Exteroceptors and proprioceptors

What is exteroception?

Touch, temperature, and pain sensation is from external stimuli 

What is proprioception?

Inside the body like at muscles and tendons. Also get proprioception from visceral organs. 

What does the sensory neurone in the spinal cord do?

* Supplies muscle and synapses with motorneurones, other supply tendons, joints and skin.

What do sensory neurones in the skin do?

Sensory neurons which have processes in the skin also in muscle which run into the spinal cord cell bodies in the dorsal root ganglion. Information on touch, pain and temperature, but also muscle length and tension.  

What do Ruffini endings do?

Measure slippage, shear stress

What do naked endings do?

Detect pain and temperature, connected to hairs

Why are many nerve endings encapsulated by glial cells just below the surface?

To modify their properties to some extent.  

What do hair follicle endings detect?

Hair movement

What do Merkel endings on finger pads do?

Sense sustained pressure, detailed mapping of texture and shape (slow adapting)

What do messier endings sense?

Touch - brief response, effective for transient contact (rapid adapting)

What do pacinian corpuscle endings detect?

Vibration

What do sensory pathways do?

* Send their axons into a number of ascending tracts run up through the spinal cord to the brain via the thalamus project to primary sensory cortex.  
* Tracts run up the thalamus run to primary sensory cortex. 
* Primary motor from the cortex to control movement.  

What is similar to ascending tract in primates?

Spinothalamic tract

What is similar to ascending tract in non-primates?

Spinocervical tract

What does the primary somatosensory cortex do?

Involved in sensation also proprioception goes there also goes to the cerebellum. 

How can we see how the sensory cortex is mapped?

Take a slice of sensory cortex

What is the sensory homunculus?

Representation of the body in the cortex 

What is the map of the body distinct to?

The face

What do muscle spindles do?

Within the muscle fibres to produce the power are small spindle shaped structures whose length is monitored by sensory and controlled by motor.

How does the muscle spindle work?

* When the muscle is stretched, the sensory axon in the spindle responds 
* When the signal reaches the spinal cord, the alpha motorneurons contract the muscle  this may make the spindle slack  so it cannot respond to further stretch 
* Intrafusal fibre inside muscle 
* Muscle fibre contracts, change in length of muscle spindle.   
* Spindle goes slack.  
* Tighten it up by contracting fibres. 
* Necessary to transmit sensation and keep the system responsive. 
* Adapted through changes in muscle length, contract via gamma motor neurons. 
* Gamma motor neurons spindle contracts to the optimal response range for new length. 

What do Golgi tendon organs?

* Monitor stretch in tendons 
* Our awareness of the relative positions of different parts of the body comes from the activity of the muscle spindles  plus the Golgi tendon organs 
* Endings sit in tendons, monitor stretch with muscle length. Gives us a subjective indication of body position.  

What do joint receptors do?

* These endings in the capsule and  the supporting ligaments of joints give sensations of pain and discomfort when the joint is moved beyond its range of normal movement 
* Give sensation of pain or discomfort when joint is moved beyond its normal range. Arm locks, leg locks. Not indicative of just their position but more their normal range.  

How is exteroception mediated in animals with exoskeletons?

Mechanosensory hairs which detect touch and air currents

What are the features of touch hairs on the exoskeleton?

* Sensitive to touch and also longer to sense air flow.
* Quite stiff, flexible sockets, have sensory neurons underneath.  

What are the features of sensory hairs?

* Quite stiff.
* Some less stiff are much longer, flexible socket, allow to move.
* Short – contact.
* Underneath, have sensory neuron.
* Contain a modified cilia.
* Monitors movement.
* Support cells make up a little package.
* Action potential then runs to the central nervous system.  

What are locus thoracic ganglion?

* Terminals within insect ganglia arising from receptors.
* Sensory neuron.
* Arise from the cuticle.
* Mechanoreceptive nerve endings 
* The axon terminals of  invertebrate mechanoreceptors end in the brain or segmental ganglia 

How do you run large volumes of ganglion?

Shave off hair, wait 24 hours, inject dye run into large volume of ganglion. 

What do campaniform sesillae do (proprioceptor)?

* The oscillating halteres act like gyroscopes  as they tend to resist changes in the direction of their movement. When the  insect turns during flight this creates strains in the shaft of each haltere that are detected by the  arrays of campaniform sensillae that lie at its base 
* 2 winged insects, the hind wings are modified to a structure called a halteres, which tell you if its turning.  Acts like a gyroscope.  
* Have a mass on the end, very thin stalk, move up and down out of phase with wings. Same frequency as wing beat. Monitor movement of cuticle. Allows animal to know its movement, like a gyroscope. Like horizon detectors in vision lecture, occili, here gyroscope.   
* Camaniform sensillae detect deformation as animal rotates. Same information as an aircraft. 

What is the chordotonal organ?

This lies inside the exoskeleton and  is a stretch receptor which has its own tendon, here attached to the next joint of the leg. When the joint is flexed, different sensory neurons detect the position of the joint and the speed of the movement 

What is the muscle receptor organ?

* The muscles in this organ are not the ones that provide the power for movement (the ones you eat!);  
* The MRO is formed from small accessory muscles that are supplied by the same nerve cells that control the power muscles (i.e. in parallel). 
* Analogous to the muscle spindle  
* The movements monitored by the MRO are the  powerful tail flips that are used for escape 
* Present in crayfish or lobster 

What does the ampulla of lorenzini do?

* Found on the sharks and rays, these sensory structures detect the electric fields associated with muscle activity in potential prey species. They also detect temperature. 
* Sensory nerve endings lie at the base of a pit filled with a conductive gel 
* Prey can be detected even when not visible (murky water or under sand/sediment) 
* Electroreceptors are generally absent from true mammals (with the possible exception of some dolphin species) however they are present in the  monotremes e.g.  duck billed platypus  (aquatic) and echidna (terrestrial – spiny anteater) which have ampullae like receptors on the beak/snout. 

What are the 2 basic types of electric fish?

1)those that use high voltages to stun prey and (electric eels and torpedo rays) 

2)weakly electric fish that use  constant fields or trains of pulses for navigation and communication in murky environments  (Mormyrids, Gymnarchus etc.) rather like the bats use their calls 

What generates electric fields in electric fish?

Modified muscle cells

Why is signal amplitude kept low?

* Producing the electric discharges is energetically expensive 
* They can also be detected by some predators 
* Only high during feeding and social interactions

What are elongated muscle cells?

* Work like battery.  
* Variety of different types, stun prey, just a big pulse.  
* Communication, series of pulses for navigation.  
* By the wall, higher frequency 

What do birds use for direction during migration?

Magnetic fields

What 3 theories suggest birds use magnetic field for navigation and what is the favoured theory?

* 1)Electroreceptors (like the ones found in some fish) might be activated by moving through a magnetic field but birds don’t appear to have such receptors 
* 2) Detection might be based on magnetic metal crystals (magnetite – similar to a compass needle) if these were present  in nerve cells. Magnetite granules are present around the beak  in some birds but in macrophages, not neurons. 
* 3) Magnetic sense seems to be linked to vision. It is only present when birds are under  full spectrum white light.  
* The current favoured theory is that the magnetic field alters the spin state of  high energy electrons generated when photopigments absorb light energy.