Download Central Nervous System

Central Nervous System
• Central nervous system (CNS)
• brain and spinal cord enclosed in bony coverings
• Functions of the spinal cord
• spinal cord reflexes
• integration (summation of inhibitory and excitatory) nerve
impulses
• highway for upward and downward travel of sensory and
motor information
• Brain functions
• sensations, memory, emotions, decision making, behavior
14-1
Overview of Spinal Cord
• Spinal cord - Information highway between brain and body
• Each pair of spinal nerves receives sensory information
and issues motor signals to muscles and glands
• Spinal cord is a component of the CNS while the spinal
nerves are part of the PNS
14-2
Functions of the Spinal Cord
• Conduction
• bundles of fibers passing information up and down spinal cord
• connecting different levels of the trunk with each other and the brain
• Neural integration - input from multiple sources, integrated, and
executed output
• Locomotion
• repetitive, coordinated actions of several muscle groups
• central pattern generators are pools of neurons providing control
of flexors and extensors (walking)
• Reflexes
• involuntary, stereotyped responses to stimuli (remove hand from
hot stove)
• involves brain, spinal cord and peripheral nerves
14-3
Anatomy of the Spinal Cord
• Spinal cord - cylinder of nervous tissue that arises from the
brainstem at the foramen magnum of the skull
• Passes through the vertebral canal (thick as a finger)
• vertebral column grows faster so in an adult the spinal cord only extends to L1
• 31 pairs of spinal nerves arise from cervical, thoracic,
lumbar and sacral regions of the cord
• First pair passes between the skull and C1
• Rest pass through intervertebral foramina
• Cord segment - part of the spinal cord supplied by each pair of
spinal nerves
14-4
Surface Anatomy
C1
Cervical
enlargement
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CervicalCompanies, Inc. Permission required for reproduction or display.
spinal
nerves
C7
Dural
sheath
Subarachnoid
space
Thoracic
spinal
nerves
Spinal cord
Vertebra (cut)
Spinal nerve
Lumbar
enlargement
T12
Spinal nerve rootlets
Medullary
cone
Posterior median sulcus
Lumbar
spinal
nerves
Cauda equina
Subarachnoid space
Epidural space
Posterior root ganglion
L5
Rib
Arachnoid mater
Terminal
filum
Sacral
spinal
nerves
Dura mater
S5
Col
(a)
(b)
Figure 13.1
13-5
Meninges of the Spinal Cord
• 3 Fibrous layers enclosing spinal cord
• Dura mater
• tough collagenous membrane surrounded by epidural space
filled with fat and blood vessels
• epidural anesthesia utilized during childbirth
• Arachnoid mater
• layer of simple squamous epithelium lining dura mater and
loose mesh of fibers filled with CSF
(creates subarachnoid space)
• Pia mater
• delicate membrane adherent to spinal cord
14-6
Meninges of Vertebra and Spinal Cord
14-7
Cross-Sectional Anatomy of the Spinal Cord
• Central area of gray matter shaped like a butterfly and surrounded
by white matter in 3 columns
• Gray matter = neuron cell bodies with little myelin
• White matter = myelinated axons
14-8
Gray Matter in the Spinal Cord
• Pair of dorsal or posterior horns
• dorsal root of spinal nerve is totally sensory fibers
• Pair of ventral or anterior horns
• ventral root of spinal nerve is totally motor fibers
• Connected by gray commissure punctured by a central canal
continuous above with 4th ventricle
14-9
White Matter in the Spinal Cord
• White matter of the spinal cord surrounds the gray matter
• White column = bundles of myelinated axons that carry signals up
and down to and from brainstem
• 3 pairs of columns or funiculi
•
•
•
•
Dorsal, lateral, and anterior columns
Tracts or fasciculi - subdivisions of each column
Ascending and descending tract head up or down
Contralateral means origin and destination are on opposite sides while ipsilateral
means on same side
14-10
Ascending Tracts
• Ascending tracts carry
sensory signals up the spinal
cord
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Somesthetic cortex
(postcentral gyrus)
Third-order
neuron
Thalamus
• Sensory signals travel across
three neurons from origin in
receptors to the destination in
the sensory areas of the brain
• First-order neurons: detect
stimulus and transmit signal to
spinal cord or brainstem
• Second-order neurons: continues
to the thalamus at the upper end
of the brainstem
• Third-order neurons: carries the
signal the rest of the way to the
sensory region of the cerebral
cortex
Cerebrum
Midbrain
Medial
lemniscus
Gracile
nucleus
Second-order
neuron
Cuneate
nucleus
Medulla
Medial
lemniscus
First-order
neuron
Gracile fasciculus
Cuneate fasciculus
Spinal cord
(a)
13-11
Receptors for body movement, limb positions,
fine touch discrimination, and pressure
Figure 13.5a
Descending Tracts
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• Descending tracts - carry motor
