• Secreted by the choroid plexuses,special capillaries of pia matercovered by ependymal cells
• Selective transfer of substancesfrom the blood to form CSF
• Nutritive and protective• Helps maintain stable ionic
concentrations in the CNS• Circulates in ventricles,
central canal of spinal cord, and subarachnoid space
• After exchanging substances,CSF is absorbed by the arachnoid granulations
• Volume is about 140 ml at any time
Cerebrospinal Fluid
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Cerebrospinal Fluid Pressure
• Continuous secretion and reabsorption of cerebrospinal fluid (CSF) keeps fluid pressure in ventricles constant
• Interference with circulation of CSF (by infection, tumor, blood clot) can increase intracranial pressure (ICP) in ventricles
• Can lead to collapse of cerebral blood vessels, injury of brain tissues compressed against skull
• Spinal tap / lumbar puncture is used to measure CSF pressure
• Pressure can be relieved by insertion of a drain into subarachnoid space
• Hydrocephalus in infant (“water on the brain”) can be treated by insertion of a shunt
Clinical Application 11.2
7
Traumatic Brain Injury
• Traumatic Brain Injury (TBI): result of mechanical force such as a fall, attack, accident, sports injury
• Concussion: a mild TBI; typically results from a one-time injury and has no lasting symptoms
• Chronic traumatic encephalopathy (CTE): sports-related, mild repetitive TBI; results from many small injuries over time; symptoms begin years later and have long-lasting effects on memory and behavior
• Blast-related brain injury: Severe TBI, resulting from explosions in combat situations; often leads to cognitive decline years after injury
Clinical Application 11.1
8
11.4: Brain
9
The brain contains centers for / performs the following functions:
• Neural centers for sensory function
• Sensations and perceptions
• Motor commands to skeletal muscles
• Higher mental functions, such as memory, reasoning
• Neural centers for coordinating muscular movement
• Neural centers for regulating visceral activities
• Personality
Major portions of the
brain:
• Cerebrum
• Diencephalon
• Cerebellum
• Brainstem
The Brain
10
• Neural tube gives rise to CNS
• Brain forms from 3 vesicles:
• Forebrain (prosencephalon)
• Midbrain (mesencephalon)
• Hindbrain (rhombencephalon)
• Forebrain divides into the
telencephalon and diencephalon
• Mesencephalon persists
• Hindbrain divides into
metencephalon and
myelencephalon
• These 5 cavities persist in
mature brain as ventricles and
• tubes that connect them
Brain Development
11
Cells in tissue
around the
ventricles
differentiate into
the various
regions of the
brain.
Midbrain, pons,
and medulla
oblongata make
up the brainstem.
Brain Development
12
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Cerebrum is largest part of brain:• Cerebral hemispheres:
2 halves, separated by falx cerebri• Corpus callosum:
Connects cerebral hemispheres• Gyri:
Ridges or convolutions• Sulci:
Shallow grooves in surface• Central sulcus
• Fissures:• Deep grooves in surface• Longitudinal: separates the
cerebral hemispheres• Transverse: separates
cerebrum from cerebellum
Structure of the Cerebrum
13
5 lobes of the cerebral hemispheres:
• Frontal lobe
• Parietal lobe
• Temporal lobe
• Occipital lobe
• Insula (Island of Reil)
4 of the lobes are
named for the bones
that they underlie.
Structure of the Cerebrum
14
Cerebral Cortex:
• Thin layer of gray matter, which makes up outermost layer of the cerebrum
• Contains almost 75% of neuron cell bodies in nervous system
White Matter of Cerebrum:
• Lies under cerebral cortex
• Makes up most of cerebrum
• Contains bundles of myelinated axons, that connect neuron cell bodies in cerebral cortex to other portions of nervous system
Structure of the Cerebrum
15
Cerebral cortex is responsible for higher mental functions:
• Interpreting impulses from sensory organs
• Initiating voluntary movements
• Storing information as memory
• Retrieving stored information
• Reasoning
• Seat of intelligence and personality
Functions of the Cerebral Cortex
16
• The cerebral cortex can be divided into sensory, association and motor areas; some overlap exists
• Each area contains a group of neurons working together to perform a particular function
Functional Regions of the Cortex
17
Cutaneous sensory area:• Parietal lobe• Interprets sensations on skinSensory speech area (Wernicke’s area):• Temporal /parietal lobe• Usually left hemisphere• Understanding and
reading)• Store memories of visual scenes, music, and complex patterns
• Occipital lobe association areas:• Analyze and combine visual images with other sensory experiences
• Insula:Translating sensory information into proper emotional responses
Association Areas
21
Motor Areas of the Cortex
22
Primary motor areas:
• Frontal lobes
• Control voluntary muscles
• Most nerve fibers
cross over in brainstem
Broca’s area:• Anterior to primary motor cortex• Usually in left hemisphere• Controls muscles needed for speech
Frontal eye field:• Above Broca’s area• Controls voluntary movements of eyes and eyelids
Motor Areas (on left)
23
Functions of the Cerebral Lobes
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The left hemisphere is dominant in most people
• Dominant hemisphere controls:
• Language skills of speech, writing, reading
• Verbal, analytical, and computational skills
• Nondominant hemisphere controls:
• Nonverbal tasks
• Motor tasks
• Understanding and interpreting musical and visual patterns
• Provides emotional and intuitive thought processes
Hemisphere Dominance
25
Memory is the consequence of learning.
