Transcript Nervous System Introduction

Bio 321 Neuroanatomy
Dr. M. Yu
Nervous System Introduction
Bio 321 Neuroanatomy
Medical Significance
• The brain and nervous system control all other
functions of the body.
• The extreme importance of the nervous system in
medicine is based on the serious nature of the
many disorders affecting its structures (more than
1000 disorders).
• Causes more hospitalization than any other
diseases, including heart diseases and cancers.
• Neurological diseases affect 50 million Americans
and costs us about $400 billions annually
Introduction
• In this country alone, the numbers are
overwhelming:
– 1. Cerebrovascular Disease - is the 3rd ranking cause of
death - vascular conditions of brain & spinal cord
annually kill ~500,000
– 2. Epileptics seizures ~ 1,500,000
– 3. Movement disorders affect another one million
people
– 4. There are ~ 2 million totally blind individuals; &
over 13 million with visual impairments
– 5. There are ~ 17 million totally or partially deaf
persons
Introduction
– 6. Over 3 million people are afflicted with Alzheimer’s
disease
– 7. At least 700,000 have cerebral palsy
– 8. More than 250,000 have multiple sclerosis
– 9. In addition, there are over 500,000 accidental head
and spine injuries annually; fortunately only a minority
of which actually injure the brain or spinal cord
– 10. Acute head injury is the leading cause of death or
disability between ages 2 & 40 (as of 1995)
Cellular Components of the Nervous System
• Neurons - the primary functional cells in the
nervous system (- approx. 100 Billion in
CNS)
– 1. responsible for initiating & conducting
electrical signals by which nervous system
communicates
– 2. size & shape varies greatly between regions of
the nervous system & with respective functions
– 3. mature neurons do NOT divide or replicate,
do NOT regenerate following injury
Anatomic features (common to all neurons)
• 1. Soma - cell body
– metabolic center of cell
– typical cellular organelles: large clear nucleus,
nucleolus, mitochondria, ER, Golgi Apparatus,
lysosomes
• 2. endoplasmic reticulum - a characteristic
feature of neurons: - high concentration of
RER (granules, form Nissl substance, stain
basic)
Neurons
• 3. Cytoplasmic fibrils - another distinguishing
feature of neurons, make up cytoskeleton
– a. Neurofilaments: intermediate cytoskeletal filaments abundant, found throughout soma & along processes
(10 nm in diameter) – a principle support system
component
– b. Neurotubules (Microtubules): especially abundant in
dendrites - found in cytoplasmic, axonal transport,
movement of organelles throughout cell - (20-30 nm in
diameter) = microtubules, oriented lengthwise
– c. Microfilaments: thinnest, associated with external
membrane & dendritic spines - anchor membrane
constituents, hold Receptors in place - (5 nm in
diameter) = microfilaments in other cells Processes of
neurons
Dendrites
• Dendrites - number, length, degree of
ramification varies between neuronal types
– a. usually many per neuron, typically branch
extensively
– b. carry incoming signals toward soma
– c. neuron signal receptors (the dendritic field of
a cell = its receptive field)
Axon
• Axon - one per neuron
– a. conducting process - carries info away from
soma, toward other neurons or effectors
– b. length varies greatly, can be very long sciatic axons > 1 meter (spine -> foot)
– c. no ribosomes, no protein synthesis, depend
on axonal cytoplasmic transport from soma
– d. collaterals = branches of axon, degree of
branching varies greatly
Axon
• Axon Hillock = Initial segment (of axon)
– a. base of axon as it leaves soma - looks pale (no Nissl
staining)
– b. specialized segment of membrane for action potential
initiation due to lower threshold than rest of cell
– c. also has an extremely high concentration of voltagegated ion channels
– d. allows action potential conduction to travel in ONE
direction
Terms
• 1. Nucleus - a group of neuronal cell bodies within
the central nervous system, eg lateral geniculate
nucleus
• 2. Ganglion (ganglia, pl) - group of cell bodies in
peripheral nervous system
• 3. Nerve - collection of axons in the PNS (tract,
fasciculus = a bundle of nerve fibers)
• 4. White matter - axons, myelinated
• 5. Gray matter - concentrations of cell bodies and
unmyelinated dendrites
Classification of neurons
• A. based on processes:
– 1. Unipolar neurons a.