signals down the brainstem and
spinal cord
• Involve two neurons
• Upper motor neuron originates in
cerebral cortex or brainstem and
terminates on a lower motor neuron
• Lower motor neuron in brainstem
or spinal cord
• Axon of lower motor neuron leads
the rest of the way to the muscle
or other target organ
13-12
Motor cortex
(precentral gyrus)
Internal
capsule
Cerebrum
Midbrain
Medulla
Spinal cord
Spinal cord
Cerebral
peduncle
Upper motor
neurons
Medullary
pyramid
Decussation
in medulla
Lateral
corticospinal
tract
Anterior
corticospinal
tract
Decussation in
spinal cord
Lower motor
neurons
To skeletal muscles
Figure 13.6
General Anatomy of Nerves and Ganglia
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Rootlets
Posterior root
Posterior root
ganglion
Anterior
root
Spinal
nerve
• Nerve - a cordlike organ composed of
numerous nerve fibers (axons) bound
together by connective tissue
• Mixed nerves contain both afferent
(sensory) and efferent (motor)
fibers
• Blood vessels penetrate connective
tissue coverings
• Nerves have high metabolic rate
and need plentiful blood supply
Blood
vessels
Fascicle
Epineurium
Perineurium
Unmyelinated nerve fibers
Myelinated nerve fibers
(a)
Endoneurium
Myelin
Figure 13.8a
13-13
General Anatomy of Nerves and Ganglia
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Direction of
signal transmission
Spinal cord
Posterior root
ganglion
Anterior root
Posterior root ganglion
Somatosensory
neurons
Sensory nerve fibers
Sensory
pathway
Figure 13.9
Spinal nerve
Posterior root
Epineurium
Blood vessels
Anterior root
Motor nerve fibers
Motor
pathway
To peripheral
To spinal cord receptors and effectors
• Ganglion—cluster of neurosomas outside the CNS
• Enveloped in an endoneurium continuous with that of the nerve
• Among neurosomas are bundles of nerve fibers leading into and out of the
ganglion
• Posterior root ganglion associated with spinal nerves
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Spinal Nerves
• 31 pairs of spinal nerves (mixed nerves)
• 8 cervical (C1–C8);
• C1 between skull and atlas
• Others exiting at intervertebral foramen
• 12 thoracic (T1–T12)
• 5 lumbar (L1–L5)
• 5 sacral (S1–S5)
• 1 coccygeal (Co)
13-15
Spinal Nerves
• Proximal branches forming spinal nerves
• Each spinal nerve has two points of attachment to the spinal cord
• Posterior (dorsal) root is sensory input to spinal cord
• Posterior (dorsal) root ganglion—contains the somas of sensory neurons carrying signals
to the spinal cord
• Six to eight rootlets that enter posterior horn of cord
• Anterior (ventral) root is motor output out of spinal cord
• Six to eight rootlets that leave spinal cord and converge to form anterior root
• These merge to form spinal nerve proper that enters intervertebral foramen
• Cauda equina: formed from roots arising from L2 to Co
• Distal branches of spinal nerves
• Distal to vertebral foramen, the nerve divides into:
• Anterior ramus: innervates the anterior and lateral skin and muscles of the trunk
• Gives rise to nerves of the limbs
• Posterior ramus: innervates the muscles and joints in that region of the spine and the
skin of the back
• Meningeal branch: reenters the vertebral canal and innervates the meninges, vertebrae,
and spinal ligaments
13-16
Spinal Nerves
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Posterior
Spinous process
of vertebra
Deep muscles of back
Posterior root
Spinal cord
Posterior ramus
Transverse process
of vertebra
Spinal nerve
Posterior root ganglion
Meningeal branch
Anterior ramus
Communicating rami
Anterior root
Sympathetic ganglion
Vertebral body
Anterior
Figure 13.11
13-17
Nerve Plexuses
• Anterior rami branch and anastomose repeatedly to form five
nerve plexuses
• Cervical plexus in the neck, C1 to C5
• Supplies neck and phrenic nerve to the diaphragm
• Brachial plexus near the shoulder, C5 to T1
• Supplies upper limb and some of shoulder and neck
• Median nerve—carpal tunnel syndrome
• Lumbar plexus in the lower back, L1 to L4
• Supplies abdominal wall, anterior thigh, and genitalia
• Sacral plexus in the pelvis, L4, L5, and S1 to S4
• Supplies remainder of lower trunk and lower limb
• Coccygeal plexus, S4, S5, and Co
13-18
The Sacral and Coccygeal Plexuses
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Lumbosacral
trunk
L4
Roots
Anterior divisions
Posterior divisions
L5
S1
S2
Superior gluteal nerve
Inferior gluteal nerve
S3
S4
S5
Co1
Figure
13.18
13-19
Sciatic nerve:
Common fibular nerve
Tibial nerve
Posterior cutaneous
nerve
Pudendal nerve
Reflexes
• Reflexes - quick, involuntary, stereotyped reactions of glands or muscle
to stimulation
• Automatic response to change in environment
• Integration center for spinal reflexes is gray matter of spinal
cord
• Examples
• somatic reflexes result in skeletal muscle contraction
• autonomic (visceral) reflexes involve smooth & cardiac muscle and
glands.
