Sensory Pathways and the Somatic Nervous System

Sensations from body reaches the opposite side of the brain Sensory pathways Physiology Sensory pathways allow us to detect, analyse and organise response to our environment Each ascending pathway (dorsal column/medial leminiscal and spinothalamic tract (carry information from sensory receptors to the brain) Sensations from body reaches the opposite side of the brain

Analyse and response Convey / mediate Detect

Special senses: vision, olfaction and hearing and balance Sensory pathways Physiology Types of sensation Somatic Special senses: vision, olfaction and hearing and balance Somatic sensation: works through mechanoreceptors, thermoreceptors, pain receptors and others

Sensory pathways Physiology Mechanoreceptors: tactile and position Thermal receptors: heat and cold Nociceptors: pain

Some types of ensory receptors A B C D A: Free nerve endings (pain, temperature) B: Pacinian corpuscle (pressure) C: Meissner’s corpuscle (touch) D: Muscle spindle (stretch)

Illustration of different receptors and sensation modalities in one image

Sensory pathways 3 order neuron Both spinothalamic and dorsal column/medial leminiscal systems are 3 order neurons 3 order neurons: means use 3 neurons or synapses until they trasduct the stimulus and finaly reach the somatosensory cortex Dorsal root ganglion Spinal cord neuron Thalamic nucleus

Dorsal column/Medial leminiscal

Spinothalamic tract Pain, temp,. Crude touch itch and sexual sensation

Spinothalamic pathway Dorsal coloumn pathway

Postcentral gyrus, somatotopic representation/mapping

Dorsal column: fine touch, joint position and pressure pathway Left spinal cord injury Loss of sense of: Fine touch proprioception vibration in left leg Dorsal column: fine touch, joint position and pressure Proprioception (pron.: /ˌproʊpri.ɵˈsɛpʃən/ PRO-pree-o-SEP-shən), from Latin proprius, meaning "one's own", "individual" and perception, is the sense of the relative position of neighboring parts of the body and strength of effort being employed in movement. Position , pressure Basis of proprioceptive sense The initiation of proprioception is the activation of a proprioreceptor in the periphery.[13] The proprioceptive sense is believed to be composed of information from sensory neurons located in the inner ear (motion and orientation) and in the stretch receptors located in the muscles and the joint-supporting ligaments (stance). There are specific nerve receptors for this form of perception termed "proprioreceptors," just as there are specific receptors for pressure, light, temperature, sound, and other sensory experiences. Conscious and unconscious proprioception In humans, a distinction is made between conscious proprioception and unconscious proprioception: Conscious proprioception is communicated by the posterior column-medial lemniscus pathway to the cerebrum.[20] Unconscious proprioception is communicated primarily via the dorsal spinocerebellar tract[21] and ventral spinocerebellar tract,[22] to the cerebellum. An unconscious reaction is seen in the human proprioceptive reflex, or righting reflex – in the event that the body tilts in any direction, the person will cock their head back to level the eyes against the horizon.

Spinothalamic: crude touch, pain, tepmrature spinothalamic pathway Left spinal cord injury Loss of sense of: Crude touch Pain Warmth/cold in right leg Crude touch (or non-discriminative touch) is a sensory modality which allows the subject to sense that something has touched them, without being able to localise where they were touched (contrasting fine touch) d) Tickle and itch. e) Sexual sensation .

Dermatomes Spinal nerves have motor fibres and sensory fibres. The motor fibres innervate certain muscles, while the sensory fibres innervate certain areas of skin. A skin area innervated by the sensory fibres of a single nerve root is known as a dermatome. A group of muscles primarily innervated by the motor fibres of a single nerve root is known as a myotome. Dermatome is a Greek word which literally means "skin cutting". A dermatome is an area of the skin supplied by nerve fibers originating from a single dorsal nerve root.  The dermatomes are named according to the spinal nerve which supplies them. The dermatomes form into bands around the trunk but in the limbs their organisation is more complex as a result of the dermatomes being "pulled out" as the limb buds form and develop into the limbs during embryological development.

Dermatomes are areas of skin supplied by same/individual nerve roots from the spinal cord

Components of the sensory examination Crude touch Pain and temperature Vibration and position Fine touch 2 point discrimination Stereognosis

Pin prick

Light touch

Temperature sensation

Joint position

Joint position

Two-Point Discrimination

Stereognosis