2 types of memory:
• Short-term (working) memory:• Neurons connected in a circuit
• Circuit is stimulated over and over
• When impulse flow ceases, memory does also unless it enters long-term memory via memory consolidation
• Long-term memory:• Holds more memory than short-term, lasts a lifetime
• Changes structure or function of neurons
• Enhances synaptic transmission
Memory
26
Basal Nuclei:• Sometimes called basal
ganglia
• Masses of gray matter deep
within cerebral hemispheres
• Consist of caudate nucleus,
putamen, and globus pallidus
• Produce dopamine
• Help control voluntary
movement
Basal Nuclei
27
Parkinson Disease (PD)
• In PD, neurons degenerate in the substantia nigra, which produces the neurotransmitter, dopamine
• Less dopamine reaches basal nuclei
• Dopamine deficiency leads to motor problems, such as tremors, shuffling gait, rigidity, small handwriting, speech difficulties, mask-like face
• No treatments can cure or slow down progression of PD
• Treated with Levodopa, precursor of dopamine, which can cross blood-brain barrier
• Surgery and deep brain stimulation have had some success
• Causes of PD: certain drugs, pesticide exposure, frequent blows to the head
• There is a genetic component, but it is not directly inherited
Clinical Application 11.3
28
• Between cerebral hemispheres and above the brainstem
• Surrounds the third ventricle
• Composed of gray matter
• Thalamus
• Hypothalamus
• Optic tracts
• Optic chiasma
• Infundibulum
• Posterior pituitary
• Mammillary bodies
• Pineal gland
Diencephalon
29
Thalamus:• Gateway for sensory impulses ascending to cerebral cortex• Receives all sensory impulses (except for sense of smell)• Channels impulses to appropriate part of cerebral cortex for interpretation
Hypothalamus:• Maintains homeostasis by regulating visceral activities, such as heart
rate, blood pressure, body temperature, water & electrolyte balance, hunger, body weight, movement and glandular secretion in digestive tract, sleep and wakefulness, pituitary gland function
• Links nervous and endocrine systems
Limbic System:• Consists of several structures in various parts of brain, including
diencephalon• Controls emotional responses, feelings, behavior oriented toward survival• Reacts to potentially life-threatening upsets (physical or psychological)
Diencephalon
30
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Brainstem connects brain to spinal cord. Consists of:
• Midbrain
• Pons
• Medulla oblongata
Brainstem
31
Midbrain:• Short section of brainstem• Lies between diencephalon
and pons• Contains bundles of fibers
that join lower parts of brainstem and spinal cord with higher part of brain
• Cerebral aqueduct• Cerebral peduncles
(bundles of nerve fibers)• Corpora quadrigemina
(centers for visual and auditory reflexes)
• Red nucleus (role inpostural reflexes)
Midbrain
32
Pons:• Rounded bulge on
underside of brainstem
• Between midbrain and
medulla oblongata
• Relays nerve impulses
between medulla oblongata
and cerebrum
• Relays impulses from
cerebrum to cerebellum
• Helps regulate rhythm of
breathing
Pons
33
Medulla oblongata:• Enlarged continuation of
spinal cord
• Conducts ascending and
descending impulses
between brain & spinal cord
• Contains cardiac, vasomotor,
and respiratory control centers
• Contains various nonvital
reflex control centers
(coughing, sneezing,
swallowing, and vomiting)
• Injuries are often fatal
Medulla Oblongata
34
Reticular Formation:• Called Reticular Activating System• Complex network of nerve fibers
scattered throughout brain stem• Extends into the diencephalon• Connects to centers of
hypothalamus, basal nuclei, cerebellum, and cerebrumwith ascending and descending tracts
• Filters incoming sensory information
• Arouses cerebral cortex into state of wakefulness
• Decreased activity causes sleep
Reticular Formation
35
Types of Sleep
36
Non-rapid Eye Movement (Non-REM) Sleep:• Slow wave sleep • Person is tired• Decreasing activity of reticular formation• Restful and dreamless• Reduced blood pressure and respiratory rate• 3 stages, ranging from light to heavy• Alternates with REM sleep
