Three Types of Neurons(fig. 17.1 p. 322; fig. 17.2 p. 323)

i. Sensory Neurons (aka Afferent Neurons)

-- a sensory neuron’s dendrites extend to the surface of the body (skin),to a sensory organ (eg. Ear, nose, eye etc…), or to other internal locations (eg. aortic bodies), where they branch into multiple dendrites, in order to increase their surface area. The cell body exists just outside the body’s midline (spinal cord/brain).

-- therefore, dendrites of a sensory neuron are long and myelinated just like most axons, and due to this, they are able to create and propagate action potentials to the cell body.

-- dendrites receive a sensorystimulus at specialized sensory receptors (located on the ends of a dendrite’s many branches specialized for light, odor, taste, pressure, temperature, etc.), generate APs, and transmit the message to the sensory neuron’s cell body, which is located just outside the body’s midline (ie. just outside the spinal cord/brain).

-- because neurons (including sensory neurons) tend to ‘bundle’ together to form nerves (in this case, sensory nerves), their respective cell bodies group together as well. A collection of sensory neuron cell bodies grouped together just outside the spinal cord/brain is known as a dorsal-root ganglion (“dorsal” due to its location just outside the dorsal side of the spinal cord).

*Ganglia (singular: Ganglion) -- a collection of neuron cell bodies within the Peripheral Nervous System (PNS).

-- the dorsal-root ganglia integrate the dendrites’ signals and transfer the impulse to axons, which propagate the message on to the Central Nervous System (CNS). The axons of sensory neurons are shorter than the axons of non-sensory neurons.

-- sensory neurons are bundled together into purely sensory nerves, or are bundled with ‘related’ motor neurons into mixed nerves.

towards CNS

ii. Interneurons(aka Association Neurons or Connector Neurons)

-- neurons that are locatedsolely within the CNS.

-- convey impulses from the sensory neuron to the subsequent motor neuron (a relatively short distance).

-- 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 (aka Efferent Neurons)

-- the dendrites of motor neurons are typically shorter in length, reside within the CNS, and synapse with the axon terminals of interneurons.

-- the signal is then integrated within a cell body that is located within the CNS.

-- a collection of motor neuron cell bodies is known as a ventral-root ganglion(located within CNS, unlike the dorsal-root ganglia which exist just outside the CNS).

-- an AP is then generated in the axon hillock and propagated along the axon (very long in length in order to reach the ‘periphery’ of the body) to an effector (muscle/gland) that is ‘away’ from the CNS.

-- the impulse can either result in a contraction/relaxation of a muscle or secretion/non-secretion of a gland/organ.

-- motor neurons are said to‘innervate’effectors (ie. cause them to ‘react’ in some manner).

-- motor neurons are bundled together into purely motor nerves, or are 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 (not on test!): 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.

-- a reflex is anautomatic, involuntary reaction to a stimulus generated either inside or outside of the body.

-- a reflex arc can be solely controlled by the brain (eg. blink of an eye), or solely controlled 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).