The Nervous System Ch. 12 & 13 Lindsey Bily Austin High School Anatomy & Physiology.

Lindsey BilyAustin High SchoolAnatomy & Physiology

Made up of the brain, spinal cord and nerves.

The purpose of the nervous system is to detect changes in internal and external environment, evaluate the information, and possibly respond by causing changes in the muscles or glands.

Divided into the Central and Peripheral Nervous System.

Central Nervous System: brain and spinal cord and nerves that lie completely within the brain and spinal cord.

Peripheral Nervous System: nerve tissues that lie in the “outer regions” of the nervous system.

Cranial Nerves: nerves that originate in the brain.

Spinal Nerves: nerves that originate in the spinal cord.

CNS

PNS

Obviously, signals go to the brain and back out of it, so we need nerves to send messages both ways.

Afferent Division: incoming or sensory pathways (usually blue in diagrams).

Efferent Division: outgoing or motor pathways (usually red in diagrams).

Somatic Nervous System: carry information to the skeletal muscle cells. Voluntary.

Autonomic Nervous System: carry information to the smooth and cardiac muscles, and glands. Involuntary. Sympathetic: “fight or flight” Parasympathetic: normal resting activities

“rest and repair”.

Glia or Glial cells: do not conduct information but support the function of neurons.

Neurons: excitable cells that conduct nerve impulses.

Neurons

Glial Cells (gray)

Glia literally means “glue”.There are about 900 billion glial cells

in the body. 9 times the number of stars in the Milky Way.

Unlike neurons, glial cells can divide throughout their life.

Susceptible to cancer due to their ability to divide. Most brain cancers are due to glial cells.

Astrocytes: “Stars of the Nervous System” They get glucose from the blood and feed it to the neurons. Help to form the Blood Brain Barrier (BBB).

Microglia: In CNS. They are usually small and stationary, but enlarge and move around when they are needed to “eat” (phagocytosis) microorganisms and cellular debris during

inflamed or degenerating nerve tissue. Ependymal cells: form thin sheets that line fluid filled cavities

in the brain and spinal cord. Similar to epithelial cells. Oligodendrocytes: similar to astrocytes but have fewer

branches. Help to hold nerve fibers together and produce the fatty myelin sheath around the nerve fibers in the CNS.

Schwann Cells: found only in the PNS. Serve the same role as oligodendrocytes.

Motor neuron (red) astrocytes (green)

Microglia (green)

Ependymal Cells Oligodendrocytes Schwann Cells

Many Schwann Cells wrap themselves around a single neuron.

Myelin is a white fatty substance that insulates the neuron like plastic on a wire.

The microscopic gaps between Schwann Cells are called Nodes of Ranvier.

Very important for nerve impulse conduction.

Cells with myelin are called white fibers and gray fibers when they are non-myelinated.

Formed by the astrocytes that wrap their “feet” around the capillaries in the brain.

Regulates the passage of ions and molecules into and out of the brain.

Water, oxygen, carbon dioxide, glucose and small lipid soluble molecules such as alcohol can cross the barrier easily.

Ions (Na+ and K+) are regulated because they could disrupt nerve impulses.

Must be taken into consideration when developing drug treatments for brain disorders.

Ex. Parkinsons need dopamine but it cannot pass BBB. They are given L-dopa which can pass and is made into dopamine by the brain cells.

Myelin disorder of the oligodendrocytes. Loss of myelin and destruction of the

oligodendrocytes. Hard plaquelike lesions replace the

myelin and causes inflammation. Impaired nerve conduction, loss of

coordination, visual impairment and speech disturbances.

Most common in women 20-40. Caused by autoimmunity or a viral

infection.

We have about 100 billion neurons. This is only about 10% of all the nervous system cells in the brain.

Neurons are also called nerve fibers. Parts of the neuron:

Cell body Dendrites: branch off from the cell body. Means

“tree”. They receive stimuli and conduct electrical signals towards the cell body and/or axon.

Axon: a single process that comes off the cell body via the axon hillock. They conduct impulses away from the cell.

Multipolar: one axon, several dendrites. Most neurons in the brain and spinal cord.

Bipolar: one axon and one highly branched dendrite. Least common type of neuron, found in retina, inner ear, and olfactory pathway (nasal).

Unipolar or Psuedounipolar: single process extending from the cell body. Always sensory neurons that conduct information to the CNS.

Multipolar

Bipolar

Unipolar

Afferent (sensory) neurons: Transmit impulses to the CNS.

Efferent (motor) neurons: transmit impulses away from CNS towards or to muscles or glands.

