Download Physiology2 - Sheet#2 - Dr.Loai Alzgoul

Physiology2 - Sheet#2 - Dr.Loai Alzgoul - Done By: Maryam Breik
Physiology of sensory system :
Sensation is transmitted from the PNS by the peripheral nerves to the spinal cord then
up to the cortex. There are many spinal nerves, and each pair is connected to a vertebra
(C2, C3, C4, L1, L2 … etc). When each spinal nerve enters the spinal cord it divides into:
1) Posterior root (Sensory)
2) Anterior root (Motor )
So as the spinal cord itself divided into white and gray matter: the spinal nerve of the
white and the gray matter of the posterior root are mainly for sensation, and the white
and gray matter of anterior root are mainly for motor actions .
Sensory neurons transfer signals: from receptors of PNS  posterior root  spinal
nerve Posterior part of the spinal cord (posterior horn) then upward directly or
synapses with other neuron upward.
Motor neuron transfer signals: from the upper parts of spinal cord the anterior part of
spinal cord (anterior horn) effectors.
Physiology2 - Sheet#2 - Dr.Loai Alzgoul - Done By: Maryam Breik
Types of sensation :
1) Special sensation : Smell , taste , Vision , Balance ( Equilibrium )
2) General sensation : All sensations other than the special sensations, and they are
divided into:
A) Visceral : from the visceral organ such as the kidney, heart, Stomach ,GI tract ,
pancreas , Lung .....)
B) Somatic : All other sensations (=other than from the visceral organ) (= from other parts
of the body ) And it's divided into :


exteroceptor (external sensation )
proprioceptor ( Sense of self )
Let's take an example to understand the different between them :
EXAMPLE:
Somatic sensation of the hand, which has no visceral organs :
1) From the outside environment (touch , temperature) is called exteroceptor
2) From the hand itself ( muscle length and tension , joint position and their
motion ) is called proprioceptor.
Physiology2 - Sheet#2 - Dr.Loai Alzgoul - Done By: Maryam Breik
The brain receives all the sensations from the whole body (zlma msh'3ol :P ) , but its
response differs according to the type of sensation. Some senses go through slow
sensation pathway, while others take the fast sensation pathway.
Why?!
Fast sensation pathway is for those sensations which need processing by the cortex ,
and they take posterior column medial lemuiscus pathway ( PCML) => high way
While , Slow sensation pathways take their response from the spinal cord (by reflexes)
and later on, the brain will know what happened and you'll feel the pain that the brain
stem controls. They take the anteriolateral system pathway (ALS( spinothalamic tract)).
Important examples:
*Slow sensation modalities: temperature, crud touch (‫ )الحكة‬,pain
Why?! Because when you burn your finger (b3eed elshr ;) ) you will raise your hand by
reflection from the spinal cord before feeling the pain (spinal cord reflex)
*Fast sensation modalities: proprioception, fine touch and pressure and vibration
Why?! Its too important for the body to know where is the hand, the muscles , their
motions , what does it touch and whether it's smooth or rough. so the body need PCML
pathway for those senses.
PCML pathway:
Peripheral nerve (1st order neuron (unipolar))
Posterior root the white matter of the spinal cord (posterior column) that's why it is
called PCML
Medulla
Posterior nucleus of the medulla which has cell body and synapses with the 2nd order
neuron and it crosses to the other side (rightleft ,and vice versa)
Physiology2 - Sheet#2 - Dr.Loai Alzgoul - Done By: Maryam Breik
Mid brain
Thalamus, and here it synapses with the 3rd order neuron
Somatosensory cortex (post central gyrus)
Check the video 
Don't forget : All the PCML pathway is for sensory neurons ;)
- Although the PCML pathway is always the same , the posterior column of the spinal
cord is divided into two parts ( Gracile and cuneate ) , the nucleus of medulla too ,, why?!!
Because the main sensations that transfer through PCML pathway are touch , vibration
and proprioception . Those sensations come mainly from the upper limb and the lower
limb of the body , when the white matter of the spinal cord contains more axons , it will be
bigger and vice versa .
The lower part of the spinal cord is smaller (posterior column is small) => contains a
lower number of nerves' axons.
*From L4 to C (cervical vertebra) of the cord there aren't a lot of neurons to transfer the
information about sensation
*From cervical vertebra and upward there are more axons that come from the hand
and the upper limb.
Gracile => for the lower limb
Cuneate => for the upper limb
So the Gracile and Cuneate are called together posterior column, and called posterior
nucleus in the medulla.
Each area of the brain has its special job (area for: hand, feet , legs …. etc) which are
divided according to the number of neurons serving the organ not to the organ's size.
Physiology2 - Sheet#2 - Dr.Loai Alzgoul - Done By: Maryam Breik
ALS pathway :
Pain sensation (for example) through the peripheral nerve (1st order,unipolar)
Spinal cord, because reflexes happen here the there'll be a synapse with the 2nd order
neuron)
Posterior horn of the grey matter (2nd order neuron)
White matter of the opposite side (crossing)
the area where the signals transfer through is anteriolateral that's why it is called
anteriolateral system
Medulla (also in the anteriolateral side),, here the axon branches because it also gives
info of pain for the brain stem
Thalamus (3rd order neuron)
Somatosensory cortex
Check the video too 
- The velocity of the pathway (fast/slow sensations) depends mainly and generally on
the number of synapses and axons in the pathway, myelination, and axon's size.
As we noticed, the number of synapses and axons in the ALS and PCML pathway are the
same (both have 3 axons ), so it's not a factor to compare between them.
Other factors: Myelination ; heavily myelinated neurons are faster than unmyelinated
neurons . Axon's size; bigger axon's size are faster than small axons.
PCML pathway is heavily myelinated and has bigger axons in diameter, that's why it's
faster than ALS pathway .
Physiology2 - Sheet#2 - Dr.Loai Alzgoul - Done By: Maryam Breik
Memorize this table which divides the types of axons according to "anatomy" and
"physiology".
The first partition as a general classification: myelinated (A) and unmyelinated(C) .then
according to the diameter ( alpha, beta, gamma,delta).That was as anatomy science.
In physiology , we concentrate on the function of the axon which is conduction , that's
why they divided them according to the speed of conduction (I , II , III , IV ).
Notice that (as we said previously) , the C region ( unmyelinated and small axon size) is too
slow (IV) => ALS pathway (tickling, pain , crude touch, temperature …. etc)
*Don't forget to memorize all the sensory function from the table .
** I or alpha  proprioception
** II , III  the rest of sensations
** IV  ALS pathway sensations
GOOD LUCK 