Download The Nervous System

The Nervous System
The nervous system is the master
controlling and communicating system
of the body. Every thought, action, and
emotion reflects its activity. Its cells
communicate by electrical and chemical
signals, which are rapid and specific, and
usually cause almost immediate
responses.
Functions of the Nervous System
 Sensory input – gathering information
 To monitor changes occurring inside and
outside the body
 Changes = stimuli
 Integration
 To process and interpret sensory input and
decide if action is needed
 Motor output
 A response to integrated stimuli
 The response activates muscles or glands
For example, when you are driving and see a
red light ahead (sensory input), your nervous
system integrates this information (red light
means “stop”), and your foot goes for the
brake (motor output).
Structural Classification of the
Nervous System
Two divisions:
 Central nervous system (CNS)
 Brain
 Spinal cord
 Peripheral nervous system (PNS)
 Cranial nerves
 Spinal nerves
Functional Classification of the PNS
It divides them into TWO subdivisions:
1-Sensory (afferent) division
 Nerve fibers that carry information to the central
nervous system from:
- sensory receptors in the skin, skeletal muscles
and joints (somatic sensory fibers).
- Sensory receptors in the visceral organs
(visceral sensory fibers)
2-Motor (efferent) division
 Nerve fibers that carry impulses away from the
central nervous system ( to Muscles &Glands).
It has two subdivisions
1-Somatic nervous system = voluntary, it
controls skeletal muscles
 N. B. skeletal muscle reflexes are involuntary
2-Autonomic nervous system = involuntary, it
controls smooth &cardiac muscles &glands
This also is divided into:
- sympathetic &
- parasympathetic
Organization of the Nervous System
Histology of Nervous Tissue
• Despite the complexity of the nervous
system, there are only two functional cell
types:
• Neurons - excitable nerve cells that
transmit electrical signals
• Neuroglia (glial) cells (Astrocytes,
Microglia, Ependymal cells,
Oligodendrocytes ) are the supporting cells.
Neuroglia cells - 2 types in the Peripheral NS
Satellite cells
- surround neuron cell bodies in the periphery
- Protective , cushioning cells
Schwann cells (neurolemmocytes)
• surround axons/dendrites and form the myelin sheath
around larger nerve fibers in the periphery
• similar to oligodendrocytes in function – insulators
Neuron (Nerve cell)
Cells specialized to transmit messages
Differ structurally but have common features, but
all have :
 A Cell body with nucleus and the usual organelles
Except centrioles
 Extensions (proceses) outside the cell
body
 Dendrites – conduct impulses toward the cell
body
 Axons – conduct impulses away from the cell
body
Neuron Anatomy
Axons and Nerve Impulses
 Axons end in axonal terminals
 Axonal terminals contain vesicles
which is full of chemical transmitter
called neurotransmitters
 Axonal terminals are separated from
the next neuron (neuroneural
junction) by a gap called Synaptic
cleft (Synapse)
Nerve Fiber Coverings
*Most long nerve fibers are
covered with a whitish, fatty
material called Myelin with
waxy appearance. It :
-insulates the fiber &
-Increases transmission rate
*Axons outside CNS are
wrapped by Schwann Cells
Figure 7.5
Neuron Cell Body Location
 Most are found in the central nervous
system so CNS appears in two colors:
 Gray matter – cell bodies and unmyelinated
fibers
 Nuclei – clusters of cell bodies within the
white matter of the central nervous system
Ganglia – collections of cell bodies
outside the central nervous system
 White matter- collection of myelinated
fibers.
Functional Classification of Neurons
1-Sensory (afferent) neurons carry impulses from :
 Cutaneous sense organs (skin receptors)
 Proprioceptors – receptors that detect stretch or
tension in muscles and tendons and joints
to the CNS. Their cell bodies are in ganglia outside
the CNS.
2-Motor (efferent) neurons carry impulses from the central
nervous system to muscles and glands ,their cell bodies
are always in CNS.
3-Interneurons (association neurons)
 Their cell bodies are always found in CNS.
 Connect sensory and motor neurons in neural
pathways.
Neuron Classification
Figure 7.6
Functional Properties of Neurons
 Irritability – ability to respond to stimuli
 Conductivity – ability to transmit an
impulse
 The plasma membrane at rest is polarized
that is:
Fewer positive ions are inside the cell than
outside the cell.
Starting a Nerve Impulse
 Depolarization – a
stimulus depolarizes the
neuron’s membrane
 A deploarized
membrane allows
sodium (Na+) to flow
inside the membrane
 The exchange of ions
initiates an action
potential in the neuron
The Action Potential
 If the action potential (nerve impulse)
starts, it is propagated over the entire
axon
 Potassium ions rush out of the neuron
after sodium ions rush in, which
repolarizes the membrane
 The sodium-potassium pump restores
the original configuration
 This action requires ATP
Continuation of the Nerve Impulse
between Neurons
 Impulses are able to cross the synapse
to another nerve
 Neurotransmitter is released from a nerve’s
axon terminal
 The dendrite of the next neuron has
receptors that are stimulated by the
neurotransmitter
 So an action potential is started in the
dendrite of the next neuron.