Rapid Eye Movement (REM) Sleep:• Paradoxical sleep (because some areas of brain are active) • Heart and respiratory rates irregular• Dreaming occurs
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Cerebellum:• Inferior to occipital lobes• Posterior to pons and medulla
oblongata• Two hemispheres separated
by falx cerebelli• Vermis connects hemispheres• Cerebellar cortex (gray matter)• Arbor vitae (white matter)• Cerebellar peduncles • Dentate nucleus (largest nucleus) • Integrates sensory information
concerning position of body parts• Coordinates skeletal muscle activity• Maintains posture
Cerebellum
37
Major Parts of the Brain
38
Brain Waves• Recordings of fluctuating electrical changes in the brain• Commonly recorded from EEG, via electrodes on scalp that detect
electrical changes in extracellular fluid of the brain• EEG based on potential changes
in large groups of neurons• 4 types of brain waves:
- Alpha: awake, resting, eyes closed
- Beta: active mentalactivity, under tension
- Theta: mostly in children- Delta: mainly duringsleep
Clinical Application 11.4
39
Sleep Disorders
40
11.5: Spinal Cord
41
• Slender column of nervous
tissue continuous with brain
and brainstem
• Extends downward through
vertebral canal
• Begins at the foramen
magnum and terminates at
the first and second lumbar
vertebrae (L1-L2) space
• Consists of 31 segments;
each gives rise to a pair
of spinal nerves
Portions of the spinal cord
(longitudinal section):• Cervical enlargement
• Lumbar enlargement
• Conus medullaris
• Filum terminale
• Cauda equina
Structure of the Spinal Cord
42
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Structure of the spinal cord (cross section):
Structure of the Spinal Cord
43
2 main functions of the spinal cord:
• Center for spinal reflexes
• Conduit (pathway) for impulses to and from the brain
Functions of the Spinal Cord
44
Reflex:Automatic, subconscious response to stimuli within or outside the body
Reflex arc:Neural pathway, consisting of a sensory receptor, 2 or more neurons, and an effector
• Simple reflex arc (contains only sensory and motor neurons)
• Most common reflex arc (sensory neuron, interneurons, motor neurons)
Reflex Arcs
45
Reflex Arc
46
Parts of a Reflex Arc
47
Monosynaptic (stretch) reflex:• Consists of 2 neurons: sensory and motor; 1 synapse in spinal cord
• Example of a stretch reflex: the knee-jerk reflex
• Helps maintain an upright posture
Reflex Behavior
48
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Withdrawal Reflex:• Occurs when person touches or steps on something painful (stove, tack)
• Prevents or limits tissue damage, by removing limb from painful stimulus
• Polysynaptic: contains sensory neuron, interneuron, motor neuron
• Reciprocal innervation: flexors contract, extensors are inhibited
Reflex Behavior
49
Crossed Extensor Reflex:• During withdrawal reflex,
flexors on affected side
contract, and extensors
are inhibited (ipsilateral)
• At same time, extensors on
opposite side (contralateral)
contract, flexors are inhibited
• Also shifts body weight, so
person remains upright
Reflex Behavior
50
Uses of Reflexes• Reflexes are used to assess condition of nervous system• Routinely performed in neurological testing• In patients with nervous system injury, location and extent of damage can
be judged by testing reflexes• Effectiveness of anesthetic during surgery can be assessed through reflex
• Ascending tracts conduct sensory impulses to the brain (pink)
• Descending tracts conduct motor impulses from the brain to motor neurons reaching muscles and glands (light brown)
• Tracts are composed of axons
Ascending and Descending Tracts
52
Major Ascending (Sensory)
Spinal Cord Tracts:
• Fasciculus gracilis and
fasciculus cuneatus
• Spinothalamic tracts
• Spinocerebellar tracts
Ascending Tracts
53
Major Descending (Motor)
Spinal Cord Tracts:
• Corticospinal tracts
• Reticulospinal tracts
• Rubrospinal tract
Descending Tracts
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Nerve Tracts of the Spinal Cord
55
Spinal Cord Injuries
• Most common causes are workplace and motor vehicle accidents
• Severity depends on extent and location of damage
• Compression or distortion of the spinal cord can result in damage to or death of neurons, leading to pain, weakness, loss of sensation, and muscular atrophy
