Lecture 23 - Somatic sensation

Question 1

Neuron forms

Answer 1

Anaxonic (no axon, so all communication (inputs and outputs) are mediated by dendrites
Unipolar
Bipolar
Multipolar

Question 2

Special sense

Answer 2

Special sense = vision, hearing, taste, smell (and pheromones) and vestibular/balance

Question 3

Somatic and visceral sensations

Answer 3

Somatic and visceral sensations = touch, pain, temperature and body position (proprioception)

Question 4

Sensory receptors

Answer 4

Sensory receptors are either …
Sensory endings of an afferent neuron - free nerve endings (it is the primary afferent neurone that is doing the sensing itself)
OR
Specialised receptor cells which synapse onto afferent neurons ( separate receptor cell that is highly specialised which is connected to an axon that then transmits this information to the brain)

They are highly sensitive to a particular stimulus (modality) - converts energy sensed (mechanical, temperature etc) into energy known as transduction

Question 5

4 types of information that describe a sensory stimulus

Answer 5

Modality, location, duration and intensity

Question 6

Modality

Answer 6

Type of sensory receptor activated determines type of information being transmitted e.g. muscle spindle for proprioception

Question 7

Intensity

Answer 7

Frequency of action potential firing in afferent neurons. Higher the frequency, higher the intensity of sensation. (For a stimulus to be register, the action potentials must be over threshold) (increasing stimulus = increasing action potential frequency)

Question 8

Duration

Answer 8

Duration of action potential firing in an afferent neuron

How long the action potentials fire for (Sensory receptors are most sensitive to change and show adaptation - decreased input over time due to continuous stimulation).

Question 9

Location

Answer 9

Location of sensory receptor(s) activated, mapped in the brain

Sensory receptors are mapped in the brain (receptive fields are a region of space in which a stimulus can lead to activity in a particular afferent neuron. Small fields = better discrimination)

Question 10

Proprioception

Answer 10

Proprioception is the sense of self-movement and body position.
Stimulus modality

Question 11

Muscle spindle sense…

Answer 11

Where our muscles/joints are in space

Question 12

Muscle spindle in depth

Answer 12

Muscle spindles are stretch receptors within the body of a muscle that primarily detect changes in the length of the muscle. They convey length information to the central nervous system via afferent nerve fibers. This information can be processed by the brain as proprioception.

Whenever the muscle changes length it operates mechanically operated receptors that pulls ion channels open, allows Na+to come in and depolarise these endings and get them to threshold

Question 13

Touch

Answer 13

Stimulus modality - by which external objects or forces are perceived through contact with the body (especially the hands)

Changes in pressure, temperature, air currents etc can be detected

Question 14

Action potential frequency

Answer 14

Below threshold - no response in the afferent neuron
Increasing stimulus increases action potential frequency

Receptors are mostly set up to be responsive to changes rather than a stead input
Mechanical input at the input zone that stretches the membrane and allows Na+ to enter and depolarise the neuron

Question 15

Stimulus duration

Answer 15

Sensory receptors are most sensitive to change
Often show adaptation - decreased output over time in response to continuous stimulation (most sensitive at the beginning of a stimulus as there is the most change at this point)

Question 16

Tonic receptors

Answer 16

A tonic receptor is a sensory receptor that adapts slowly to a stimulus and continues to produce action potentials over the duration of the stimulus. … Examples of such tonic receptors are pain receptors, joint capsule, and muscle spindle.

Question 17

Phasic receptors

Answer 17

A phasic receptor is a sensory receptor that adapts rapidly to a stimulus. e.g. touch receptors

Question 18

Adaptation

Answer 18

decreased output over time in response to continuous stimulation

Question 19

Receptive field

Answer 19

Region of space in which a stimulus can lead to activity in a particular afferent neuron

Small fields and dense innervation gives good discrimination. Gives you fine detail, stronger ability to discriminate where a stimulus is coming from

Many receptive fields = many axons

Receptive field of a sensory axon in a peripheral nerve is the area of the body that, if stimulated, results in the firing of the axon. Receptive fields can be characterised by size and also by density.

The size of receptive fields affects how accurately the location of a stimulus can be determined. A small receptive field allows for good localisation of a stimulus, in contrast, a larger receptive field provides less specific information about a stimulus location. The density of receptive fields determines how accurately two distinct stimuli can be discriminated. Two stimuli can be more easily detected in areas of the body we are receptive fields are more densely packed. This is because to stimulate close together on the skin will touch different receptive field and therefore will be felt is two points. Any time to stimuli touch only one receptive field they will be felt is one not two points.