• a single primary process extends from soma, which can branch
into dendrites & axon
• b. the principle neuronal cell type of invertebrates
• c. Pseudounipolar neurons, e.g. dorsal root ganglion cells,
appear unipolar, in embryo are actually bipolar; axon &
dendrite extensions fuse
– 2. Bipolar neurons - e.g. bipolar cells in retina
• - axon & dendrite both extend from soma, from opposite ends
– 3. Multipolar neurons
• a. multiple dendritic branches from all parts of soma, & one
axon
• b. many variations due to length, number of dendrites, length
of axon
• c. e.g. pyramidal neuron, spinal motor neuron, purkinje neuron
Classification of neurons
• B. Based on functions
– Sensory neurons: carry sensory information
from
– Motor neurons: carry motor information to
muscle sand glands
– Interneurons: in between sensory and motor
– Projection neuron / relay neuron: has long
fibers and project information from on region to
another
Classification of neurons
• C. Based on orientation
– 1. Afferent - refers to the neuron or process
extending toward the cell in question
– 2. Efferent - refers to the neuron or process
extending away from the cell in question
Neuroglial (Glia)
• supporting cells - Do not conduct action
potential, body has 10-15 times more glia
cells than neurons (about one trillion)
• Derived from Neuroectodermal and
mesodermal origin
• Neuroectodermal origin: #1-4
– 1. Schwann cells
• - form myelin sheath which insulates an axon in
peripheral nerves
• - cell winds around axon, inside its own layers,
piling up layers of lipid/protein cell membranes
• - one Schwann cell associates with and myelinates a
segment of only one axon
• - Schwann cell, myelin, axon are all surrounded by a
basement membrane (covers whole unit)
• - help to buffer excess extracellular K+ (prevent
rampant depolarization)
• - myelin sheath insulation greatly speeds conduction
• 2. Oligodendroglia
– same function as Schwann cells, but for axons
of neurons in central nervous system
– have small, round, dense nuclei
– unlike Schwann cells, can myelinate a segment
of several axons
– no basement membrane surrounds the axon like
Schwann cells do in PNS
– these features affect ability of CNS cells to
regenerate following injury (vs. PNS)
• 3. Astrocytes
– named from ‘star’ appearance of processes
radiating out from soma
– oval nuclei (larger & much less dense than
oligodendroglia)
Function of Astrocytes
– i. nutritive: form significant portion of brain blood
barrier (BBB), surrounding CNS vasculature
• thought to be important communicators between neurons and
capillaries
• transport nutrients, ions, molecules from capillary to neurons
– ii. healing: act as scavengers, remove neuronal debris,
seal off area for healing
– iii. buffers excess extracellular K+ , preventing excess
depolarization
– iv. remove chemical neurotransmitters from synaptic
clefts - high affinity for specific neurotransmitters
– v. support: provide structure, stability, act similarly to
connective tissues
– vi. in development: act as guides for outgrowth &
migration in the developing NS
Types of astrocytes
• Distinguished by localization, not on any
functional difference
– i. fibrous astrocytes - name for those in, around
white matter (areas rich in axons)
– ii. protoplasmic astrocytes - those in gray
matter, near synapses, dendrites
• 4. Ependymal cells
– layer of ciliated columnar epithelial cells with
tight junctions which line cavities of the neural
tube (cerebral ventricles, spinal cord central
canal)
• a. this layer forms a selective barrier
between nervous tissue & ventricular fluid
• b. also forms choroid plexus - produces
cerebral spinal fluid by filtering its
components from blood
Glial cells of mesodermal origin
• Microglia
• - cells from mesoderm which migrate into
CNS
• - normally in small numbers, proliferate as
needed
• - become macrophages in response to injury
or damage,
• - act as scavengers, have a phagocytic role