• CNS neurons do not regenerate
• Injury to ascending tracts can result in loss of sensation
• Injury to descending tracts can result in loss of motor function, paralysis
• Research is looking into ways of limiting damage after injury: growth factors, stem cells
Clinical Application 11.6
56
11.6: Peripheral Nervous System
57
Peripheral Nervous System (PNS):Consists of nerves that connect CNS to other body parts; PNS includes:
• Cranial nerves arising from the brain
• Spinal nerves arising from the spinal cord
PNS can also be divided into:
• Somatic nervous system: cranial & spinal nerves that connect CNS to the skin and skeletal muscles (conscious activities)
• Autonomic nervous system: cranial & spinal nerves that connect CNS to viscera (subconscious activities)
Subdivisions of the Nervous System
58
• Nerves are bundles of axons
• Connective tissue coverings:
- Endoneurium
- Perineurium
- Epineurium
Structure of Peripheral Nerves
59
Sensory nerves:Conduct impulses into brain or spinal cord
Motor nerves:Conduct impulses to muscles or glands
Mixed nerves:• Contain both sensory and motor nerve fibers
• Most nerves are mixed nerves
• All spinal nerves are mixed nerves (except the first pair)
Nerve and Nerve Fiber Classification
60
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Cranial and spinal nerves are subdivided into these types:• General somatic efferent fibers:
Carry motor impulses from CNS to skeletal muscles
• General visceral efferent fibers:Carry motor impulses from CNS to smooth muscles and glands
• General somatic afferent fibers:Carry sensory impulses to CNS from skin and skeletal muscles
• General visceral afferent fibers:Carry sensory impulses to CNS from blood vessels and internal organs
“General” indicates that fibers are associated with general structures, as opposed to those of the special senses.
Nerve Fiber Classification
61
“Special” fibers are associated with specialized structures, and are found only in cranial nerves:
• Special somatic efferent fibers:
Carry motor impulses from brain to muscles used in chewing, swallowing,
speaking and forming facial expressions
• Special visceral afferent fibers:
Carry sensory impulses to brain from olfactory and taste receptors
• Special somatic afferent fibers:
Carry sensory impulses to brain from receptors of sight, hearing and
equilibrium
Nerve Fiber Classification
62
Cranial Nerves:
• 12 pairs on underside of brain
• Most are mixed nerves
• Some are sensory, associated with special senses
• Some are primarily motor, innervate muscles or glands
• Most are attached to the brainstem, with 2 exceptions
- First pair has fibers that start in the nasal cavity
- Second pair originates in eyes, fibers synapse in thalamus
• Cranial nerves are numbered with Roman numerals, from anterior to posterior (I – XII)
Cranial Nerves
63
Cranial Nerves
64
• Olfactory nerve (I):• Sensory nerve only
• Associated with sense of smell
• Bipolar neurons; pass through cribriform plate of ethmoid bone, and enter olfactory bulbs
• Optic nerve (II):• Sensory nerve only
• Associated with sense of vision
• Neuron cell bodies form ganglion layers of retina, and pass through optic foramina of the orbits
Cranial Nerves I and II
65
• Oculomotor nerve (III):• Primarily motor nerve
• Motor impulses to several voluntary muscles that raise eyelids, move the eyes
• Motor impulses to involuntary muscles that focus lens, adjust light entering eye (part of autonomic nervous system)
• Small sensory component (proprioceptive fibers)
• Trochlear nerve (IV):• Smallest pair of cranial nerves
• Primarily motor nerve
• Motor impulses to one pair of muscles that move the eyes
• Small sensory component (proprioceptive fibers)
Cranial Nerves III and IV
66
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Trigeminal nerve (V):• Mixed nerve• Largest pair of cranial nerves• 3 large sensory branches:
(1) Ophthalmic division:Sensory from surface of eyes, tear glands, scalp, forehead, and upper eyelids
(2) Maxillary division:Sensory from upper teeth, upper gum, upper lip, palate, and skin of face