How Neurons Communicate at
Synapses
Figure 7.10
The Reflex Arc
 Reflex – rapid, predictable, and
involuntary responses to stimuli.
 Reflex arc – direct route from a sensory
neuron, to an interneuron, to an effector
 Ref. arc have a minimum 5 elements:
Simple Reflex Arc
Figure 7.11b, c
Types of Reflexes
 Autonomic reflexes as:
Salivary gland secretion
Heart and blood pressure regulation
Changes in size of the pupil
 Digestive system regulation
 Somatic reflexes as:
Activation of skeletal muscles
REFEXES MAY USE :
• SPINAL CORD, as flexor reflex and knee jerk
.
• BRAIN ( for proper evaluation) as response
of pupils to light
• N.B.1, Exaggerated, Distorted or Absent
reflexes indicate nervous system disorder.
• N.B.2, Reflex changes often occur before the
pathological condition become obvious
Central Nervous System (CNS)
 CNS develops from the embryonic
neural tube
 By the fourth week the anterior end begins
to expand and brain formation begins, The
rest of the tube becomes the spinal cord
 The central canal becomes enlarged in 4
regions of the brain to form the ventricles:
-Four chambers within the brain
-Filled with cerebrospinal fluid
Regions of the Brain
 Cerebral
hemispheres
 Diencephalon
 Brain stem
 Cerebellum
Figure 7.12
Cerebral Hemispheres (Cerebrum)
 Paired (left
and right)
superior parts
of the brain
 Include more
than half of
the brain
mass
Figure 7.13a
Layers of the Cerebrum
 Gray matter(outside)
 Outer layer
 Composed mostly of
neuron cell bodies
 White matter(inside)
 Axons(tracts)
 Example: corpus
callosum connects
the two hemispheres
 Basal nuclei –
internal islands in the
gray matter
Figure 7.13a
Cerebral Hemispheres (Cerebrum)
 The surface
is made of
ridges (gyri)
and grooves
(sulci)
Figure 7.13a
Specialized Area of the Cerebrum
 Cerebral areas involved in special senses
 Gustatory area (taste)
 Visual area
 Auditory area
 Olfactory area
 Interpretation areas of the cerebrum
 Speech/language region
 Language comprehension region
 General interpretation area
Specialized Area of the Cerebrum
Figure 7.13c
Diencephalon
 Sits on top of the brain stem
 Enclosed by the cerebral heispheres
 Made of three parts
 Thalamus
 Hypothalamus
 Epithalamus
Diencephalon
Figure 7.15
Thalamus
 Surrounds the third ventricle
 The relay station for sensory impulses (except olfaction)
 Transfers impulses to the correct part of the cortex for
localization and interpretation
Hypothalamus (Under the thalamus)
Important autonomic nervous system center
 Helps regulate body temperature
 Controls water balance
 Regulates metabolism
 An important part of the limbic system (emotions)
 The pituitary gland is attached to the hypothalamus
Epithalamus
 Forms the roof of the third ventricle
 Houses the pineal body (an endocrine
gland)
 Includes the choroid plexus which forms the
cerebrospinal fluid
Brain Stem
Attaches to the spinal cord and is formed of:
Midbrain
Pons
Medulla oblongata
Brain Stem
Figure 7.15a
Midbrain
 Reflex centers for vision and hearing
Pons
 The bulging middle part of the brain stem
 Mostly composed of fiber tracts
 Includes nuclei involved in the control of
breathing
Medulla Oblongata




The lowest part of the brain stem
Merges into the spinal cord
Includes important fiber tracts
Contains important control centers
 Heart rate control
 Blood pressure regulation
 Breathing
 Swallowing
 Vomiting
Cerebellum
 Two hemispheres with convoluted
surfaces
 Provides involuntary coordination of
body movements
Protection of the Central Nervous
System
 Scalp and skin
 Skull and vertebral column
 Meninges
 Cerebrospinal
fluid
 Blood brain
barrier
Figure 7.16a
Meninges
• Three connective tissue membranes lie external to the
CNS
– dura mater,
-arachnoid mater, and
- pia mater
• Functions of the meninges
• Cover and protect the CNS
• Protect blood vessels and enclose venous sinuses
• Contain cerebrospinal fluid (CSF)
• Form partitions within the skull
Meninges
Figure 12.24a
Dura Mater
• Leathery, strong meninx composed of two
fibrous connective tissue layers
• The two layers separate in certain areas and
form dural sinuses
• Double-layered external covering
• Periosteum – attached to surface of the
skull
• Meningeal layer – outer covering of the
brain
• Folds inward in several areas
Arachnoid Mater
• The middle meninx, which forms a loose brain covering
• It is separated from the dura mater by the subdural space
• Beneath the arachnoid is a wide subarachnoid space filled
with CSF and large blood vessels
• Arachnoid villi protrude superiorly and permit CSF to be
absorbed into venous blood
Pia Mater
Deep meninx composed of delicate connective tissue
that clings tightly to the brain
Cerebrospinal Fluid
 Similar to blood plasma composition
 Formed by the choroid plexus
 Forms a watery cushion to protect the
brain
 Circulated in subarachnoid space,
ventricles, and central canal of the
spinal cord
Ventricles and Location of the
Cerebrospinal Fluid
Figure 7.17a
Blood Brain Barrier
 Includes the least permeable capillaries
of the body to exclude many potentially
harmful substances
 It can not prevent some substances
 Fats and fat soluble molecules
 Respiratory gases
 Alcohol
 Nicotine
 Anesthesia
Traumatic Brain Injuries
 Concussion
 Slight brain injury
 No permanent brain damage
 Contusion
 Nervous tissue destruction occurs
 Nervous tissue does not regenerate
 Cerebral edema
 Swelling due to inflammation of the brain
 May compress brain tissue causing death
Cerebrovascular Accident (CVA)
 Commonly called a stroke
 Caused by ruptured or obstructed blood vessel
supplying a region of the brain
 Brain tissue supplied with oxygen from that blood
source will die.
