Chpt 6 Section A: Cells of the Nervous System

central nervous system

(CNS), composed of the brain and spinal cord

peripheral nervous system

(PNS), consisting of the nerves that connect the brain and spinal cord with the body's muscles, glands, sense organs, and other tissues.

sensory receptors

At their peripheral ends (the ends farthest from the CNS) of afferent neurons. respond to various physical or chemical changes in their environment by generating electrical signals in the neuron. The receptor region may be a specialized portion of the plasma membrane or a separate cell closely associated with the neuron ending.

astrocyte

CNS glial cell, the astrocyte, helps regulate the composition of the extracellular fluid in the CNS by removing potassium ions and neurotransmitters around synapses. Also, they stimulate the formation of tight junctions between the cells that make up the walls of capillaries found in the CNS. This forms the blood-brain barrier, which is a selective filter. Astrocytes also sustain the neurons metabolically— for example, by providing glucose and removing the secreted metabolic waste product ammonia. In embryos, astrocytes guide CNS neurons as they migrate to their ultimate destination, and they stimulate neuronal growth by secreting growth factors. Have many neuronlike characteristics. For example, they have ion channels, receptors for certain neurotrans- mitters and the enzymes for processing them, and the capability of generating weak electrical responses.

retrograde

Dynein movement is in the other direction, from the axon terminals to the cell body, carrying recycled membrane vesicles, growth factors, and other chemical signals that can affect the neuron's morphology, biochemistry, and connectivity. Retrograde transport is also the route by which some harmful agents invade the CNS, including tetanus toxin and the herpes simplex, rabies, and polio viruses.

nerves of PNS

Groups of afferent and efferent neuron axons, together with myelin, connective tissue, and blood vessels, form the nerves of the PNS

oligodendrocytes

In the brain and spinal cord, these myelin- forming cells are a type of glial cell . Each oligodendrocyte may branch to form myelin on as many as 40 axons.

anterograde

Kinesin transport mainly occurs from the cell body toward the axon terminals

Interneurons

Lie ONLY in the CNS and connect neurons within the CNS. As a rough estimate, for each afferent neuron entering the CNS, there are 10 efferent neurons and 200,000 interneurons. Thus, they account for over 99% of all neurons. The number of interneurons interposed between specific afferent and efferent neurons varies according to the complexity of the action they control.

how do neurons operate?

Neurons operate by generating electrical signals that move from one part of the cell to another part (a) of the same cell or to neighboring cells. Most neu- rons serve as integrators because their output reflects the balance of inputs they receive from up to hundreds of thousands of other neurons.

varicosities

Sometimes axon terminal isn't responsible for sending chemical messages, but rather some neurons release their chemical messengers from a series of bulging areas along the axon known as varicosities.

glial cells

The other major cell types of the nervous system are non- neuronal cells called glial cells. These cells generally do not participate directly in electrical communication from cell to cell as do neurons, but they are very important in various supportive functions for neurons. Unlike most neurons, glial cells retain the capacity to divide throughout life (larger cause of cancer)

nodes of Ranvier

The spaces between adjacent sections of myelin where the axon's plasma membrane is exposed to extracellular fluid.

axonal transport

To maintain the structure and function of the axon, vari- ous organelles and other materials must move as far as 1 meter between the cell body and the axon terminals. This movement, termed axonal transport, depends on a scaffolding of microtubule "rails" running the length of the axon and specialized types of motor proteins known as kinesins and dyneins

axon central process vs. peripheral process

afferent neurons have a distinct shape. Shortly after leaving the cell body, the axon divides. One branch, the peripheral process, begins where the afferent terminal branches converge from the receptor endings. The other branch, the central process, enters the CNS to form junctions with other neurons. (note that the blob is the cell body)

three types of neurons

afferent, efferent, and interneurons

axon hillock

also called initial segment, The region of the axon that arises from the cell body.

plasticity

brain has an amazing abil- ity to modify its structure and function in response to stimulation or injury, involve the generation of new neurons, but particularly involves the remodeling of synaptic connections. depends on age

collaterals

branches of the axon. Near their ends, both the axon and its collaterals undergo further branching (see Figure 6.1). The greater the degree of branching of the axon and axon collaterals, the greater the cell's sphere of influence.

neuron structure

cell body (or soma) contains the nucleus and ribosomes and thus has the genetic information and machinery necessary for protein synthesis. dendrites are a series of highly branched outgrowths. receive incoming information from other neurons; increase a cell's surface area. dendritic spines increase surface area of dendrites futher. axon has branches called axon collateral and axon terminals.

Efferent neurons

convey information away from the CNS to EFFECTOR cells like muscle, gland, or other cell types

Afferent neurons

convey information from the tissues and organs of the body toward the CNS

sheaths of myelin

cover the axons of many neurons. usually consists of 20 to 200 layers of highly modified plasma membrane wrapped around the axon by a nearby supporting cell. speeds up conduction of the electrical signals along the axon and conserves energy.

axon terminal

each axon branch ends in an axon terminal. This is responsible for releasing neurotransmitters from the axon.

Regeneration of Axons

f axons are severed, they can repair themselves and restore signif- icant function provided that the damage occurs outside the CNS and does not affect the neuron's cell body. After such an injury, the axon segment that is separated from the cell body degenerates. The part of the axon still attached to the cell body then gives rise to a growth cone, which grows out to the effector organ so that function can be restored.

neuron

functional unit of the nervous system, the individual cell.

Schwann cells

glial cells in the PNS. form individual myelin sheaths surrounding 1- to 1.5-mm-long segments at regular intervals along some axons.

the growth cone

in the embryo after last cell division when each neuronal daughter cell differentiates/migrates to final location, this SPECIALIZED ENLARGMENT forms the tip of each extending axon and is involved in finding the correct route and final target for the process. Once the target of the advancing growth cone is reached, synapses form.

synapse

junction between two neurons. signal transmitted with neurotransmitters goes through the synapse. postsynaptic neuron may have thou- sands of synaptic junctions on the surface of its dendrites and cell body, so that signals from many presynaptic neurons can affect it.

ependymal cells

line the fluid-filled cavities within the brain and spinal cord and regulate the production and flow of cerebrospinal fluid, which will be described later.

axon, also called nerve fiber

long process that extends from the cell body and carries outgoing signals to its target cells. range in length depending on purpose.

kinesins and dyneins

motor proteins that move along microtubules in neurons to allow for axonal transport. At one end, these double-headed motor proteins bind to their cellular cargo, and the other end uses energy derived from the hydrolysis of ATP to "walk" along the microtubules.

nerve vs. nerve fiber

nerve: a bundle of axons. nerve fiber: used to refer to a single axon,

postsynaptic neuron

neuron conducting signals away from a synapse

presynaptic neuron

neuron that conducts a signal toward a synapse

cell adhesion molecules

reside on the membranes of the glia and embryonic neurons, help neurons find correct place in embryo

microglia

specialized, macrophage-like cells that perform immune functions in the CNS, and may also contribute to synapse remodeling and plasticity

neurotransmitters

these chemical messengers are released by electrochemical signal from neuron; used to communicate with other cells. diffuse across an extracellular gap (synapse) to the cell opposite the terminal. The neurotransmitters released from one neuron alter the receiving neuron by binding with specific protein receptors on the membrane of the receiving neuron.