Lecture 9
PowerPoint® Lecture Slides are prepared by Dr. Isaac Barjis, Biology Instructor
MCAT Prep Exam
Nervous System
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Nervous System Overview
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Nervous system
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Endocrine system
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Provides swift, brief responses to stimuli
Adjusts metabolic operations and directs longterm changes
Nervous system includes
All the neural tissue of the body
 Basic unit = neuron
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Divisions of the Nervous system
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CNS (Central Nervous system)
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Brain and spinal cord
PNS (Peripheral Nervous system)
Neural tissue outside CNS
 Afferent division brings sensory information from
receptors to the CNS
 Efferent division carries motor commands from
CNS to effectors
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Efferent division includes somatic nervous system and
autonomic nervous system
Functional Overview of the Nervous
System
An Introduction to the Anatomical
Organization of the Nervous System
Neuron
Neuron structure
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Soma or Perikaryon - The cell body of a neuron, containing the nucleus and
organelles
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Axon hillock - is a specialized part of the cell body (or soma) of a neuron that
connects to the axon
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Neurofilaments, neurotubules, neurofibrils
Axon hillock is the last site in the soma where membrane potentials propagated from
synaptic inputs are summated before being transmitted to the axon
Once a triggering threshold is exceeded at axon hillock then an action potential
propagates through the rest of the axon
Axon - an axon or nerve fiber is a long, slender projection of a nerve cell that
conducts electrical impulses away from the neuron's cell body or soma.
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Most mammalian axons are ensheathed by an insulating substance called myelin
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Myelin is produced by glial cells: Oligodendrocytes (in the CNS) and Schwann Cell (in the PNS)
Myelin allows axon to conduct impulses faster
Telodendria - terminal or smaller branches of an axon that terminates with
specialized ending that releases neurotransmitters
The Anatomy of a Multipolar Neuron
Synapse
Synapse or synaptic cleft is site of
intercellular communication
 Neurotransmitters released from synaptic
knob of presynaptic neuron and delivered to
the dendrites of postsynaptic neuron.
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The Structure of a Typical Synpase
Functional
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Sensory neurons
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Motor neurons
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deliver information from exteroceptors, interoceptors,
or proprioceptors
Form the efferent division of the PNS
Interneurons (association neurons)
Located entirely within the CNS
 Distribute sensory input and coordinate motor output
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A Functional Classification of Neurons
Neuroglia
Neuroglia of the Central Nervous System
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Four types of neuroglia in the CNS
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Ependymal cells
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Astrocytes
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Largest and most numerous
Oligodendrocytes
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Related to cerebrospinal fluid
Myelination of CNS axons
Microglia
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Phagocytic cells
Neuroglia of the Peripheral Nervous
System
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Two types of neuroglia in the PNS
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Satellite cells
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Surround neuron cell bodies within ganglia
Schwann cells
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Ensheath axons in the PNS
Neurophysiology: Ions and Electrical
Signals
The transmembrane potential
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Electrochemical gradient
Sum of all chemical and electrical forces acting
across the cell membrane
 Sodium-potassium exchange pump stabilizes
resting potential at ~70 mV
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An Introduction to the Resting
Potential
A typical resting membrane
potential is -70 millivolts
(mV) i.e. inside the neuron
is more negative than
outside.
 This membrane potential is
due to selective ionic
permeability and it is
maintained by Na/K pump
(also called Na/K ATPase)
 As seen from the diagram
concentration K is higher
inside than outside and
concentration of Na is
higher outside than inside.
