Download The Nervous System

Chapter 8: The Nervous System – Part 1
The Nervous System
Two Organ Systems Control All the Other Organ Systems
• Nervous system characteristics
• Rapid response, brief duration
• Endocrine system characteristics
• Slower response, long duration
Two Anatomical Divisions
• Central nervous system (CNS)
• Brain, spinal cord
• Peripheral nervous system (PNS)
• All the neural tissue outside CNS
• Afferent division (sensory input)
• Efferent division (motor output)
• Somatic nervous system and Autonomic nervous system
Neural Tissue Organization
Two Classes of Neural Cells
• Neurons – basic units of the NS
• For information transfer, processing, and storage
• Neuroglia (smaller, outnumber neurons, can divide)
• Support framework for neurons, regulate environment, phagocytic
Three Classes of Neurons
• Sensory neurons (afferent)
• Deliver information to CNS
• Can be categorized according to the information they detect
•3 types of somatic sensory receptors
•External receptors – touch, temperature, pressure, sight,
hearing
•Proprioceptors – monitor the position and movement of
skeletal muscles and joints
•Visceral (internal) receptors – monitor digestive, respiratory,
cardiovascular, urinary, and reproductive systems, provide
sensations of taste, deep pressure, and pain
• Motor neurons (efferent)
• Stimulate or inhibit peripheral tissues
• 2 types
•Somatic motor neurons – (of the somatic NS) innervate the
skeletal muscles
•Visceral motor neurons – (of the autonomic NS) innervate all other
effectors
• Interneurons
• Located between sensory and motor neurons, analyze inputs, coordinate
outputs
Neuron Anatomy (*cannot divide)
• Cell body
• Nucleus, Mitochondria, RER, Nissl bodies
• Dendrites
• Signal reception (inward), several branches
• Axon
• Signal propagation (outward) from the hillock toward the synaptic
terminals, may branch into collaterals
Structural Classes of Neurons (sketch to the right!)
• Unipolar
• Dendrite, axon continuous
• Afferent neurons
• Sensory neurons of the PNS
• Multipolar
• Many dendrites, one axon
• Most common class of neuron
• Motor neurons that control skeletal muscles
• Bipolar
• One dendrite, one axon
• Very rare, usually in special sense organs
Types of Neuroglia (glia) in the CNS
• Astrocytes
• Part of blood-brain barrier, largest, most numerous, repair damage
• Oligodendrocytes
• Cytoplasmic extensions wrap around causing myelination, gaps
between cells are called nodes of Ranvier while the covered portions are
called internodes, white matter consists of myelinated axons, gray matter
is dominated by cell bodies
• Microglia
• Phagocytic defense cells
• Ependymal cells
• Lining of brain, spinal cord cavities, source of cerebrospinal fluid (CSF)
Types of Neuroglia in the PNS
• Satellite cells
• Surround and support cell bodies
• Schwann cells
• Surround all peripheral axons, its outer surface is the neurilemma
• Form myelin sheath on myelinated axons
Anatomic Organization of CNS Neurons
• Center: Collection of neurons with a shared function
• Nucleus: A center with a discrete anatomical boundary
• Neural cortex – gray matter covering of brain portions
• White matter - Bundles of axons (tracts) that share origins, destinations,
and functions, tracts in the spinal cord form larger groups called columns
Anatomic Organization of PNS Neurons
• Ganglia: Groupings of neuron cell bodies (gray matter)
• Nerve: Bundle of axons (white matter) supported by connective tissue, both
sensory and motor axons may be present in the same nerve
• Spinal nerves
• To/from spinal cord
• Cranial nerves
• To/from brain
Pathways in the CNS (link brain to body)
• Ascending pathways
Carry information from sensory receptors to processing centers in the
brain
• Descending pathways
Carry commands from specialized CNS centers to skeletal muscles
Neuron Function
The Membrane Potential (transmembrane potential)
• Resting potential (the membrane potential of an undisturbed cell)
• Excess negative charge inside the neuron
• Created and maintained by Na-K ion pump
• Negative voltage (potential) inside
• -70 mV (0.07 Volts)
Please review “Factors
Responsible for the Membrane
Potential” on page 244 on your
own time.