• heart rate, respiration, digestion, urination, etc
• Reflexes can be:
• simple
• involve peripheral nerves and the spinal cord
• spinal reflexes
• learned (acquired)
• involve peripheral nerves and the brain
14-20
Reflex Arc
• Specific nerve impulse pathway -5 components of reflex arc
• Somatic Receptor
• Registers stimuli
• Sensory neuron - afferent nerve fibers
• Carry information from receptors to posterior horn of spinal cord or the
brainstem
• Integrating center
• Part of the CNS that processes the information and generates response
• Motor neuron
• Transmits the response to the effector
• Effector
• Muscle or gland
• 4 important somatic spinal reflexes
• Stretch reflex - when a muscle is stretched, it “fights back” and contracts
• Tendon reflex - contraction of a muscle when its tendon is tapped
• in response to excessive tension on the tendon
14-21
The Patellar Tendon Reflex Arc
14-22
Central Nervous System-The Brain
•
Evolution of the central nervous system shows spinal cord has
changed very little while brain has changed a great deal
• Greatest growth in areas of vision, memory, and motor control of the
prehensile hand
14-23
The Brain
• Longitudinal fissure - cerebral hemispheres.
• gyri = folds; sulci = grooves
• cortex = surface layer of gray matter
• nuclei = deeper masses of gray matter
• tracts = bundles of axons (white matter)
14-24
Median Section of the Brain
• Three major portions of the
brain
• Cerebrum is 83% of brain
volume; cerebral hemispheres, gyri
and sulci, longitudinal fissure,
corpus callosum
• Cerebellum contains 50% of the
neurons; second largest brain
region, located in posterior cranial
fossa
• Brainstem is the portion of the
brain that remains if the cerebrum
and cerebellum are removed;
diencephalon, midbrain, pons, and
medulla oblongata
14-25
Gray and White Matter
• Gray matter = neuron cell bodies, dendrites, and synapses
• forms cortex over cerebrum and cerebellum
• forms nuclei deep within brain
• White matter = bundles of axons
• forms tracts that connect parts of brain
14-26
Meninges of the Brain
14-27
Brain Ventricles
• Ventricles - four internal chambers within the brain
• Two lateral ventricles: one in each cerebral hemisphere
• Interventricular foramen—a tiny pore that connects to third ventricle
• Third ventricle: single narrow medial space beneath corpus callosum
• Cerebral aqueduct runs through midbrain and connects third to fourth ventricle
• Fourth ventricle: small triangular chamber between pons and cerebellum
• Connects to central canal, runs down through spinal cord
• Ependyma - neuroglia that lines the ventricles and covers choroid plexus
• Produces cerebrospinal fluid
• Choroid plexus - spongy mass of blood capillaries on the floor of each ventricle
14-28
Cerebrospinal Fluid
• Fills ventricles and subarachnoid space
• Brain produces and absorbs 500 ml/day
• choroid plexus creates by filtration of blood
• Functions
• floats brain so it is neutrally buoyant
• cushions from hitting inside of skull
• chemical stability -- rinses away wastes
• Escapes (4th ventricle) to surround brain
• Absorbed into venous sinus by arachnoid villi
14-29
Ventricles and Cerebrospinal Fluid
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Arachnoid villus
8
Superior
sagittal
sinus
Arachnoid mater
1 CSF is secreted by
choroid plexus in
each lateral ventricle.
Subarachnoid
space
Dura mater
1
2 CSF flows through
interventricular foramina
into third ventricle.
2
Choroid plexus
Third ventricle
3
3 Choroid plexus in third
ventricle adds more CSF.
7
4
Cerebral
aqueduct
4 CSF flows down cerebral
aqueduct to fourth ventricle.
Lateral aperture
5 Choroid plexus in fourth
ventricle adds more CSF.
