Sistema Nervioso

13.1 Overview of the Nervous System
The nervous system has two major divisions, the central nervous system consisting of the brain and spinal cord, and the peripheral nervous system consisting of nerves. The nervous system has three specific functions: receive sensory input, perform integration, and generate motor output.

Nervous Tissue
The nervous system contains neurons that conductimpulses and neuroglial cells that service neurons.

Neuron Structure
Sensory neurons take information from sensory receptors to the central nervous system (CNS); interneurons occur within the CNS, and motor neurons take information from the CNS to effectors (muscles or glands). Neurons are composed of dendrites, a cell body, and an axon. Long axons are covered by a myelin sheath.

Myelin SheathLong axons are covered by a myelin sheath formed by neuroglial cells called Schwann cells, interrupted by gaps called nodes of Ranvier. A myelin sheath gives nerve fibers their white, glistening appearance and plays an important role in nerve regeneration within the peripheral nervous system (PNS).

The Nerve Signal
Nerve signals convey information within the nervous system.

Resting PotentialWhen an axon is not conducting a nerve impulse, the inside of an axon is negative (-65mV) compared to the outside. The sodium-potassium pump actively transports Na+ out of an axon and K+ to inside an axon. The resting potential is due to the leakage of K+ to the outside of the neuron.

Action Potential
An action potential is a rapid change in polarity as the nerve signal occurs. It is anall-or-none phenomenon and occurs only when threshold is reached.

Sodium Gates Open
The gates of sodium channels open first and Na+ flows into the axon. The membrane potential depolarizes.

Potassium Gates Open
The gates of potassium channels open and K+ flows into the axon. The membrane potential repolarizes.

Visualizing an Action Potential
Researchers find it useful to plot the voltagechanges over time.

Propagation of an Action Potential
The action potential occurs in each successive portion of an axon. A refractory period ensures that the action potential will not move backwards. In myelinated fibers the action potential only occurs at the nodes of Ranvier. This is called saltatory conduction.

The Synapse
Transmission of the nerve impulse from one neuron to another takesplace at a synapse when a neurotransmitter molecule is released from an axon bulb into a synaptic cleft. The binding of the neurotransmitter to receptors in the postsynaptic membrane causes either excitation or inhibition. Once a neurotransmitter has been released into a synaptic cleft and has initiated a response, it is removed from the cleft.


Neurotransmitter Molecules
Two well knownneurotransmitters are acetylcholine (Ach) and norepinephrine (NE) but at least 100 different neurotransmitters are known or suspected. Many drugs work by interfering with the action of neurotransmitters.

Synaptic Integration
Excitatory signals have a depolarizing effect, and inhibitory signals have a hyperpolarizing effect on the post synaptic membrane. Integration is the summing up of thesesignals.


13.2 The Central Nervous System
The CNS consists of the spinal cord and brain, which are both protected by bone, meninges, and cerebrospinal fluid. The CNS receives and integrates sensory input and formulates motor output. The CNS is composed of short, nonmyelinated gray matter and myelinated tracts called white matter.


The Spinal Cord
The spinal cord extends from the baseof the brain and is located in the vertebral canal formed by the vertebrae.


Structure of the Spinal Cord
The gray matter of the spinal cord contains neuron cell bodies; the white matter consists of myelinated axons that occur in bundles called tracts. Because these tracts cross over, the left side of the brain controls the right side of the body and vice versa. The spinal cord also has...