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Electrochemical Gradients
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Because K is higher inside than outside, K chemical
gradient is to get out of neuron i.e. K diffuses down its
concentration gradient out of the cell
Because K is positive and inside the cell is negative the
K electrical gradient is to stay inside the cell i.e. positive
is attracted to negative
Because Na is higher outside than inside, Na chemical
gradient is to get into neuron i.e. diffuses down its
concentration gradient into the cell
Because Na is positive and inside the cell is negative the
Na electrical gradient is to get inside the cell i.e. positive
is attracted to negative
The total net charge inside the cell is negative because in
addition to negatively charged proteins, the cell
membrane is more permeable to K, the to Na, thus more
K leave the cell
Changes in the transmembrane potential
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Membrane contains
Passive (leak) channels that are always open
 Active (gated) channels that open and close in
response to stimuli
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Three types of active channels
Chemically regulated channels
 Voltage-regulated channels
 Mechanically regulated channels
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Action Potential
Appears when region of excitable
membrane depolarizes to threshold
 Steps involved
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Membrane depolarization and sodium
channel activation
 Sodium channel inactivation
 Potassium channel activation
 Return to normal permeability
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The Generation of an Action
Potential
Characteristics of action potentials
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Generation of action potential follows allor-none principle
Refractory period lasts from time action
potential begins until normal resting
potential returns
Continuous propagation
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spread of action potential across entire
membrane in series of small steps
salutatory propagation
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action potential spreads from node to node,
skipping internodal membrane
Muscle action potential versus neural
action potential
Muscle tissue has higher resting potential
 Muscle tissue action potentials are longer
lasting
 Muscle tissue has slower propagation of
action potentials
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Synaptic Activity
Nerve impulse
Action potential travels along an axon
 Information passes from presynaptic neuron
to postsynaptic cell
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General properties of synapses
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Electrical
 Rare
 Pre- and postsynaptic cells are bound by
interlocking membrane proteins
General properties of synapses
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Chemical synapses
 More common
 In chemical synapses the nerve terminal contains
thousands of membrane bound vesicles that are
filled with chemical messengers called
Neurotransmitters
 Excitatory neurotransmitters cause depolarization
and promote action potential generation
 Inhibitory neurotransmitters cause hyperpolarization
and suppress action potentials
Chemical Synapse: Cholinergic
synapses
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Arrival of action potential at the nerve terminal causes
vesicles containing neurotransimtter (e.g Acetylcholine)
to fuse with the membrane
Fuse of vesiccle with the membrane release
acetylcholine (ACh) in the synaptic cleft
Information flows across synaptic cleft
Synaptic delay occurs as calcium influx and
neurotransmitter release take appreciable amounts of
time
ACh broken down
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Choline reabsorbed by presynaptic neurons and recycled
Synaptic fatigue occurs when stores of ACh are
exhausted
The Function of a Cholinergic Synapse
Other neurotransmitters
Adrenergic synapses release norepinephrine
(NE)
 Other important neurotransmitters include
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Dopamine
 Serotonin
 GABA (gamma aminobutyric acid)
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Neurotransmitter Functions
An Introduction to the Organization of
the Brain
Major regions and landmarks
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Six regions in the adult brain
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Cerebrum
Diencephalon
Mesencephalon
Pons
Cerebellum
Medulla oblongata
Brain contains extensive areas of neural cortex
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Layer of gray matter on the surface of the
cerebellum and cerebrum
An Introduction to Brain Functions
Embryology of the brain
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Brain forms from three swellings at the tip of the
developing neural tube
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Prosencephalon
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Mesencephalon
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Forms the telencephalon and eventually the cerebrum and
diencephalon
Telecephalon will become cerebrum
Mesencephalon will become midbrain
Rhombencephalon
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Forms the metencephalon (cerebellum and pons) and
myelencephalon (medulla oblongata)
Ventricles of the brain
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Central passageway of the brain enlarges to
form ventricles (cavity)
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Contain cerebrospinal fluid (CSF)
Cerebrospinal fluid (CSF)
CSF cushions delicate neural structures
 Supports the brain
 Transports nutrients, chemical
messengers, and waste products
 Pathway of CSF
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Produced at the Choroid plexus,
 Travels through the lateral and medial
apertures to the subarachnoid space,
 Diffuses across the arachnoid granulations
into the superior sagittal sinus
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Blood supply to the brain
Blood brain barrier isolates neural tissue from
general circulation
 Incomplete barrier in areas
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Parts of the hypothalamus
 Pituitary gland
 Pineal gland
 Choroid plexus
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The pons
The pons contains
 Sensory and motor nuclei for four cranial
nerves
 Nuclei that help control respiration
 Connects part of the brain with one another:
Nuclei and tracts linking the cerebellum with the
brain stem, cerebrum and spinal cord
 Connections are provided by bundles of axons
(tracts).
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Ascending, descending and transverse tracts
The cerebellum
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Adjusts postural muscles and tunes on-going
movements
Cerebellar hemispheres
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Anterior and posterior lobes – govern subconscious
aspects of skeletal muscle.
Vermis – the central area
Flocculonodular lobe – contributes to equilibrium
and balance.
Superior, middle and inferior cerebellar
peduncles link cerebellum with brain stem,
diencephalon, cerebrum, and spinal cord
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Interconnects the two cerebellar hemispheres
The mesencephalon
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The tectum (roof) contains the corpora
quadrigemina (four rounded elevations)
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Two Superior - they serve as a reflex centers
for some visual activities.
