Chapter 12

leak channels

channels that are always open and allow ions to move along their gradient. Help maintain resting membrane potential

Functional Classification of Neurons 1

Sensory neurons or afferent neurons either contain sensory receptors at their distal ends (dendrites) or are located just after sensory receptors that are separate cells. Once an appropriate stimulus activates a sensory receptor, the sensory neuron forms an action potential in its axon and the action potential is conveyed into the CNS through cranial or spinal nerves. Most sensory neurons are unipolar in structure.

voltage gated channels

open and close in response to changes in membrane potential

mechanically gated channels

open and close in response to physical deformation of receptors

Factors that affect the speed of propagation

Amount of myelination. As you have just learned, action potentials propagate more rapidly along myelinated axons than along unmyelinated axons. Axon diameter. Larger diameter axons propagate action potentials faster than smaller ones due to their larger surface areas. Temperature. Axons propagate action potentials at lower speeds when cooled.

Functional Classification of Neurons 3

Interneurons or association neurons are mainly located within the CNS between sensory and motor neurons. Interneurons integrate (process) incoming sensory information from sensory neurons and then elicit a motor response by activating the appropriate motor neurons. Most interneurons are multipolar in structure.

Ion channels

Ion channels open and close due to the presence of "gates." The gate is a part of the channel protein that can seal the channel pore shut or move aside to open the pore. The electrical signals produced by neurons and muscle fibers rely on four types of ion channels: leak channels, ligand-gated channels, mechanically-gated channels, and voltage-gated channels:

Excitatory and Inhibitory Postsynaptic Potentials 2

A neurotransmitter that causes hyperpolarization of the postsynaptic membrane is inhibitory. During hyperpolarization, generation of an action potential is more difficult than usual because the membrane potential becomes inside more negative and thus even farther from threshold than in its resting state. A hyperpolarizing postsynaptic potential is termed an inhibitory postsynaptic potential (IPSP).

neural circuits 1

A number of neurons that are connected by synapses. A single presynaptic neuron may synapse with several postsynaptic neurons. Such an arrangement, called divergence, permits one presynaptic neuron to influence several postsynaptic neurons (or several muscle fibers or gland cells) at the same time. In a diverging circuit, the nerve impulse from a single presynaptic neuron causes the stimulation of increasing numbers of cells along the circuit

electrical excitability

Like muscle cells, neurons possess electrical excitability, the ability to respond to a stimulus and convert it into an action potential

Functional Classification of Neurons 2

Motor neurons or efferent neurons convey action potentials away from the CNS to effectors (muscles and glands) in the periphery (PNS) through cranial or spinal nerves. Motor neurons are multipolar in structure.

Lamellated (Pacinian) corpuscles

They are nerve endings in the skin responsible for sensitivity to vibration and pressure.

Corpuscle of touch (Meissner corpuscle)

This corpuscle is a type of nerve ending in the skin that is responsible for sensitivity to light touch

Nociceptors

sensory neuron that responds to damaging or potentially damaging stimuli by sending "possible threat" signals to the spinal cord and the brain. If the brain perceives the threat as credible, it creates the sensation of pain to direct attention to the body part, so the threat can hopefully be mitigated; this process is called nociception.

Peripheral Nervous System (PNS)

consists of all nervous tissue outside the CNS. nerves and sensory receptors. divided into sensory and motor divisions. Sensory or afferent division of the PNS conveys input into the CNS from sensory receptors in the body. This division provides the CNS with sensory information about the somatic senses (tactile, thermal, pain, and proprioceptive sensations) and special senses (smell, taste, vision, hearing, and equilibrium). motor or efferent division of the PNS conveys output from the CNS to effectors (muscles and glands.) This division is further subdivided into a somatic nervous system and an autonomic nervous system

Central Nervous System (CNS)

consists of the brain and spinal cord. Brain contains about 85 billion neurons. Spinal cord is connected to the brain through the foramen magnum of the occipital bone and is encircled by the bones of the vertebral column. The spinal cord contains about 100 million neurons. Source of thoughts, emotions, and memories. Most signals that stimulate muscles to contract and glands to secrete originate in the CNS.

