Neurons
I. Introduction
A. The basic structural and functional unit of the nervous system.
Neurons are able to store memory, think, communicate with other
neurons and regulate organs and glands because they:
1. are excitable
2. conduct impulses
3. release specific chemical regulators (neurotransmitters)
B. Neurons are single cells.
C. Neurons are very sensitive to decreased oxygen.
D. Neurons cannot divide mitotically.
1. The number is fixed at birth.
2. The number of connections (synapses) can change.
II. Anatomy
A. Cell body (aka) perikaryon and soma
1. nucleus with nucleolus
2. organells
3. micotubules
4. Nissl bodies (chromatophilic substances–rough E.R.)
5. neurofibrils
Clusters of cell bodies in the CNS are called _________________ .
Clusters of cell bodies in the PNS are called__________________.
B. Dendrites
C. Axon
D. Telodendria
E. Synaptic terminal (synaptic knob)
F. Myelin
1. lipid (white):
a. insulates (“saltatory conduction”)
b. neurolemmal sheath
2. nodes of Ranvier
4. In PNS myelin is made by _______________________.
In CNS myelin is made by_______________________.
III. Classification
A. STRUCTURAL: based on the number of processes that come off the
cell body.
1. Bipolar
2. Pseudounipolar (unipolar)
3. Multipolar
4. Anaxonic
B. FUNCTIONAL: based on the direction of the nerve impulse
(action potential).
1. Afferent neuron = Sensory neuron
2. Efferent neuron = Motor neuron
3. Interneuron = Association neuron
C. SOURCE of stimulus (if sensory)
a. Somatic afferent
b. Visceral afferent
D. Type of TARGET organ (if motor)
a. Somatic efferent
b. Visceral efferent
E. LOCATION
1. Afferent: (fig, 11.43, page 375:
a. 1st order
b. 2nd order
c. 3rd order
fig. 15.4, page 478)
2. Efferent: (fig. 11.42, page 375)
a. Upper motor neuron
b. Lower motor neuron
1. Somatic efferent
2. Visceral efferent
a. presynaptic neuron = preganglionic neuron = 1st order lower
motor neuron
b. postsynaptic neuron = postganglionic neuron = 2nd order
lower motor neuron
3. Connections in and to the brain (fig. 11.24, page 359)
a. Association fibers
ex. Arcuate fibers
b. Commissural fibers
ex. Corpus Callosum, Anterior commissure
c. Projection fibers
ex. Internal Capsule (in brain) forms tracts and columns in spinal
cord.
NERVOUS SYSTEM
Functions:
1.
2.
3.
4.
(P. 334)
Orientation of the body to INTERNAL and EXTERNAL environments;
coordination and control of body activities;
assimilation of experiences requisite to memory, learning, & intelligence &
programming of instinctual behavior.
Sensory –monitors changes (stimuli) inside and outside the body. (Energy forms.)
Integrative –interprets the changes (“integration”)
Motor–controls responses by activating muscles and glands
There are two Anatomic divisions.
I. Central Nervous System (C.N.S.)
A) Brain: “higher functions,” learning, memory, intelligence, reasoning, personality
integrates and coordinates sensory data and motor commands.
B) Spinal Cord:
II. Peripheral Nervous System (P.N.S.)
–nervous tissue outside the C.N.S.
–conveys information to and from the C.N.S.
A) receptors
B) neurons
C) nerves
D) ganglia
E) plexus
Stimuli: forms of ENERGY: Everything we perceive is a form of energy: light, sound,
chemical, mechanical, temperature.
TRANSDUCTION: changing energy from one form to another.
A) RECEPTORS: transduce energy. They change these forms of energy to a type the
body can use. “Action potential” = “nerve impulse”
“Law of Specific Nerve Energy” Each receptor responds to a certain type of energy.
“Sensation” takes place at the receptor.
“Perception” takes place in the brain.
