Nervous
System
Classification of Neurons and Supporting
Cells
Part 4
Updated Schedule
• Today – Neurons and Neuroglial Cells
• Monday– Reflexes, Neurological Diseases
▫ Lab activity
• Wednesday – Study Guide
• Friday – Exam
Nervous
Tissue
Neurons
(nerve cells): transmit
messages/stimuli
Anatomy of a Neuron:
▫
▫
▫
▫
▫
Cell body – contains nucleus; metabolic
center
Dendrite – fiber that conveys messages
toward cell body
Axon – conduct nerve impulses away from
the cell body
Axon terminals – end of axon; contain
neurotransmitters & release them
Synaptic cleft/synapse – gap between
neurons
Nervous Tissue
Neuroglial cells - supporting cells
• CNS neuroglial cells:
 Astrocytes: perform a variety of tasks, from
axon guidance, synaptic support, to the control of
blood flow (nutrients)
 Most abundant
 Ependymal: forms the epithelial lining of the
ventricles and the central canal of the spinal
cord; helps in the formation of CSF
 Oligodendrocytes: provide support and
insulation to axons in the CNS; create the myelin
sheath
 Microglial: macrophages of the brain and spinal
cord; act as the first/main form of immune
defense in CNS; 10-15% of cells in CNS
• PNS: Schwann cells, satellite cells


surround large neurons
protect & cushion
•
Myelin: whitish, fatty material that covers nerve
fibers to speed up nerve impulses
▫ 80% lipid, 20% protein
•
•
•
Schwann cells: surround axons and form myelin
sheath
Myelin sheath: tight coil of wrapped membranes
Nodes of Ranvier: gaps between Schwann cells
Structural Classification: # processes extending from cell
body
Multipolar
Bipolar
Unipolar
1 axon, several
dendrites
1 axon, 1 dendrite
1 process
Rare
Short with 2
branches (sensory,
CNS)
Most common (99%)
Ex. Motor neurons,
Ex. retina, nose, ear
interneurons
Ex. PNS ganglia
Functional Classification: direction nerve impulse is
traveling
Sensory
neurons
Motor
neurons
Interneurons
carry impulses
from sensory
receptors to CNS
carry impulses
from CNS to
muscles & glands
connect sensory &
motor neurons
Functional Classification
• Direction in which the nerve impulse travels relative to the CNS
▫ Sensory/Afferent: dendrites are connected to receptors where
stimulus is initiated in skin/organs and carry impulse
toward CNS; axons are connected to other neuron dendrites;
unipolar except for bipolar neurons in special sense organs; cell
bodies in sensory ganglia outside CNS Receptors:
▫ extroceptors (pain, temperature, touch)
▫ interoceptors (organ sensation)
▫ proprioceptors (muscle sense, position, movement)
▫ Motor/Efferent: carry messages from CNS to effectors; dendrites
are stimulated by other neurons and axons are connected to
effectors (muscles and glands); multipolar
▫ Association/Interneurons: carry impulses from one neuron to
another (afferent to efferent); found only in CNS; lie between
sensory and motor neurons; shuttle signals; 99% of neurons in
body
Neuron Function
1. Irritability:
ability to respond
to & convert to
nerve impulse
2. Conductivity:
transmit impulse
to other neurons,
muscles, or glands
Irritability of a Neuron:
• Cell membrane at rest = polarized
▫ Neurons in a resting state normally have a
membrane potential around -70mV
▫ Na+ outside cell, K+ inside cell
 Inside is (-) compared to outside
• Stimulus  excited neuron (Na+ rushes in) 
becomes depolarized
• Depolarization activates neuron to transmit an
action potential (nerve impulse)
▫ All-or-none response
▫ Impulse conducts down entire axon
• K+ diffuses out  repolarization of membrane
• Na+/K+ ion concentrations restored by sodiumpotassium pump (uses ATP)
Depolarization
Nerve Conduction
• Action potential reaches
axon terminal  vesicles
release
neurotransmitters (NT)
into synaptic cleft
• NT diffuse across
synapse  bind to
receptors of next neuron
• Transmission of a nerve
impulse =
electrochemical event
Gated Ion Channels (Na+ and K+)
Neurotransmitters
• Exact numbers are unknown but more than 100 have
been identified
• Excitatory: cause depolarization
• Inhibitory: reduce ability to cause action potential
▫ Examples: acetylcholine, serotonin, endorphins
Neurotransmitters
• Acetylcholine: most common, it excites skeletal muscle,
but inhibits cardiac muscle; is also involved with memory;
deficiency of ACh could be a cause of Alzheimer’s.
