comd 474 PPT 6 Neural Parhways Governing Sensation

Anatomy of sensation

• Cranial nerves

• Spinal nerve/spinal cord

Cranial nerves

These supply the head, neck, and vocal mechanisms, such as the larynx, pharynx, velum and tongue, with sensation.

Spinal nerve/spinal cord

These provide the general body with sensation.

Somatic sensation

This involves  a 3-neuron pathway that is responsible for transmitting sensory information such as light touch and pressure, pain, temperature, and proprioception. 

What does the Lateral Spinothalamatic Tract do?

This transmits pain, temperature, and itch sensations from the body to the brain

What is the Lateral Spinothalamatic Tract?

• Special receptors in the skin and muscles register a sensation and send it to the first-order neurons in the spinal root ganglion, which enter the spinal cord from peripheral receptors and synapse in the dorsal horn with the second-order neuron.

• The second-order neuron crosses to the other side of the spinal cord and enters the lateral white column before carrying the signal up the tract to the thalamus, where it synapses with the third-order neuron and travels to the postcentral gyrus in the parietal lobe 

Perception of pain - Fast

These are myelinated axons that produce a sharp prickly sensation

Perception of pain - Slow

These are unmyelinated axons that have a slower onset, greater persistence, and burning sensation 

Perception of pain - Visceral

This pain comes from internal organs 

Perception of pain - Referred

This is pain you feel in a different part of your body, away from the actual injury or sick organ, and presents as an achy and sometimes burning sensation.

What does the Ventral Spinothalamic Tract do?

This transmits crude touch (general or light touch that lacks precise localization) and superficial pressure 

What is the Ventral or Anterior Spinothalamatic Tract Pathway

• The first-order neuron picks up the stimuli and travels to the dorsal gray horn of the spinal cord, where it synapses and decussates with the second-order neurons. 

• The signal travels up the spinal cord to the brainstem and the posterior ventral nucleus of the thalamus.

• The signal ends at the postcentral gyrus of the parietal lobe

Proprioception 

The ability to determine the body’s position in space

Two-point discrimination

The ability to discriminate the shortest distance between two tactile points on the body 

Vibration 

The ability to recognize vibration from touch

Stereognosis/Form perception

The ability to recognize objects by touch 

Spinocerebellar Tract

• The unconscious perception/already formed muscle patterns 

• Lesser importance for humans

• Tracts end in the Cerebellum 

  • Allows proprioceptive impulses from all over the body to integrate in the cerebellum 

Dorsal Columns

• The first-order neuron is found in the dorsal root ganglion. The axon enters the spinal cord and ascends to the medulla in the dorsal white columns. It consists of two fiber bundles, or fasciculi. Each fasciculus ends in the medulla, where the second-order neuron axons decussate and synapse with the medial lemniscus. The medial lemniscus then ascends to the ventral posterior nucleus of the Thalamus, goes through the internal capsule, and ends in the sensory cortex of the parietal lobe. 

Proprioception Pathways 

• There are nerves connecting to the skin, muscles, and joints

• The two fasciculi cross over at the medulla and synapse to the medial lemniscus, and from that point, it turns from ipsilateral to contralateral 

•  One medial lemniscus receives information from the fasciculus gracilis and the fasciculus cuneatus during synapse 

Lesions of Proprioception

Damage to  the precentral gyrus of the parietal lobe, dorsal columns, or dorsal root ganglion may produce a loss of proprioception, astereognosis, loss of two-point discrimination in the trunk or extremities 

Astereognosis

This is a type of agnosia that specifically affects tactile recognition 

Lesions of Proprioception - Damage below the level of the medulla

Loss is ipsilateral 

Lesions of Proprioception - Damage above the level of the medulla

Loss is contralateral 

Sensory Examination

Light touch, pain, and temperature are mediated by fibers of the dorsal root of the spinal cord which come from dermatomes

Dermatomes

Distinct areas of skin supplied by sensory nerve fibers from a single spinal nerve root 

Sensory Examination - In peripheral nerve injuries

• Impairment of touch corresponds to dermatomal zones

• In pain and temperature, the boundary of each segment has an overlap area with the adjacent nerves

Sensory Examination - Light touch - Light touch

Anesthesia

Sensory Examination - Light touch - Decreased loss

Hypoasthesia 

Sensory Examination - Light touch - Increased loss

Hyperasthesia

Sensory Examination - Light touch - Inability to localize

Atopognosis 

Sensory Examination - Two-point discrimination 

• The ability to discriminate the shortest distance between two tactile points on skin

• Utilize a caliper

• Compare right and left sides of the body

• Loss suggests parietal lobe lesion 

Sensory Examination - Double stimulation 

Sensory Examination - Pain and Temperature - Complete loss

 Analgesia  - think pain reliever

Sensory Examination - Pain and Temperature - Decreased

Hypoalgesia - think high pain tolerance

Sensory Examination - Pain and Temperature - Increased

Hyperalgesia - think low pain tolerance

Sensory Examination - Proprioception 

• Test by closing your eyes and determining if the joint of an arm, hand, or leg is in flexion or extension  

Sensory Examination - Stereognosis 

• The ability to perceive the weight, form, and other details of a body by touch

• Compared to -

• Tactile agnosia - a recognition disorder caused by a cortical sensory reason rather than a pathway

Sensory Examination - Vibratory

• The ability to perceive vibration 

• Test with a vibrating tuning fork on a bony prominence vs. nonvibrating

Sensory Examination - Body Sway

Romberg’s test

Romberg’s test

• Stand with feet together

• Observe sway with eyes open

• Compare sway with eyes closed

• Increased sway with eyes closed or loss of balance is positive