Nervous System – General Nervous System Structure
cells, tissues and organs of body are all working for
every cell in our body responds to stimuli by changing
organisms survival
its metabolism in one way or another
need to integrate all body activities for homeostasis
but cells of the nervous system are highly specialized
for receiving stimuli and conducting impulses to
need good communication and control:
various parts of the body
Nervous System
Neuroendocrine in humans, these nerve cells have become organized
Endocrine System System into the most complex and least understood of the
body’s systems
receptor ! integration ! effector
CNS: brain PNS: cranial nerves
General Functions of the Nervous System spinal cord spinal nerves
1. receive and process sensory information from two major cell/tissue types in Nervous System:
internal and external environment
neurons – impulse conduction
2. maintain homeostasis by transmitting the
appropriate responses through muscles and glands communicates by:
electrochemical impulses (=nerve impulses)
3. Integrate rapid reflex responses with slower cell-to-cell chemicals (=neurotransmitters)
hormonal responses
~1 Trillion neurons
generally no mitosis
4. generate complex neural pathways of all higher
brain functions: neuroglia (=glial cells) – support, protection,
self awareness insulation, aid in function of neurons
thinking, learning
speech, communication
emotions [need specialized cells because of unique sensitivity of
neurons to their environment]
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10-50 times more neuroglia than neurons
some mitosis
two types; axons and dendrites
Neurons Dendrites
highly specialized to: shorter; branching
respond to stimuli receptor regions
conduct messages in the form of nerve impulses
! each neuron receives info from dozens to 10’s of
1000’s of other neurons
generally don’t divide after birth
specialized for information collection
!live up to 100 years (eg. dendritic spines)
very high metabolic rate: thinly insulated
require glucose, can’t use alternate fuels convey messages toward cell body
= graded potentials (not nerve impulses)
require lots of O2 – only aerobic metabolism
can’t survive more than a few minutes without large surface area for reception of signals
O2 from other neurons
all neurons have cell body and 1 or more processes contain all organelles (except nucleus) as in cell body
cell body: Axons
contains: most cytoplasm each neuron has a single axon
nucleus
most organelles
no centrioles (don’t divide) long, slender process
neurofibrils
up to 3-4 feet long (eg. motor neuron of toe)
processes:
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axons may give off a few collateral branches sensory neurons in CNS are multipolar
(=axon collaterals)
b. interneurons (association)
sometimes with branching axon collaterals
in CNS
where integration occurs
thick insulation 99% of neurons in body
lots of variation in structure
at terminus, axon branches profusely
(up to 10,000 branches) c. motor neurons (efferent)
outside CNS
each branch ends in enlarged bulb
multipolar
= synaptic knob (=axonal terminal) all cell bodies of somatic and some autonomic are inside CNS
has all organelles except rough ER 2. Structure
! gets proteins via microtubules and
microfilaments
a. unipolar (=pseudounipolar)
difference between nerve and neuron:
single short process that splits into two longer
processes that together act as an axon
neuron = individual nerve cell
originate as bipolar neurons
nerve = bundle of axons outside CNS surrounded functionally one long fiber carries impulse that bypasses cell
by layers of connective tissue body
the dendrites are considered the receptive branches at the
neurons can be classified by: beginning of the single process
1. function especially in ganglia of PNS
2. structure (# of processes)
most are sensory neurons
1. Function b. bipolar
a. sensory neurons (afferent) 2 processes; 1 axon, 1 dendrite
outside CNS, almost all are unipolar, a few are bipolar only in some sense organs
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5. Schwann cells
eg. olfactory cells in nose, some retinal neurons, sensory
neurons of inner ear
1. Astrocytes
c. multipolar
have numerous branches producing a starlike
!3 processes; 1 axon, many dendrites shape
most common
largest and most abundant type
most neurons in CNS (interneurons)
! comprise >90% of the tissue in some parts of the brain
