Download CHAPTER 7 - Nervous Tissue

Biol 2401 – Human Anatomy & Physiology Lecture Notes - Sloan
CHAPTER 7 - Nervous Tissue
Dec 2010
I. NS General Functions – Maintaining homeostasis!
A. Monitors changes (sensory input)
B. Processes & interprets sensory input (integration)
C. Effects a response (motor output)
II. NS Organization
A. Central Nervous System (CNS) - brain & spinal cord
B. Peripheral Nervous System (PNS)
1. Physical subdivisions
a. Spinal Nerves
b. Cranial nerves
2. Functional subdivisions
a. Afferent (sensory) - from body to brain
1.) Somatic – skin, skeletal muscle & joints
2.) Visceral - internal organs
b. Efferent (motor) - from brain to body
1.) Voluntary (somatic) – to skeletal muscle
2.) Involuntary (autonomic) – to smooth & cardiac muscle & glands
a.) Sympathetic
b.) Parasympathetic
III. NS Histology
A. Supporting Cells - neuroglia or glial cells
1. Function - assist, segregate, & insulate neurons
2. Able to reproduce throughout life
3. Six Types
a. Schwann cells
1.) Wraps around axons
2.) Outer area with cytoplasm is neurilemma
3.) Inner, tightly wrapped area is myelin sheath
a.) protects & insulates axon
b.) increases speed of impulses
4.) space between Schwann cells is Node of Ranvier
5.) Rid area of debris after injury
b. Astrocytes
1.) Star shaped
2.) Most abundant supporting cells
3.) Anchor & form barrier between neurons & capillaries (blood-brain barrier)
c. Microglia
1.) oval shaped w/long processes
2.) macrophages
3.) protect & can replace astrocytes & oligodendrocytes
d. Ependymal cells
1.) line cavities of the brain & spinal canal
2.) form & circulate CSF
e. Oligodendrocytes
1.) form myelin sheaths of CNS axons
2.) have multiple flat extensions that coil around many neurons
f. Satellite cells
1.) found w/in ganglia
2.) are thought to help regulate chemical environment of cell
Pg 1
A. Neurons
1. Three special characteristics
a. Extreme longevity
b. Amitotic
c. High metabolic rate
2. Structure - p 206
a. Cell Body - biosynthetic center of cell - well developed rough ER & Golgi
1.) clustered in nuclei in CNS
2.) clustered in ganglia in PNS
b. Neuron Processes - cytoplasmic extensions, fibers
1.) Bundles are tracts in CNS, nerves in PNS
2.) Dendrites
a.) short, thick, branched
b.) hundreds per cell body
c.) receptive sites
d.) conduct impulses toward cell body
3.) Axons
a.) arise from cell body at axon hillock (trigger zone)
b.) may be long or short
c.) only one per neuron
d.) may branch (collaterals), branch at end into 104 or more axon terminals
e.) transmit impulses away from cell body
f.) may be wrapped in myelin sheath
1.) Schwann cells (PNS)
2.) Oligodendrocytes (CNS)
g.) Multiple Sclerosis (MS) results from deterioration of the myelin sheath
3. Functional Classes of Neurons
a. sensory or afferent
1. Dendrites are part of specialized receptors
a.) Special senses – hearing, vision, taste, smell, equilibrium
b.) Cutaneous receptors in skin
1.)Naked nerve ending (pain & temp)
2.)Meissner’s corpuscle (touch)
3.) Pacinian corpuscle (deep pressure)
c.) Proprioceptors (muscles & tendons)
