Cell Membrane Potential
- Neuron Structure
- a neuron includes a cell body, cell processes, and
organelles usually found in cells.
- dendrites, axons and the cell body provide receptive
- a single axon arises from the cell body and may be
enclosed in a myelin sheath and a neurilemma.
- Neuroglial Cells
- neuroglial cells are accessory cells
- they fill spaces, support neurons, hold nerve tissue
ion place, produce myelin and carry on phagocytosis.
- They include the neurolemmacyte, astrocytes,
oligodendrocytes, microglia, and ependyma.
- functions of each include the following:
astrocyte : structural support, part of blood brain
* oligodendrocyte : produces myelin for several neurons
in the central nervous system.
microglia : support, phagocytic
ependyma : epithelial like tissue, cover ventricles of
brain, produces cerebral spinal fluid ( CSF )
* Myelin forms about the 14th week of gestation.
- Regeneration of a Nerve Fiber
- if a cell body is injured, the neuron is likely to
- if a peripheral nerve is severed, its distal portion
will die, but the proximal portion may regenerate and
establish its former connections provided it has a
tube of connective tissue to guide it.
- Significant regeneration is unlikely in the central
A cell membrane is usually polarized as a result of an
unequal distribution of ions on either size. This
distribution is due to the presence of pores and channels in
the membrane that allow passage of selected ions.
Classification of Synapses
- Resting Potential
- there is a high concentration of sodium ions on the
outside of the membrane and a high concentration of
potassium ions inside.
- there are large numbers of negatively charges ions on
the inside of the cell.
- in a resting cell, more positive ions leave the cell
than enter it, so the outside develops a positive
charge relative to the inside.
- Local Potential Changes
- stimulation of a membrane affects its resting
potential in a local region.
- the membrane is depolarized if it becomes less
negative; hyperpolarized if it becomes more negative.
- local potential changes are graded and subject to
- if threshold is reached, and action potential will be
- Action Potentials
- at threshold, sodium channels open and sodium
diffuses into the cell causing depolarization.
- about the same instant, potassium channels open and
potassium diffuses out causing repolarization.
- this rapid change in potential is an action
- many action potentials may occur before active
transport reestablishes the resting potential.
- the propagation of action potentials across a
nerve fiber is an impulse .
- Refractory Period
- the refractory period is a brief time following an
action potential that the membrane is unresponsive to
- during absolute refractory the membrane cant be
stimulated; during relative refractory it may be
stimulated with a stimulus of high intensity.
- All or None Response
- a nerve impulse is conducted in an all or none
- all impulses conducted on a fiber are the same
- Impulse Conduction
- unmyelinated fibers conduct impulses over their
- myelinated fibers conduct impulses that travel from
node to node ( saltatory )
- impulse conduction is faster on large, myelinated.
possibly 100,000 presynaptic terminals lie on dendrites and
soma ( 80 - 95 % on dendrites )
Types of Common Neurotransmitters
- Synaptic Transmission
- impulses travel from dendrite or cell to the axon to
a presynaptic terminal.
- axons have synaptic knobs at their ends that secrete
a neuro transmitter.
- the neurotransmitter is released when a nerve impulse
reaches the end of an axon.
- when the neurotransmitter reaches the nerve fiber on
the distal side of cleft it triggers an action
potential in the post synaptic neuron.
- Neurotransmitter Substances
- about 30 neurotransmitters have been identified.
- calcium ions diffuse into synaptic knobs in response
to action potentials, causing release of
- neurotransmitters are quickly decomposed or removed
from synaptic clefts.
- neuropeptides are composed of amino acids in chains.
- some are neurotransmitters and some neuromodulators.
- there are possibly 200 +
- neuropeptides include : enkephalins, endorphins and
- Synaptic Potentials
- some neurotransmitters cause action potentials
- others cause the membrane to become hyperpolarized
and inhibit action impulses.
Monoamines : epinepherine, norepinepherine, dopamine,
Amino Acids : glycine, aspartic acid, glutamic acid
Neuropeptides : hypothalamic releasing hormones
pituitary peptides ( ADH )
Processing of Impulses
Excitatory : Na channels opened - depolarization
ex. norepinepherine, dopamine, serotonin
- Neuronal Pools
- neurons are organized into pools within the central
- each pool receives impulses, processes them, and
continues the impulses.
- each neuron in a pool may receive excitatory and
- a neuron is facilitated when it receives sub
threshold stimuli and becomes more excitable
( ex. pain stimuli )
- impulses from 2 or more neurons may converge on a
- convergence makes possible a neuron to summate
impulses from different sources
- . impulses leaving a pool may diverge by passing into
several output fibers
- divergence allows impulses to be amplified
Inhibitory : increase K permeability - hyperpolarized
ex. glycine, GABA, amino acids
Classification of Nerves and Nerve Fibers
- Classification Based on Structure
- bipolar : 1 axon, 1 dendrite
found: eye, ear, nose
- multipolar : 1 axon, many dendrites
found : neurons with cell bodies in CNS
- unipolar : one process that divides into axon - dendrite
found : ganglia cells
- Functional Classification
- General Efferent Somatic : brain to skeletal muscle
- General Afferent Somatic : muscle/skin to CNS
- General Efferent Visceral : CNS to organs
( autonomic fibers )
- General Afferent Visceral : organs to CNS
- Special Visceral Efferent : CNS to muscles of speech,
- Special Visceral Afferent : olfactory and taste to
- Special Somatic Afferent : sight and hearing to CNS
- Reflex Arc
- a reflex arc includes a sensory neuron, interneuron,
and a motor neuron
- the reflex arc is the behavorial unit of the CNS
- Reflex Behavior
- reflexes are automatic, unconscious response to
- they assist to maintain homeostasis
- knee jerk reflex involves sensory and motor neurons
- withdrawl reflexes involve interneurons as well as
sensory and motor
- Structure of the Spinal Cord
- spinal cord is composed of 31 segments
- each segment gives rise to a pair of spinal nerves
- characterized by a cervical enlargement, lumbar
enlargement, and two deep groves that divide it into
right and left halves.
- has a central core of gray matter that is surrounded
by white matter.
- the white matter is composed of myelinated nerve
- Functions of the Spinal cord
- provides a two way communication between brain and
rest of the body.
- ascending tracts carry impulses to the brain;
descending tracts carry impulses to muscles and
- many of the tracts in the ascending and descending
tracts crossover in the spinal cord or brain.
- dura mater : outer layer, forms periosteum of cranial
bones, many blood vessels and nerves, extends inward
into brain to form partitions, continues down the
spinal cord, does not contact the vertebra ( epidural
space ), epidural space filled with fat and
- arachnoid mater : net like membrane, no blood
vessels, between arachnoid and pia is the
subarachnoid space which is filled with CSF .
- pia mater : thin, blood vessels, contacts surface of