1. Neuron Structure

    1. a neuron includes a cell body, cell processes, and organelles usually found in cells.

    2. dendrites, axons and the cell body provide receptive surfaces.

    3. a single axon arises from the cell body and may be enclosed in a myelin sheath and a neurilemma.

  2. Neuroglial Cells

    1. neuroglial cells are accessory cells

    2. they fill spaces, support neurons, hold nerve tissue ion place, produce myelin and carry on phagocytosis.

    3. They include the neurolemmacyte, astrocytes, oligodendrocytes, microglia, and ependyma.

    4. functions of each include the following:

      astrocyte : structural support, part of blood brain barrier

      * 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.

  3. Regeneration of a Nerve Fiber

    1. if a cell body is injured, the neuron is likely to die.

    2. 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.

    3. Significant regeneration is unlikely in the central nervous system.

Cell Membrane Potential

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.

  1. Resting Potential

    1. there is a high concentration of sodium ions on the outside of the membrane and a high concentration of potassium ions inside.

    2. there are large numbers of negatively charges ions on the inside of the cell.

    3. in a resting cell, more positive ions leave the cell than enter it, so the outside develops a positive charge relative to the inside.

  2. Local Potential Changes

    1. stimulation of a membrane affects its resting potential in a local region.

    2. the membrane is depolarized if it becomes less negative; hyperpolarized if it becomes more negative.

    3. local potential changes are graded and subject to summation.

    4. if threshold is reached, and action potential will be triggered.

  3. Action Potentials

    1. at threshold, sodium channels open and sodium diffuses into the cell causing depolarization.

    2. about the same instant, potassium channels open and potassium diffuses out causing repolarization.

    3. this rapid change in potential is an action potential.

    4. many action potentials may occur before active transport reestablishes the resting potential.

    5. the propagation of action potentials across a nerve fiber is an impulse .

  4. Refractory Period

    1. the refractory period is a brief time following an action potential that the membrane is unresponsive to ordinary stimuli.

    2. during absolute refractory the membrane cant be stimulated; during relative refractory it may be stimulated with a stimulus of high intensity.

  5. All or None Response

    1. a nerve impulse is conducted in an all or none manner.

    2. all impulses conducted on a fiber are the same

  6. Impulse Conduction

    1. unmyelinated fibers conduct impulses over their entire surface.

    2. myelinated fibers conduct impulses that travel from node to node ( saltatory )

    3. impulse conduction is faster on large, myelinated. fibers.

Classification of Synapses

axosomatic
axoaxonic
axodendritic

possibly 100,000 presynaptic terminals lie on dendrites and soma ( 80 - 95 % on dendrites )

Synapse

  1. Synaptic Transmission

    1. impulses travel from dendrite or cell to the axon to a presynaptic terminal.

    2. axons have synaptic knobs at their ends that secrete a neuro transmitter.

    3. the neurotransmitter is released when a nerve impulse reaches the end of an axon.

    4. when the neurotransmitter reaches the nerve fiber on the distal side of cleft it triggers an action potential in the post synaptic neuron.

  2. Neurotransmitter Substances

    1. about 30 neurotransmitters have been identified.

    2. calcium ions diffuse into synaptic knobs in response to action potentials, causing release of neurotransmitter.

    3. neurotransmitters are quickly decomposed or removed from synaptic clefts.

  3. Neuropeptides

    1. neuropeptides are composed of amino acids in chains.

    2. some are neurotransmitters and some neuromodulators.

    3. there are possibly 200 +

    4. neuropeptides include : enkephalins, endorphins and substance P

  4. Synaptic Potentials

    1. some neurotransmitters cause action potentials

    2. others cause the membrane to become hyperpolarized and inhibit action impulses.

Types of Common Neurotransmitters

Monoamines : epinepherine, norepinepherine, dopamine, serotonin

Amino Acids : glycine, aspartic acid, glutamic acid

Neuropeptides : hypothalamic releasing hormones pituitary peptides ( ADH ) sleep peptides


Processing of Impulses

  1. Neuronal Pools

    1. neurons are organized into pools within the central nervous system

    2. each pool receives impulses, processes them, and continues the impulses.

  2. Facilitation

    1. each neuron in a pool may receive excitatory and inhibitory stimuli

    2. a neuron is facilitated when it receives sub threshold stimuli and becomes more excitable ( ex. pain stimuli )

  3. Convergence

    1. impulses from 2 or more neurons may converge on a single neuron.

    2. convergence makes possible a neuron to summate impulses from different sources

  4. Divergence

    1. . impulses leaving a pool may diverge by passing into several output fibers

    2. divergence allows impulses to be amplified

Excitatory : Na channels opened - depolarization ex. norepinepherine, dopamine, serotonin

Inhibitory : increase K permeability - hyperpolarized ex. glycine, GABA, amino acids


    Classification of Nerves and Nerve Fibers

  1. 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

  2. Functional Classification

    1. General Efferent Somatic : brain to skeletal muscle

    2. General Afferent Somatic : muscle/skin to CNS

    3. General Efferent Visceral : CNS to organs ( autonomic fibers )

    4. General Afferent Visceral : organs to CNS

    5. Special Visceral Efferent : CNS to muscles of speech, chewing, swallowing

    6. Special Visceral Afferent : olfactory and taste to the CNS

    7. Special Somatic Afferent : sight and hearing to CNS

Nerve Pathways

  1. Reflex Arc

    1. a reflex arc includes a sensory neuron, interneuron, and a motor neuron

    2. the reflex arc is the behavorial unit of the CNS

  2. Reflex Behavior

    1. reflexes are automatic, unconscious response to changes

    2. they assist to maintain homeostasis

    3. knee jerk reflex involves sensory and motor neurons

    4. withdrawl reflexes involve interneurons as well as sensory and motor

Spinal Cord

  1. Structure of the Spinal Cord

    1. spinal cord is composed of 31 segments

    2. each segment gives rise to a pair of spinal nerves

    3. characterized by a cervical enlargement, lumbar enlargement, and two deep groves that divide it into right and left halves.

    4. has a central core of gray matter that is surrounded by white matter.

    5. the white matter is composed of myelinated nerve fibers

  2. Functions of the Spinal cord

    1. provides a two way communication between brain and rest of the body.

    2. ascending tracts carry impulses to the brain; descending tracts carry impulses to muscles and glands.

    3. many of the tracts in the ascending and descending tracts crossover in the spinal cord or brain.

  3. Meninges

    1. 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 connective tissue.

    2. arachnoid mater : net like membrane, no blood vessels, between arachnoid and pia is the subarachnoid space which is filled with CSF .

    3. pia mater : thin, blood vessels, contacts surface of brain