Neurobiology Chapter 9: Somatic Sensory System

What are the four regions of the postcentral gyrus and what sensations do they mediate?

-3a: proprioception -3b: tactile -1: tactcile -2: proprioception and tactile

Which two central pathways convey information from receptors to the brain?

-Dorsal column system -Spinocerebellar tract

What is the sensory function of Merkel discs? What are their stimuli? What kind of indentation do they respond to?

-Form and texture perception -Edges, points, corners, curvature -Sustained (slow adaptation)

What are the five types of sensory receptors? What are their functions?

-Mechanoreceptors (detect deformation) -Thermoreceptors (detect change in temperature) -Nociceptors (detect damage - pain receptors) -Electromagnetic (detect light, electric fields, magnetic fields) -Chemoreceptors (taste, smell, CO2, O2, etc.)

The dorsal columns of the spinal cord are topographically organized such that the fibers conveying information from lower limbs lie mostly __________. The fibers that convey information from the upper limbs, trunk and neck like in a more __________ bundle.

-Medial -Lateral

Sense of touch is based on the interpretation of complex spatial-temporal sensory input obtained from a number of distinct receptors. What are these four receptors?

-Merkel -Meissner -Pacinian -Ruffini

What is the sensory function of Meissner corpuscles? What are their stimuli? What kind of indentation do they respond to?

-Motion detecting; grip control -Skin motion -None (rapid adaptation)

What is the sensory function of Pacinian corpuscles? What are their stimuli? What kind of indentation do they respond to?

-Perception of distant events through transmitted vibrations; tool use -Vibration -None (rapid adaptation)

What is the sensory function of Ruffini endings? What are their stimuli? What kind of indentation do they respond to?

-Tangential force; hand shape; motion direction -Skin stretch -Sustained (slow adaptation)

The somatosensory system mediates which sensations?

-Touch -Pressure -Vibration -Proprioception -Heat -Cold -Pain

(Somatic input to the thalamus) Where does the VPL receive projections from? Where does the VPM receive projections from?

-VPL: lower body and posterior head -VPM: face

Narrate the pathway depicted in Figure B.

1. A sensory receptor is activated and an action potential is generated 2. The action potential passes through the dorsal root ganglion and enters the dorsal root of the spinal cord 3. The neuron synapses with an alpha motor neuron and an interneuron 4. The interneuron inhibits the effector muscle and alpha motor neuron causes contraction of the flexor muscle

How a sensation is perceived is determined by what two things?

1. Characteristics of the receptor 2. Central connections of the axon connected to the receptor

Gradiations in signal intensity can be achieved by what two things?

1. Increasing the number of fibers (spatial summation) 2. Increasing the rate of firing in a limited number of fibers (temporal summation)

Put these receptors in order from rapidly adapting to slowly adapting: hair receptor, muscle spindle, pacinian corpuscle, joint capsule receptor

1. Pacinian corpuscle 2. Hair receptor 3. Muscle spindle 4. Joint capsule receptor

Which of the four regions of the postcentral gyrus is the primary area for processing somatosensory information coming from the VPC of the thalamus?

3b

What is a receptor potential?

A graded potential produced by activation of a sensory receptor.

Explain this diagram.

A lesion study was performed in which digit 3 of a monkey was amputated. Immediately after the amputation, the cortical map did not change. After a few months, the region in the cortex that had formerly responded to digit 3 stimulation started responding to digit 2 and 4 stimulation.

What is a phasic receptor?

A rapidly adapting receptor

What is a tonic receptor?

A slow adapting receptor

How does a stimulus cause a receptor potential and possibly an action potential?

A stimulus alters the permeability of cation channels in the afferent nerve, generating a depolarizing current. If the depolarizing current is large enough, the membrane may reach threshold and initiate an action potential.

What allows you to drop something hot before you've realized that it hurts to hold?

Afferent axons that mediate pain and temperature have a very small diameter and may be myelinated or unmyelinated.

Where does the secondary somatosensory cortex project to?

Amygdala and hippocampus

Explain this diagram.

An animal was trained in a task that requires extensive use of digits 2 and 3. After several months of training, the cortical map changed so that a larger region of the cortex contained neurons activated by the digits used in the task.

Where does Bromann's area 3b project to? Where do projections go after that?

Areas 1 and 2. They then project to the secondary somatosensory cortex and parietal areas 5 and 7.

What is the significance of the postcentral gyrus in somatic input to the thalamus?

Axons arising from neurons in the VPC of the thalamus project to cortical neurons in the primary somatosensory cortex. This cortex is located in the postcentral gyrus, and comprises four different regions which represent different sensory modalities.

Explain these graphs.

Deformation of a Pacinian corpuscle causes the membrane of the afferent fiber to stretch, and results in the opening of mechanoreceptor channels. Sodium enters these channels and causes depolarization. If the stimulus is strong enough, threshold will be reached and an action potential will occur.

Why does testing for pain sensation provide a more precise assessment of a segmental nerve injury than does testing for responses to touch, pressure or vibration?

Dermatones overlap more extensively for sensations of touch, pressure and vibration.

What is the take home from this chart?

Different types of sensory input are detected by different receptor types. The receptor types are associated with different types of axons. The axons are of different diameter and may be myelinated or unmyelinated. Axon diameter and myelination determine conduction velocity.

The dorsal column pathway receives information from sensory receptors throughout the body, and carries this in nerve tracts in the white matter of the _________ _________ of the spinal cord, to the medulla where it is continued on to the VPL nucleus of the thalamus and relayed from there to the somatosensory cortex.

Dorsal columns

Where are cell bodies of sensory neurons located?

Dorsal root ganglion and cranial ganglia

Explain this diagram. What does each dot represent?

