Download The Nervous System

The Nervous
System
Chapter 7
The Nervous System
• Every thought, action, or emotion you have is
controlled by your nervous system
• Communicates with your body through a
series of specific and rapid electrical impulses
Functions
• Uses sensory receptors to monitor internal and
external changes (stimuli and sensory input)
• Processes and interprets the sensory input and
makes decisions about what should be done
(integration)
• Effects a response by activating muscles or glands
(motor output)
Structural Classification
Central Nervous System= the brain + spinal cord.
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Occupies the dorsal body cavity
Acts as the integrating and command center of the
nervous system.
Structural Classification
Peripheral Nervous System= spinal nerves + cranial
nerves
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Consists mainly of nerves that extend from the brain
and spinal cord.
Links all the parts of the body by carrying impulses
from the sensory receptors
to the CNS and from the
CNS to appropriate glands
and muscles.
Functional Classification (PNS)
1. Sensory (afferent) Division- consists of nerve
fibers that convey impulses to the CNS.
-Somatic sensory fibers – from the skin, skeletal muscle
and joints
- Visceral sensory fibers- from the visceral organs.
2. Motor (efferent) Division –carries impulses
from the CNS to effector organs, muscles, and
glands.
a. Somatic/voluntary nervous system
b. Automatic/involuntary nervous system
Functional Classification (PNS)
• Somatic nervous system: (aka voluntary nervous
system)
– Consciously or voluntarily control our muscles
– Not necessarily all muscles: ex: reflexes
• Autonomic nervous system: (aka involuntary
nervous system)
– Activity of smooth or cardiac muscle
Nervous Tissue
• Even though nervous tissue is complex, it
is made up of just two principal types of
cells:
1. supporting cells
2. neurons.
Nervous Tissue
• Neurons
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Cell body
Nissl Substance/Neurofibrils
Dendrites
Axons
Axon Terminals
Myelin Sheath
Neurilemma
Nodes of Ranvier
Ganglia
Tracts/Nerves
Nervous Tissue
• Cell body- metabolic center of the neuron
containing organelles.
• Nissl substance- rough ER
• Neurofibrils- intermediate filaments
important in maintaining cell shape.
• Dendrites- convey incoming messages toward
the cell body.
• Axons – generate nerve impulses and conduct
them away from the cell body.
Nervous Tissue
• Axon Terminals- contain hundreds of tiny vesicles
that contain neurotransmitters
• Myelin Sheath – protects and insulates the axon
and increases the transmission rate of nerve
impulses.
• Neurilemma- external to myelin sheath
• Nodes of Ranvier- gaps or indentations on myelin
sheath
Nervous Tissue
Supporting Cells of the CNS – Neuroglia
• Astrocytes- abundant star-shaped cells that account
for nearly half of neural tissue.
– Make exchanges between capillaries and neurons
• Microglia- spiderlike phagocytes that dispose
of debris, including dead brain cells and
bacteria.
• Ependymal Cells- line cavities of brain and
spinal cord.
– Circulate cerebralspinal fluid.
• Oligodendrocytes- produce myelin sheaths.
Nervous Tissue
supporting cells and neurons
• Supporting Cells of the PNS
1. Schwann Cells
1. Satellite Cells
Functional Classification of
Neurons
• Groups neurons according to the direction the
nerve impulse is traveling
• Sensory or afferent neurons – carry impulses
from sensory receptors to the CNS. Their
dendrites are associated with specialized
receptors. Their cell bodies are in ganglion outside
the CNS.
• Motor or efferent neurons – carry impulses form
the CNS to the viscera and/or muscles and glands.
Their cell bodies are in the CNS.
Structural Classification of
Neurons
• Based on the number of processes extending
from the cell body.
• Multipolar
• Bipolar
• Unipolar
Physiology of Neurons
Irritability and Conductivity
Irritability – the ability to respond to a stimulus and convert it
to a nerve impulse.
• A resting neuron is polarized. There are more Na+ outside
and less K+ inside.
• Stimuli causes the permeability of the plasma membrane to
change, changing the polarity.
• This depolarization activates the neuron to initiate and
transmit an action potential.
• Repolarization allows the cell to conduct the next impulse.
Physiology of Neurons
Irritability and Conductivity
Conductivity – the ability to transmit the impulse to
other nerve cells, muscles, or glands.
Reflexes
• Rapid, predictable and involuntary responses to
stimuli that occur over neural pathways called
reflex arcs.
• Autonomic Reflexes – regulate the activity of
smooth muscle, the heart, and glands.
• Somatic Reflexes – includes all reflexes that
stimulate the skeletal muscles.
Reflex Arcs
• All reflex arcs have a minimum of five
elements
1. Sensory receptor
2. Afferent neurons
3. CNS integration center
4. Efferent neurons
5. Effector organ
Reflex Arcs
• There is always a delay at the synapse.
• Some reflexes involve only the spinal cord
neurons to function.
• Some reflexes require that the brain become
involved because many different types of
information need to be evaluated in order to
arrive at the correct response.
Central Nervous System
the brain
• The brain is commonly discussed in terms of 4
major regions…
1. Cerebral Hemisphere
2. Diencephalon
3. Brain Stem
4. Cerebellum
Cerebral Hemispheres
• The pair of cerebral hemispheres are the most
superior and largest part of the brain.