Changes in Membrane Potential
• Result from changes in ion movement (remember that any stimulus that
alters the membrane permeability to Na or K or alters the activity of the
exchange pump will disturb the resting potential of the cell!)
• Ions move in channels or proteins which can open or close
• If Na+ channels open  positive charges outside cell  membrane
potential moves towards depolarization
• If K+ channels open  positive charges inside cell  membrane potential
moves towards hyperpolarization
• Information transfer between neurons involves action potentials and
graded potentials (changes in the membrane potential that cannot spread
far from the site of stimulation so it affects only a limited portion of the cell
membrane)
Propagation of an Action Potential
• Continuous propagation
• Involves entire membrane surface
• Proceeds in series of small steps (slower)
• Occurs in unmyelinated axons
• Saltatory propagation
• Involves patches of membrane exposed at nodes
• Proceeds in series of large steps (faster)
• Occurs in myelinated axons
Neural Communication
Synapse Basics
• Intercellular communication
• Action potential arrives at the axon terminal
• Transfer the input to next cell
• Chemical signaling does the information transfer
• Neurotransmitter release
Structure of a Synapse
• Presynaptic components
• Axon terminal
• Synaptic knob – rounded shape of the presynaptic neuron
• Synaptic vesicles – contain neurotransmitters, release their material in
the cleft
• Synaptic cleft – narrow space
• Postsynaptic components
• Neurotransmitter receptors – where the neurotransmitters bind
Synaptic Function and Neurotransmitters
• Cholinergic synapses
• Release neurotransmitter acetylcholine
• Enzyme in synaptic cleft (acetylcholinesterase) breaks it down
• Adrenergic synapses
• Release neurotransmitter norepinephrine
• Dopaminergic synapses
• Release neurotransmitter dopamine
•
•
•
Neuronal pools
Groups of interconnected neurons with specific functions, has limited inputs
and outputs
Divergence
Spread of information from one neuron to several others (from one pool to
many others)
Convergence
Several neurons send information to one other (from many pools to one)
The Central Nervous System
Meninges—Layers that surround and protect the brain and spinal cord (CNS)
• Dura mater (“tough mother”)
• Tough, fibrous outer layer
• Epidural space above dura of spinal cord – sometimes used for
injections of anesthetic causing an epidural block during childbirth
• Arachnoid (“spidery”)
• Subarachnoid space – a web of collagen and elastic fibers
• Cerebrospinal fluid – shock absorber, transports dissolved gases,
nutrients, chemical messengers, and waste products
• Pia mater (“delicate mother”)
• Thin inner layer, highly vascular
Spinal Cord Basics
• Relays information to/from brain
• Processes some information on its own
• Divided into 31 segments
• Each segment has a pair of:
• Dorsal root ganglia (contains the cell bodies of sensory neurons)
• Dorsal roots (contain the axons of those neurons, bring sensory
information to the spinal cord)
• Ventral roots (contain the axons of CNS motor neurons that control
muscles and glands)
• Gray matter appears as horns
• White matter organized into columns (ascending tracts carry sensory
information toward the brain, descending tracts convey motor commands
into the spinal cord)
Brain - The four hollow chambers in the center of the brain filled with
cerebrospinal fluid (CSF)
• CSF produced by choroid plexus
• CSF circulates
• From ventricles and central canal
• To subarachnoid space
• Accessible by lumbar puncture
• To blood stream
Ventricles = fluid filled chambers in the cerebral hemispheres
Brain Regions
• Cerebrum
• Diencephalon
• Midbrain
• Pons
• Medulla oblongata
• Cerebellum
Functions of the Cerebrum (largest region)
• Conscious thought
• Intellectual activity
• Memory
• Origin of complex patterns of movement
Anatomy of Cerebral Cortex
• Highly folded surface
• Elevated ridges (gyri)
• Shallow depressions (sulci)
• Cerebral Hemispheres
• Longitudinal fissure (from anterior to posterior)
• Central sulcus
• Boundary between frontal and parietal
• Other lobes (temporal, occipital)
Functions of the Cerebral Cortex
• Hemispheres serve opposite body sides
• Primary motor cortex (precentral gyrus of the frontal lobe)
• Directs voluntary movement
• Primary sensory cortex (postcentral gyrus of the parietal lobe)
• Receives somatic sensation (touch, pain, pressure, temperature)
• Association areas
• Interpret sensation
• Coordinate movement
Hemispheric lateralization - held together by the corpus callosum
• Categorical hemisphere (usually left)
• General interpretative and speech centers (Wernicke’s area – receives
info from all sensory association areas, integrates sensory to visual and
auditory memories)
• Language-based skills (speech center = Broca’s area)
• Representational Hemisphere (usually right)
• Spatial relationships
• Logical analysis
Brain Waves (electroencephalogram= EEG)
A printed record of electrical activity over time in the brain.