1.) Golgi tendon organs
2.) Muscle spindles
b. association
c. motor
3. Structural Classes of Neurons - based on # of processes extending from cell body (p 211)
Characteristic/Type
Multipolar
Bipolar
Unipolar
>2
motor, association
neurons of CNS
2
sensory neurons
associated w/special
senses
1
sensory
Drawing
# of Processes
Locations
IV. Neurophysiology
A. Two main properties of neurons
1. Irritability – the ability to respond to a stimulus
2. Conductivity – ability to transmit impulses
Pg 2
B. Resting Membrane Potential (voltage across membrane)
1. Inside of cell is slightly more negative than the outside
2. Due to fewer positive ions inside cell
3. Membrane is said to be polarized
3. From sodium/potassium pump (3 sodium ions out for every 2 potassium ions in).
4. Membrane freely permeable to Cl-, slightly less permeable to K+, not as permeable to Na+
B. Transmission of Signals
1. Caused by a change in membrane potential
2. May be due to
a. change in membrane permeability to an ion
b. alteration in ion concentrations on the 2 sides of the membrane
3. Gated Channels
a. Voltage gated channels - open or close in response to changes in membrane voltage
b. Chemically gated channels - open in response to neurotransmitter
4. Direction of change
a. Depolarization
1.) Reduction in membrane potential
2.) Inside becomes less negative
3.) Increases probability of impulse
b. Hyperpolarization
1.) Increase in membrane potential
2.) Inside becomes more negative
3.) Decreases the probability of impulse
5. Two types of signals
a. Graded potentials
1.) Short-lived, local change in membrane potential
2.) May be depolarization or hyperpolarization
3.) Decrease with distance traveled
4.) Magnitude increases w/ increase in stimulus (graded)
5.) Caused by stimulus that causes opening of gated channels
6.) Propagated for short distances by attractions to oppositely charged ions inside
& outside membrane
7.) Named for where they occur and what they do
a.) Receptor potential - occurs in sensory receptor such as retina
b.) Postsynaptic potential - occurs in response to a neurotransmitter released from another
cell across the synapse
b. Action potentials = Nerve impulses
1.) Brief, large depolarizations
2.) Occur only in axons, usually starts at axon hillock
3.) Propagated in one direction entire
length of axon
4.) All or none (above threshold point or not)
a.) More frequent for bigger stimuli
5.) Faster in larger diameter axons & w/myelin sheath
C. The Synapse
1. Junction of one neuron to next neuron or to effector cell
2. Events at synapse
1.) axon terminal releases neurotransmitters that travel across synaptic cleft
2.) results in change permeability of postsynaptic membrane by binding with channel proteins
3.) slower than electrical
4.) synaptic delay - slowest part of transmission
Pg 3
D. Neurotransmitters - communicator molecules
1. Criteria for NT status
a. present in presynaptic terminal
b. produces IPSP or EPSP in postsynaptic membrane
c. must be removed naturally (enzyme degradation or reuptake or diffusion away)
2. Classification
a. Acetylcholine (Ach)
1.) in PNS are released at all NM junctions and by some neurons of autonomic NS
2.) also common in CNS
3.) degraded by acetylcholinesterase
b. Biogenic amines
1.) made from amino acids
2.) control emotions & biological clock
3.) two types
a.)) catecholamines - dopamine, norepinephrine, epinephrine
b.)) indolamines - serotonin, histamine
c.)) hallucinogens bind to same sites (LSD, mescaline)
c. Amino acids - in CNS, GABA, glycine, aspartate & glutamate
d. Neuropeptides (chains of amino acids)
1.) endorphins, enkephalins (natural opiates or euphorics)
2.) opiates bind to same sites
e. Novel messengers
1.) ATP
2.) NO
VIII. Reflexes
A. Rapid automatic response, somatic or autonomic, in reflex arc (path)
B. Parts of a reflex arc
1. Receptor - dendritic endings
2. Sensory neuron
3. Integration center in CNS (spinal cord or brain)
4. Motor neuron
5. Effector - muscle fiber or gland
C. Absent, exaggerated, or distorted reflexes indicate Nervous System disorders
E. Examples
1. Knee-jerk (spinal cord)
2. Withdrawal (spinal cord)
3. Pupillary (brain)
VIII. Central Nervous System
A. Develops from the neural tube, part of the ectoderm
B. The Brain – Cerebral Hemispheres (Cerebrum), Diencephalon, Brain Stem, Cerebellum
1. Cerebral Hemispheres – largest part of brain
