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Nerve Pathways
• A reflex (the simplest of nerve pathways)
is an automatic, involuntary (made without
conscious thought) response to stimuli
(changes within or without the body).
reflexes help maintain homeostasis by
controlling involuntary processes such as
heart rate & breathing
reflexes also carry out the automatic actions
of swallowing, sneezing, coughing, & vomiting
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Reflex Arc
A reflex arc begins with a sensory receptor
(dendrites of a sensory neuron or a
specialized receptor cell in a sensory
organ)  a sensory neuron carries the
impulse to the CNS  an interneuron in
the CNS conducts the impulse to a motor
neuron  the motor neuron transmits
impulse from CNS out to an effector
which responds by producing a reflex
(behavioral action).
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Knee Jerk!
• The knee-jerk reflex is an example or a
simple reflex that utilizes only 2 neurons:
a sensory neuron communicates directly
with a motor neuron (no interneuron as a
go-between!!)
sensory neuron  motor neuron
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XII. Bony Coverings
• The brain is covered by the cranial cavity
of the skull.
• The spinal cord is covered by the spinal or
vertebral column,
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Meninges – Dura Mater
• The dura mater is the outermost layer of
the meninges; it is formed of tough, white
fibrous connective tissue
is attached to the inside of the skull
contains blood vessels & nerves
forms partitions that support & protect the
brain & spinal cord.
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Meninges – Arachniod Mater
• The arachnoid mater is the middle layer
of the meninges; it is a thin, web-like
membrane that spreads over the surface
of the brain & spinal cord
• The subarachnoid space (between this
mater and pia mater) contains the
cerebrospinal fluid (CSF)
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Meninges – Pia Mater
• The innermost meninges is the pia mater.
• It is very thin; contains numerous nerves &
blood vessels.
• Nourishes the cells of the brain
Spinal Cord
• The spinal cord is continuous with the
brain, but…
• The superior boundary of the spinal cord is
the inferior end of the cranium.
• The inferior boundary of the spinal cord is
at the disk between the L1 (1st lumbar) and
L2 (2nd lumbar) vertebrae.
• There are 31 pairs of spinal nerves.
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Spinal Cord
•
The spinal cord has 2 major functions:
1. conducting nerve impulses
2. Serving as a center for spinal reflexes
•
The nerve tracts that carry sensory
information to the brain are called
ascending tracts.
 Injury to ascending tracts results in a loss of
sensation in body parts distal to the injury
Spinal Cord
•
The nerve tracts that conduct motor
impulses from the brain to muscles and
glands are called descending tracts.
 Injury to descending tracts results in a loss of
motor function in body parts distal to the injury
2 Types of Descending
Spinal Tracts
• Pyramidal tracts
(aka corticospinal
tracts)
• Carry impulses that
control skeletal
muscle movements.
• Extrapyramidal tracts
control motor activities
associated with
maintaining balance
and posture
The Brain
The major parts of the brain are:
• the cerebrum (the largest portion of the
brain)
• the cerebellum
• the diencephalon
• the brain stem
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Structure of the Cerebrum
• Two large masses – left & right cerebral
hemispheres connected by a bridge of
neural fibers called the corpus callosum.
• Surface has many ridges or convolutions
(aka gyri) separated by grooves.
• A shallow groove is called a sulcus; a
deep groove is called a fissure.
Lobes of the Cerebrum
• Frontal lobe
• Parietal lobe
• Temporal lobe
• Occipital lobe
• Insula
The first 4 lobes listed correspond to the
bones of the skull with the same names
Cerebrum
• The outermost layer of the cerebrum is the
cerebral cortex, a thin layer of gray
matter (myelinated or unmyelinated?).
• The cerebral cortex contains nearly 75%
of all the neuron cell bodies in the nervous
system.
• Beneath the cortex is a mass of white
matter that makes up the bulk of the
cerebrum.