Classified:
1. Exteroceptors: sense external environment. ex. touch, tempt., pressure
“Special senses” (more complicated) sight, smell, taste
2. Interoceptors: sense internal environment
“Proprioceptors” sense position of muscle and joints
C) NERVES: cablelike collections of neurons (axons) in P.N.S.
Classified: 1. “Mixed” Contain motor and sensory neurons. All spinal
nerves are mixed.
2. “Sensory only” three cranial nerves are sensory only.
3. “Motor only” ???
D) GANGLIA: collections of nerve cell bodies in the P.N.S.
1) sensory: visceral and somatic: dorsal root ganglia
2) motor: somatic None
visceral (A.N.S.)
a) parasympathetic “terminal ganglia”
b) sympathetic: sympathetic chain ganglia=paravertebral g.
collateral ganglia
E) PLEXUS: network of interwoven nerves in P.N.S.
F) NUCLEUS: collection of nerve cell bodies in the C.N.S.
G) TRACT: bundle of axons in the C.N.S. with a common origin,
destination and function.
H) COLUMN: several tracts bundled together in the spinal cord.
I) GRAY MATTER: dendrites, cell bodies, unmyelinated axons:
In the brain: “cortex” and “nucleus
In the spinal cord: “H” or “butterfly”
J) WHITE MATTER: myelinated axons
K) NEUROGLIA: supporting cells. They do NOT conduct “action
potentials.”
SYNAPSE
How the neurons communicate. Neurons can have 20,000 synapses.
A) Chemical synapse: use neurotransmitter (N.T.)
1. axon terminal of presynaptic neuron (synaptic knob)
contains vessicles with N.T.
2. synaptic cleft (gap)
3. dendrite or cell body of post synaptic neuron
contains receptors for N.T., channels & enzymes
An “action potential” (nerve impulse) reaching the synaptic knob causes
exocytosis of the N.T. The N.T. diffuses across the synaptic cleft, binds
to the receptor, which causes the channels to open or close. This sets up
changes in the 2nd (postsynaptic) neuron.
Two of the neurotransmitters: Ach (acetylcholine)
N.E. (norepinephrine)
Synapse= neuron to neuron
Neuromuscular junction= neuron to muscle cell
B) Electrical synapse (junction)
Gap junction (intercalated disc)
Protect the CNS
1) Bone: cranial vault, vertebral canal
2) Meninges
3) Cerebrospinal fluid (CSF)
4) Blood brain barrier (BBB)
P.367-372
II. MENINGES
A) Dura Mater (“tough mother”)
Functions: 1) protection
2) helps anchor brain
3) venous drainage (“dural sinus” = vein)
4) partition (wall)
It is the outside layer, can be seen with the naked eye and is tough.
There are two layers around the brain (periosteal and meningeal).
Anchor: periosteal layer is attached to the inside of the cranium.
“Dural sinus”: The two layers form the wall of the vein.
ex. superior sagittal sinus
Partitions: Falx cerebri
Falx cerebelli
table 11.5 p. 369
Tentorium cerebelli
Diaphragma sella
B) Arachnoid
subarachnoid space (contains CSF)
C) Pia Mater (tender mother)
thin, delicate, follows contour of brain, highly vascular
III. CSF: cushion, supports (brain is suspended in CSF), nourish and
helps remove metabolic wastes
Made: choroid plexus (specialized capillaries) and ependymal cells
Absorbed: arachnoid villi and then into venous circulation
P.N.S.
Conveys impulses to and from the brain and spinal cord.
Receptors:
Nerves:
A) Cranial
B) Spinal
(p. 416–dermatome)
Ganglia:
Nerve plexuses:
I. Spinal nerves (p.400)
Mixed–contain afferent and efferent neurons.
31 pairs: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal.
(p. 344–endoneurium, fasciculus–perineurium–epineurium)
ROOTS: attach nerves to spinal cord.
Dorsal root:
Ventral root:
SPINAL NERVE: where roots come together
BRANCHES: (Ramus)
Meningeal branch:
Posterior (dorsal) ramus:
Anterior (ventral) ramus:
White rami communicantes:
Gray rami communicantes:
Sympathetic trunk (“chain”) ganglion
II. Nerve Plexuses: “Network of interlaced nerves.”
Formed by anterior (ventral) rami.