a. Divided into lobes named for bone under which they lie
1.) Frontal
2.) Parietal
3.) Occipital
4.) Temporal
b. Covered with
1.) gyri – ridges
2.) sulci – shallow grooves
a.) Central sulcus separates frontal from parietal lobes
3.) fissures – deeper grooves
a.) Longitudinal fissure separates right from left hemispheres
Pg 4
c. Functions
1.) Somatic sensory area (see diagram pg 218)
a.) in postcentral gyrus of parietal lobe
b.) processes sensory info from skin and muscles
2.) Primary motor area (see diagram pg 218)
a.) in precentral gyrus of frontal lobe
b.) allows conscious movement of skeletal muscles
3.) Visual Cortex
a.) processes light info from eyes
b.) in occipital lobe
4.). Auditory Cortex
a.) processes info from inner ear regarding sound
b.) in temporal lobe
5.) Olfactory Area
a.) Processes chemical info regarding smell
b.) Deep inside temporal lobe
6.) Broca’s area
a.) at base of precentral gyrus
b.) allows verbalization of speech
d. Cerebral Cortex
1.) Outermost gray layer
2.) Unmyelinated axons and cell bodies
e. Cerebral White Matter
1.) Inner white tissue
2.) Myelinated axons coming and going from cortex
3.) Eg., Corpus collosum
f. Basal Nuclei
1.) Islands of gray matter within cerebral white matter
2.) Synapses here modify info leaving cortex
3.) Problems here cause (see pages 224-225)
a.) Parkinson’s Disease – tremor at rest, shuffling gate, stiff facial expression
b.) Huntington’s Chorea – wild, jerky movements and later metal deterioration
2. Diencephalon
a. Surrounded by cerebral hemispheres
b. Three parts
1.) Thalamus – relay center for sensory impulses
2.) Hypothalamus
a.) Ties CNS with endocrine system via pituitary gland
b.) Closely tied to limbic system (emotions)
c.) Regulates temperature, water balance, and metabolism
d.) Center for thirst, appetite, sex, pain, and pleasure
3.) Epithalamus
a.) Pineal body – day/night cycles
b.) Choroid plexus – capillaries surrounded by ependymal cells, make CSF
3. Brain Stem
a. Three Parts
1.) Midbrain
a.) Cerebral peduncles – tracts to and from cerebrum
b.) Corpora quadrigemina – (four cheeks) – reflex centers for vision and hearing
2.) Pons – controls rate and depth of breathing
3.) Medulla Oblongata – continuous with spinal cord, controls BP, breathing, heart rate, swallowing
b. Reticular Formation
1.) Diffuse areas of gray matter throughout brain stem
2.) Control consciousness and sleep/wake cycles
3.) Damage here causes coma
Pg 5
4. Cerebellum
a. Inferior to occipital lobe of cerebrum
b. Inner white matter called “arbor vitae”
c. Coordinates muscle activity, balance, and equilibrium
d. Ataxia – clumsy movements and inability to maintain balance, due to malfunctioning of cerebellum
C. Ventricles of Brain
1. Hollow spaces filled with CSF
2. Four
a. Lateral Ventricles (1 & 2) – in cerebral hemispheres
b. Third Ventricle – within diencephalon
c. Cerebral Aquaduct – thin canal connecting third and fourth ventricles
d. Fourth Ventricle – in brain stem, has openings into central canal of spinal cord and subarachnoid
space
D. Protection of CNS
1. Meninges
a. Dura Mater
1.) Outer, periostial meninx
b. Arachnoid Mater
1.) Middle layer, like spider web
2.) CSF circulates just under this layer in subarachnoid space
c. Pia Mater
1.) Inner layers, on surface of brain & spinal cord
d. Meningitis
1.) inflammation of meninges due to virus or bacteria, can spread to brain to cause encephalitis
2. Cerebrospinal Fluid (CSF)
a. Similar to plasma with less protein and different ion composition
b. Circulation of CSF
1.) Formed from blood in choroids plexuses in ventricles of brain
2.) Through ventricles out from 4th ventricle into subarachnoid space
3.) Up to Superior Sagittal Sinus in Longitudinal Fissure
4.) Into arachnoid villi and reabsorbed into bloodstream
c. Testing of CSF done via spinal tap (lumbar puncture)
d. Hydrocephalus
1.) enlarged head due to obstruction of flow of CSF
2.) treated by insertion of shunt to send excess fluid to neck or chest
E. Brain Dysfunction
1. Concussion
a. Dizziness, “stars”, or momentary loss of consciousness
b. No permanent damage
2. Contusion
a. Marked tissue destruction
b. Symptoms depend on area of brain injured
3. Cerebral edema
a. Swelling of brain due to inflammatory response
4. Subdural hematoma
a. Bleeding into area between meninges or between meninges & skull