also some sensory and some motor neurons in PNS
astrocytes cover the entire brain surface and most
d. anaxonic neurons of the nonsynaptic regions of the neurons in
the gray matter of CNS
have multiple dendrites but no axons
do not produce action potentials also most functionally diverse type
found in brain and retina form supportive framework for nervous tissue
in very general terms, shape is related to direct the formation of tight webs of cells around
function: brain’s capillaries
many unipolar
sensory neurons =blood/brain barrier
& bipolar
interneurons
mostly multipolar
because of “irritability” of nervous tissue and
motor neurons sensitivity to 02, glucose etc neurons are
isolated into their own “fluid compartment”
Neuroglia
1. astrocytes this blockage of free exchange between
2. microglia capillaries and tissues is unique for nervous
3. ependymal cells tissue
4. oligodendrocytes
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! prevents sudden and extreme fluctuations in ciliated cells ! resemble cuboidal epithelium
composition of tissue fluid in CNS
line ventricles and spinal canal
! protects irreplaceable neurons from damage
help to produce and circulate CerebroSpinal Fluid
capillaries in brain are much less leaky than
normal capillaries 4. Oligodendrocytes (oligodendroglia) (CNS)
! tight junctions: materials must pass smaller cells, fewer (up to 15) processes
through cells
clustered around nerve cell bodies
astrocytes form an additional layer around
these capillaries to further restrict each process reaches out to nerve fiber and wraps
exchange around it to produce myelin sheath (electrical
insulation) around neurons in CNS
[myelin=fatty substance]
! astrocytes help regulate flow into CSF
myelin (in CNS and PNS) can be:
small molecules (O2 , CO2 , alcohol) diffuse rapidly
larger molecules penetrate slowly or not at all thick = “myelinated fibers”, “white matter”
thin = “unmyelinated fibers”, “gray matter”
2. Microglia (CNS)
Multiple Sclerosis
small macrophage cells autoimmune disease possibly triggered by a virus in genetically
susceptible individuals
oligodendrocytes and myelin sheaths of CNS deteriorate and are
in inflamed or degenerating brain tissue they replaced by hardened scar tissue
occur esp between 20-40 yrs of age
nerve fibers are severed
carry out phagocytosis of microbes and cellular & myelin sheaths in CNS are gradually destroyed
debris ! short circuits; loss of impulse conduction
affects mostly young adults
common symptoms:visual problems, muscle weakness, clumsiness
3. Ependymal Cells (CNS) eventual paralysis
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5. Schwann Cells (PNS) Synapses
found only in PNS meeting point between a neuron and any other cells
= synapse
form a segmental wrapping around nerve fibers
each segment is produced by 1 Schwann cell neurons generally are not directly connected to
gaps between cells = Nodes of Ranvier each other but are separated by a small gap
form neurilemma and myelin sheath in PNS synapses are the functional connection between
neurons neurons and a few other cells (eg. muscles, glands)
outermost coil of Schwann cell with most of CNS:
neuron ! neuron
cytoplasm & organelles forms neurilemma
PNS:
! only in PNS neurons sensory cell!neuron
neuron ! neuron
! plays essential role in regeneration of neuron ! muscle fiber [=neuromuscular jct]
neuron ! gland [=neuroglandular jct]
cut or injured neurons
neuron ! epithelial cells
[CNS neurons don’t regenerate]
each neuron synapses with 1000 – 10,000 axonal
terminals
! ~1 quadrillion synapses in human brain
at birth brain has ~50 trillion synapses
! 1 month later has 1 quadrillion synapses
at synapse the electrical signal is converted to a
chemical signal that must diffuse across the
synapse to have an effect on the next neuron
synapses make neural integration possible
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! each synapse is a “decision making” device that determines
whether the second cell will respond to the signal from the
first
General Function of a Synapse:
1. nerve impulse reaches end of axon at synapse and
triggers release of chemical (=neurotransmitter)
! exocytosis
2. NT diffuses across synapse and binds to receptor
proteins in cell membrane of target cell
3. triggers response in target cell
whole process takes 0.3 – 5.0 ms
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