Two inferior colliculi – are part of auditory
pathway
The mesencephalon contains many nuclei
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Red nucleus
Substantia nigra
Cerebral peduncles
RAS (Reticular Activating System)
headquarters
The diencephalon
The diencephalon is composed of
Epithalamus
 Hypothalamus
 Thalamus
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The thalamus
Final relay point for ascending sensory
information
 Coordinates the activities of the cerebral
cortex and basal nuclei
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The hypothalamus
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Controls somatic motor activities at the
subconscious level
 Controls autonomic function
 Coordinates activities of the endocrine and
nervous systems
 Secretes hormones
 Produces emotions and behavioral drives
 Coordinates voluntary and autonomic functions
 Regulates body temperature
 Coordinates circadian cycles of activity
The cerebral
The cerebral cortex
 Surface contains gyri and sulci or
fissures
Longitudinal fissure separates two
cerebral hemispheres
 Central sulcus separates frontal and
parietal lobes
 Temporal and occipital lobes also
bounded by sulci
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Gross Anatomy of the Spinal Cord
Adult spinal cord
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Localized enlargements provide innervation to
limbs
 31 segments
 each segment has a pair of dorsal roots and a
pair of ventral roots
Spinal meninges
Provide physical stability and shock
absorption
 Three layers
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Dura mater
 Arachnoid
 Pia mater
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Dura mater
Covers spinal cord
 Tapers to coccygeal ligament
 Epidural space separates dura mater from
walls of vertebral canal
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Arachnoid
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Interior to dura mater are the subdural
space, the arachnoid and the subarachnoid
space
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Subarachnoid space contains CSF
Pia mater
Meshwork of elastin and collagen fibers
 Innermost meningeal layer
 Denticulate ligaments extend from pia
mater to dura mater
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Sectional anatomy of the spinal cord
White matter is myelinated and
unmyelinated axons
 Gray matter is cell bodies, unmyelinated
axons and neuroglia
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Projections of gray matter toward outer
surface of cord are horns
The Sectional Organization of the
Spinal Cord
Horns of spinal cord
Posterior gray horn contains somatic and
visceral sensory nuclei
 Anterior gray horns deal with somatic motor
control
 Lateral gray horns contain visceral motor
neurons
 Gray commissures contain axons that cross
from one side to the other
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White matter
Divided into six columns (funiculi) containing
tracts
 Ascending tracts relay information from the
spinal cord to the brain
 Descending tracts carry information from the
brain to the spinal cord
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Peripheral Distribution of Spinal
Nerves
Principles of Functional Organization
General organization
 Sensory neurons
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Motor neurons
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Deliver information to CNS
Distribute commands to peripheral effectors
Interneurons
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Interpret information and coordinate responses
An introduction to reflexes
Reflexes are rapid automatic responses to
stimuli
 Neural reflex involves sensory fibers to CNS
and motor fibers to effectors
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Reflex arc
Wiring of a neural reflex
 Five steps
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Arrival of stimulus and activation of receptor
 Activation of sensory neuron
 Information processing
 Activation of motor neuron
 Response by effector
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Components of a Reflex Arc
Reflex classification
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According to
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development
Site of information processing
 Nature of resulting motor response
 Complexity of neural circuit
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reflex classifications
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Innate reflexes
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Result from connections that form between
neurons during development
Acquired reflexes
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Learned, and typically more complex
More reflex classifications
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Cranial reflexes
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Reflexes processed in the brain
Spinal reflexes
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Interconnections and processing events occur in
the spinal cord
still more reflex classifications
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Somatic reflexes
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Control skeletal muscle
Visceral reflexes (autonomic reflexes)
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Control activities of other systems
Monosynaptic Reflexes
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Stretch reflex automatically monitors skeletal
muscle length and tone
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Patellar (knee jerk) reflex
Sensory receptors are muscle spindles
 Postural reflex maintains upright position
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An Overview of Sensory Pathways and the Somatic Nervous
System
Neural pathways
 Afferent pathways
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Sensory information coming from the sensory
receptors through peripheral nerves to the
spinal cord and on to the brain
Efferent pathways
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Motor commands coming from the brain and
spinal cord, through peripheral nerves to
effecter organs
An Overview of the Autonomic
Nervous System (ANS)
ANS
Coordinates cardiovascular, respiratory,
digestive, urinary and reproductive functions
 Preganglionic neurons in the CNS send
axons to synapse on ganglionic neurons in
autonomic ganglia outside the CNS
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Divisions of the ANS
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Sympathetic division (thoracolumbar, “fight