Interneurons: Transmit impulses from afferent neurons to efferent neurons. Lie completely within the CNS.

Neurons are often arranged in a pattern called a reflex arc. It’s a signal conduction route.

Most common form is a 3-neuron arc (sensory interneuron motor)

2-neuron arc (sensory motor) Synapse: place where nerve information

is passed from one neuron to another. Passed from the synaptic knobs of one neuron to the dendrites of the other.

Nerves: bundles of nerve fibers in the Peripheral nervous system held together by several layers of connective tissue. (ex. Sciatic nerve)

Tracts: bundles of nerve fibers in the central nervous system. (ex. Corticospinal tract) White matter: myelinated nerve

fibers Gray matter: unmyelinated nerve

fibers and cell bodies

Mature neurons cannot divide, damaged neurons cannot be replaced.

They can sometimes repair themselves in the PNS if the damage is not too severe. 1. After the damage has occurred, the distal

portion of the axon degenerates. 2. macrophages move in and remove the debris.

3. the neurolemma (nerve sheath formed by Schwann Cells) forms a tunnel from the point of injury to the effector. 4. new Schwann cells grow within the tunnel to

support axon growth.

The skeletal muscle that is innervated to the damaged nerve atrophies as it is not being stimulated.

If the damaged axon doesn’t repair itself, sometimes a nearby healthy neuron will establish a connection with the muscle.

One damaged axon in a single neuron can shut down an entire nerve pathway if not repaired.

Cells in the CNS hardly ever repair themselves. They lack a neurolemma to build a tunnel and astrocytes fill in damaged areas and form scar tissue.

Most spinal cord injuries involve crushing or bruising of the nerves.

Inflammation after the accident causes more damage to surrounding nerves.

Early treatment of the antiinflammatory drug, methylprednisolone is prescribed within 8 hours of injury to reduce the swelling.

Neurons exhibit excitability and conductivity.

A nerve impulse is a wave of electrical fluctuation that travels along the plasma membrane.

Membrane potential: Cells have slightly more (-) charges inside the cell than on the outside (extracellular fluid is more +).

This difference in ion concentration across the plasma membrane has potential energy.

A membrane is polarized if it has a membrane potential.

We can measure the potential difference between the two sides of the polarized membrane in (V=volts or mV= millivolts).

-70 mV tells us that the difference in charge is 70 mV and that the inside of the cell is negative (-).

+30 mV tells us that the difference in charge is 30 mV and the inside of the cell is positive (+).

A neuron is “resting” when it is not conducting nerve impulses.

Stays about -70mV (RMP-resting membrane potential)

There are no gates or they are closed to not allow anions (-) in or out of the cell.

Cations (+) Na+ and K+can move in and out of the cell through gates.

K+ gates are usually open and Na+ gates are usually closed.

There are also Na+ and K+ pumps that are active transport mechanisms. Pumps 3 Na+ out for every 2 K+ in and at

different rates. If 100 K+ are pumped inside the cell, 150 Na+

are pumped out. This maintains a difference in + charges inside

and out of the cell. Slightly more positive outside the cell.

Very little Na+ diffuses through the membrane. The pump maintains a imbalance of ions inside and outside of the cell.

The resting membrane potentials (RMP) of neurons can fluctuate due to certain stimuli.

Local Potential: slight change in the RMP. Stimulus-gated Na+ channels open in

response to a sensory stimulus or stimulus from another neuron (excitation)

When they open, more Na+ rushes into the cell, causing it to become more +. Depolarization (movement of the membrane potential to zero mV).

Stimulus-gated K+ channels open during inhibition. Causing the outside of the cell to become more +. Hyperpolarization (movement of the membrane potential away from zero mV.) Now we are below the RMP.

Local potentials are graded potentials meaning they can be large or small depending on the strength of the stimulus. They are also isolated to a particular location on the plasma membrane and do not travel down the axon.

Small depolarizations or hyperpolarizations applied to certain dendrites on a neuron.

Action Potential is the membrane potential of a neuron that is conducting an impulse. Also called a nerve impulse.

There are 6 steps in conducting an action potential.

http://www.metope.org/neuron/

1. An adequate stimulus must be applied and the stimulus-gated Na+ channels will open to allow Na+ in (depolarization).

2. If the level of depolarization surpasses the threshold potential (usually -59 mV) voltage-gated Na+ channels will open allowing MORE Na+ in the cell.

3. As more Na+ comes inside, the voltage inside the cell gets closer and closer to 0 mV and will continue to +30 mV. Means we now have more + ions in the cell than outside of the cell.