• Amines: synthesized from amino acid molecules
▫ Serotonin: CNS inhibitory; moods, emotions, sleep, appetite
 Antidepressants  boost serotonin levels
▫ Histamine: CNS stimulant; play a major role in allergic reactions,
dilating blood vessels and making the vessel walls abnormally
permeable.
 Antihistamines work by preventing the release of histamine
▫ Dopamine: movement, reward-motivated behavior
 Parkinson’s = reduction of dopamine
▫ Epinephrine: autonomic nervous response; increases cardiac
activity, blood pressure, glycogen breakdown, blood glucose levels
▫ Norepinephrine: produces vasoconstriction, heart rate increase,
and blood pressure increase
 Antidepressants  boost norepinephrine levels
Neurotransmitters
• Amino acids
▫ Glutamate: CNS excitatory; cognition, memory and
learning
▫ Glycine: CNS inhibitory
▫ GABA: inhibitory; calms and relaxes
• Neuropeptides: short strands of amino acids
called polypeptides
▫ Enkephalins/endorphins: pituitary gland;
inhibitory, block pain
▫ VIP: vasoactive intestinal peptide; smooth muscle
relaxation
▫ CCK: cholecystokinin; Studies have linked CCK with
anxiety and panic attacks in people with panic disorder
▫ Substance P: excitatory, transmits pain information
Neurotransmitters
Neurotransmitter
Action
Affected by:
Acetylcholine
muscle contraction
botulism, nicotine
Dopamine
“feeling good”
cocaine,
amphetamines
Serotonin
sleep, appetite, nausea,
mood, migraines
Prozac, LSD,
ecstasy
Endorphins
ease pain, cause
pleasure
morphine, heroin,
methadone
GABA
main inhibitory
NT
alcohol, Valium,
barbiturates
The Nervous
System
Part 5
Reflexes, Neurological Diseases
Spinal Nerves
• Humans have 31 left-right
pairs of spinal nerves, each
roughly corresponding to a
segment of the vertebral
column:
▫ 8 cervical spinal nerve pairs
(C1-C8)
▫ 12 thoracic pairs (T1-T12)
▫ 5 lumbar pairs (L1-L5)
▫ 5 sacral pairs (S1-S5)
▫ 1 coccygeal pair.
• The spinal nerves are part of
the PNS.
Spinal Nerve ~ Composition
• Each spinal nerve is formed from
the combination of nerve fibers
▫ The posterior/dorsal root is
the afferent sensory root and carries
sensory information to the brain.
▫ The anterior/ventral root is
the efferent motor root and carries
motor information from the brain.
• Outside the spinal column, the
nerves branch to various parts of
the body
▫ Approximately 46 miles of nerves
throughout the human body
Reflexes
• Rapid, predictable, involuntary responses to
stimuli
1. Somatic Reflexes: stimulate skeletal muscles
▫
Ex.) Pulling away hand from hot object
2. Autonomic Reflexes: regulate smooth
muscles, heart, glands
▫
Ex.) salivation, digestion, blood pressure,
sweating
Reflex Arc (neural pathway)
Five elements:
1. Receptor – reacts to stimulus
2. Sensory neuron
3. CNS integration center (interneurons)
4. Motor neuron
5. Effector organ – muscle or gland
Reflex Activities
Patellar (Knee-jerk)
Reflex
Pupillary Reflex
Patellar (Knee-jerk)
Reflex
• Stretch reflex
• Tapping patellar
ligament causes
quadriceps to contract
 knee extends
• Exaggeration or
absence of the reaction
suggests that there
may be damage to the
central nervous
system
Pupillary Reflex
• Optic nerve 
brain stem 
muscles constrict
pupil
• Useful for
checking brain
stem function and
drug use
Achilles reflex:
Tap Achilles tendon 
movement of toes
▫ Positive result would be jerking
of the foot towards its plantar
surface
Plantar reflex:
Draw object down sole of foot 
curling of toes
▫ Babinski’s sign: check to see if
motor cortex or spinal tract is
damaged
PTSD
• Develops after traumatic events; war, assault, etc.