or flight”)
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Thoracic and lumbar segments
Parasympathetic division (craniosacral, “rest
and repose”)
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Preganglionic fibers leaving the brain and sacral
segments
Sympathetic division
Sympathetic division anatomy
Preganglionic neurons between
segments T1 and L2 of spinal cord
 Ganglionic neurons in ganglia near
vertebral column
 Specialized neurons in adrenal glands
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Sympathetic activation
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In crises, the entire sympathetic division
responds
Sympathetic activation
 Affects include increased alertness, energy and
euphoria, increased cardiovascular and
respiratory activities, elevation in muscle tone,
mobilization of energy resources
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Neurotransmitters and sympathetic
function
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Stimulation of sympathetic division has two
distinct results
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Release of ACh or NE at specific locations
Secretion of E and NE into general circulation
Most postganglionic fibers are adrenergic, a few
are cholinergic or nitroxidergic
 Two types of receptors are alpha receptors and
beta receptors
 Sympathetic ganglionic neurons end in
telodendria studded with varicosities filled with
neurotransmitter
Parasympathetic division
Preganglionic neurons in the brainstem and
sacral segments of spinal cord
 Ganglionic neurons in peripheral ganglia
located within or near target organs
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Organization and anatomy of the
parasympathetic division
Preganglionic fibers leave the brain as
cranial nerves III, VI, IX, X
 Sacral neurons form the pelvic nerves
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Parasympathetic activation
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Effects produced by the parasympathetic
division
relaxation
 food processing
 energy absorption
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Olfaction
Olfactory organs
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Contain olfactory epithelium with olfactory
receptors, supporting cells, basal cells
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Olfactory receptors are modified neurons
Surfaces are coated with secretions from
olfactory glands
 Olfactory reception involved detecting
dissolved chemicals as they interact with
odorant binding proteins
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Olfaction
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Olfactory pathways
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No synapse in the thalamus for arriving
information
Olfactory discrimination
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Can distinguish thousands of chemical stimuli
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CNS interprets smells by pattern of receptor activity
Olfactory receptor population shows
considerable turnover
 Number of receptors declines with age
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Gustation
Taste receptors
Clustered in taste buds
 Associated with lingual papillae
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Taste buds
Contain basal cells which appear to be stem
cells
 Gustatory cells extend taste hairs through a
narrow taste pore
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Gustatory pathways
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Taste buds are monitored by cranial nerves
Synapse within the solitary nucleus of the
medulla oblongata
 Then on to the thalamus and the primary
sensory cortex
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Gustatory discrimination
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Primary taste sensations
Sweet, sour, salty, bitter
 Receptors also exist for umami and water
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Taste sensitivity shows significant individual
differences, some of which are inherited
 The number of taste buds declines with age
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Vision
Accessory structures of the eye
Eyelids (palpebrae) separated by
the palpebral fissue
 Eyelashes
 Tarsal glands
 Lacrimal apparatus
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external structures of the eye
Conjunctiva covers most of eye
 Cornea is transparent anterior portion
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The eye
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Three layers
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Outer fibrous tunic
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Middle vascular tunic
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Sclera, cornea, limbus
Iris, ciliary body, choroid
Inner nervous tunic
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Retina
The Sectional Anatomy of the Eye
internal structures of the eye
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Ciliary body
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Ciliary muscles and ciliary processes, which
attach to suspensory ligaments of lens
Retina
Outer pigmented portion
 Inner neural part
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Rods and cones
retina
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Retina contains rods and cones
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Cones densely packed at fovea (center of the
macula lutea)
Retinal pathway
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Photoreceptors to bipolar cells to ganglion cells,
to the brain via the optic nerve
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Axons of ganglion cells converge at blind spot (optic
disc)
Horizontal cells and amacrine cells modify the
signal passed along the retinal neurons
Rods and Cones
Photoreceptor structure
Outer segment with membranous discs
 Narrow stalk connecting outer segment to
inner segment
 Light absorption occurs in the visual
pigments
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Derivatives of rhodopsin
Equilibrium and Hearing
Both equilibrium and hearing are
provided by
receptors of the inner ear
Anatomy of the ear – External Ear
Auricle or pinnae surrounds the ear
 External acoustic meatus ends on tympanic
membrane
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The Anatomy of the Ear
Middle ear
Communicates with pharynx via
pharyngotympanic membrane
 Middle ear encloses and protects the
auditory ossicles
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