b. Progressive neurological deterioration as bleeding continues
Pg 6
4. Cerebrovascular accident (CVA)
a. Stroke
b. Due to lack of blood flow to brain tissue (ischemia)
1.) Hemorrhagic – burst blood vessel
2.) Obstructive – Blockage of blood flow due to emboli or plaques
c. Aphasia – inability to speak due to stroke
d. Recovery is due to neurons moving into dead areas or growth of dendrites/axons into damaged
areas
5. Transient Ischemic Attacks
a. temporary ischemia without permanent damage
b. May forewarn of future strokes
c. Often associated with smoking
F. Spinal Cord
1. 17 inches long – from foramen magnum to ~ L1-2, encased in vertebral column
2. Meninges extend beyond L2 – spinal taps done at L3 or lower
3. Cauda equina – “horses’ tail” – collection of spinal nerves in canal below cord
4. Central canal – thin canal through center of cord containing CSF
5. Gray matter in butterfly shape INSIDE cord – efferent, afferent, and interneurons
a. Ventral gray matter – motor – neurons leave cord via ventral root to get to muscle (polio, ALS)
b. Dorsal gray matter – sensory – neurons enter cord via dorsal root, cell bodies are in dorsal root
ganglia
6. White matter on OUTSIDE – no interneurons
7. Paralysis – loss of motor function
a. Quadriplegia – transaction or crush of cervical spinal cord; all four limbs paralyzed
b. Paraplegia – transaction or crush of thoracic-L1 spinal cord; legs paralyzed
c. Hemiplegia – injury to brain or brain stem; one side of body paralyzed
d. Spinal Shock – transient dysfunction of cord due to injury lasting 48 hrs or less
PERIPHERAL NERVOUS SYSTEM
I. General
A. Includes all nervous tissue outside brain & spinal cord
B. Peripheral nerves, ganglia (location of nerve cell bodies) , & sensory receptors
C. Divided functionally into
1. Sensory (afferent)
2. Motor (efferent)
a. Somatic - skeletal muscle
b. Autonomic - smooth & cardiac muscle & glands
1. Sympathetic
2. Parasympathetic
3. Mixed – both sensory and motor
II. :Structure of a Nerve
A. Bundle of neurons outside CNS with connective tissue wrappings and blood vessels
B. Connective tissue wrappings
1. Endoneurium – surrounds individual axons and their Schwann cells
2. Perineurium – surround a small bundle of axons known as a fascicle
3. Epineurium – surrounds the entire nerve
Pg 7
II. Structural Divisions
A. Cranial Nerves
1. Arise directly from brain
2. 12 pr, numbered in order from anterior to posterior
I. Olfactory, II Optic, III Oculomotor, IV Trochlear, V Trigeminal, VI Abducens, VII Facial,
III Vestibulocochlear, IX Glossopharyngeal, X Vagus, XI Accessory, XII Hypoglossal
3. Serve only head & neck structures, except for CN IX (Vagus)
4. Names related to function
5. All but olfactory, optic, and vestibulocochlear are mixed
B. Spinal Nerves
1. Arise directly from spinal cord
2. 31 pr, all mixed, named for point where they exit cord
a. 8 cervical
d. 5 sacral
b. 12 thoracic
e. 1 coccygeal
c. 5 lumbar
3. Connected to spinal cord by dorsal & ventral roots
a. Ventral - motor from anterior horn
b. Dorsal - sensory to posterior horn
4. Roots merge to form a spinal nerve just before exiting spinal column
5. Roots shortest in cervical region, longest in coccygeal (cauda equina)
6. Spinal nerves branch almost immediately after exiting column
a. Ventral ramus - supply anterior & lateral trunk & limbs
1. T2 - T12 supply intercostal muscles (breathing)
2. Others interlace to form plexuses that serve limbs
b. Dorsal ramus - supply posterior neck & trunk
c. Meningeal branch - innervates meninges & vertebrae
7. Dermatome - area of skin innervated by the cutaneous branches of a single spinal nerve
8. Plexuses - branching nerve networks formed by ventral rami
a. General
1. each plexus branch has fibers from several different nerve roots
2. fibers from each nerve root are carried to the body periphery via several different pathways
3. advantage - damage to one spinal segment or root does not lead to complete motor or sensory
loss in the limb region served
b. Specific
1. Cervical Plexus - formed by ventral rami of upper 4 cervical nerves, deep in neck
a. Phrenic nerve - diaphragm
b. others
2. Brachial Plexus - in neck & axilla, innervates shoulder & arm, terminal branches are
a. axillary nerve
b. musculocutaneous nerve
c. median nerve - thumb opposition (grasping)
d. ulnar nerve - “funny bone”
