Chapter 12
Nervous Tissue

Controls and integrates all body activities within limits that maintain life

Three basic functions

sensing changes with sensory receptors

fullness of stomach or sun on your face

interpreting and remembering those changes

reacting to those changes with effectors

muscular contractions

glandular secretions

Major Structures of the Nervous System

Brain, cranial nerves, spinal cord, spinal nerves, ganglia, enteric plexuses and sensory receptors

Organization of the Nervous System

CNS is brain and spinal cord

PNS is everything else

Nervous System Divisions

Central nervous system (CNS)

consists of the brain and spinal cord

Peripheral nervous system (PNS)

consists of cranial and spinal nerves that contain both sensory and motor fibers

connects CNS to muscles, glands & all sensory receptors

Subdivisions of the PNS

Somatic (voluntary) nervous system (SNS)

neurons from cutaneous and special sensory receptors to the CNS

motor neurons to skeletal muscle tissue

Autonomic (involuntary) nervous systems

sensory neurons from visceral organs to CNS

motor neurons to smooth & cardiac muscle and glands

sympathetic division (speeds up heart rate)

parasympathetic division (slow down heart rate)

Enteric nervous system (ENS)

involuntary sensory & motor neurons control GI tract

neurons function independently of ANS & CNS

Neurons

Functional unit of nervous system

Have capacity to produce action potentials

electrical excitability

Cell body

single nucleus with prominent nucleolus

Nissl bodies (chromatophilic substance)

rough ER & free ribosomes for protein synthesis

neurofilaments give cell shape and support

microtubules move material inside cell

lipofuscin pigment clumps (harmless aging)

Cell processes = dendrites & axons

Parts of a Neuron

Dendrites

Conducts impulses towards the cell body

Typically short, highly branched & unmyelinated

Surfaces specialized for contact with other neurons

Contains neurofibrils & Nissl bodies

Axons

Conduct impulses away from cell body

Long, thin cylindrical process of cell

Arises at axon hillock

Impulses arise from initial segment (trigger zone)

Swollen tips called synaptic end bulbs contain vesicles filled with neurotransmitters

Axonal Transport

Cell body is location for most protein synthesis

neurotransmitters & repair proteins

Axonal transport system moves substances

slow axonal flow

movement at 1-5 mm per day

movement in one direction only -- away from cell body

fast axonal flow

moves organelles & materials along surface of microtubules

at 200-400 mm per day

transports in either direction

for use or for recycling in cell body

Axonal Transport & Disease

Fast axonal transport route by which toxins or pathogens reach neuron cell bodies

tetanus (Clostridium tetani bacteria)

disrupts motor neurons causing painful muscle spasms

Bacteria enter the body through a laceration or puncture injury

more serious if wound is in head or neck because of shorter transit time

Functional Classification of Neurons

Sensory (afferent) neurons

transport sensory information from skin, muscles, joints, sense organs & viscera to CNS

Motor (efferent) neurons

send motor nerve impulses to muscles & glands

Interneurons (association) neurons

connect sensory to motor neurons

90% of neurons in the body

Structural Classification of Neurons

Based on number of processes found on cell body

multipolar = several dendrites & one axon

most common cell type

bipolar neurons = one main dendrite & one axon

found in retina, inner ear & olfactory

unipolar neurons = one process only(develops from a bipolar)

are always sensory neurons

Association or Interneurons

Named for histologist that first described them or their appearance

Neuroglial Cells

Half of the volume of the CNS

Smaller cells than neurons

50X more numerous

Cells can divide

rapid mitosis in tumor formation (gliomas)

4 cell types in CNS

astrocytes, oligodendrocytes, microglia & ependymal

2 cell types in PNS

schwann and satellite cells

Astrocytes

Star-shaped cells

Form blood-brain barrier by covering blood capillaries

Metabolize neurotransmitters

Regulate K+ balance

Provide structural support

Oligodendrocytes

Most common glial cell type

Each forms myelin sheath around more than one axons in CNS

Analogous to Schwann cells of PNS

Microglia

Small cells found near blood vessels

Phagocytic role -- clear away dead cells

Derived from cells that also gave rise to macrophages & monocytes

Ependymal cells

Form epithelial membrane lining cerebral cavities & central canal

Produce cerebrospinal fluid (CSF)

Satellite Cells

Flat cells surrounding neuronal cell bodies in peripheral ganglia

Support neurons in the PNS ganglia

Schwann Cell

Cells encircling PNS axons

Each cell produces part of the myelin sheath surrounding an axon in the PNS

Axon Coverings in PNS

All axons surrounded by a lipid & protein covering (myelin sheath) produced by Schwann cells

Neurilemma is cytoplasm & nucleus
of Schwann cell

gaps called nodes of Ranvier

Myelinated fibers appear white

jelly-roll like wrappings made of lipoprotein = myelin

acts as electrical insulator

speeds conduction of nerve impulses

Unmyelinated fibers

slow, small diameter fibers

only surrounded by neurilemma but no myelin sheath wrapping

Gray and White Matter

White matter = myelinated processes (white in color)

Gray matter = nerve cell bodies, dendrites, axon terminals, bundles of unmyelinated axons and neuroglia (gray color)

In the spinal cord = gray matter forms an H-shaped inner core surrounded by white matter

