Download Chapter 12 Nervous Tissue

THE NERVOUS SYSTEM
HEADING
VOCABULARY
IMPORTANT INFO
Major Structures of the Nervous System
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Brain, cranial nerves, spinal cord, spinal nerves,
ganglia, enteric plexuses and sensory receptors
Nervous System Divisions
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Central nervous system
(CNS)
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consists of the brain and
spinal cord
Peripheral nervous system
(PNS)
consists of cranial and spinal nerves that contain
both sensory and motor fibers
 connects CNS to muscles, glands & all sensory
receptors
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Subdivisions of the PNS
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Somatic (voluntary) nervous system (SNS)
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neurons from cutaneous and special sensory receptors to
the CNS
motor neurons to skeletal muscle tissue
Autonomic (involuntary) nervous systems
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sensory neurons from visceral organs to CNS
motor neurons to smooth & cardiac muscle and glands
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sympathetic division (speeds up heart rate)
parasympathetic division (slow down heart rate)
Enteric nervous system (ENS)
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involuntary sensory & motor neurons control GI tract
neurons function independently of ANS & CNS
Neurons 
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Functional unit of nervous system
Have capacity to produce action
potentials
 electrical excitability
Cell body
 single nucleus with prominent
nucleolus
 Nissl bodies (chromatophilic
substance)
 rough ER & free ribosomes for
protein synthesis
 Neurofilaments give cell shape
and support
 Microtubules move material
inside cell
Cell processes = dendrites & axons
The basic unit of the nervous
system = neuron?
Dendrites
receive stimuli
 Nerve cell body
@ nucleus
transmits the
stimuli
 Axon transmits
the impulse to
another dendrite
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Parts of a Neuron
Neuroglial cells
Nucleus with
Nucleolus
Axons or
Dendrites
Cell body
Dendrites
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Conducts impulses
towards cell body
Typically short, highly
branched & unmyelinated
Surfaces specialized for
contact with other
neurons
Contains neurofibrils &
Nissl bodies
Axons
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Conduct impulses away
from cell body
Long, thin cylindrical
process of cell
Impulses arise from initial
segment (trigger zone)
Side branches (collaterals)
end in fine processes
called axon terminals
Swollen tips called
synaptic end bulbs contain
vesicles filled with
neurotransmitters
Functional Classification of Neurons
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Sensory (Afferent) neurons
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Motor (Efferent) neurons
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transport sensory information
from skin, muscles, joints,
sense organs & viscera to
CNS
send motor nerve impulses to
muscles & glands
Interneurons (Association)
neurons
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connect sensory to motor
neurons
90% of neurons in the body
What is a synapse ?
 Junction
of two
neurons
 Neurotransmitter
convert the
electrical impulse
into a chemical
message
Axon ending: terminal bud
Transfers the
electrical nerve
impulse
 By chemical
neurontransmitters
 From one
neuron to the
next

The Action Potential: Summarized
Chemical
Synapses
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Action potential reaches end bulb
and voltage-gated Ca+ 2
channels open
Ca+2 flows inward triggering
release of neurotransmitter
Neurotransmitter crosses
synaptic cleft & binding to
receptors
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the more neurotransmitter released
the greater the change in potential
of the postsynaptic cell
Synaptic delay is 0.5 msec
One-way information transfer
The Central Nervous System:
1) Spinal Cord
2) Brain
 medulla for
breathing
 cerebellum for
balance
 cerebrum for
higher thinking
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Spinal Cord Protection
•By the vertebral column, meninges,
cerebrospinal fluid, and vertebral ligaments.
External Anatomy of Spinal Cord
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Flattened cylinder
16-18 Inches long &
3/4 inch diameter
In adult ends at L2
In newborn ends at L4
Growth of cord stops at
age 5
Cervical enlargement
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upper limbs
Lumbar enlargement
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lower limbs
Spinal Cord & Spinal Nerves
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Spinal nerves begin as roots
Dorsal or posterior root is incoming sensory fibers
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dorsal root ganglion (swelling) = cell bodies of sensory
nerves
Ventral or anterior root is outgoing motor fibers
Spinal Nerves
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31 Pairs of spinal nerves
Named & numbered by the
cord level of their origin
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8 pairs of cervical nerves
(C1 to C8)
12 pairs of thoracic nerves
(T1 to T12)
5 pairs of lumbar nerves
(L1 to L5)
5 pairs of sacral nerves
(S1 to S5)
1 pair of coccygeal nerves
Mixed sensory & motor
nerves
The Brain and Cranial Nerves
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almost 3 lb.
