Download The Nervous System

Reflex Arcs
• Much of what the body must do every day is programmed as
reflexes.
• Reflexes are rapid, predictable, involuntary responses to
stimuli.
• Reflexes are categorized as autonomic or somatic depending
on what they regulate
The Components of a Reflex Arc
• All reflex arcs have a minimum of five elements:
– A sensory receptor: reacts to a stimulus
– An effector organ: muscle or gland being stimulated
– Afferent & efferent neurons to connect the two
– The CNS integration center
• Most reflexes are processed directly in the spinal
cord, but some, such as rapid response of pupils to
light involve the brain
Formation of the Neural Tube
• In a 3 week embryo, the ectoderm
thickens along the dorsal midline
axis to form the neural plate
• The neural plate folds in to form a
groove flanked by neural folds
• As the neural groove deepens,
superior ends of the neural folds
fuse to for the neural tube.
• The tube detaches from surface ectoderm and sinks.
• The brain will develop from this tube at the anterior end and
the spinal cord from the caudal end.
• Small groups of neural fold cells migrate laterally between the
surface ectoderm and neural tube to form the neural crest
which will give rise to neurons destined to ganglia.
The Brain: Embryonic Development
• Encephalos means “brain”
• The brain develops from neural tube
• By week four, the brain subdivides into
– Forebrain – prosencephalon
– Midbrain – mesencephalon
– Hindbrain – rhombencephalon
• These further divide, each with a fluid filled region:
ventricle, aqueduct or canal
– Spinal cord also has a canal
• Two major bends, or flexures, occur (midbrain and
cervical)
Brain Development
Effect of Space Restriction on Development
• Cerebral hemispheres are forced, to grow posteriorly over rest of
brain, enveloping it, and the hemispheres grow into horseshoe
shape (b and c)
• Continued growth causes creases, folds and wrinkles
Anatomical Classification
• Cerebrum
– Cerebral hemispheres
– (cortex, white matter, basal ganglia)
• Diencephalon
– Thalamus
– Hypothalamus
– Epithalamus
• Brain stem
– Midbrain
– Pons
– Medulla
• Cerebellum
• Spinal cord
Adult Brain Regions
(Medical Scheme)
Basic Pattern of the CNS
• Central cavity surrounded by
a gray matter core, which is
then surrounded by white
matter (myelinated fiber
tracts)
• The brain has additional
regions of gray matter,
consisting of neuron cell
bodies, in an outer sheet,
called a cortex, around both
cerebral hemispheres and the
cerebellum
• The cortex disappears with
descent to the brain stem,
but scattered gray matter
nuclei are seen within the
white matter.
Cerebral Hemispheres
• Most superior part of the brain
• Many brain-stem structures are obscured by the
hemispheres and can only be seen if a sagittal section
is made
• Surface exhibits elevated ridges called gyri (gyrus)
separated by shallow grooves called sulci (sulcus)
• Deeper grooves called fissures separate the large
regions of the brain
– The longitudinal fissure separates the right and left
hemisphere
– Other fissures divide the brain into lobes names for the
cranial bones that lie over them
Lobes of the Cerebrum
Functional Areas of the Cerebral Cortex
Diencephalon (Interbrain)
• Sits on top of the brain stem;
enclosed by cerebral hemispheres
• Thalamus: relay station for
sensory impulses passing upward
to the sensory cortex
• Hypothalamus: important
autonomic nervous centers;
regulates body temperature, water
balance and metabolism and plays
important part of limbic system
(“emotional-visceral” brain)
including thirst, appetite, sex,
pain & pleasure centers
• Epithalamus: includes the pineal
body & choroid plexus
Brain Stem
• About the size of a thumb in diameter and approximately 3 inches long
• Contains many small gray matter areas that form cranial nerves and
control vital activities such as breathing & B.P.
• Midbrain: rounded protrusions called corpora quadrigemina are reflex
centers involved in vision & hearing
• Pons: mostly fiber tracts; also involved in breathing control
• Medulla oblongata: merges into spinal cord; regulated visceral activities
– HR, BP, breathing, swallowing, vomiting
The Cerebellum
• Large & cauliflower-like; projects dorsally
from under the occipital lobe
• Two hemispheres and a convoluted surface
• Outer cortex of gray matter and inner region
of white matter
• Provides precise timing for skeletal muscle
activity and controls balance and equilibrium
Protection of the CNS
• Irreplaceable neurons can be injured by even
the slightest pressure
• Brain & spinal cord are protected by bone
(skull & vertebrae), membranes (meninges) &
a watery cushion (cerebrospinal fluid)
• The blood-brain barrier provides protection
from harmful substances in the blood.