Each dot represents an action potential recorded from a single mechanosensory afferent fiber innervating the human finger as it moves across a row of Braille. Only slowly adapting Merkel cell afferents provide a high-fidelity representation of the Braille pattern.

(True/false) Dermatome overlap is more extensive for sensations of pain and temperature than for touch, pressure and vibration.

False, it's the other way around.

How does the information supplied by different somatosensory receptors remain segregated in their passage to the thalamus?

Inputs carrying different types of somatosensory information (mechanoreceptors, muscle spindle afferents, golgi tendon organs) terminate on separate populations of relay cells within the VPC of the thalamus.

If you touch something with the right side of your body, that sensory information ends up on the left side of your brain. How?

Internal arcuate fibers cross over in the caudal medulla.

What is the relationship between the segmental distribution of proprioceptors and the dermatomal map?

It does not follow the dermatomal map but is more closely allied with the pattern of muscle innervation.

What is the dorsal column system?

It's a sensory pathway of the CNS that conveys sensation of deep touch, vibration, pressure and proprioception from the skin and joints.

What are parallel pathways?

Labeled lines that remain separate through several stages of processing.

Explain this diagram.

Most activity occurs in the center of the receptive field.

Where do parietal areas 5 and 7 project to?

Motor and premotor cortical areas

What are these images showing?

Neurons with similar response properties are clustered together into functionally distinct columns that traverse the depth of the cortex.

How do receptors perceive an overload of stimulus?

Pain

Neurons in the dorsal root ganglia are pseudounipolar. What does this mean?

Peripheral and central components of afferent fibers are continuous, attached to the cell body in the ganglia by a single process.

How come when you get hit in the head, you see stars?

Photoreceptors normally respond to light. However, they can respond to pressure if enough stimulus is applied.

Explain this diagram.

Projections from the thalamus to the somatosensory cortex are arranged topographically.

Describe this diagram.

Pseudounipolar neurons in the dorsal root ganglia synapse with efferent neurons that go to the skin or muscle, as well as central processes that synapse with neurons located in the spinal cord and at higher levels of the nervous system.

(Slowly or rapidly adapting receptors?) Important for balance and movement.

Rapidly

(Slowly or rapidly adapting receptors?) Rate and strength of the response is related to the rate and intensity of the stimulus.

Rapidly

(Slowly or rapidly adapting receptors?) Respond only when change is taking place.

Rapidly

Signals are specific. Individual afferents respond to a limited set of stimuli. Why is this?

Receptors are specialized to depolarize only in response to a limited set of stimuli.

Where are receptors located in the somatosensory system?

Skin or muscle

(Rapidly/slowly) adapting receptors can encode duration of a stimulus as well as the size and shape of a stimulus.

Slowly

(Slowly or rapidly adapting receptors?) Continue to transmit impulses to the brain for long periods of time while the stimulus is present.

Slowly

(Slowly or rapidly adapting receptors?) Include muscle spindle, golgi tendon apparatus, ruffini's endings, merkels discs.

Slowly

(Slowly or rapidly adapting receptors?) Keep brain appraised of the status of the body with respect to its surroundings.

Slowly

(Slowly or rapidly adapting receptors?) Provide information about the spatial attributes of a stimulus (size and shape).

Slowly

Explain this diagram.

Slowly adapting receptors continue responding to a stimulus whereas rapidly adapting receptors respond only at the onset.

The receptive fields in regions with dense innervation (fingers, lips, toes) are relatively (small/large) and densely packed compared with those in the forearm or back.

Small

Why is it important that dermatomes overlap?

So that injury to an individual dorsal root does not lead to complete loss of sensation in the relevant skin region.

Explain this graph.

Stimulus strength for graded potentials is encoded by size, and stimulus strength for action potentials is encoded by frequency. The size of an action potential does not change!

What synapses are missing in the spinal cord in this diagram?

Synapses with interneurons

What is a receptive field?

The area of the skin surface over which stimulation results in a significant change in the rate of action potentials.

The somatosensory system consists of a map in which different regions of the cortex correspond to stimulation of different body parts. How does this compare to the auditory and olfactory system?

The auditory system does not have a map. It relies on frequency disparity and interaural delay to encode sound. The olfactory system also does not have a map. There is no "sour" or "sweet" region of the olfactory system.

The size of a receptive field is largely a function of what?

The branching characteristics of the afferent within the skin. Smaller arborizations result in smaller receptive fields.

What is two-point discrimination?

The minimum interstimulus distance required to perceive two simultaneously applied stimuli as distinct.

Where does the dorsal column project to?

The primary somatosensory cortex

What is sensory transduction?

The process of converting the energy of a stimulus into an electrical signal.

What is a dermatome?

The territory innervated by each spinal nerve.

Explain this diagram.

This is a map of the somatic sensory cortex, created by stimulating the periphery and seeing which areas of the brain experience electrical activity. Slowly adapting and rapidly adapting receptive fields overlap in digit 4, however they are partially segregated within the D IV region.

What sensation travels faster, touch or pain/temperature?

Touch. These axons are larger in diameter and are more insulated.

Somatic sensory pathways from the spinal chord synapse in cells in which region of the thalamus?

Ventral posterior complex (VPC)

Explain this graph.

When a continuous stimulus is applied, impulses per second decreases over time due to adaptation. Difference receptors adapt at different rates. For example, Pacinian corpuscles adapt very quickly.

What is receptor adaptation?

When a continuous stimulus is applied, receptors respond rapidly at first, but response declines.

Explain this diagram.

You can sense the difference between two different points that are very close together in places like fingers, lips and toes. Note that the shorter bars indicates more sensory neurons in a smaller receptive field.