• Evidence that susceptibility to PTSD is hereditary.
Approximately 30% of the variance in PTSD is
caused from genetics alone.
• 3 areas of the brain in which function may be
altered in PTSD have been identified:
▫ Prefrontal cortex: personality, social skills/behavior
▫ Amygdala: memory, decision making, emotional
reactions
▫ Hippocampus: center of emotion, memory, and the
autonomic nervous system (elongated ridges on the floor of
each lateral ventricle)
Alzheimer’s Disease
• Discovered in 1906, Dr. Alois Alzheimer noticed changes in the
brain tissue of a woman who had died of an unusual mental
illness.
• It is a chronic neurodegenerative disease; starts slow and gets
worse over time
• Alzheimer's disease is currently ranked as the sixth leading cause
of death in the United States
• Most common early symptom is short term memory loss.
• As the disease advances, symptoms can include: language
problems, disorientation, mood swings, loss of motivation, lack of
self-care
• The average life expectancy following diagnosis is 3-10 years.
• The cause of Alzheimer's disease is poorly understood.
▫ About 70% of the risk is believed to be genetic with other risk
factors including head injuries, depression, or hypertension.
ACh and Alzheimer’s
• Acetylcholine is found in cells called
cholinergic neurons. These neurons
are involved in a variety of functions,
including cognitive processing and
motor function
• About 50 years ago, discovered that
drugs that block acetylcholine release
can block memory functions
▫ Later shown that one of the enzymes
needed to form acetylcholine  choline
acetyltransferase, or ChAT  drops to
significantly lower levels in people with
Alzheimer's disease
• A significant reduction in the number
of cholinergic neurons in the forebrain
of Alzheimer's sufferers was found in
the 1980s
• Overall  lower production of ACh,
lower number of cells that contain it
and its receptor sites
Adrenoleukodystrophy (ALD)
• X-linked metabolic disorder, characterized by
progressive neurologic deterioration due to
demyelination of the cerebral white matter.
• Brain function declines as the protective
myelin sheath is gradually stripped from the
brain’s nerve cells.
▫ Without that sheath, the neurons cannot
conduct action potentials
▫ This sequence of events appears to be related to
an abnormal accumulation of saturated fatty
acid chains in the CNS, which sets off an
abnormal immune response that leads to
demyelination
Facts About ALD
• Affects approximately 1 in
20,000 people from all races
▫ Mainly men, X-linked
• Three Categories:
▫ Childhood cerebral form:
appears in mid-childhood (at
ages 4 - 8)
▫ Adrenomyelopathy: occurs
in men in their 20s or later in
life
▫ Addison disease: adrenal
gland does not produce
enough steroid hormones
Symptoms
• Childhood form:
▫ Changes in muscle tone, especially muscle spasms and spasticity
▫ Hearing loss
▫ Worsening nervous system deterioration, including coma, decreased
fine motor control, and paralysis
▫ Seizures
▫ Swallowing difficulties
▫ Visual impairment or blindness
• Later form:
▫ Difficulty controlling urination
▫ Possible worsening muscle weakness or leg stiffness
▫ Problems with thinking speed and visual memory
• Adrenal gland failure:
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▫
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Coma
Decreased appetite
Loss of weight, muscle mass
Muscle weakness
Vomiting