Meninges
• Three connective tissue membranes that
cover and protect CNS structures
• Outermost leathery dura mater is double
layered where it surrounds the brain
– Inner periosteal layer forms periosteum in skull
– Outer meningeal layer forms outermost covering
& continues as spinal dura mater
• Web-like middle arachnoid mater, threadlike
extensions span subarachnoid space
• Innermost pia mater clings tightly to the
surface of brain and spinal cord
Cranial Meninges
Cerebrospinal Fluid
• Similar make-up to the
plasma in which it forms
• Continually formed from
blood in the choroid
plexuses
• Forms a watery cushion
protecting nerves from
physical trauma
• Inside the brain CSF is
constantly circulating
Blood Brain Barrier
• Most body tissues can withstand small fluctuations in
hormone, ion & nutrient concentration but in the brain
this may result in uncontrolled neural activity
• The blood brain barrier is composed of the least
permeable capillaries in the whole body, only water,
glucose and essential amino acids easily pass through
• Metabolic wastes (urea), toxins, proteins & most drugs
are prevented from entering the brain tissue
• Non-essential amino acids & potassium ions are
actively pumped from brain into blood across capillary
walls
• The barrier however, is virtually useless against fats &
fat soluble molecules and respiratory gases
The Spinal Cord
• Approximately 17 inches long; an
extension of the brain stem
• Extends from foramen magnum of the
skull to 1st/2nd lumbar vertebrae,
ending just below the ribs
• Two-way conduction pathway to and
from the brain & a major reflex center
• Cushioned and protected by meninges;
extend beyond spinal cord in vertebral
canal
• 31 pairs of spinal nerves arise from
cord and exit vertebral column; cauda
equina refers to the collection of spinal
nerves at end of vertebral canal
Gray Matter of Spinal Cord & Spinal Roots
• In a cross-section, gray matter looks similar to a butterfly
& surrounds central canal which contains the CNS
• Neurons with specific functions are found here:
– Sensory neuron fibers enter via dorsal root and are found in
dorsal root ganglion
– Motor neuron axons exit via the ventral root
– Dorsal and ventral roots fuse to form the spinal nerves
White Matter of the Spinal Cord
• Composed of myelinated fiber tracts:
running to higher centers, traveling
from brain to cord & conducting
impulses from one side to another
• Each of the columns contains fiber
tracts made up of axons with same
destination and function
– Sensory or afferent: conduct impulses
to the brain
– Motor or efferent: carry impulses from
brain to skeletal muscles
• The posterior column contains only
ascending tracts; the lateral and
descending column contains both
ascending and descending tracts
Structure of a Nerve
• A nerve is a bundle of
neuron fibers found outside
the CNS
• Within a nerve, neuron
fibers, or processes, are
wrapped in protective
connective tissue coverings
• Like neurons, nerves are
classified based on the
direction they carry
impulses as afferent,
efferent or mixed.
Cranial Nerves
• 12 pairs; primarily serve head & neck; only 1 pair, vagus
nerves, extends to thoracic and abdominal cavity
• Most are mixed, but optic, ofactory & vestibulocochlear
are purely sensory
• “Oh, oh, oh, to touch and feel very good velvet, ah”
Olfactory
Optic
Oculomotor
Trochlear
Trigeminal
Abducens
Facial
Vestibulocochlear
Glossopharyngeal
Vagus
Accessory
Hypoglossal
Spinal Nerves & Nerve Plexuses
• 31 pairs of spinal nerves; formed by combination of dorsal
and ventral roots of the spinal cord
• Named for region of the cord from which they arise
• Each only app. ½ inch long, b/c quickly divides into dorsal &
ventral rami
– Smaller dorsal rami serve skin & muscles of posterior trunk
– Ventral rami of T1-T12 form intercostal nerves, serving
muscles between ribs & skin and of the anterior & lateral trunk
– All other ventral rami form networks called plexuses which
serve the limbs
•
•
•
•
Cervical
Brachial
Lumbar
Sacral
The Autonomic Nervous System
• Motor sub-division of the PNS that controls the bodies
activities automatically
– Special groups of neurons that regulate cardiac & smooth
muscle
– Largest contributor to the maintenance of homeostasis
• Patterns of efferent pathways differs from that of
somatic nervous system
– Somatic: cell bodies of motor neurons are inside CNS & axons
(spinal nerves) extend to the skeletal muscle served
– Autonomic: chain of two motor neurons
• The first is in the brain/spinal cord and its axon (pre-ganglionic
axon) leaves to synapse with second motor neuron
• Post-ganglionic axon of second neuron extends to the organ
served
Autonomic v. Somatic Nervous Systems
Sympathetic v. Parasympathetic
• The ANS is sub-divided into the sympathetic and
parasympathetic nervous systems
• Both divisions serve the same organs, but have
opposite effects, counterbalancing each others
activities to maintain homeostasis.