• The entire surface of these hemispheres has
elevated ridges called gyri, separated by
shallow grooves called sulci.
• Deeper grooves called fissures separate the
brain into lobes, named for the cranial bone
that lies over them.
Cerebral Hemispheres
1.
2.
3.
4.
Lobes
Parietal Lobe
Occipital Lobe
Temporal Lobe
Frontal Lobe
1.
2.
3.
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6.
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Functional Areas
Somatic Sensory Area
Specialty Senses
Primary Motor Area
Corticospinal Tract
Broca’s Area
Speech Area
Corpus Callosum
Basal Nuclei
Diencephalon
• The diencephalon sits atop the brain stem and
is enclosed by the cerebral hemispheres.
1. Thalamus
2. Hypothalamus
3. Epithalamus
Brain Stem
• Provides a pathway for ascending and
descending tracts and has many small gray
matter areas.
1. Midbrain
2. Pons
3. Medulla Oblongata
4. Reticular Formation
Cerebellum
• It projects dorsally from under the occipital
lobe.
• The cerebellum provides the precise timing for
skeletal muscle activity and controls our
balance and equilibrium.
• Because of it’s activity, our body movements
are smooth and coordinated.
Protection of the CNS
• The CNS is protected by bone (skull and
vertebral column) , membranes (the
meninges), and a watery cushion
(cerebrospinal fluid).
• Protection from toxins in the blood is provided
by the blood-brain barrier.
Protection of the CNS
meninges
• The three connective tissue membranes that
cover and protect the CNS structures.
Protection of the CNS
cerebrospinal fluid
• A watery broth similar to the make up of the
blood plasma from which it is continually formed.
• The CSF is formed in clusters of capillaries that
hang in the diencephalon called the choroid
plexuses. It circulates through the brain ventricles
and returns to the blood, constantly draining as
new CSF forms, keeping the overall volume and
pressure relatively constant
Protection of the CNS
blood-brain barrier
• The brain is absolutely dependent on a constant
internal environment.
• It could not function if exposed to the types of
fluctuations of hormones, ions, and nutrients that
continually occur elsewhere in the body,
especially after eating and exercising.
• Neurons are kept separate from blood borne
substances by the BBB.
• Remember how nerve impulses are initiated
Peripheral Nervous System
• The PNS consists of nerves and scattered
groups of neuronal cell bodies, called ganglia,
found outside the CNS
What is a “Nerve”?
- nerve structure• A nerve is a a bundle of neuron fibers found
outside the CNS. Neuron fibers are wrapped in
protective tissue coverings. (which should look
very familiar)
• Like neurons, nerves are classified according
to the direction in which they transmit
impulses. (afferent- sensory, efferent-motor,
or mixed –both directions)
PNS- Cranial Nerves
• There are 12 pairs of cranial nerves primarily
serve the head and neck. Only one pair
extends to the thoracic and abdominal
cavities.
• The cranial nerves are mostly mixed nerves
(except for those associated with special
senses) and are numbered in order and given
a name that revels their major function.
PNS- Spinal Nerves and Nerve
Plexuses
• The 31 pairs of spinal nerves are formed by a
combination of the ventral and dorsal roots of
the spinal cord.
• They are named for the region of the cord
from which they arise. (Cervical, Thoracic,
Lumbar, Sacral)
• Almost immediately after they are formed
they divide into dorsal and ventral rami
(branches)
PNS- Spinal Nerves and Nerve
Plexuses
• Dorsal Rami serve the posterior body trunk
• The Ventral Thoracic Rami serve the anterior
and lateral trunk.
• The other ventral rami form the 4 nerve
plexuses which service the limbs.
– Cervical
– Brachial
– Lumbar
– Sacral
PNS- Autonomic Nervous
System
• The ANS is a division of the PNS that regulates
cardiac muscle, smooth muscle and glands
automatically.
• The ANS is the major force in maintaining
homeostasis.
• It is divided into two main arms which
counterbalance each other. The sympathetic
mobilizes the body during extreme situations
while the parasympathetic allows us to “unwind”.
Developmental Aspects of the Nervous
System
• Formed during the first month of embryonic
development.
• Any maternal infection in early pregnancy can
have harmful effects on the nervous system of
the fetus.
Developmental Aspects of the Nervous
System
• Maternal measles- causes deafness and CNS
damage.
• Radiation
• Drugs (alcohol, cocaine, opiates)
• Lack of oxygen can cause the death of
neurons.
– Example: smoking decreases amount of oxygen to
blood.
Cerebral Palsy
• Neuromuscular disability in which voluntary
muscles are poorly controlled and spastic.
• Can be caused by a temporary lack of oxygen.
Congenital Malformations
• Anacephaly- failure of cerebrum to develop
• Hydrocephaly- abnormal accumulation of
cerebrospinal fluid on brain
• Spina bifida- vertebrae form incompletely
Developmental Aspects continued
• One of last areas to mature- hypothalamus
– Babies have issues regulating body temperature
• No more neurons formed after birth, but
growth and maturation of nervous system
continues through childhood.
• Young adult- when brain reaches maximum
weight.
Developmental Aspects continued
• Over next 60 years neurons are damaged and
die.
• They cannot reproduce themselves.
• However, we can continue to learn through
life