Types of Neurons (fig. 17.1 p. 322; fig. 17.2 p. 323)
Three Types:
i. Sensory Neurons
-- dendrites of a sensory neuron extend to the surface of the body (skin) or
to a sensory organ (eg. Ear, nose, eye etc…), where they branch into
multiple dendrites, while the cell body exists just outside the body’s midline
(spinal cord/brain).
-- dendrites of a sensory neuron are long and myelinated just like axons;
they are able to create and propagate action potentials to the cell body.
-- dendrites receive a sensory stimulus at specialized sensory receptors
(located on the ends of a dendrite’s many branches; they are specialized for
light, odour, taste, pressure, temperature, etc.), generate APs, and transmit
the message to the neuron’s cell body (again, located just outside the spinal
cord/brain) where it is integrated and transferred to an axon, which then
continues the message into the CNS (Central Nervous System = spinal
cord/brain).
** remember, neurons are bundled together into nerves, so a
collection of sensory neuron cell bodies is referred to as a dorsal-root
ganglion.
Ganglia (singular: Ganglion) -- a collection of neuron cell bodies within
the Peripheral NS (eg. the Dorsal-Root Ganglion of sensory neuron cell
bodies lies just outside the dorsal side of the spinal cord).
-- a sensory neuron typically possesses very long dendrite(s) and a shorter
than usual axon.
-- sometimes referred to as an afferent neuron (ie. ‘running to’ the CNS).
-- can be bundled together into purely sensory nerves, or can be bundled
with ‘related’ motor neurons into mixed nerves.
ii. Interneurons (aka Association Neurons or Connector Neurons)
-- exist completely within the CNS.
-- convey impulses from the sensory neuron to the subsequent motor
neuron (a relatively short distance); do so in a ‘perpendicular’ manner.
-- may be comprised of non-myelinated axons (gray neurons/nerves) since
many impulses travel a relatively short distance, or myelinated axons (white
neurons/nerves) that carry messages to and from the brain (longer distance).
-- typically possess short dendrites, and either long or short axons
depending on the range that it covers (longer axons are myelinated).
- it is possible for more than one interneuron to be involved in
‘connecting’ a sensory neuron to a motor neuron
iii. Motor Neurons
-- the dendrites of motor neurons are typically short in length, reside within
the CNS, and synapse with synaptic endings of interneurons.
-- the signal is carried to a cell body that is located within the CNS where
the signal is integrated and relayed to the axon (very long in length), which
leads to an effector (muscle/gland) that is ‘away’ from the CNS.
-- impulse can cause either contraction/relaxation of a muscle or
secretion/non-secretion of a gland/organ.
-- again, short dendrites, long axons.
** a collection of motor neuron cell bodies is loosely known as a
ventral-root ganglion (located within CNS).
-- motor neurons innervate effectors (ie. cause them to ‘react’ in some
manner).
-- sometimes referred to as efferent neurons (ie. ‘running away from’ the
CNS).
-- can be bundled together into purely motor nerves, or can be bundled with
‘related’ sensory neurons into mixed nerves.
-- motor and sensory neurons’ axons and dendrites are myelinated (making
them white in colour (aka white matter)); interneurons and any ganglia are
unmyelinated (making them gray in colour (aka gray matter)).
FYI: humans possess 12 pairs of cranial (brain) nerves that are strictly
sensory or motor nerves, or mixed nerves. Most cranial nerves innervate the
head, neck, and facial regions, but one nerve (vagus) innervates the
pharynx, larynx, and most internal organs (communication route for medulla
oblongata). As well, humans have 31 pairs of spinal nerves that are all
mixed nerves (although they do branch off of the spinal cord as individual
sensory (dorsal root) and motor nerves (ventral root) that then merge into
mixed nerves just past the dorsal-root ganglion). Each spinal nerve
innervates structures of the body near their spinal cord connection (fig. 17.7
p. 328, fig. 17.9 p. 330).
Reflex Arc (fig. 17.8 p. 329)
-- A typical reflex involves all three of the neuron types mentioned above.
-- reflexes are automatic, involuntary reactions to stimuli generated either
inside or outside the body.
-- can be solely controlled by the brain (eg. blink of an eye), or by the spinal
cord (eg. pin-prick or ‘hot potato’).
Example Reflex (pin-prick on arm) (fig. 17.8 p. 329):
i. pin pricks the skin on the arm
ii. sensory pressure receptors (on the dendrites of sensory neurons) in the
skin receive the sensory stimulus.
iii. the stimulus reaches threshold and generates an AP that is sent via the
sensory nerve dendrites to the sensory nerve cell bodies (ie. dorsal-root
ganglion) located just outside the spinal cord. The dorsal-root ganglion
integrates the signal and sends it out on short axons to the spinal cord.
iv. the signal now passes to the dendrites of interneurons within the spinal
cord (remember, the passing of a signal from neuron to neuron involves
transmission across a synapse). The interneurons’ cell bodies continue the
signal to their axons and eventually to the dendrites of a motor nerve.
v. the motor nerve’s dendrites carry the impulse through the ventral-root
ganglion and into the ventral root (motor) axons where the signal
eventually, through the propagation of APs, reaches the appropriate muscle
in the arm.
vi. muscles in the arm contract to pull arm away from the pin-prick.
Many different interneurons are signaled by the sensory nerve impulse.
Why?
Because there are many different responses to a pin-prick that occur
simultaneously or within a very short span of each other:
a. pull arm away
b. look towards arm
c. jump back
d. utter shouts
e. feel pain
*different interneurons carry the original signal to different parts of the
CNS, even to the brain, making the person conscious of the stimulus
(delayed pain response).