In the brain = a thin outer shell of gray matter covers the surface & is found in clusters called nuclei inside the CNS

Electrical Signals in Neurons

Neurons are electrically excitable due to the voltage difference across their membrane

Communicate with 2 types of electric signals

action potentials that can travel long distances

graded potentials that are local membrane changes only

In living cells, a flow of ions occurs through ion channels in the cell membrane

Two Types of Ion Channels

Leakage (nongated) channels are always open

Gated channels open and close in response to a stimulus results in neuron excitability

Gated Ion Channels

Local Anesthetics

Prevent opening of voltage-gated Na+ channels

Nerve impulses cannot pass the anesthetized region

Novocaine and lidocaine

Encoding of Stimulus Intensity

How do we differentiate a light touch from a firmer touch?

frequency of impulses

firm pressure generates impulses at a higher frequency

number of sensory neurons activated

firm pressure stimulates more neurons than does a light touch

Signal Transmission at Synapses

2 Types of synapses

electrical

ionic current spreads to next cell through gap junctions

faster, two-way transmission & capable of synchronizing groups of neurons

chemical

one-way information transfer from a presynaptic neuron to a postsynaptic neuron

Chemical Synapses

Action potential reaches end bulb and voltage-gated Ca+ 2 channels open

Ca+2 flows inward triggering release of neurotransmitter

Neurotransmitter crosses synaptic cleft & binding to ligand-gated receptors

the more neurotransmitter released the greater the change in potential of the postsynaptic cell

Synaptic delay is 0.5 msec

One-way information transfer

Removal of Neurotransmitter

Diffusion

move down concentration gradient

Enzymatic degradation

acetylcholinesterase

Uptake by neurons or glia cells

neurotransmitter transporters

Prozac = serotonin reuptake
inhibitor

Strychnine Poisoning

In spinal cord, Renshaw cells normally release an inhibitory neurotransmitter (glycine) onto motor neurons preventing excessive muscle contraction

Strychnine binds to and blocks glycine receptors in the spinal cord

Massive tetanic contractions of all skeletal muscles are produced

when the diaphragm contracts & remains contracted, breathing can not occur

Neurotransmitter Effects

Neurotransmitter effects can be modified

synthesis can be stimulated or inhibited

release can be blocked or enhanced

removal can be stimulated or blocked

receptor site can be blocked or activated

Agonist

anything that enhances a transmitters effects

Antagonist

anything that blocks the action of a neurotranmitter

Small-Molecule Neurotransmitters

Acetylcholine (ACh)

released by many PNS neurons & some CNS

excitatory on NMJ but inhibitory at others

inactivated by acetylcholinesterase

Amino Acids

glutamate released by nearly all excitatory neurons in the brain ---- inactivated by glutamate specific transporters

GABA is inhibitory neurotransmitter for 1/3 of all brain synapses (Valium is a GABA agonist -- enhancing its inhibitory effect)

Small-Molecule Neurotransmitters (2)

Biogenic Amines

modified amino acids (tyrosine)

norepinephrine -- regulates mood, dreaming, awakening from deep sleep

dopamine -- regulating skeletal muscle tone

serotonin -- control of mood, temperature regulation, & induction of sleep

removed from synapse & recycled or destroyed by enzymes (monoamine oxidase or catechol-0-methyltransferase)

Neuropeptides

3-40 amino acids linked by peptide bonds

Substance P -- enhances our perception of pain

Pain relief

enkephalins -- pain-relieving effect by blocking the release of substance P

acupuncture may produce loss of pain sensation because of release of opioids-like substances such as endorphins or dynorphins

Regeneration & Repair

Plasticity maintained throughout life

sprouting of new dendrites

synthesis of new proteins

changes in synaptic contacts with other neurons

Limited ability for regeneration (repair)

PNS can repair damaged dendrites or axons

CNS no repairs are possible

Neurogenesis in the CNS

Formation of new neurons from stem cells was not thought to occur in humans

1992 a growth factor was found that stimulates adult mice brain cells to multiply

1998 new neurons found to form within adult human hippocampus (area important for learning)

Factors preventing neurogenesis in CNS

inhibition by neuroglial cells, absence of growth stimulating factors, lack of neurolemmas, and rapid formation of scar tissue

Repair within the PNS

Axons & dendrites may be repaired if

neuron cell body remains intact

schwann cells remain active and form a tube

scar tissue does not form too rapidly

Chromatolysis

24-48 hours after injury, Nissl bodies break up into fine granular masses

Multiple Sclerosis (MS)

Autoimmune disorder causing destruction of myelin sheaths in CNS

sheaths becomes scars or plaques

1/2 million people in the United States

appears between ages 20 and 40

females twice as often as males

Symptoms include muscular weakness, abnormal sensations or double vision

Remissions & relapses result in progressive, cumulative loss of function

Epilepsy

The second most common neurological disorder

affects 1% of population

Characterized by short, recurrent attacks initiated by electrical discharges in the brain

lights, noise, or smells may be sensed

skeletal muscles may contract involuntarily

loss of consciousness

Epilepsy has many causes, including;

brain damage at birth, metabolic disturbances, infections, toxins, vascular disturbances, head injuries, and tumors

Neuronal Structure & Function