Anatomy exam ch 12

Central Nervous System (CNS)

Central Nervous System (CNS)

Brain and spinal cord

Peripheral Nervous System (PNS)

Cranial nerves, spinal nerves, enteric plexuses in small intestine, and sensory receptors in skin

Functions of the nervous system

Sensory, motor, and integrative

Organizations of PNS

Somatic nervous system, autonomic system

Organization of autonomic sytem

Sympathetic, parasympathetic, and enteric

Sensory function

Sensory receptors detect internal stimuli, such as an increase in blood pressure, or external stimuli. 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 through cranial and spinal nerves. Stimulation of the effectors causes muscles to contract and glands to secrete.

Dendrites

Receiving or input portions of a neuron

Cell body

Contains a nucleus surrounded by cytoplasm that includes typical cellular organelles such as lysosomes, mitochondria, and a Golgi complex

Nissl bodies

Free ribosomes and prominent clusters of rough endoplasmic reticulum and produces proteins

Axon

A long, thin, cylindrical projection that often joints to the cell body at a cone-shaped elevation called the axon hillock

Myelin sheath

Multilayered lipid and protein covering around some axons that insulates them and increases the speed of nerve impulse conduction

Myelin sheath gaps (nodes of Ranvier)

gaps in the myelin sheath that appear at intervals along the axon- each schwann cell wraps one axon segment between two myelin sheath gaps

Axon terminal

Axon and its collaterals end by dividing into these fine processes

Synaptic end bulb

Tips of some axon terminals that swell into bulb-shaped structures

Multipolar neurons

Have several dendrites and one axon- most neurons in the brain and spinal cord are of this type

Bipolar neurons

Have one main dendrite and one axon- found in the retina of the eye, the inner ear, and the olfactory area

Unipolar neurons

Have dendrites and one axon that are fused together to form a continuous process that emerges from the cell body

Sensory/afferent neuron

Contain sensory receptors at their distal ends or are located just after sensory receptors that are separate cells. Once an appropriate stimulus activates a sensory receptor, the sensory neuron forms a nerve impulse in its axon and the nerve impulse is conveyed into the CNS through cranial or spinal nerves. (usually unipolar)

Motor neurons/efferent neurons

Convey nerve impulses away from the CNS to effectors in the PNS through cranial or spinal nerves. (usually multipolar)

Interneurons/association neurons

Mainly located within the CNS between sensory and motor neurons. Interneurons integrate incoming sensory information from sensory neurons and then elicit a motor response by activating the appropriate motor neurons. (usually multipolar)

Protoplasmic astrocytes

Have many short branching processes and are found in gray matter

Fibrous astrocytes

Have many long unbranched processes and are located mainly in white matter

Atrocytes function

Contain microfilaments that give them strength and support neurons; isolate neurons of the CNS from various potentially harmful substances in blood and create blood brain barrier; regulate growth, migration, and interconnection among neurons; maintain chemical environment and learning and memory

Neuroglia found in CNS

Astrocytes, Oligodendrocytes, microglial cells, ependymal cells

Neuroglia of the PNS

Schwann cells, satellite cells

Oligodendrocytes

Responsible for forming and maintaining the myelin sheath around CNS axons

Microglial cells

Function as phagocytes, they remove cellular debris formed during normal development of the nervous system

Ependymal cells

Cuboidal to columnar cells arranged in a single layer that possess microvilli and cilia; line the ventricles of the brain and central canal of the spinal cord

Schwann cells

Encircle PNS axons, form myelin sheath around axons; each schwann cell myelinates a single axon

Satellite cells

Regulate the exchanges of materials between neuronal cell bodies and interstitial fluid

White matter

Composed primarily of myelinated axons

Gray matter

Contains neuronal cell bodies, dendrites, unmyelinated axons, axon terminals, and neuroglia. Nissl bodies impart a gray color and there is little to no myelin in these areas

Graded potentials

Used for short-distance communication

Action potentials

Allow communication over long distances within the body

Leak channels

These gates randomly alternate between open and closed positions

Ligand-gated channel

These gates opens and closes in response to the binding of a ligand stimulus- located in the dendrites of some sensory neurons, such as pain receptors and dendrites and cell bodies of interneurons and motor neurons

Mechanically gated channel

Gates open or close in response to mechanical stimulation in the form of vibration, touch, pressure, or tissue stretching. Found in auditory receptors in the ears

Voltage gated channel

Opens in response to a change in membrane potential

Factors that maintain a resting membrane potential

Unequal distribution of ions, inability of most ions to leave the cell, electrogenic nature of Na-K ATPase

Unequal distribution of ions

Extracellular fluid is rich in Na+ ions, cytosol is rich in K+ ions. The plasma membrane has more K+ leak channels, the number of potassium ions that diffuse down their concentration gradient out of the cell into the ECF is greater than the number of sodium ions that diffuse down their concentration gradient from the ECF into the cell.

Inability of most anions to leave the cell

Most anions inside the cell are not free to leave, they cannot follow the K+ out of the cell because they are attached to nondiffusible molecules such as ATP and large proteins

Electrogenic nature of the Na+/K+ ATPases

The pumps help maintain the resting membrane potential by pumping out Na+ as fast as it leaks in. The pumps remove more positive charges from the cell than they bring into the cell, they are electrogenic

Hyperpolarizing graded potential

The response makes the membrane more polarized

Depolarizing graded potential

When the response makes the membrane less polarized

Graded potential occurrence

A stimulus causes mechanically gated or ligand-gated channels to open or close in an excitable cell's plasma membrane- occur mainly in the dendrites and cell body of a neuron

Depolarizing phase

Voltage-gated Na+ channels open rapidly- both the electrical and chemical gradients favor the inward movement of Na+; inflow of Na+ changes membrane potential from -55mV to +30mV

Repolarizing phase

Shortly after the activation gates of the voltage-gated Na+ channels open, the inactivation gates close. A threshold-level depolarization also opens voltage-gated K+ channels. Slowing of Na+ inflow and acceleration of K+ outflow cause the membrane potential to change from +30mV to -70mV

After-hyperpolarizing phase

While the voltage-gated K+ channels are open, outflow of K+ may be large enough to cause an after-hyperpolarizing phase of the nerve impulse. During this phase, the voltage-gated K+ channels remain open and the membrane potential becomes even more negative (about -90mV)

Refractory Period

The period of time after nerve impulse begins during which an excitable cell cannot generate another nerve impulse in response to a normal threshold stimulus.

Factors that affect the speed of propagation

amount of myelination, axon diameter, temperature

A fibers

largest diameter axons, myelinated. The axons of sensory neurons that propagate impulses associated with touch, pressure, position of joints, and some thermal and pain sensations are these fibers

B fibers

myelinated and exhibit saltatory conduction. have a longer absolute refractory period than A fibers; conduct sensory nerve impulses from the viscera to the brain and spinal cord

C fibers

smallest diameter axons, and are unmyelinated. exhibit the longest absolute refractory periods. conduct some sensory impulses for pain, touch, pressure, heat, and cold from the skin. autonomic motor fibers that extend from autonomic ganglia to stimulate the heart, smooth muscle, and glands are C fibers

Excitatory postsynaptic potential

depolarizing postsynaptic potential

Inhibitory postsynaptic potential

a hyperpolarizing postsynaptic potential

Spatial summation

summation of postsynaptic potentials in response to stimuli that occur at different locations in the membrane of a postsynaptic cell at the same time

Temporal summation

summation of postsynaptic potentials in response to stimuli that occur at the same location in the membrane of the postsynaptic cell but at different times