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The Nervous System
-- the nervous system is comprised of two types of cells:
1. NEURONS – cells that actually transmit nerve impulses; main
focus of this unit. There are three general types:
a. Sensory Neurons – receive info from ‘periphery’ of body and send
to the Central Nervous System (CNS) (ie. brain and/or spinal cord);
b. Somatic Motor Neurons – send signals from CNS to skeletal
muscles;
c. Autonomic Motor Neurons – send signals from CNS to smooth or
cardiac muscles and/or glands.
Refer to fig. 17.1 p. 322.
2. NEUROGLIAL CELLS – cells that support and/or service neurons
eg. microglial cells – phagocytes that clean up debris;
astrocytes – cells that lie between neurons and capillaries that serve to
nourish neurons; as well, they produce a hormone (glial-derived
growth factor) that aids in the regeneration of neurons.
oligodendrocytes (Schwann cells) – create the special lipid, myelin,
that serves to protect and speed up the functioning of neurons.
Refer to fig. 11.6 p. 200.
Neurons (Nerve Cells) – Functions
Each neuron must perform five functions:
1. Receive information from the internal environment (including organs,
muscles, other neurons) and/or the external environment (including stretch,
pressure, temperature receptors);
2. Integrate the information it receives and produce an appropriate output
signal (excitatory or inhibitory);
3. Conduct the output signal to its terminal ending, which may be some
distance away;
4. Transmit the signal to other neurons, glands, or muscles;
5. Coordinate most of the metabolic activities that maintain the integrity of
the cell (ie. it must keep itself alive); although, astrocytes help in this
regard.
Neurons – Structure
A typical neuron has three distinct structural regions (fig. 17.2 p. 323):
i. Dendrites
ii. Cell Body
iii. Axon (including axoplasm, axomembrane, axon hillock, synaptic
endings/terminals (aka axon bulbs).
i. Dendrites (aka ‘Receptive Regions’)
-- typically short (except in most sensory neurons), numerous, and
extensively branched (name derived from Greek dendron = “tree”).
-- structures that receive signals from a variety of possible locations:
- other neurons' synaptic terminals;
- the internal environment (brain, spinal cord, glands, chemicals
(hormones) etc;
- the external environment (skin, stretch receptors, temperature (heat)
receptors, pressure receptors)
-- dendrites branch off from the cell body of the neuron in order to cover a
relatively large surface area.
-- dendrites send the received signals TOWARDS the cell body.
-- dendrites that receive signals in chemical form (eg. from other neurons’
synaptic terminals) possess specific receptors that can bind specifically
(‘lock & key’) to the chemical neurotransmitters released by these synaptic
terminals.
-- dendrites that receive infrared (heat) or pressure signals possess different
receptors that are able to synthesize the information received into a signal
for the cell body to integrate.
ii. Cell Body (aka ‘Integration Centre’)
-- the cell body receives the signal, which was initially received by the
dendrites, from the dendrites.
-- numerous dendrites serve one cell body, each sending their own signal in
a near simultaneous nature.
-- the cell body ‘adds up’ (integrates) the dendrites' signals and 'decides'
whether or not to produce an action potential (aka nerve impulse).
-- the ‘sum’ of the signals must be greater than a certain threshold in order
for an action potential to be generated.
-- the cell body’s integrated signal is passed on to the axon by way of the
axon hillock, a cone-shaped region of the cell body that funnels into the
fiber-like axon.
-- in addition to its integration function, the cell body possesses a nucleus
and other organelles, and functions like most other cells.
iii. Axon
-- an axon carries nerve impulses AWAY from the cell body.
-- if an action potential is generated, it will originate within the axon
hillock, which will then pass the signal on to the axon.
-- the axon carries the action potential from the cell body/axon hillock to its
bulb-like synaptic endings (located at the end of an axon).
-- axons are typically long, thin fibers (can be up to 3 feet in length in the
leg (Sciatic nerve - base of spinal cord to toe muscles)).
-- remember, they are still a part of the neuron, thus they possess a cell
membrane known as the axomembrane.
-- an axon’s cytoplasm is specifically referred to as the axoplasm.
-- usually, axons are bundled together into NERVES (like wire in cables).
- axons that occur in nerves are also referred to as nerve fibers.
- the cell bodies that associate due to the formation of nerves are
known as ganglia (singular: ganglion) if they exist outside of the
CNS (thus, in the Peripheral Nervous System (PNS)), or they simply
exist within, and contribute to the structure of the CNS itself.
-- the action potential (nerve impulse) does NOT diminish in strength as its
journey along an axon persists.
-- synaptic endings are swellings at the end of an axon.
-- synaptic endings serve as the link between the neuron and either a gland,
a muscle cell, or the dendrites of another neuron – this because nerve cells
do not actually touch each other when they pass signals on, nor do they
touch other cells that they receive/send signals from/to.
-- most synaptic endings contain chemicals called neurotransmitters, which
somehow relay the impulse across the ‘gap’ to the next structure.
-- this ‘gap’ is known as the synaptic cleft.