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Nervous System
A. Introduction- two groups of organs
1. Central nervous system- brain & spinal cord--CNS
2. Peripheral nervous system- nerves which connect CNS to rest of body--PNS
B. General Functions
1. Sensory function: sensory receptors—at ends of peripheral nerves; gather info = sensory input
a. Converted into nerve impulses
b. Directed to central nervous system
2. Integrative function
a. Impulses brought together –sensory impulses integrated into brain as perceptions
b. Create sensations, etc
c. Make decisions—conscious or unconscious
3. Motor functions—produce motor output—nervous system EFFECTs a response
a. Responses by effectors (organs and glands)
b. Carried by peripheral nervous system
C. Nervous Tissue (neuro = Greek for nerve; neuron= nerve cell, neurotransmitter: sends chemical messages from
one neuron to another)
1. Introduction
a. Structural and functional units—neurons
b. Neuroglial cells—like connective tissue (aid neurons; protect them)
2. Neuron Structure—several common features
a. Cell body--swelling
(1) Organelles—where metabolic processes take place; mitochondria, lysosomes, golgi bodies,
chromophilic substances, etc.
(2) Neurofibrils—network of fine threads
(3) Nucleus
b. Nerve fibers (two kinds: axons & dendrites)
(1) Dendrites
(a) Usually short & highly branched (dendro=tree in Greek)
(b) Provide main receptive surfaces; go TOWARDS cell body from other neurons
(2) Axons—usually one and larger
(a) Arises from slight elevation of cell body called Axonal hillock
(b) Conducts impulses AWAY from cell body
(c) Many side branches (called collaterals)
(d) Larger axons often enclosed in sheaths composed of Schwann cells—outside CNS
i.
Wrapped like insulation
ii.
Lipid-protein (high lipid proportion)
iii.
Forms myelin sheath-insulates axon
iv.
Outside portion—neurilemma
v.
Myelinated fibers—white matter
vi.
Unmyelinated fibers—gray matter
vii.
Myelin sheaths have many gaps, called nodes, where axon membrane is exposed. As
impulse moves along axon, it jumps from one node
3. Neuroglial cells: nerve glue—supporting cells, accessory cells, “glia”
a. Functions (fill spaces, support neurons, framework, produce myelin, phagocytosis)
b. Examples:
(1) Schwann cells (PNS)—myelin producing glia of the periphery
(2) Astrocytes (CNS)—star shaped; between nerve & blood vessels (support & remove debris from
blood to help nerve cells)
(3) Oligodendrocytes (CNS)—rows along nerve fibers
-form myelin in brain &cord, but NO NEURILEMMA; that’s why they can’t regenerate
(4) Microglia (CNS)—small, spiderlike, scattered; function is support & phagocytosis—“eat” dead brain
cells and bacteria
(5) Ependymal (CNS)—epithelial-like membrane which covers specialized parts &line spaces like brain &
spinal cord cavity
D. Nervous Conduction
1. Resting potential—separation of charge, or “potential difference”
a. Outside of fiber is electrically charged or polarized with respect to inside
b. Unequal distribution of ions
c. At rest, membrane impermeable to Na+ with a greater concentration of Na+ on the outside (because of
active transport)
d. Greater concentration of K+ on the inside because of active transport (potassium can pass through
easier)
e. At rest, the outside is positive
2. Action Potential
a. Membranes can be stimulated & can trigger an impulse if threshold is reached
b. Stimulation causes depolarization (change in permeability)—membrane potential changes
c. Movement causes flow of ions; Na+ in and K+ out
d. After depolarization, membrane repolarizes and resting potential is re-established
e. Initiation of impulse causes membrane adjacent to it to change & the impulse moves
f. Change in polarity is called the action potential & the movement of it is a nerve impulse; (actually a
wave of electronegativity)
3. Impulse Conduction
a. Unmyelinated fibers conduct impulses the entire length of their surface
b. Myelinated fibers—constrictions called nodes of Ravier—allow the impulse to jump
c. Inpulse jumps from node to node—faster conduction than unmyelinated fibers; called saltatory
conduction
4. All-or-None response
a. If a nerve fiber responds at all to a stimulus, it responds completely; threshold intensity causes impulse
b. Strength of stimulus does not increase strength of impulse
E. Synapse—space between neurons
1. Transmission—called synaptic transmission. Impulse in presynaptic neuron is transmitted across the
synaptic cleft to the postsynaptic neuron
