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Nerves
Shoot with dart gun, how long to react, Olympic male sprinter reacts to starter pistil
12/100 of a second. 100 billion nerve cells in brain, greater than number of visible stars
in Milky Way. size - if neuron were the size of a baseball, axon would extend 1.6 km
one cm3 of human brain may contain several million nerve cells, nerve transmission 100m/see or 200 mph. How different from endocrine system
In many cases, the nervous system and endocrine system cooperate and interact in regulation
body functions and behavior though they play different roles. Eg. With its complexity, the
nervous system can integrate vast amounts of information, such as that required for human
thought and speech. Endocrine doesn't have the complexity. Timing also key. Endocrine can
take minutes hours or days due to time taken to make and carried hormones to target organs,
nerves are fast.
In general, the nervous system can be divided into the central nervous system and the
peripheral nervous system. Flowchart.
Electrical impulses generated by receptors or the brain travel along the nerves conductors. .
The neuron - term given to the complete nerve cell and its processes, that is, it's ability to
conduct electrochemical impulses over a distance. Nerves usually mean a group of neurons
and their supporting cells. Tend to be used interchangeably.
Neurons vary in length but motor neurons all have some common features and structures.
Know how to draw. Axon should be 3 times longer than cell body and have at least 5
dendrites, cell body, nucleus, axon, myelin sheath, nodes of Ranvier, motor end plate that
doesn’t have myelin and ends in a button or dot.
By the age of 2, each neuron has about 15,000 synapses or connective points. Some will
be reinforced by sensory input and others, left unused, will be wither away.
There are 1000 to 10000 synapses for a typical neuron. Neurons multiply at a
rate of 250,000 neurons/min during early pregnancy - can do mitosis.
Axon of sciatic nerve - from lower part of spine to muscle of lower leg - is one
meter or more.
The nerve impulse is generated in the dendrites (Greek for tree). Dendrites are structural
adaptations that increase the surface area of the neuron. The impulse is conducted down the
axon. Many neuron axons are covered with a lipid sheath – the myelin sheath - produced by
Schwann cells - on of the supporting cells. It acts as a type of insulation and is believed to
increase speed of impulse. Where one sheath meets another, part of the axon is unprotected,
called the Nodes of Ranvier.
There are three types of neurons based on what they do;
1) sensory - runs from receptors to CNS
2) relay (association) - found in spinal cord, stimulated by sensory or brain, runs up and down
spinal cord.
3) motor - transmits impulses from CNS to effectors. Effectors are usually skeletal muscles or
glands
Astrocytes - encircle capillaries in the brain and contribute to the blood-brain barrier
which restricts passage of most substances into the brain- giving tight control of the ECF.
Most drugs can’t pass this blood-brain barrier.
A nerve impulse
The cytoplasm of the neuron is negatively charged with respect to its outside ECF. The cell
membrane is said to be polarized - having one side a different charge than the other - called the
resting potential. This charge or difference in electrical potential, can actually be
measured at -70mV,about 5% of the voltage of AA battery. How does it achieve this
potential?
The resting potential arises from two things - difference in the ionic composition of the
intracellular and intracellular fluids and the selectively permeability of the plasma membrane.
The plasma membrane is a phospholipid bilayer with membrane proteins. Ions, being
electrically charged, cannot dissolve in the lipid and can't directly diffuse across. In order to
cross, they must either be transported by proteins or move through ion channels or pores gated ion channels. The ion channels are specific to ions - ones for Na+, others for CI- and K +.
Cell has a greater permeability to K+ than to Na+ so there are probably more channels.
There is limited diffusion of Na+ through sodium channels. There is a free movement of K+,
though the channels. And there is the sodium-potassium pump, which actively transports ions
using ATP. Three Na+ are moved out for every two K+ moved in. Ion channels for -ve ions are
closed. Most -ve ions are large and can't get out. So, Na+ are actively pumped out and not
allowed back in. K+ are pumped in but at a slower rate are allowed back out. Negative ions like
CI-, proteins, sulfates, phosphates are not allowed to move out of the cell. The cell is polarized.
When a stimulus reaches threshold level, this will cause the neural membrane to become
permeable to sodium ions. The voltage-gated sodium channels open up and Na+ can move into
the cell. Na+ enters by facilitated diffusion and causes depolarization. Some Na+ ions drift
over to the next part of the neuron and trigger an action potential there It involves positive
feedback as Na+ ions move, this causes more ion channels to open up and so on. Also, K +
ions are not allowed to diffuse out, channels are closed. This causes a change in electrical
potential (which can be measures at about 110m V). This cell is depolarized or
depolarization. This depolarization will sweep down the cell due to positive feedback - starting
at the dendrites and moving to the axon. Threshold level impulse is one that is strong enough
to initiate an impulse, that is, depolarizes the cell, which also means strong enough to open the
voltage gated channels. Once that starts, the action potential continues. Strong impulses do
NOT initiate a stronger response. If threshold is reached its all-or-none. But stronger impulses
must be recognized by the brain. They will initiate or depolarize more neurons and/or at a
greater frequency. This principle is also seen in muscles.
In mylinated neurons, depolarization only occurs at the nodes of Ranvier. The impulse jumps
from node to node - much faster.
After depolarization, there will be a refractory period when the cell will repolarize itself.
(about 1 to 2 milliseconds). This is done by the sodium-potassium pump pumping out the Na+
ions and K+ channels open up. Because the cell is not polarized yet, if a stimulus is received
during this period, no depolarization will occur.
Depolarizing and repolarizing is an action potential. Impulses are not weakened with distance
traveled.
thicker axons transmit faster. Different axons have different threshold levels so
stimulated by different voltages.
Local anesthetics temporarily inactivates the voltage gated channels.
A piece of string and match – hold the string up and light the bottom end. Flame will
travel upward. Demos the energy needed to start the impulse but then the reaction
travels along ‘on its own’ along the neuron.
Mousetrap – demo of the idea of threshold all-or-none.
The impulse is an electro-chemical transmission. This is the electrical part. The chemical is
what distinguished this from electrical wires.
The synapse - the axon of one neuron does not touch the dendrites of the next. There is a space
or gap, called the synaptic cleft or synapse. The membrane surrounding the axon with the
action potential is called the pre-synaptic membrane and the membrane surrounding the
dendrites waiting to pick up the signal are called the post-synaptic membrane.
When an action' potential depolarizes the membrane of the synaptic it triggers a movement of
calcium ions. Ca2+ are in the synapse, when the impulse reaches the synapse, the ions go into
the pre-synaptic knob. This causes synaptic vesicles to fuse with the presynaptic membrane.
When they do that, they release neurotransmitter molecules into the synaptic cleft. These
molecules diffuse across the cleft and bind to receptors on the postsynaptic membrane. The
receptors control selective ion channels; the binding of neurotransmitters to a specific receptor
opens the ion channel. The resulting ion inf1ux changes the voltage the postsynaptic
membrane in one of two ways. Either moving the membrane potential towards threshold level
- excitatory post-synaptic potential and causing depolarization or keeping the channels closed
even more, hyperpolarization - inhibitory post-synaptic potentials. In either case, the
neurotransmitter molecules are quickly degraded by enzymes, closing the ion channels,
causing Ca2+ to leave the pre-synaptic knob and terminating the synaptic response.
The post-synaptic membranes are chemical gated in contrast to the voltage-gated channel in
the pre-synaptic axon.
The neurotransmitters are released by exocytosis, requiring a large amount of
mitochondria.........