Cerebrum
Cerebral functions include:
• Sensory – interpret sensory input & give
rise to sensations or feelings
• Motor – generate nerve impulses that
control muscle activity
• Associative – interpreting, reasoning,
memory, & other higher brain functions
Cerebrum
Functional areas include:
• Motor
major motor areas in frontal lobe just in front
of central sulcus control skeletal muscles
speech movements controlled by Broca’s
area
frontal eye field controls voluntary movements
of eyes and eyelids
Cerebrum
Functional areas include:
• Sensory – parietal lobes, posterior
occipital lobes, temporal lobes
cutaneous sensations from parietal lobe
 visual areas in posterior occipital lobe
auditory areas in the temporal lobe
taste areas are near the base of central sulci
smell arises from areas deep within the
cerebrum
Cerebrum
Functional areas include:
• Associative – anterior portions of frontal
lobe, lateral portions of parietal, temporal,
& occipital lobes
memory, reasoning & problem solving,
judgment, emotion
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Dominant or Non-dominant
Dominant Hemisphere
• Used for languagerelated activities such as
speech, reading, writing,
and for complex
intellectual skills that
require verbal, analytical,
and computational skills.
• In 90% of population left
hemisphere is dominant.
Non-dominant Hemisphere
• Used for motor tasks that
require orientation of the
body in space,
understanding &
interpreting musical
patterns, and non-verbal
visual experiences.
• Also controls intuitive
thinking and emotional
thinking.
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Cerebral Spinal Fluid (CSF)
• The pia mater contains specialized
cauliflower-like masses of specialized
capillaries that secrete CSF.
• The brain & spinal cord float in the CSF
which supports and protects them by
acting as shock absorbers.
• CSF also maintains electrolyte balance
and serves as a pathway to the blood for
excretion of cellular waste.
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Diencephalon
Thalamus
• Functions in sorting & directing sensory
information arriving from other parts of the
nervous system.
• Perform services of both messenger &
editor. (It is the executive secretary for the
cerebrum!)
• Produces an awareness of pain, touch, &
temperature
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Diencephalon
Hypothalamus
• Maintains homeostasis by regulating heart
rate & rhythm, hunger & weight gain,
various other visceral functions
• Regulates secretions of various hormones,
including those that stimulate sleep and
wakefulness
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Limbic System
• The limbic system includes the thalamus,
hypothalamus. It:
controls emotional experience and expression
recognizes threats to the organisms and
produces emotions such as fear, anger, &
pleasure, which leads to behaviors that
increase chances for survival
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Brain Stem
• Lies between the brain and spinal cord
and connects the two structures.
• Three parts:
midbrain
pons
medulla oblongata
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Midbrain
• It is like a doorman to the cerebrum by
conveying nerve impulses to & from the
cerebrum.
• Several visual reflexes located here,
including those that turn the eyes in
concert with the head and auditory
reflexes such as those that turn the head
in the direction of a sound.
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Pons
• Helps regulated rate & depth of breathing
• Relays impulses to the cerebellum and
from the periphery to the higher brain
centers
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Medulla Oblongata
• Transmits all ascending and descending
impulses to the spinal cord
• Control center for vital reflexes that an
individual alive (heart, breathing)
• Also the center for reflexes like sneezing
and coughing
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Cerebellum
• Cerebellum is the reflex control center – it
integrates sensory information
concerning the position of the body in space,
and
for coordinating complex skeletal movements
• Damage to the cerebellum results in
tremor, loss of precision in skeletal
muscles, loss of muscle tone, abnormal
gait, & loss of balance.
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PNS – 2 parts: Somatic &
Autonomic
• The somatic system is under conscious
control
• The autonomic system controls effectors
that are involuntary
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Cranial Nerves
There are 12 pair of cranial nerves.
I
Olfactory………………. On
II
Optic…………………… Old
III
Oculomotor……………. Olympus
IV
Trochlear………………. Towering
V
Trigeminal……………… Tops
VI
Abducens…………….... A
VII
Facial…………………… Friendly
VIII Vestibulocochlear……... Viking
IX
Glossopharyngeal……... Grew
X
Vagus……………………. Vines
XI
Accessory……………….. And
XII
Hypoglossal…………...... Hops
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Divisions of the Autonomic
Nervous System
• The sympathetic division – prepares you
for energy-expending, stressful, or
emergency situations (fight or flight)
• The parasympathetic division – is most
active under ordinary, restful conditions
(rest / digest)
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