Biology: Action Potential and Spinal Cord

Topics: Action potential, Nervous system, Neuron Pages: 9 (2528 words) Published: June 27, 2013
ckingmotherCoordination by Neural Signaling

26.1 Invertebrates reflect an evolutionary trend toward bilateral symmetry and cephalization * Invertebrate Nervous Organization
* In simple animals, such as sponges, the most common observable response is closure of the osculum (central opening) * Hydras (cnidarians) have a nerve net that is composed of neurons * Planarians, (flatworms) have a ladderlike nervous system * In annelids (earthworm), arthropods (crab), and molluscs (squid) the nervous system shows further advances * Cephalization - concentration of ganglia and sensory receptors in a head region * Ganglion (pl. ganglia) - cluster of neurons

Vertebrate Nervous Organization
* Cephalization, and bilateral symmetry, results in paired sensory receptors to gather information about environment * Eyes, ears, and olfactory structures
* Central nervous system (CNS)
* Spinal cord and brain and develops from an embryonic dorsal neural tube * Ascending tracts carry sensory information to the brain, and descending tracts carry motor commands to the neurons in the spinal cord that control the muscles * Vertebrate brain divided into three parts

* Hindbrain - most ancient part and regulates motor activity below the level of consciousness * Midbrain - optic lobes are part of the midbrain and was a center for coordinating reflexes involving the eyes and ears * Forebrain - originally dealt mainly with smell. Later, the thalamus evolved to receive sensory input from the midbrain and the hindbrain and to pass it on to cerebrum * Cerebrum integrates sensory and motor input and is particularly associated with higher mental capabilities

26.2 Humans have well-developed central and peripheral nervous systems * Peripheral nervous system (PNS) consists of all the nerves and ganglia that lie outside the CNS * All signals that enter and leave the CNS travel through paired nerves * Those connected to spinal cord are spinal nerves * Those attached to brain are cranial nerves

* Somatic sensory axons (fibers) send signals from the skin and special sense organs * Visceral sensory fibers convey information from the internal organs * CNS and PNS must work in harmony to carry out three primary functions * Receive sensory input

* Perform integration
* Generate motor output

26.3 Neurons are the functional units of a nervous system
* Neurons (nerve cells) receive sensory information and convey information to an integration center * Three major parts
* Cell body - contains a nucleus and a variety of organelles * Dendrites - short, highly branched processes receive signals from sensory receptors or other neurons and transmit them to cell body * Axon - portion of the neuron that conveys information to another neuron or to other cells * Axons bundle together to form nerves and are often called nerve fibers * Axons are covered by a white insulating layer called the myelin sheath * Neuroglia - cells that provide support and nourishment to the neurons * Myelin sheath is formed from membranes of tightly spiraled neuroglia * In PNS, Schwann cells perform this function, leaving gaps called nodes of Ranvier, or neurofibril nodes

Types of Neurons
* Motor (efferent) neurons carry nerve impulses from CNS to muscles or glands * Have many dendrites and a single axon
* Cause muscle to contract or glands to secrete
* Sensory (afferent) neurons take nerve impulses from sensory receptors to the CNS * Sensory receptors may be the end of a sensory neuron itself (a pain or touch receptor), or may be a specialized cell that forms a synapse with a sensory neuron * Interneurons (association neurons) occur entirely within the CNS * Parallel the structure of motor neurons and convey nerve impulses between various...
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