(3) Mandibular division:Sensory from scalp, skin of jaw, lower teeth, lower gum, and lower lip
• Motor to muscles of mastication
Cranial Nerve V
67
• Abducens nerve (VI):• Primarily motor nerve
• Motor impulses to one pair
of muscles that move the eyes
• Some sensory (proprioceptive
fibers)
• Facial nerve (VII):• Mixed nerve
• Sensory from taste receptors
• Motor to muscles of facial
expression, tear glands, and
salivary glands
Cranial Nerves VI and VII
68
• Vestibulocochlear nerve (VIII):• Acoustic or auditory nerve
• Sensory nerve only
• 2 branches:
• Vestibular branch:
Sensory from equilibrium receptors of ear
• Cochlear branch:
Sensory from hearing receptors
• Glossopharyngeal nerve (IX):• Mixed nerve
• Sensory from pharynx, tonsils, part of tongue
• Motor to salivary glands and muscles of pharynx (for swallowing)
Cranial Nerves VIII and IX
69
Vagus nerve (X):
• Mixed nerve
• Somatic motor to muscles
of speech and swallowing
• Autonomic motor to heart,
other viscera of thorax and
abdomen
• Sensory from pharynx, larynx,
esophagus, and viscera of
thorax and abdomen
Cranial Nerve X
70
• Accessory nerve (XI):• Primarily motor nerve• Formerly called “Spinal Accessory”• Contain cranial and spinal branches:
• Cranial branch:• Join Vagus N.; motor to muscles of soft palate, pharynx and
larynx• Spinal branch:
• Motor to muscles of neck and back• Small sensory component (proprioceptive fibers)
• Hypoglossal nerve (XII):• Primarily motor• Motor to muscles of the tongue for speaking, chewing, swallowing• Small sensory component (proprioceptive fibers)
Cranial Nerves XI and XII
71
Functions of Cranial Nerves
72
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• All are mixed nerves, except the first pair
• Originate from spinal cord• 31 pairs of spinal nerves:
• 8 cervical nerves, (C1 to C8)• 12 thoracic nerves (T1 to T12)• 5 lumbar nerves (L1 to L5)• 5 sacral nerves (S1 to S5)• 1 coccygeal nerve (Co)
• Cauda equina:Formed by descending roots oflumbar, sacral, and coccygealnerves
Spinal Nerves
73
Each spinal nerve splits into a dorsal and ventral root inside the vertebral column:
Dorsal (posterior) root:• Sensory root
• Dorsal root ganglion:
Contains cell bodies of sensory
neurons whose axons conduct
impulses from peripheral body
parts into the spinal cord
Spinal Nerves
74
Dermatome: An area of skin innervated by the sensory nerve fibers of a particular spinal nerve
Spinal Nerves and Dermatomes
75
• Ventral (anterior) root: • Motor root• Axons of motor neurons
• Whiplash: sudden bending of the neck, compression of cervical plexus nerves; leads to persistent headache, pain in neck
• Broken or dislocated vertebra in neck can sever or damage axons leading to the phrenic nerves can result in paralysis of diaphragm
• Thoracic outlet syndrome: pressure on brachial plexus, due to continuous flexion of arm (as in painting or typing); results in pain in neck, shoulder, upper limb
• Sciatica: compression of intervertebral disc in lumbar region; results in pain in lower back, gluteal region, and perhaps thigh, calf, foot
• Carpal tunnel syndrome: repeated movements of hand inflame tendons that pass through carpal tunnel (space between bones in wrist); swelling in tendons compresses median nerve, resulting in pain in the arm, wrist
Clinical Application 11.7
81
11.7: Autonomic Nervous System
82
Autonomic Nervous System (ANS):• Part of the peripheral nervous system (PNS)
• Functions without conscious effort
• Controls visceral activities
• Regulates smooth muscle, cardiac muscle, and glands
• Helps maintain homeostasis
• Helps body respond to stress
• Prepares body for exercise, intense physical activity
• Sensory (afferent) nerve fibers transmit signals from the viscera and skin to neural centers in CNS
• Motor (efferent) impulses travel along 2 efferent nerve fibers which synapse in ganglia outside CNS
• Muscles or glands respond to nerve impulses by contracting, secreting, or being inhibited
• 2 divisions of the autonomic nervous system:
• Sympathetic division:
- Prepares body for ‘fight or flight’ situations; speeds body up
- Most active under energy-requiring, stressful, emergency situations
• Parasympathetic division:
- Prepares body for ‘resting and digesting’ activities; slows body down