Other neurons

some neurons are named for the histologist who first described them or for an aspect of their shape or appearance; examples include Purkinje cells in the cerebellum and pyramidal cells found in the cerebral cortex of the brain, which have pyramid-shaped cell bodies

ligand-gated channels

open in the presence of a specific binding substance, usually a hormone or neurotransmitter

continuous conduction

the step-by-step depolarization and repolarization of each adjacent segment of the plasma membrane. slow conduction that occurs in nonmyelinated axons

Classification of nerve fibers

A fibers are the largest diameter axons (5-20 μm) and are myelinated. A fibers have a brief absolute refractory period and conduct nerve impulses (action potentials) at speeds of 12 to 130 m/sec (27-290 mi/hr). The axons of sensory neurons that propagate impulses associated with touch, pressure, position of joints, and some thermal and pain sensations are A fibers, as are the axons of motor neurons that conduct impulses to skeletal muscles. B fibers are axons with diameters of 2-3 μm. Like A fibers, B fibers are myelinated and exhibit saltatory conduction at speeds up to 15 m/sec (34 mi/hr). B fibers have a somewhat longer absolute refractory period than A fibers. B fibers conduct sensory nerve impulses from the viscera to the brain and spinal cord. They also constitute all of the axons of the autonomic motor neurons that extend from the brain and spinal cord to the ANS relay stations called autonomic ganglia. C fibers are the smallest diameter axons (0.5-1.5 μm) and all are unmyelinated. Nerve impulse propagation along a C fiber ranges from 0.5 to 2 m/sec (1-4 mi/hr). C fibers exhibit the longest absolute refractory periods. These unmyelinated axons conduct some sensory impulses for pain, touch, pressure, heat, and cold from the skin, and pain impulses from the viscera. Autonomic motor fibers that extend from autonomic ganglia to stimulate the heart, smooth muscle, and glands are C fibers. Examples of motor functions of B and C fibers are constricting and dilating the pupils, increasing and decreasing the heart rate, and contracting and relaxing the urinary bladder.

neural circuits 3

A fourth type of circuit is the parallel after-discharge circuit. In this circuit, a single presynaptic cell stimulates a group of neurons, each of which synapses with a common postsynaptic cell. A differing number of synapses between the first and last neurons imposes varying synaptic delays, so that the last neuron exhibits multiple EPSPs or IPSPs. If the input is excitatory, the postsynaptic neuron then can send out a stream of impulses in quick succession. Parallel after-discharge circuits may be involved in precise activities such as mathematical calculations.

Excitatory and Inhibitory Postsynaptic Potentials 1

A neurotransmitter causes either an excitatory or an inhibitory graded potential. A neurotransmitter that causes depolarization of the postsynaptic membrane is excitatory because it brings the membrane closer to threshold. A depolarizing postsynaptic potential is called an excitatory postsynaptic potential (EPSP). Although a single EPSP normally does not initiate a nerve impulse, the postsynaptic cell does become more excitable. Because it is partially depolarized, it is more likely to reach threshold when the next EPSP occurs.

Different types of neurons

Multipolar neurons usually have several dendrites and one axon. Most neurons in the brain and spinal cord are of this type, as well as all motor neurons. Bipolar neurons have one main dendrite and one axon. They are found in the retina of the eye, the inner ear, and the olfactory area (olfact = to smell) of the brain. Unipolar neurons have dendrites and one axon that are fused together to form a continuous process that emerges from the cell body. These neurons are more appropriately called pseudounipolar neurons because they begin in the embryo as bipolar neurons. During development, the dendrites and axon fuse together and become a single process. The dendrites of most unipolar neurons function as sensory receptors that detect a sensory stimulus such as touch, pressure, pain, or thermal stimuli. The trigger zone for nerve impulses in a unipolar neuron is at the junction of the dendrites and axon. The impulses then propagate toward the synaptic end bulbs. The cell bodies of most unipolar neurons are located in the ganglia of spinal and cranial nerves.

saltatory conduction

Rapid transmission of a nerve impulse along an axon, resulting from the action potential jumping from one node of Ranvier to another, skipping the myelin-sheathed regions of membrane. the special mode of action potential propagation that occurs along myelinated axons, occurs because of the uneven distribution of voltage-gated channels. Few voltage-gated channels are present in regions where a myelin sheath covers the axolemma. By contrast, at the nodes of Ranvier (where there is no myelin sheath), the axolemma has many voltage-gated channels. Hence, current carried by Na+ and K+ flows across the membrane mainly at the nodes. The flow of current across the membrane only at the nodes of Ranvier has two consequences: action potential appears to "leap" from node to node as each nodal area depolarizes to threshold, thus the name "saltatory." Because an action potential leaps across long segments of the myelinated axolemma as current flows from one node to the next, it travels much faster than it would in an unmyelinated axon of the same diameter. Opening a smaller number of channels only at the nodes, rather than many channels in each adjacent segment of membrane, represents a more energy-efficient mode of conduction. Because only small regions of the membrane depolarize and repolarize, minimal inflow of Na+ and outflow of K+ occurs each time an action potential passes by. Thus, less ATP is used by sodium-potassium pumps to maintain the low intracellular concentration of Na+ and the low extracellular concentration of K+.