Brain functions in sensations, memory, emotions, decision
making, behavior
Principal Parts of the Brain
Cerebrum
 Diencephalon
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thalamus &
hypothalamus
Cerebellum
 Brainstem
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medulla, pons &
midbrain
Medulla
Oblongata
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Continuation of spinal cord
Ascending sensory tracts
Descending motor tracts
Nuclei of 5 cranial nerves
Cardiovascular center
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Respiratory center
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force & rate of heart beat
diameter of blood vessels
medullary rhythmicity area sets basic rhythm of breathing
Information in & out of cerebellum
Reflex centers for coughing, sneezing, swallowing etc
One inch long
 White fiber tracts
ascend and
descend
 Pneumotaxic &
apneustic areas
help control
breathing
 Middle cerebellar
peduncles carry
sensory info to the
cerebellum
 Cranial nerves 5-7
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Pons
Midbrain
One inch in
length
 Extends from
pons to
diencephalon
 Cerebral
aqueduct
connects 3rd
ventricle above
to 4th ventricle
below
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Cerebellum
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2 cerebellar hemispheres and Vermis (central area)
Function
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correct voluntary muscle contraction and posture based on
sensory data from body about actual movements
sense of equilibrium
Thalamus
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1 inch long mass of gray mater in each half of brain
(connected across 3rd ventricle by intermediate mass)
Relay station for sensory information on way to cortex
Crude perception of some sensations
Hypothalamus
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Dozen or so nuclei in 4 major regions
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mammillary bodies are relay station for olfactory reflexes;
infundibulum suspends the pituitary gland
Major regulator of homeostasis
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receives somatic and visceral input, taste, smell & hearing
information; monitors osmotic pressure, temperature of
blood
Functions of Hypothalamus
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Controls and integrates activities of ANS which
regulates smooth, cardiac
muscle and glands
Synthesizes regulatory
hormones that control the
anterior pituitary
Contains cell bodies of axons
that end in posterior pituitary
where they secrete hormones
Regulates rage, aggression, pain, pleasure & arousal
Feeding, thirst & satiety centers
Controls body temperature
Regulates daily patterns of sleep
Epithalamus
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Pineal Gland
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endocrine gland
the size of small
pea
secretes
melatonin during
darkness
promotes
sleepiness & sets
biological clock
Habenular Nuclei
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emotional
responses to
odors
Cerebrum
(Cerebral Hemispheres)
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Cerebral cortex is gray matter
overlying white matter
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2-4 mm thick containing billions
of cells
grew so quickly formed folds
(Gyri) and grooves (Sulci or Fissures)
Longitudinal fissure separates left & right cerebral
hemispheres
Corpus Callosum is band of white matter connecting
left and right cerebral hemispheres
Each hemisphere is subdivided into 4 lobes
Right versus left Cerebrum
Limbic System
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Emotional brain--intense pleasure & intense pain
Strong emotions increase efficiency of memory
2 Types of Nervous Responses?
A. Voluntary
 the brain &
 spinal cord
B. Involuntary or
Autonomic System
 Sympathetic
 Parasympathetic
Somatic Sensory Pathways
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First-order neuron conduct impulses to
brainstem or spinal cord
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either spinal or cranial nerves
Second-order neurons conducts impulses from
spinal cord or brainstem to thalamus--cross
over to opposite side before reaching thalamus
 Third-order neuron conducts impulses from
thalamus to primary somatosensory cortex
(postcentral gyrus of parietal lobe)
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Somatic Motor Pathways
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Control of body movement
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motor portions of cerebral cortex
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initiate & control precise movements
Basal Ganglia help establish muscle tone & integrate
semivoluntary automatic movements
Cerebellum helps make movements smooth & helps maintain
posture & balance
Somatic motor pathways
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direct pathway
from cerebral
cortex to spinal
cord & out to
muscles
indirect pathway
includes synapses
in basal ganglia,
thalamus, reticular
formation &
cerebellum
The Autonomic Nervous System
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Regulate activity of smooth
muscle, cardiac muscle &
certain glands
Structures involved
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general visceral afferent
neurons (Sensory)
general visceral efferent
neurons (Motor)
integration center within the
brain
Receives input from limbic
system and other regions of
the cerebrum
The Autonomic Nervous System
Autonomic versus Somatic NS
•Autonomic NS
–unconsciously
perceived visceral
sensations
–involuntary inhibition or
excitation of smooth
muscle, cardiac muscle
or glandular secretion
–2 neurons needed to
connect CNS to organ
–Preganglionic and
Postganglionic neurons
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Somatic NS
consciously
perceived
sensations
 excitation of
skeletal muscle
 one neuron
connects CNS to
organ
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Autonomic versus Somatic NS
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Notice that the ANS pathway is a 2 neuron
pathway while the Somatic NS only contains
one neuron.
Divisions of the ANS
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2 major divisions
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Parasympathetic
Sympathetic
Dual innervation
(2 nerve supplies)
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1 speeds up organ
1 slows down
organ
Sympathetic NS
increases heart
rate
Parasympathetic
NS decreases heart
rate
Sympathetic Responses
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Dominance by the sympathetic system is
caused by physical or emotional stress –
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exercise
Alarm Reaction = Flight or Fight Response
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“E situations”: emergency, embarrassment, excitement,
dilation of pupils, increase of heart rate, force of
contraction & BP
decrease in blood flow to nonessential organs
increase in blood flow to skeletal & cardiac muscle
airways dilate & respiratory rate increases
blood glucose level increase
Long lasting due to lingering of NE in synaptic
gap and release of norepinephrine by the
adrenal gland
Parasympathetic Responses
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Enhance “Rest-and-Digest” activities
Mechanisms that help conserve and restore body
energy during times of rest
Normally dominate over sympathetic impulses
SLUDD Type Responses = salivation, lacrimation,
urination, digestion & defecation and 3 “decreases”--decreased HR, diameter of airways and diameter of
pupil
Paradoxical Fear when there is no escape route or no
way to win
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causes massive activation of parasympathetic division
loss of control over urination and defecation
Control of Autonomic NS
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Not aware of autonomic responses because
control center is in lower regions of the brain
Hypothalamus is major control center
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Input: emotions and visceral sensory information
 smell, taste, temperature, osmolarity of blood, etc
Output: to nuclei in brainstem and spinal cord
posterior & lateral portions control sympathetic NS
 increase heart rate, inhibition GI tract, increase
temperature
anterior & medial portions control parasympathetic
NS
 decrease in heart rate, lower blood pressure,
increased GI tract secretion and mobility