A&p Ch. 11

When a nerve is injured, there are 2 possible outcomes.

1. When the two ends of an injured axon are ALLIGNED in close proximity, healing and regeneration of the axon are likely to occur.

2. When the two ends of an injured axon are NOT ALLIGNED in close proximity, regeneration is unlikely to occur.

* surgically can bring the ends of the severed nerve close together.

* regeneration of damaged nerve tracts within the CNS are very limited, especially when compared to the regeneration of nerves in the PNS.

1. Astrocytes: provide structural support for neurons & blood vessels. Influence the functioning of the blood-brain barrier and process substances that pass through it. Isolate damaged tissue & limit the spread of inflammation. Also help to maintain synaptic function.

2. Ependymal cells: line the ventricles of the brain & the central canal of the spinal cord. Some are specialized to produce CSF fluid. These plus astrocytes both help in the blood-brain barrier.

3. Microglia: phagocitize microorganisms, foreign substances & necrotic tissue. Machrophages of the brain, also protect neurons.

4. Oligodendrocytes: (all over branching cells) form myelin sheaths around the axons of several CNS neurons. *supportive.

1. Schwann cells: form a myelin sheath around the axons of several CNS neurons. *protect

2. Satellite cells: support and nourish neuron cell bodies within ganglia.

A grades potential is a relatively small change in the membrane potential. The local disturbances in the membrane potential are called local potentials because the potential change can vary from small to large. The detection of stimulus input to the neuron, causes sodium channels (ligand-gated in channels) to open.

If enough NA+ enters, the graded potentials can sumate at the trigger zone in the axon hillock. When the graded potentials sumate to a level called threshold (-60mv), an action potential occurs. Threshold is the membrane potential at which voltage-gated NA+ channels open, AP's are initiated here. Precisely, AP's occur when a neuron sends information down an axon (electrical signals produced). Graded potentials can be either a depolarization or a hyperpolarization. Depolarizing graded potentials that sumate to to threshold, produce an AP, but hyperpolarizing graded potentials can never reach threshold and do not produce AP's.

An AP has a depolarization phase in which the membrane potential moves away from the resting state and becomes MORE POSITIVE. The inside of the membrane becomes more positive because of the inward movement of NA+ due to the opening of NA+ channels & K+ channels also begin to open.

A repolarization phase is in which the membrane potential returns toward the resting state and becomes MORE NEGATIVE. NA+ channels close and additional K+ channels open. Na+ movement into the cells stops and K+ movement out of the cell increases, causing the repolarization.

Hyperpolarization occurs after the repolarization phase and the plasma membrane may be slightly hyperpolarized for a short period of time, called the after potential. (K+ is leaving through voltage-gated K+ channels- slow to close).

The refractory period: once an AP is produced at a given point on the plasma membrane, that area becomes less sensitive to further stimulation.

The first part of the refractory period, during which complete insensitivity exists to another stimulus is called the absolute refractory period. Time during which an AP when a second stimulus, no matter how strong, CANNOT INITIATE another AP.

The second part of the refractory period, is called the relative refractory period, follows the absolute refractory period. Time during which a stronger than threshold stimulus CAN EVOKE another AP.

Summation of graded potentials occurs when the effects produced by one graded potential combine with the effects produced by a different graded potential elsewhere on the plasma membrane, which could lead to an AP.

1. Spatial summation: occurs when multiple AP's from separate neurons arrive simultaneously at the same postsynaptic neuron.

2. Temporal summation: results when two or more AP's arrive very close together at the postsynaptic cell from the presynaptic terminal of a particular neuron.

We see that AP's can propagate (promote) or spread across the plasma membrane. In a neuron, AP's are normally produced at the trigger zone and propagate along the axon. The location at which the next AP is generated is different for unmyelinated axons and myelinated axons.

In unmyelinated axons, the next AP is generated in the membrane immediately adjacent to the previous AP.

When an AP is produced, the inside of the membrane becomes more positive than the outside. This process is called the continuous conduction, which involves the sequential opening of voltage-gated NA+ and K+ channels located within the axon membranes along the entire length of the axon to propagate an impulse.

This is the slower conduction because the nerve impulse travels down the whole unmyelinated neuron.

In satatory conduction the nerve impulse will jump between the nodes of ranvier, therefore, the local current (movement of positively charged ions) quickly flows to a node and stimulates the voltage-gated NA+ channels to open. Resulting in the production of an AP. Myelinated axons are wrapped by several layers of plasma membrane from schwann cells, making them larger in diameter.

Quicker conduction.

Astrocytes and ependymal cells

Microglia

4. Oligodendrocytes: (all over branching cells) form myelin sheaths around the axons of several CNS neurons. *supportive.