Merkel discs

The Merkel disc, a main type of tactile end organ consisting of Merkel cells (MCs) and Aβ-afferent endings, are highly abundant in fingertips, touch domes, and whisker hair follicles of mammals.

What are functions of the nervous system?

Sensory function. Sensory receptors detect internal stimuli, such as an increase in blood pressure, or external stimuli (for example, a raindrop landing on your arm). This sensory information is then carried into the brain and spinal cord through cranial and spinal nerves. Integrative function. The nervous system processes sensory information by analyzing it and making decisions for appropriate responses—an activity known as integration. Motor function. Once sensory information is integrated, the nervous system may elicit an appropriate motor response by activating effectors (muscles and glands) through cranial and spinal nerves. Stimulation of the effectors causes muscles to contract and glands to secrete.

White matter/Grey matter

White matter is composed primarily of myelinated axons. The whitish color of myelin gives white matter its name. gray matter of the nervous system contains neuronal cell bodies, dendrites, unmyelinated axons, axon terminals, and neuroglia. It appears grayish, rather than white, because the Nissl bodies impart a gray color and there is little or no myelin in these areas. Blood vessels are present in both white and gray matter. In the spinal cord, the white matter surrounds an inner core of gray matter that, depending on how imaginative you are, is shaped like a butterfly or the letter H in transverse section; in the brain, a thin shell of gray matter covers the surface of the largest portions of the brain, the cerebrum and cerebellum

Neuron

a nerve cell; the basic building block of the nervous system. Have three parts: (1) a cell body, (2) dendrites, and (3) an axon. perikaryon or soma, contains a nucleus surrounded by cytoplasm that includes typical cellular organelles such as lysosomes, mitochondria, and a Golgi complex. Neuronal cell bodies also contain free ribosomes and prominent clusters of rough endoplasmic reticulum, termed Nissl bodies. Newly synthesized proteins produced by Nissl bodies are used to replace cellular components, as material for growth of neurons, and to regenerate damaged axons in the PNS. The cytoskeleton includes both neurofibrils, composed of bundles of intermediate filaments that provide the cell shape and support, and microtubules, which assist in moving materials between the cell body and axon. Aging neurons also contain lipofuscin, a pigment that occurs as clumps of yellowish brown granules in the cytoplasm. Lipofuscin is a product of neuronal lysosomes that accumulates as the neuron ages, but does not seem to harm the neuron. A collection of neuron cell bodies outside the CNS is called a ganglion. (ganglia is plural).

neural circuits 2

convergence, several presynaptic neurons synapse with a single postsynaptic neuron. This arrangement permits more effective stimulation or inhibition of the postsynaptic neuron. In a converging circuit (Figure 12.28b), the postsynaptic neuron receives nerve impulses from several different sources. Some circuits are organized so that stimulation of the presynaptic cell causes the postsynaptic cell to transmit a series of nerve impulses. One such circuit is called a reverberating circuit. In this pattern, the incoming impulse stimulates the first neuron, which stimulates the second, which stimulates the third, and so on. Branches from later neurons synapse with earlier ones. This arrangement sends impulses back through the circuit again and again. The output signal may last from a few seconds to many hours, depending on the number of synapses and the arrangement of neurons in the circuit. Inhibitory neurons may turn off a reverberating circuit after a period of time. Among the body responses thought to be the result of output signals from reverberating circuits are breathing, coordinated muscular activities, waking up, and short-term memory.

Somatic nervous system

conveys output from the CNS to skeletal muscles only. Because its motor responses can be consciously controlled, the action of this part of the PNS is voluntary.

Autonomic nervous system

conveys output from the CNS to smooth muscle, cardiac muscle, and glands. Because its motor responses are not normally under conscious control, the action of the ANS is involuntary. Comprised of two main branches, the sympathetic nervous system and the parasympathetic nervous system. With a few exceptions, effectors receive innervation from both of these branches, and usually the two branches have opposing actions. For example, neurons of the sympathetic nervous system increase heart rate, and neurons of the parasympathetic nervous system slow it down. In general, the parasympathetic nervous system takes care of "rest-and-digest" activities, and the sympathetic nervous system helps support exercise or emergency actions—the so-called "fight-or-flight" responses. A third branch of the autonomic nervous system is the enteric nervous system (ENS) (en-TER-ik; enteron = intestines), an extensive network of over 100 million neurons confined to the wall of the gastrointestinal (GI) tract. The ENS helps regulate the activity of the smooth muscle and glands of the GI tract. Although the ENS can function independently, it communicates with and is regulated by the other branches of the ANS.