CERVICAL PLEXUS: ( C1-C4) ex. Phrenic nerve
BRACHIAL PLEXUS: ( C5-T1) ex. Radial & Ulnar nerve
( roots–trunks–divisions–cords–branches) Randy Travis drinks cold beer.
LUMBAR PLEXUS: (L1-L4) ex. Femoral nerve
SACRAL PLEXUS: (L4-S4) ex. Sciatic nerve
Reflex Arc: receptor–sensory neuron–“center” in CNS (interneuron)-motor neuron–effector.
A) Visceral (autonomic) reflex: cardiac muscle, smooth muscle and
glands
B) Somatic reflex: skeletal muscles
Sensory
Motor
Afferent
Efferent
Unipolar
Multipolar
SOMATIC
1st order
2nd order
3rd order
VISCERAL
1st order
2nd order
3rd order
SOMATIC
Upper motor n.
Lower motor n.
VISCERAL
Upper motor n.
*Lower motor n.
preganglionic n.
postganglionic n.
____________________________________________________________
table 13.1 p. 421 Compare Somatic Motor and Autonomic Motor Systems
fig. 13.1 (these are referring to the lower motor portions)
Feature
Effector
# neurons from CNS
to effector
Ganglia
Somatic motor
Autonomic motor
Skeletal muscle
Cardiac m., smooth m.
& glandular epithelium
Two
One
No
Yes
Type of neuromuscular Specialized localized area
Junction
“Motor end plate”
Receptors throughout
Effect of nerve impulse Excite only
Excite or inhibit
Neurotransmitter
at effector:
Acetylcholine
Acetylcholine &
Norepinephrine
Type of nerve fibers
Fast conducting
thick, myelinated
Slow conducting
Effect of denervation Flaccid paralysis &
atrophy
Denervation hypersensitivity
A.N.S.
Target organs: Cardiac muscle: heart
Smooth muscle: (non-striated, “involuntary,” around tubes–
blood vessels, bronchioles, G.I. tract, ureters, vas deferens,
uterine tubes, some sphincters)
Glandular epithelium
Dual innervation:
DIVISIONS OF A.N.S.
SYMPATHETIC
“Thoracolumbar”
***Theme:
Fight or flight
PARASYMPATHETIC
“Crainosacral”
Rest and digest
(table 13.6)
“ peace or piece”
Location of
PREganglionic n.
cell body
Lateral horn of thoracic
& lumbar region of
spinal cord
Location of
ganglia
“Paravertebral” *
“Prevertebral” *
Brain (C.N. III,
VII, IX, X) &
sacral region of
spinal cord
“Terminal g.”
Length of PRE & POST
ganglionic n.
Pre–short
Post–long
Pre–long
Post–short
Distribution of
Postganglionic n.
Throughout the
body
Head & viscera
Divergence
Pre to postg.
Great divergence
1 pre—20 post
Very little
Mass discharge
“Mass action”
Usually
Not normally
Neurotransmitter:
Sympathetic
Parasympathetic
PREganglionic n. to
POSTganglionic n.
Acetylcholine
Acetylcholine
POSTganglionic n. to
Target organ
* Norepinephrine
(noradrenaline)
“Adrenergic”
Acetylcholine
“Cholinergic”
Question: If N.E. is released by (most) postganglionic sympathetic neurons,
how does it cause opposite effects?
Ex. bronchial smooth muscle relaxes (dilates)
cardiac muscle beat harder & faster
Answer: RECEPTORS
____________________________________________________________
Neurotransmitter rules:
I. Ach:
1. All preganglionic neurons (sympathetic & parasympathetic)
2. All parasympathetic postganglionic neurons
3. Lower somatic motor neurons–to skeletal muscles
4. Neurons to stimulate the Adrenal Medulla
**All preganglionic neurons or paths where there is only one neuron.
II. N.E.
1. Most postganglionic sympathetic neurons.