4. Voltage-gated Na+ channels only stay open for about 1 millisecond before they close. Action potentials are all-or-none, either they will occur or not at all.

5. Once the peak of the action potential is reached , it starts to move back to -70 mV (resting potential). This is called repolarization. The reaching of the threshold potential causes voltage-gated K+ channels to open as well, but they are slow to respond. So they don’t open until the +30 mV potential is reached. Then K+ pours out and Na+ goes back out.

6. Because so much K+ pours out of the cell, the voltage goes past -70 mV for a brief period of hyperpolarization, but then it gets back to the resting state.

Brief period during which a local area on the axon’s membrane can not be restimulated. Absolute refractory period: ½ a millisecond after

the threshold potential is surpassed. Axon will not respond to any stimulus.

Relative refractory period: few milliseconds after the absolute refractory period. Can only be stimulated if the stimulus is really strong.

A strong stimulus causes more action potentials vs. a weak stimulus. However, the strength of each action potential is the same.

The action potential causes an electrical current to flow down segments of the axon’s membrane.

It will never move backward due to the refractory period of the membrane before the AP.

In myelinated fibers, the myelin sheath prevents ion movement, so electrical changes only occur in the gaps between myelin (Nodes of Ranvier).

The AP seems to “leap” from node to node. This is called saltatory conduction. (Latin-saltare-”to leap”)

The larger the diameter of the fiber, the faster it conducts impulses.

Myelinated fibers conduct impulses faster than unmyelinated fibers.

Fastest fibers innervate skeletal muscles and can fire impulses close to 300 mph.

Slowest fibers, such as sensory receptors in the skin, conduct impulses at less than 1 mph.

Block pain. Inhibit the opening of Na+ channels

so the nerve cannot conduct impulses. Bupivacaine (Marcaine): used in dental

procedures. Procaine used to block signals in sensory

pathways of the spinal cord. Benzocaine and phenol: found in over the

counter products that release pain associated with teething, sore throat pain, and other ailments.

Synapse is where signals are sent from one neuron to another (presynaptic neuron to the postsynaptic neuron)

Types of Synapses Electrical: occur where two cells are joined at

gap junctions (cardiac muscle, some smooth muscle). The impulse goes from one plasma membrane to the other.

Chemical: Use chemicals (neurotransmitters) to send a signal from the pre- to the postsynaptic cell.

Structures of the chemical synapse. 1. Synaptic knob- tiny bulge at the end of the

presynaptic neuron’s axon. Contains numerous small sacs or vesicles that contain neurotransmitter.

2. synaptic cleft- space between the synaptic knob and the plasma membrane of the postsynaptic neuron, 1 millionth of an inch wide!

3. the plasma membrane of the postsynaptic neuron- has protein receptors embedded in which neurotransmitters bind.

Electrical Synapse Chemical Synapse

Action potentials cannot cross synaptic clefts even though the spaces are so tiny.

Instead, neurotransmitters (NT) are released in cause a response in the postsynaptic neuron.

Excitatory NT cause depolarization and inhibitory NT cause hyperpolarization.

1. Action potential (AP) reaches a synaptic knob, causing voltage-gated Ca 2+ channels to open and allow Ca 2+ to diffuse into the knob rapidly.

2. Increase in Ca2+ causes NT to be released into the synaptic cleft.

3. The NT binds to receptors on the postsynaptic membrane which causes the ion gates to open.

4. The NT will either cause an excitatory postsynaptic potential (EPSP) or an inhibitory postsynaptic potential (IPSP).

5. Once the NT binds to the receptor its action is terminated.

Neurotransmitters are how neurons talk to one another.

Can be excitatory or inhibitory. Their affect is determined by the

receptor, not the actual NT.

Acetylcholine- excites skeletal muscles but inhibits cardiac muscles.

Amines- (seratonin, histamine, dopamine, epinephrine and norepinephrine). Affect learning, motor control, emotions, etc.

Amino Acids- (glutamate, GABA, glycine). Some are excitatory and some are inhibitory in the CNS.

Other small molecule transmitters- (nitric oxide NO and carbon monoxide CO).

Neuropeptides- short strands of amino acids. Include enkephalins and endorphins. Inhibitory and block pain.

Severe psychic depression occurs when there is a lack of norepinephrine, dopamine, serotonin and other amines.

Antidepressants work several different ways. They may inhibit enzymes that are used to

inactivate the NT. They may block the reuptake of the NT by the

neuron, keeping them in the synapse longer.

Cocaine blocks the reuptake of dopamine, giving a temporary feeling of well-being.