The activity changes as nuclei and cortical areas are stimulated or quiet down.
The patterns are called brain waves, which can be correlated with an individual’s
level of consciousness.
The Basal Nuclei
• Masses of gray matter that lie deep within central white matter of the brain
• Responsible for muscle tone
• Coordinate learned movements
• Coordinate rhythmic movements (e.g., walking)
Functions of the Limbic System
• Includes the olfactory cortex, several basal nuclei, gyri, and tracts along the
border between the cerebrum and the diencephalon
• Establish emotions and related drives
• Link cerebral cortex intellectual functions to brain stem autonomic functions
• Control reflexes associated with eating
• Store and retrieve long-term memories
The Diencephalon
• Switching and relay center
• Integration of conscious and unconscious motor and sensory pathways
• Components include:
• Epithalamus
• Choroid plexus
• Pineal body
• Thalamus
• Hypothalamus
Functions of the Thalamus
• Relay and filter all ascending (sensory) information
• Relay a small proportion to cerebral cortex (conscious perception)
• Relay most to basal nuclei and brain stem centers
• Coordinate voluntary and involuntary motor behavior
Functions of the Hypothalamus
• Produce emotions and behavioral drives
• Coordinate nervous and endocrine systems
• Secrete hormones
• Coordinate voluntary and autonomic functions
• Regulate body temperature
Anatomy and Function of the Brain Stem
• Midbrain
• Process visual, auditory information
• Generate involuntary movements
• Pons
• Links to cerebellum
• Involved in control of movement
• Medulla oblongata
• Relay sensory information
• Regulate autonomic function
Anatomy and Function of the Cerebellum
• Oversees postural muscles
• Stores patterns of movement
• Fine tunes most movements
• Links to brain stem, cerebrum, spinal cord
• Communicates over cerebellar peduncles
Functions of the Medulla Oblongata
• Links brain and spinal cord
• Relays ascending information to cerebral cortex
• Controls crucial organ systems by reflex
• Cardiovascular centers – adjust heart rate, cardiac contraction strength,
flow of blood
• Respiratory rhythmicity centers
Chapter 8: The Nervous System – Part 2
The Peripheral Nervous System
PNS Basics
• Links the CNS with the body
• Carries all sensory information and motor commands
• Axons bundled in nerves
•
•
Cell bodies grouped into ganglia
Includes cranial and spinal nerves
The Cranial Nerves
• 12 Pairs
• Connect to brain not the cord
 I - olfactory
 II - optic
 III - oculomotor
 IV - trochlear
 V - trigeminal
 VI - abducens
 VII - facial
 VIII - vestibulocochlear
 IX - glossopharyngeal
 X - vagus
 XI – accessory (spinal accessory)
 XII - hypoglossal
The Spinal Nerves
• 31 Pairs
• 8 Cervical
• 12 Thoracic
• 5 Lumbar
• 5 Sacral
• 1 Coccygeal
Nerve plexus—A complex, interwoven network of nerves
• Four Large Plexuses
• Cervical plexus
• Brachial plexus
• Lumbar plexus
• Sacral plexus
•
Dermatome—Region of the body surface monitored by a pair of spinal
nerves
Reflex—an automatic involuntary motor response to a specific stimulus
• The 5 steps in a reflex arc
• Arrival of stimulus and activation of receptor
• Activation of a sensory neuron
• Information processing in CNS
•
•
Activation of a motor neuron
Response by effector