 Loss of some functions or death may result
Alzheimer’s Disease
• Progressive degenerative brain disease
 Mostly seen in the elderly, but may begin in middle
age leading to memory loss, irritability, confusion and
ultimately, hallucinations and death
Spinal Cord
 Extends from the
medulla oblongata to
below T12
 Below T12 is the cauda
equina (a collection of
spinal nerves)
 Enlargements present
in the cervical and
lumbar regions
Figure 7.18
Spinal Cord Anatomy
 Exterior white mater – conduction tracts
 Internal gray matter - mostly cell bodies
 Dorsal (posterior) horns
 Anterior (ventral) horns
 Central canal filled with cerebrospinal fluid
Figure 7.19
Peripheral Nervous System
 Nerves and ganglia outside the CNS
 Nerve = bundle of neuron fibers outside
CNS bundled by connective tissue, may be:
-Afferent (sensory) nerves – carry impulses
toward the CNS
-Efferent (motor) nerves – carry impulses away
from the CNS
-Mixed nerves – both sensory and motor fibers
Cranial Nerves
 12 pairs of nerves that mostly serve the
head and neck
 Numbered in order, front to back
 Most are mixed nerves, but three are
sensory only.
Distribution of Cranial Nerves
Figure 7.21
Cranial Nerves
 I Olfactory nerve – sensory for smell
 II Optic nerve – sensory for vision
 III Oculomotor nerve – motor fibers to
eye muscles
 IV Trochlear – motor fiber to eye
muscles
Cranial Nerves
 V Trigeminal nerve – sensory for the
face; motor fibers to chewing muscles
 VI Abducens nerve –
motor fibers to eye muscles
 VII Facial nerve – sensory for taste;
motor fibers to the face
 VIII Vestibulocochlear nerve –
sensory for balance and hearing
Cranial Nerves
 IX Glossopharyngeal nerve – sensory
for taste; motor fibers to the pharynx
 X Vagus nerves – sensory and motor
fibers for pharynx, larynx, and
abdominal viscera
 XI Accessory nerve – motor fibers to
neck and upper back
 XII Hypoglossal nerve – motor fibers to
tongue
Spinal Nerves
 There is a pair of spinal nerves at the
level of each vertebrae for a total of 31
pairs
 Spinal nerves are formed by the
combination of the ventral and dorsal
roots of the spinal cord
 Spinal nerves are named for the region
from which they arise.
Spinal Nerves
Figure 7.22a
Examples of Nerve Distribution
Figure 7.23
Autonomic Nervous System
 The involuntary part of the peripheral
nervous system
 Consists of motor nerves only
 Divided into two divisions
 Sympathetic division
 Parasympathetic division
Differences Between Somatic and
Autonomic Nervous Systems
 Nerves
 Somatic – one motor neuron
 Autonomic – two neurons ,preganglionic and
postganglionic nerves
 Effector organs
 Somatic – skeletal muscle
 Autonomic – smooth muscle, cardiac muscle,and
glands
 Nerurotransmitters
 Somatic – always use acetylcholine
 Autominic – use acetylcholine, epinephrine, or
norepinephrine
Figure 7.24
Sympathetic Division (thoracolumber)
 Originates from T1 through L2
 Norepinephrine and epinephrine are
neurotransmitters to the effector organs
Parasympathetic Division(craniosacral)
 Originates from the brain stem and S1
through S4
 Always uses acetylcholine as a
neurotransmitter
Slide 7.70
Sympathetic Pathways
Figure 7.26
Slide 7.71
Anatomy of the Autonomic Nervous
System
Figure 7.25
Autonomic Functioning
 Sympathetic – “fight-or-flight”
 Increase activities
 Remember the “E” division = exercise,
excitement, emergency, and embarrassment
 Parasympathetic – housekeeping activities
 Conserves energy
 Maintains daily necessary body functions
 Remember as the “D” division - digestion,
defecation, and diuresis
Development Aspects of the Nervous System
 The nervous system is formed during the first
month of embryonic development so any early
maternal infection can have extremely harmful
effects on the baby.
 The hypothalamus is one of the last areas of
the brain to develop.
 No more neurons are formed after birth, but
growth and maturation continues for several
years.
 The brain reaches maximum weight in young
adults (1300-1400gr.)