• Sympathetic: mobilizes the body during extreme
situations (fear, rage, exercise)
• Parasympathetic: allows us to “unwind” and conserve
energy
Anatomy of the ANS
Anatomy of the Sympathetic NS
• A.K.A. the “thoracolumbar division” b/c its first neurons are in
the gray matter of the spinal cord from T1 to L2
• Preganglionic axons leave the cord in ventral root, enter spinal
nerve & pass through ramus communicans to enter a
sympathetic chain ganglion.
• Sympathetic trunk lies alongside the vertebral column on each
side.
• Axon may synapse with second neuron at same or different
level & postganglionic axon reenters the spinal nerve to travel
to the skin or may pass through ganglion without synapsing to
form part of the splanchnic nerves, which travel to the viscera
to synapse with the second neuron in the collateral ganglion
(anterior to the vertebral column)
• The postganglionic axon leaves the collateral ganglion &
travels to nearby visceral organ.
Anatomy of the Parasympathetic Division
• The first neruons are located in brain nuclei of several
cranial nerves and in S2-S4 level of the spinal cord
• Neurons of the cranial regions send their axons out in
cranial nerves to serve the head & neck organs
– Synapse with second motor neuron in terminal ganglion
– From the terminal ganglion, the postganglionic axon extends a
short distance to the organ it serves
• In the sacral region, the preganglionic axons leave the
spinal cord and form the pelvic (pelvic splanchnic)
nerves which travel to the pelvic cavity
– preganglionc axons synapse with the second motor neurons in
the terminal ganglia on, or close to the organs they serve.
Autonomic Functioning
• Body structures served by the autonomic nervous
system generally receive fibers from both divisions
• Cause antagonistic effects because the postganglionic
axons release different neurotransmitters
– Parasympathetic (cholinergic): release acetylcholine
– Sympathetic (adrenergic): release norepinephrine
– Preganglionic axons of both release acetylcholine
• Exceptions are most blood vessels, most structures of
the skin, some glands, & the adrenal medulla which
only receive sympathetic fibers
When the ANS is in Control
• Often referred to as the “fight or flight” system; activity is evident
when we are excited, or in an emergency/life-threatening
situation
• Pounding heart; rapid, deep breathing; cold, sweaty skin; prickly
scalp; dilated eye pupils are all symptoms of sympathetic nervous
activity
• Additional, less obvious symptoms include increased blood
pressure & blood glucose levels, dilated bronchioles of the lungs,
dilation of blood vessels & withdrawal of blood from digestive
organs (to serve heart, brain & skeletal muscles)
• The ANS is at also work when you are physically stressed - after
surgery or running a marathon, adrenal glands will be pumping
out epinephrine & norepinephrine
• Hormone effects of the ANS result in effects that linger beyond the
nerve impulses
When the PNS is in Control
• Most active when the body is at rest and not threatened in any
way; sometimes called the “resting & digesting” system
• Chiefly concerned with promoting normal digestion & elimination
of feces & urine and with conserving body energy (by decreasing
cardiovascular demands)
• EX) It is a good idea to rest after a heavy meal to avoid inhibiting
or disturbing digestion by sympathetic activity
– At rest BP and HR are regulated at normal low levels & the digestive tract is
actively digesting food
– The skin is warm indicating there is no need to divert blood to skeletal
muscles or vital organs
– Pupils are constricted to protect retinas from excessive damaging light &
lenses of the eye are “set” for close vision
PNS & D/ SNS & E
• Associate the PNS with “D”
– Digesting
– Defecation
– Diuresis (urination)
• Associate the SNS with “E”
– Exercise
– Excitement
– Emergency
– Embarrrasment