Question 20

How does an arriving stimulus in a receptive field work?

Answer 20

Receptor is of a particular type which determines the modality of the input. The stimulus activates nerve endings within a particular receptor field, it is transduced in a particular way depending on what this ending is specialised to transduce. Action potential progresses along labelled line where it is to be processed in the CNS. Think of each individual axon carrying signals as a labelled line that the brain can read and interpret (imagine old phone system where people had to be manually plugged into lines to talk to people)

Question 21

Conscious sensations

Answer 21

Sensory receptor - sensory stimulus converted into action potentials (transduction)

Afferent neurons - peripheral nerve, tract or pathway

Integration - at the cerebral cortex, causes conscious sensation and perception (pain and touch associated together)

Our sensation is transduced into action potentials at the sensory receptor and carried up to the brain by the peripheral afferent neurons
Integration occurs in the cerebral cortex
This is when a stimulus is consciously sensed (primary somatosensory cortex) and perceived (meaningful interpretation at secondary/association region of the cortex)

Question 22

Somatosensory cortex

Answer 22

Postcentral gyrus - primary somatosensory cortex
Parietal lobe - somatic sensory association cortex (it is here that sensation of the environment in context is generated for us)

Sensation - conscious identification of ‘what and where’. Primary region of cortex

Perception - meaningful interpretation, association (secondary) region of the cortex

Question 23

Somatotopic organisation

Answer 23

Areas of cortex correspond to areas of the body

Densely innervated areas of the body occupy larger regions of the cortex (e.g. mouth and hands)

Left cortex represents right body and vice versa

Areas of the body with higher concentration of sensory neurons (small, densely packed receptive fields) are represented on larger portions of the somatosensory cortex.

Question 24

Sensory motor integration

Answer 24

Feedback from the cerebellum in particular, is an important system in order to give us a smooth generation of motor control that is sensitive to what we are doing to the environment and what the environment is doing to us in response to our output

Question 25

Alpha motor neuron

Answer 25

Alpha motor neurons are lower motor neurons whose cell bodies are found in the anterior horn of the spinal cord and whose axons travel down to the body to innervate skeletal muscle to cause muscle contraction.

Question 26

Excitable tissue and action potentials

Answer 26

Excitable tissues such as neuronsand muscles have a resting voltage across their membranes, the resting potential. These cells respond to stimuli with changes in membrane potential. When the cells are excited, the membrane potential changes, depolarise, and may reach threshold at which point are transient, abrupt change in the membrane potential is initiated. This is known as the action potential. The action potential generated in a neuron propagates by regenerating itself along the axon membrane until the action potential reaches synapse. The neuron me then released a neurotransmitter that defuses across the synapse to influence the membrane potential of the postsynaptic tissue.

Question 27

Information in the nervous system is coded in three ways

Answer 27

1- within a single icon or motor unit, by the frequency of action potentials
2- by the number of icons or motor units active (recruitment)
3- By the selection of particular axons or motor units to be active along a specific pathway (modality)

1 and 2 together code intensity and 3 codes location and modality

Question 28

Sensory neurons and sensory receptor

Answer 28

It’s a neurons consist of a cell body and the dorsal root ganglion and an axon with two branches, peripheral and central. The central branch terminates on synapses of neurons in the central nervous system. Peripheral branch terminates in the skin as sensory receptors. These can be unspecialised, or can be associated with specific structures. Action potentials are initiated at the sensory endings in response to a stimulus and propagate along the axon to the central synapse.

Question 29

Sensory receptors

Answer 29

Receptor endings of one sensory neuron or sensory unit are all the same, and are sensitive to one type of sensation (modality).

Some neurons are Touch Sensitive, while others are temperature sensitive. Different touch receptors are sensitive to light touch or depression.

Question 30

How does the brain identify a heavier object using only action potentials? How is modality involved in this?

Answer 30

A heavy object causes more action potentials to be sent to the somatosensory cortex, causing the perception of a more intense sensation in two ways.
1- activates more sensory neurons in the area of skin increase recruitment of sensory units
2- causes those activated sensory neurons to fire action potential at a higher frequency than the light object

Modality - a different state of touch receptors, the deeper receptors, have been activated by the lead or heavier weight, which means more receptors activated, more sensory neurons sending more action potentials to the brain therefore increased perceived intensity

Question 31

What have the light touch receptors done after responding to the initial change?