a. Ends of axons are called synaptic knobs
b. Synaptic vesicles release neurotransmitters
c. Neurotransmitters diffuse across synaptic space & react with specific receptors on post synaptic
membranes
d. Acetylcholine is released by most axons outside the brain & spinal cord
e. Cholinesterase neutralizes acetylcholine
2. Excitatory & inhibitory actions
a. Excitatory neurotransmitters increase permeability of post synaptic membrane to Na & trigger impulses;
examples: serotonin, dopamine, norepinephrine
b. Inhibitory neurotransmitters decrease permeability to Na & raise threshold
examples: gamma-aminobutyric acid (GABA) & glycine (both are amino acids)
c. Action of some drugs work by influencing the neurotransmitters (LSD & Valium)
F. Processing of Impulses
1. Neuron pools
a. Receive impulses from input fibers
b. Neuron pools process those impulses
c. Then, output fibers direct impulses out of pool
2. Facilitation
a. Occurs when excitatory impulses received are sub-threshold
b. Makes neurons more excitable to stimulation that before
3. Convergence
a. Impulses received from various sources converging at one neuron in a pool
4. Divergence
a. Impulses can pass from one member of a pool to many others
b. In this way, impulses can be amplified
G. Types of Neurons—various shapes & functions
1. Classification by structure
a. Multipolar—one axon many dendrites; brain & spinal cord
b. Bipolar—two processes (one axon & one dendrite); see in parts of eyes, nose & ears
c. Unipolar—one process, then branches
(1) Found in ganglia
(2) One goes to periphery (dendrite); other to brain (axon)
2. Classification by function
a. Sensory (afferent)-carry impulses into brain or spinal cord
(1) Receptors at end of dendrites
b. Interneurons (association)—in brain & spinal cord; form links between neurons
c. Motor (efferent) out of brain & cord
(1) Carry impulses to effectors
3. Types of nerves (nerve= groups or bundle of nerve fibers)
a. Sensory (afferent)
b. Motor (efferent)
c. Mixed (most)
H. Nerve pathways
1. Reflex arc—simplest nerve path
a. Receptors on ends of sensory nerve fibers
b. Leads to several interneurons
(1) To other parts of nervous system
(2) To motor neurons
2. Reflex behavior—simplest acts (reflexes)
a. Automatic, unconscious responses; unlearned
b. Knee jerk reflex
(1) Two neurons (sensory to motor)
(2) Maintaining upright posture
I.
J.
c. Withdrawal reflex (uses 3 neurons)
(1) Sensory impulses to cord to interneurons
(2) Interneurons to motor neurons (effectors) & also send impulses to brain (pain)
Covering of the Central Nervous System (CNS)
1. Bones & membranes
2. Meninges—membranes (singular=meninx) 3 layers
a. Dura mater—outer, tough, white fibrous tissue
(1) Many blood vessels & nerves
(2) Epidural space—between dura mater & walls of vertebrae around the cord;
(a) contains fat & connective tissue; protective pad for spinal cord
b. Arachnoid mater—middle, thin, netlike
(1) Lack blood vessels
(2) Subarachnoid space—between inner pia mater and arachnoid mater; contains cerebrospinal fluid—
clear and watery
c. Pia mater—inner, thin
(1) Many nerves & blood vessels
(2) Directly attaches to surface
Spinal Cord
1. Longest nerve from brain to the disk between the 1st & 2nd lumbar vertebrae
2. Structure
a. 31 segments, each gives rise to a pair of spinal nerves
b. Cervical enlargement—bulge, gives rise to nerves in arms
c. Lumbar enlargement—bulge, gives rise to nerves in legs
d. Two grooves: divide cord into right &left halves
(1) Anterior median fissure (deep)
(2) Posterior median sulcus (shallow)
e. Spinal cord—core of gray matter surrounded by white matter
(1) Gray matter—wings of butterfly
(a) Posterior & anterior horns
(b) Lateral horn—between anterior & posterior horns
(c) Gray commissure—connects right & left sides (central canal in center)
(d) Anterior horns gives rise to motor neurons, but most are interneurons
(2) White matter –3 sections
(a) Anterior, lateral & posterior funiculi (myelinated fibers)
(b) Major pathways called tracts
3. Functions of cord—two functions: conduct impulses to and from brain & is the center for spinal reflexes
a. Ascending tracts—sensory info to brain (all axons)
b. Descending tracts—motor impulses out of brain (all axons)
c. Example of tracts
(1) Spinothalamic—(ascending) begins in spinal cord & carries sensory impulses to thalamus
(2) Coticospinal—(descending) origin: cerebral cortex of brain & carries motor impulses to effectors
d. Center for spinal reflexes (both knee-jerk & withdrawal)