Fourth ventricle
6
5
6 CSF flows out two lateral apertures
and one median aperture.
Median aperture
7 CSF fills subarachnoid space and
bathes external surfaces of brain
and spinal cord.
7
8 At arachnoid villi, CSF is reabsorbed
into venous blood of dural
venous sinuses.
Central canal
of spinal cord
Subarachnoid
space of
spinal cord
14-30
Blood Supply and the Brain Barrier System
• Brain is only 2% of the adult body weight, and receives 15% of the
blood - 750 mL/min
• Blood is also a source of antibodies, macrophages, bacterial
toxins, and other harmful agents
• Brain barrier system—strictly regulates what substances can get
from the bloodstream into the tissue fluid of the brain
• Two points of entry must be guarded
• Blood capillaries throughout the brain tissue
• Capillaries of the choroid plexus
14-31
Hindbrain - Medulla Oblongata
• Cardiac center
• adjusts rate and force of heart
• Vasomotor center
• adjusts blood vessel diameter
• Respiratory centers
• control rate and depth of breathing
• Reflex centers
• for coughing, sneezing, gagging, swallowing,
vomiting, salivation, sweating, movements of
tongue and head
• Most of the nerve fibers are crossing over
• Left cortex controls right side of body
14-32
Pons
• Bulge in brainstem, superior to medulla
• Ascending sensory tracts
• Descending motor tracts
• Pathways in and out of cerebellum
• Nuclei
• concerned with posture, sleep, hearing,
balance, taste, eye movements, facial
expression, facial sensation, respiration,
swallowing, and bladder control
14-33
Cerebellum
• Two hemispheres connected by
vermis
• Cortex = surface folds
called folia
• Output comes from deep
gray nuclei
• granule and purkinje cells
• White matter (arbor vitae)
visible in sagittal section
• Evaluation of sensory input
• coordination and locomotor ability
• spatial perception
• Timekeeping center
• predicting movement of objects
14-34
Midbrain - Cross Section
• Tegmentum
• connects to cerebellum and
helps control fine movements
through red nucleus
• Substantia nigra
• sends inhibitory signals to
basal ganglia and thalamus
(degeneration leads to tremors
and Parkinson disease)
• Central gray matter = pain
awareness
14-35
Reticular Activating System
• Scattered nuclei in medulla, pons &
midbrain
• Reticular activating system
• alerts cerebral cortex to sensory signals (sound of
alarm, flash light, smoke or intruder) to awaken
from sleep
• maintains consciousness & helps keep you awake
with stimuli from ears, eyes, skin and muscles
• Motor function is involvement with
maintaining muscle tone
• Somatic motor control
• Cardiovascular control
• Pain modulation
14-36
Diencephalon: Thalamus and Hypothalamus
• Thalamus Functions
• Relays signals from cerebellum to motor
cortex
• Emotional and memory functions
• Hypothalamus Functions
• hormone secretion
• autonomic NS control
• thermoregulation
• food and water intake (hunger and satiety)
• sleep and circadian rhythms
• memory (mammillary bodies)
• emotional behavior
14-37
Cerebrum -- Gross Anatomy
• Cerebral cortex - 3mm layer of gray matter
• extensive folds increase surface area - divided into lobes
14-38
Functions of Cerebrum - Lobes
• Frontal lobe
• Voluntary motor functions
• Motivation, foresight, planning, memory, mood, emotion, social judgment, and
aggression
• Parietal lobe
• Receives and integrates general sensory information, taste, and some visual
processing
• Occipital lobe
• Primary visual center of brain
• Temporal lobe
• Areas for hearing, smell, learning, memory, and some aspects of vision and
emotion
• Insula (hidden by other regions)
• Understanding spoken language, taste and sensory information from visceral
receptors
14-39
Tracts of Cerebral White Matter
• Most of cerebrum is white matter
• Types of tracts
• projection tracts
• from brain to spinal cord, forms internal
capsule
• commissural tracts
• cross to opposite hemisphere
• corpus callosum
• anterior and posterior commissures
• association tracts
• connect lobes and gyri within a hemisphere
14-40
Limbic System
• Loop of cortical structures
• amygdala, hippocampus and cingulate gyrus (arches over the top of the corpus callosum
in the frontal and parietal lobes)
• Role in emotion and memory
• pleasure and aversion centers
14-41
Memory
• Information management
• requires learning, memory and forgetting
• Amnesia
• anterograde amnesia - no new memories
• retrograde amnesia – can’t remember old ones
• Hippocampus
• organizes sensory and cognitive information into a new memory
• Cerebellum
• helps learn motor skills
• Amygdala
• emotional memory
14-42
General Properties of the Autonomic Nervous