in removing debris, damaged cells
Synapse
• A. Chemical synapse
– Presynaptic membrane, postsynaptic membrane, and
synaptic cleft
– can also be classified as: axodendritic, axosomatic,
axoaxonal, and dendrodendritic
– neurotransmitter (adrenaline, acetylcholine, dopamine,
serotonin, glutamic acid…)
– neuro-modulators (excitable or inhibitive
neurotransmitters)
• B. Electrical synapse
– neuron close together, rare in mammalian nervous
system
Meninges
• There is very little connective tissue in brain
tissue, except in linings of vasculature
– A. CNS = meninges: Connective tissue
membranes surrounding the NS
• 1. pia mater - immediately next to the nervous
tissue, thin & delicate
• 2. arachnoid - middle layer
• 3. dura mater - outermost meninges, thicker & very
tough
PNS:
• 1. endoneurium: thin collagenous layer,
immediately surrounds a myelinated nerve fiber
• 2. perineurium: Connective tissue covering
surrounding a fascicle of nerve fibers
• 3. epineurium: thick connective tissue layer
surrounding many fascicles which make up a
nerve trunk
• * the 3 CNS meningial layers are continuous with
the connective tissue layers around PNS nerves
covering around all the
fascicles is the
epineurium
covering around fascicle
is the perineurium
fascicle
Axonal Transport
• Fast Axonal Transport - involves
microtubules as a track -some organelles
move along in stepwise fashion - especially
synaptic vesicles, precursors, etc - requires
oxidative metabolism, energy utilization
• 1. Anterograde: = orthograde, from cell body to
terminal
– a. mechanism by which synaptic vesicles move from
soma to terminal along microtubules
– b. speed = 410 mm/day
• 2. Retrograde: from terminal to soma
– a. mechanism for returning worn out materials to cell
body for degradation or recycling
– b. returns to cell body information from, about the axon
- communicates with neighbors
– c. nerve growth factor (NGF) travels from source to
soma, stimulates neuronal growth
– d. toxin transport, e.g. neurotropic viruses get to
CNS: herpes simplex, rabies, polio
– e. speed = approx 1/2 - 2/3 that of anterograde transport
Axonal Transport
• Slow Axoplasmic Flow
– 1. predominant means of organelle movement
within the neuron
– 2. movement is through cytoplasm of axon, not
along microtubules
– 3. speed = approx. 10 mm/day
Main regions of the central
nervous system
• Cerebrum
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Frontal, parietal, occipital, temporal and insula
Interpretation
initiating voluntary movements
storing memory
retrieving memory
Reasoning
center for intelligence and personality
• Cerebellum
– Large in human, receiving information from sensory
systems and the cerebral cortex, main function to
maintain balance, posture, mostly not initiated from the
cerebral cortex.
Brain stem
• Medullla, pons and midbrain are generally
referred as brain stem
• Medulla Oblongata
– Continuation from spinal cord into the brain, part of the
brain stem.
• Pons
– Dorsal part consists of sensory and motor tracts
– Ventral part contains connection between two
hemispheres, contribute to motor efficiency
• Midbrain
– Involved with visual and auditory system, red nucleus
and substantia nigra are also located here (more later)
• Diencephalon:
– Forms the central core of the cerebrum, including
thalamus, hypothalamus, epithalamus, and
subthalamus, controls automatic nervous system,
endocrine function via hormones and nervous impulses.
• Telencephalon
– Includes cerebral cortex, corpus striatum, and
medullary center. Areas of cerebral cortex (paleocortex)
receive primitive function from olfactory system, which
is common to lower vertebrates. Other areas are called
archicortex, includes limbic system (emotions, and
some memories, early vertebrates). Most areas of the
cerebral cortex (90%) in human are referred as
neocortex, which controls all sensations (except smell),
involves emotions, memories and intellectual activities.