• Sensation entails the ability to transduce, encode, and perceive information about the outside world.
• Although the 5 senses are very different from one another, they have similar mode of organization and operation.
• They all have highly specialized cells called receptors, that convert the energy associated with the stimuli into neural signals.
• The signal is carried by afferent nerves to the brain, where the nature of the stimulus is interpreted (Fig. 8.1).

Overview (P. 147)

• The somatic sensory system has two major components:
  1. Detection of mechanical stimuli such as touch, vibration, and pressure.
  2. Detection of painful stimuli and temperature (next Chapter).

Detection Of Mechanical Stimuli (P. 147)

• A variety of morphologically different sensory receptors are used for detection of mechanical stimuli.
• Fig. 8.3 shows the structure of these mechanoreceptors, and Table 8.1 summarizes their characteristics.
• Despite their variety the mechanoreceptors work in fundamentally the same way:
  1. Stimuli applied to the skin deform or otherwise change the nerve endings.
  2. Deformation of nerve endings in turn affects the ionic permeability of the receptor memb.
  3. Changes in permeability generate a depolarizing current in the nerve endings called receptor potential.
  4. The receptor potential then triggers AP, which is relayed to the brain for processing.
• This overall process, in which the energy of the stimulus is converted into AP in the sensory neuron, is called sensory transduction.
• The quality of a perceived stimulus is determined by the properties of the relevant receptors and the location of their central targets.
• The strength of the stimulus is conveyed by the rate of AP discharge, although this relationship is not always true (see next point).
• Some mechanoreceptors change their response to stimuli over time, a property called adaptation (Fig. 8.2).
• Rapidly adapting receptors first respond maximally to stimuli, and then they do not respond if the stimulus is continuously present.

E.g. Joint receptors

• Slow adapting receptors keep firing as long as the stimulus lasts.

E.g. Touch

Sensory Stimuli Discrimination (P. 151)

• The accuracy with which sensory stimuli can be sensed varies from one region of the body to another.
• Fig. 8.4 illustrates the results of two-point discrimination test across the body surface.
• Stimuli discrimination depends on two major factors:
  1. Receptor density - More the number of receptor, better the discrimination ability.
  2. Receptive field - Smaller the receptive field, better the discrimination ability.
• Discrimination of stimuli vary with practice, fatigue, stress, importance of stimuli, and processing quality of the CNS.

Proprioceptors (P. 152)

• A subclass of mechanoreceptors that are involved in providing information of mechanical forces arising from the body itself. For what?
• To give detailed and continuos information about the position of the limbs and other parts in space.

E.g. Muscle spindles, joint receptors, etc.

Mechanosensory Pathways (P. 154)

• There are two major pathways by which mechanoreceptors communicate with the CNS.
  1. The dorsal column-medial lemniscus pathway
  2. The trigeminal pathway

The Dorsal Column-Medial Lemniscus Pathway (P. 154)

• Fig. 8.6 A shows this pathway.
• The receptors are the peripheral axons of dorsal root ganglion (DRG) cells (which are known as the first-order neurons, because they initiate the sensory process).
• These ganglion cells also have a second axon that projects into the dorsolateral region of the spinal cord via the dorsal roots.
• The major branches of the incoming axons ascends ipsilaterally through the dorsal columns, all the way to the lower medulla, where they terminate in the gracile and cuneate nuclei.
• Pathways from the lower body terminate in the gracile nucleus, whereas pathways from the upper body terminate in the cuneate nucleus.
• The axons of the second-order neuron, called the internal arcuate tract, cross the midline to form a large tract called the medial lemniscus.
• The axons of the medial lemniscus reach the ventral posterior lateral (VPL) nucleus of the thalamus, whose cells form the third-order neurons.
• The third-order neurons take the info. to the primary somatic cortex.
• Each dorsal root (spinal nerve) controls a skin region called a dermatome (Box A)

The Trigeminal Pathway (P. 156)

• Tactile and proprioceptive function from the face is conveyed to the brain by this pathway.
• The first-order neurons are located in the trigeminal (cranial nerve V) ganglion.
• The axons of these nerve enters the brain stem at the level of the pons to form trigeminal brainstem complex.
• The second-order neurons from this complex give off axons that cross the midline and ascend to the ventral posterior medial nucleus of the thalamus through the trigeminothalmic tract.

Sensory Nuclei Of The Thalamus (P. 158)

• Fig. 8.7 shows the sensory nuclei of the thalamus.

Sensory Cortex (P. 159)

• The primary somatosensory cortex (post central gyrus) is illustrated in Fig. 8.8.
• Also shown are somatotopic maps (cortical arrangement of sensory pathways), and homunculus (= little man; representation of the human body in the sensory cortex).