- Most active under resting, non-stressful conditions
• Most organs receive input from both divisions
General Characteristics of the ANS
83
• All of the neurons are motor (efferent)
• Somatic motor pathways linkthe CNS and a skeletal musclefiber via 1 neuron
• synapse with a postganglionic neuron in a paravertebral ganglion
• continue through a paravertebral ganglion, and synapse at another sympathetic ganglion
• pass through to collateral ganglia to synapse there
Postganglionic fibers:
• extend from sympathetic ganglia to visceral effector organs
• Postganglionic fibers that originate at paravertebral ganglia usually pass through gray rami and return to a spinal nerve before proceeding to an effector
• Exception: preganglionic fibers pass through sympathetic ganglia and extend to adrenal medulla; these terminate on hormone‐secreting cells that release epinephrine and norepinephrine
Sympathetic Division
87
Sympathetic Division
88
Parasympathetic division:• Craniosacral division
• Preganglionic neurons originate in brainstem and S2 – S4 spinal levels
• Preganglionic fibers extend out on cranial or sacral nerves to terminalganglia (near or in visceral organs)
• Short postganglionic fibers continue to specific muscles or glands
• Preganglionic fibers of the head are included in oculomotor (III), facial (VII), and glossopharyngeal (IX) nerves
• Preganglionic fibers of thorax and abdomen are parts of vagus (X) nerve, which contains ~75% of all parasympathetic fibers
• Preganglionic fibers of sacral (S2 – S4) region of spinal cord carry impulses to pelvic viscera
Parasympathetic Division
89
Parasympathetic Division
90
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Cholinergic neurons:• Release acetylcholine
• All preganglionic sympathetic
and parasympathetic fibers
• Postganglionic
parasympathetic fibers
Adrenergic neurons:• Release norepinephrine
(noradrenaline)
• Most postganglionic
sympathetic fibers
Autonomic Neurotransmitters
91
Actions result from binding to protein receptors in the membrane of effector cells in synapses or neuromuscular junctions:• Cholinergic receptors:
Bind to acetylcholine; 2 types:- Muscarinic receptors: Excitatory, slow, also activated by fungal toxin, muscarine
- Nicotinic receptors: Excitatory, rapid, also activated by tobacco toxin, nicotine
• Adrenergic receptors:• Bind to epinephrine and norepinephrine; 2 types:
- Alpha and beta receptors: Elicit different responses on various effectors
Actions of Autonomic Neurotransmitters
92
After acting at a synapse or neuromuscular junction, neurotransmitters must be removed/inactivated, to prevent continued stimulation of the postsynaptic cell:
• The enzyme acetylcholinesterase (AChE) rapidly decomposes the acetylcholine that cholinergic fibers release.
• Norepinephrine from adrenergic fibers is removed from synapse by active transport, and then inactivated by the enzyme monoamine oxidase (MAO)
Terminating Autonomic Neurotransmitter Actions
93
• Controlled largely by the hypothalamus• Control of the autonomic nervous system (ANS) is involuntary
• Medulla oblongata regulates cardiac, vasomotor and respiratory activities
• Hypothalamus regulates visceral functions, such as body temperature, hunger, thirst, and water and electrolyte balance
• Autonomic reflex centers occur in medulla oblongata and spinal cord
• Reflex centers in medulla oblongata control cardiac, vasomotor, respiratory activities
• Limbic system and cerebral cortex control emotional responses
Control of Autonomic Activity
94
11.8: Life-Span Changes
95
• Brain cells begin to die before birth, due to apoptosis, a form of normal programmed cell death
• Over average lifetime, brain shrinks 10%
• More gray matter than white matter is lost with aging
• Many cells die in temporal lobes, but few in brainstem
• By age 90, frontal cortex has lost half its neurons
• Number of dendritic branches in cerebral cortex decreases
• Decreased levels of neurotransmitters
• Action potentials propagation rate declines by 5-10%
• Fading memory
• Slowed responses and reflexes
• Increased risk of fainting, falling
• Changes in sleep patterns that result in fewer sleeping hours
