The Nervous System (Pgs. 167-176)

The Nervous System Organs of the nervous system are divided into Central Nervous System (CNS) Peripheral Nervous System (PNS)

Functions of the Nervous System Sensory Integrative Motor

Functions of the Nervous System Sensory functions detect changes Sensory receptors at the end of peripheral nerves detect changes inside/outside of the body External – light, sound touch Internal – pH levels, oxygen/carbon dioxide concentrations Information gathered is converted to nerve impulses of the PNS which sends the information to the CNS

Functions of the Nervous System Integrative functions bring sensory information together and makes decisions that are acted upon by using motor function

Functions of the Nervous System Motor functions are responses to sensory information Employ peripheral nerves that carry impulses from CNS to responsive parts called effectors Effectors are outside the NS and include a. Muscles – contract when stimulated b. Glands – secrete hormones when stimulated (glandular epithelial tissue)

Glandular Epithelium

Functional Organization

Anatomical and Functional Organizaton

Nervous Tissue Neurons (nerve cells) are the functional unit; Specialized to react to physical & chemical changes in their surroundings and conduct nerve impulses Neuroglia support the physiological needs of neurons Fig. 8-2, 8-3, 3-21 pg. 70

Anatomy of a Neuron

Anatomy of a Neuron dendrite – provide receptive surfaces to carry impulses toward cell body; relatively short & highly branched cell body – contains various organelles axon – a single axon arises from cell body; transmit impulses away from the cell body; some are surrounded by specialized glia cells called Schwann cells that form myelin sheath

Functional Differences of Neurons motor neurons – transmit impulses form brain to an effector (efferent neurons) (multipolar) sensory neurons – transmit impulses to spinal cord & brain (afferent neurons) (most unipolar, some bipolar) interneurons – carry impulses from sensor neurons to motor neurons (multipolar)

Refex Arc

Structural Differences of Neurons bipoloar – only 2 nerve fibers, one arising from either end; found in specialized parts of the eyes, nose, & ears

Structural Differences of Neurons unipolar/monopolar – single nerve fiber extending from its cell body; found in ganglia outside the brain or spinal cord

Structural Differences of Neurons multipolar – many nerve fibers arising from their cell bodies; most common type of neuron in the brain & spinal cord (ganglia – a mass of neuron cell bodies, usually outside the CNS)

Neuroglia Cells Accessory cells – Schwann cells, astrocytes, microglia, oligodendrocytes, ependymal (We will discuss the functions of each of these types of cell during the lab.) Fill spaces, support neurons, hold nervous tissue together; play a role in the metabolism of glucose, help regulate K+ concentration, produce myelin, and carry on phagocytosis Fig. 8-3

Types of Neuroglia Cells

Regeneration of Nerve Fibers If a neuron cell body is injured, the neuron is likely to die If the axon of a peripheral nerve fiber is severed, its distal portion will die, but the proximal portion may regenerate & re-establish its former connections Significant regeneration is unlikely to take place in the CNS

  Structure of Peripheral Nerves – consists of bundles of nerve fibers surrounded by connective tissue – Fig. 8-4 Epineurium – outermost layer; dense and include many collagenous fibers Fasicicle – a bundle of nerve fibers Perineurium – less dense connective tissue surrounding fascicle Endoneurium – small amount of loose connective tissue that surrounds individual nerve fibers

Cell Membrane Potential and Nerve Impulses – Fig. 8-6 & 8-7

Resting Potential http://www.dnatube.com/video/5035/Neuron-Resting-Potential

  A cell membrane is usually electrically charges or polarized so that outside is + and inside is - Resting Potential (-70 mvolts) Nerve cell is not conducting impulses [Na+] is greater on the outside of the cell and [K+] is greater on the inside of the cell There is a large number of negatively charged ions inside the cell which can’t diffuse out At rest inside stays negative because K+ can diffuse easily out of the cell through open channels; Na+ can’t diffuse as easily into cells through “their” protein channels Na+ /K+ pump (active transport) – maintains system so equilibrium is not reached; therefore, Na+ is always being pumped back out and K+ is being pumped back in

Local Potential Changes

Local Potential Changes Stimulation of a membrane affects its resting potential in a local region (light, temp., other neurons Membrane starts to become depolarized (moves toward zero) Threshold potential is reached which causes an action potential

Action Potential http://www.dnatube.com/video/1105/Understanding-Action-Potential-and-Nerve-Impulses

Action Potential 1/1000 sec. or less At threshold, Na+ channels open and Na+ diffuse inward causing depolarization About the same time K+ channels open and K+ diffuses outward, causing repolarization This rapid change in potential is an action potential Many action potentials can occur before an active transport mechanism re-establishes the original resting potential The propagation of actions potentials along a nerve fiber is an impulse

Refractory Period Refractory Period A brief time (10-30 m/sec.) following the passage of a nerve impulse when the membrane is unresponsive to ordinary stimuli Membrane must return to resting potential before it can be stimulated again All – or – None Response If a nerve fiber responds at all, it responds completely All impulses carried on that fiber will be of the same strength

Coding & Interpretation of Messages Frequency of action potentials – a weak stimulus initiates only a few action potentials/sec., a strong stimulus initiates many (upper limit because of refractory period) Duration of a burst of action potentials – a weak stimulus may give rise to a short burst of pulses in the neuron, a strong stimulus a longer burst Number & kinds of neurons firing – The threshold needed to initiate a nerve impulse varies from one neuron to another. Thus a weak stimulus will cause only a few neurons to fire, strong will fire all of these neurons, plus others with higher thresholds.

Impulse Conduction Myelinated vs Unmyelinated

Impulse Conduction Unmyelinated fibers conduct impulses that travel over their entire surface Myelinated – impulses travel from node to node Impulse conduction is more rapid on myelinated fibers with large diameters http://www.youtube.com/watch?v=DJe3_3XsBOg

The Synapse – The junction between 2 neurons The Synapse – The junction between 2 neurons. A synaptic cleft is the gap between parts of two neurons at a synapse. Fig. 8-7 Impulses usually travel from a dendrite or cell body, then along the axon to a synapse Axons have synaptic knobs at their distal ends that secrete neurotransmitters The neurotransmitter is released when a nerve impulse reaches the end of an axon and the neurotransmitter diffuses across the synaptic cleft. When the neurotransmitter reaches the nerve fiber on the distal side of the cleft, a nerve impulse is triggered.

The Synapse http://www.dnatube.com/video/261/Neural-Synapse

Nerve Pathways – the route followed by an impulse as it travels through the nervous system, Fig. 8-5 Reflex Arc – simplest nerve pathway A reflex arc usually includes a sensory neuron, a reflex center composed of interneurons, and a motor neuron Reflex arc is the behavioral unit of the nervous system Reflex Behavior Reflexes are automatic unconscious responses to changes Help maintain homeostasis Knee jerk – 2 neurons Withdrawal reflexes are protective actions

Reflex Arc http://www.youtube.com/watch?v=Y5nj3ZfeYDQ&feature=related