e. radial nerve - wrist drop from improper crutch usage
3. Lumbar plexus - from L1 - L4
a. Femoral nerve
b. Obturator nerve
4. Sacral Plexus - L4 - S4
a. Sciatic nerve - longest & largest nerve in body, divides into
b. tibial nerve
c. sural nerve
d. plantar nerve
e. common peroneal nerve
Pg 8
AUTONOMIC NERVOUS SYSTEM
I. Function - maintaining homeostasis!
II. Comparison of ANS and Somatic NS
A. Effectors
1. Somatic - stimulates skeletal muscle
2. ANS - stimulates cardiac muscle, smooth muscle, and glands
B. Efferent Pathways & Ganglia
1. Somatic
a. Cell bodies of motor neurons in CNS, axons in spinal nerves
b. fibers thick & heavily myelinated
c. no ganglia for motor division
2. ANS - two motor neurons
a. First motor neuron
1.) cell body resides in brain or spinal cord (preganglionic neuron)
2.) is lightly myelinated, thin
b. Second motor neuron
1.) cell body resides in ganglion outside CNS (postganglionic neuron) with axon extending
to effector
2.) is not myelinated, very thin (slow conduction)
C. Neurotransmitter Effects
1. Somatic - acetylcholine at neuromuscular junction, effect always excitatory to muscle
2. ANS - may cause excitation or inhibition of effector
a. Norepinephrine (sympathetic & parasympathetic)
b. Acetylcholine (parasympathetic)
D. Overlap of Somatic and ANS
1. Regulated by higher brain centers
2. Both contained in almost all of spinal nerves
III. Divisions of ANS - counterbalance each other’s activities (dynamic antagonism), see table pg 241)
A. Sympathetic
1. mobilizes body during extreme situations (fear, exertion, rage)
2. “accelerator”, “fight-or-flight”
3. Effects (E’s – exercise, excitement, emergency, embarrassment)
a. pounding heart
b. rapid deep breathing
c. sweaty skin
d. dilated pupils
e. blood shunted to skeletal muscles
f. digestion put on hold
B. Parasympathetic
1. relaxes systems mobilized by sympathetic division & conserves body energy
2. “brakes”, “resting & digesting” system”
3. Effects (D’s – digestion, defecation, diuresis)
a. blood pressure & pulse low normal
b. GI tract active
c. skin warm
d. pupils constricted
e. lens of eye accommodated for close vision
IV. Functions unique to Sym Division - innervation by Sym fibers only
A. Thermoregulation - sweat glands & arrector pili muscles & blood vessels in skin
B. Renin-Angiotensin mechanism - production of renin by kidneys to control blood pressure
C. Metabolic effects - increass in metabolic rate, blood sugar, breakdown of fat, mental alertness
Pg 9
V. Local vs Diffuse Effects
A. Parasympathetic - effects of acetylcholine short-lived and local
B. Sympathetic - effects of NE and E long-lived and widespread (why it takes a while to “wind down”
after a stressful event)
VI. CNS control
A. Main integration center is hypothalamus
B. Cortical control possible by meditation and biofeedback
VII Homeostatic imbalances of ANS
A. Raynaud’s disease - exaggerated vasoconstriction in response to cold, possibly treated by severing
preganglionic sympathetic fibers serving affected areas
B. Hypertension - high blood pressure, overactivity of sympathetic NS, causes wear & tear on heart &
blood vessels, treated with adrenergic-blocking drugs
DEVELOPMENTAL ASPECTS OF NERVOUS SYSTEM
I. Brain starts development in 1st month after conception
A. Infections such as measles(rubella) can damage nervous tissue (deafness)
B. Medications
C. Smoking decreases O2 levels
D. Radiation
II. Cerebral palsy
A. Thought to be caused by infant not receiving enough O2 during delivery and other problems
B. Poor control over skeletal muscles with spasticity
C. Seizures
D. Metal retardation
E. Hearing and vision difficulties
III. Anencephaly
A. Cerebrum fails to develop
IV. Spina Bifida
A. Vertebrae do not close over spinal canal
V. Myelination starts superiorly and moves inferiorly, proximally and moves distally, females before males
VI. Orthostatic Hypotension – in elderly due to deterioration of sympathetic responses
VII. Alzheimer’s Disease – memory loss, short attention span, disorientation eventually ending with irritability,
confusion, sometimes violence, and death;, due to shortage of ACh, abnormal proteins deposits and neurofibrillar
tangles of unknown etiology
Pg 10