System
• Autonomic nervous system (ANS)—a motor nervous system that
controls glands, cardiac muscle, and smooth muscle
• Also called visceral motor system
• Primary organs of the ANS
• Viscera of thoracic and abdominal cavities
• Some structures of the body wall
• Cutaneous blood vessels
• Sweat glands
• Piloerector muscles
• Carries out actions involuntarily: without our conscious intent
or awareness
• Visceral effectors do not depend on the ANS to function; only to adjust their
activity to the body’s changing needs
• Denervation hypersensitivity - exaggerated response of cardiac and smooth
muscle if autonomic nerves are severed
Visceral Reflexes
•
Visceral reflexes - unconscious, automatic, stereotyped
responses to stimulation involving visceral receptors and
effectors and somewhat slower responses
•
Visceral reflex arc
• Receptors: nerve endings that detect stretch, tissue damage, blood
chemicals, body temperature, and other internal stimuli
• Afferent neurons: leading to the CNS
• Interneurons: in the CNS
• Efferent neurons: carry motor signals away from the CNS
• Effectors: that make adjustments
•
ANS modifies effector activity
15-44
Visceral Reflexes
• Example of homeostatic
negative feedback loop
• High blood pressure
detected by arterial stretch
receptors (1), afferent
neuron (2) carries signal to
CNS, efferent (3) signals
travel to the heart, then (4)
heart slows reducing blood
pressure
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2 Glossopharyngeal
nerve transmits signals
to medulla oblongata
1
3 Vagus nerve
transmits
inhibitory
signals
to cardiac
pacemaker
Baroreceptors
sense increased
blood pressure
Common carotid
artery
Terminal
ganglion
4 Heart rate
decreases
Figure 15.1
15-45
Autonomic Nervous System
• Visceral motor neurons control
•
•
•
•
•
heart rate
breathing rate
digestion
blood pressure
salivation
• Nerve impulses of these motor neurons start in the CNS
(medulla oblongata and pons)
• Pathway through:
• Spinal cord
• Cranial nerves
14-46
Divisions of the ANS
•
Two divisions innervate same target organ
• May have cooperative or contrasting effect
• Prepares body for physical activity: exercise, trauma, arousal,
competition, anger, or fear
• Increases heart rate, BP, airflow, blood glucose levels, etc.
• Reduces blood flow to the skin and digestive tract
15-47
Sympathetic Division
• The sympathetic division is called the “fight or flight”
system
• when the body needs to generate energy
• exercise, excitement, emergency, and embarrassment
• Fight or flight response
• increases heart rate, blood pressure, respiration rate, blood flow
to skeletal muscles, glucose metabolism
• decreases the activities that are not essential at the moment
(digestive system organs are subdued- decreased blood flow to
that system
14-48
Parasympathetic Division
• The parasympathetic division is called the “rest and
digest”
• activated when the body needs to conserve energy
• digestion, defecation, and diuresis (urination)
• Promotes necessary changes during these activities
• decreases heart rate, blood pressure, respiration rate, blood flow
to skeletal muscles, glucose metabolism
• increases the activity of and blood flow to the digestive system
organs
14-49
Autonomic Output Pathways
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Somatic efferent innervation
ACh
Myelinated
fiber
Somatic effectors
(skeletal muscles)
Autonomic efferent innervation
ACh
Myelinated
preganglionic fiber
ACh or NE
Unmyelinated
postganglionic fiber
Visceral effectors
(cardiac muscle,
smooth muscle,
glands)
Autonomic
ganglion
ANS—two neurons from CNS to effectors
• Presynaptic neuron cell body is in CNS
• Postsynaptic neuron cell body is in peripheral ganglion
Figure 15.2
Efferent Sympathetic vs. Parasympathetic
NE – Norepinephrine – Adrenergic
fibers – adrenergic receptors
Ach – Acetylcholine – Cholinergic
fibers-cholinergic reseptors
14-51
Organization of the Sympathetic and
Parasympathetic Division
14-52
Effects of Neurotransmitters of the Autonomic
Nervous System
• The cells of each organ controlled by the ANS have both ACh and NE
receptors
• organs are dually controlled
• The response of the organ is determined by the identity of the
neurotransmitter released
• the binding of ACh to its receptor will cause the effector to respond
in one way
• the binding of NE to its receptor will cause the effector to respond in
the opposite way
• The effect of ACh and NE is effector specific
• NE increases heart rate, ACh decreases heart rate
• NE decreases the secretion of saliva, ACh increases the secretion of
saliva
14-53