Examples of Reflexes
• Monosynaptic: Simplest reflex arc; sensory neuron synapses directly on
motor neuron
• Stretch reflex: Monosynaptic reflex to regulate muscle length and tension
(example: patellar reflex)
• Muscle spindle: Sensory receptor in a muscle that stimulates the stretch
reflex
Polysynaptic reflex—a reflex arc with at least one interneuron between the
sensory afferent and motor efferent
• Has a longer delay than a monosynaptic reflex (more synapses)
• Can produce more complex response
• Example: flexor reflex, a withdrawal reflex
• Brain can modify spinal reflexes
Sensory Pathway
• Afferent axon signals from a sensory receptor
• Posterior column pathway
• Carries fine touch, pressure, proprioception (position)
• Ascending neurons synapse in medulla oblongata
• Axons cross over and synapse in thalamus
• Thalamus sends axons to primary sensory cortex
• Organized as sensory homunculus
Motor Pathways
• Corticospinal pathway (tract)
• Provides conscious voluntary muscle control
• Organized as motor homunculus
• Medial & lateral pathways (tract)
• Provide subconscious involuntary muscle control
The Autonomic Nervous System
Autonomic nervous system
Branch of nervous system that coordinates cardiovascular, digestive,
excretory, and reproductive functions
Anatomy of ANS
• Preganglionic neuron (ANS motor neurons in the CNS)
• Cell body resides in CNS
• Axon synapses in PNS in autonomic ganglion
•
Postganglionic axons (postganglionic fibers) synapse on peripheral
effectors and innervate…
• Cardiac muscle, Smooth muscle, Glands, Adipose tissues
Divisions of the ANS
• Sympathetic
• Preganglionic neurons in the thoracic and lumbar segments of the spinal
cord, stimulates metabolism, increase alertness, deal with emergency
• “Fight or flight” system
• Parasympathetic
• Preganglionic neurons in the brain and sacral segments, conserves
energy, promotes sedentary activities
• “Rest and digest” system
Sympathetic Division Organization
• Preganglionic neurons in segments T 1 to L2
• Ganglia near the vertebral column
• Sympathetic ganglia
• Paired sympathetic chain ganglia – control effectors in the body wall and
inside the thoracic cavity
• Unpaired collateral ganglia – anterior to vertebral column, innervate
tissues and organs in the abdominopelvic cavity
• Preganglionic fibers to adrenal medullae (center of the adrenal gland)
• Epinephrine (adrenalin) released into blood stream when stimulated
Effects of Sympathetic Activation
• Generalized response in crises
• Increased alertness
• Feeling of euphoria and energy
• Increased cardiovascular activity
• Increased respiratory activity
• Increased muscle tone
Parasympathetic Division Organization
• Preganglionic neurons in brain stem and sacral spinal segment
• Ganglionic neurons (peripheral ganglia) in or near target organ
• Sacral fibers form pelvic nerves
Effects of Parasympathetic Activation
• Relaxation
• Food processing
•
•
Energy absorption
Brief effects at specific sites
Relationship between the Two Divisions
• Sympathetic division reaches visceral and somatic structures throughout
the body
• Parasympathetic division reaches only visceral structures via cranial nerves
or in the abdominopelvic cavity
• Many organs receive dual innervation
• In general, the two divisions produce opposite effects on the their target
organs