Answer 31

They have adapted. They have stopped phone action potentials (gradually reduce their response to the constant stimulus)

Question 32

Thermoreceptors

Answer 32

Heat in the hand

Temperature receptors are all bare, unspecialised, endings. Although the endings all look the same, sensory neurons for temperature fall into two groups, warm sensitive and cold sensitive. Skin sensory neurons sample from a discrete region of skin known as the receptive field of that neuron.

Thermo receptors cause of action potentials to fire along sensory neurons initially but then get adapted as temperature stays constant.

Thermo receptors report the relative change in temperature not the absolute temperature.

Question 33

Possible advantages and disadvantages of rapid sensory adaptation in the context of daily living and survival

Answer 33

Advantages
1- enables rapid detection of acute changes
2- allows us to ‘ignore’ less important information so we can focus on anything that may cause us harm

Disadvantages
1- slow rates of change your undetected and may lead to dangerous situations for example hypothermic in cold water as your skin thermo receptors adapt and therefore you are less aware of the cold

Question 34

Why do you become hypothermic in water more quickly than air?

Answer 34

Conducts heat more efficiently than air and therefore will draw heat from the body quicker

Question 35

Why does a windy day feel colder than a calm one, even if the air temperature is the same?

Answer 35

Greater convective heat transfer occurs with the wind. Peripheral thermoreceptors are sensitive to the increase rate of heat transfer from the body to the air.

Question 36

Conduction

Answer 36

Induction occurs through direct contact between neighbouring molecules, with heat flowing from the warmer to the cooler molecules. Different materials conduct heat at different rates and better the objects at conducting heat away from our bodies the colder the object with feel

Question 37

Sensory vs motor neurons

Answer 37

Sensory or afferent neurons transmit information about stimuli acting on the periphery to the central nervous system. Remember that a sensor unit is made up of the sensory neuron and its associated sensory receptors.

Motor or efferent neurons transmit information from the central nervous system to control the skeletal muscle contraction or other effector outputs

Question 38

Skeletal muscle fibres

Answer 38

Skeletal muscle is made up of individual muscle cells, called muscle fibres. The axon of each motor neuron branches to control several muscle fibres. A motor neuron, its axon, and the muscle fibres it controls are called a motor unit. In skeletal muscle, the action potential generated in a motor neuron propagates by regenerating itself along the axonal membrane until the action potential reaches a specialised synapse called the NMJ. The motor neuron releases a neurotransmitter that diffuses across the synapse. The neurotransmitter interacts with the muscle fibre membrane to generate an action potential. The action potential propagates along the muscle fibre membrane and causes the release of calcium ions from intracellular stores. This triggers the contraction of that muscle fibre. A single action potential in a single motor unit will invoke the brief development of tension, known as a muscle twitch. The force generated by a twitch of a single motor unit is very small.

Question 39

Explain how the perceived location of the sensation can be different from the actual location of the stimulus

Answer 39

The brain interprets information from certain axon pathways as coming from the origin, sensory receptor, of that pathway, even when the axon is activated part way along its length. Usually if the brain receives action potentials from the sensory neuron it is because it’s associated receptor has been activated. That is why you may perceive sensation in the head even though you’re activating the ulna nerve at the elbow.

Question 40

What is mechanical summation?

Answer 40

What is the formation of more cross bridges and calcium buildup in the sarcoplasm. Not the same as summation, we talk about summation occurring at neurons with more than one local/graded potential occurs on the same neuron cell body.

When stimuli are close together, calcium builds up in the cytosol, causing more cross bridges to form therefore increasing the force production therefore resulting in a stronger contraction

Question 41

Tetanus

Answer 41

State of sustained maximal muscle contraction

Smooth, sustained contractions of muscles die to the constant presence of calcium in the cytosol. Tetanus is the kind of contraction normally exhibited by skeletal muscle in the human body.

Question 42

Increasing current (strength of stimulation) increases…

Answer 42

Recruitment

Question 43

Decreasing the interval between stimuli causes an increase in

Answer 43

Frequency cause there is more stimuli per second therefore increasing frequency

Question 44

Conduction velocity

Answer 44

The speed of conduction of the action potential along a neuronal axon depends on the extent of myelination of the axon. Peripheral nerves are made up of many axons so the measurement of conduction velocity of a nerve is an average conduction velocity of many axons.

Question 45

What physiological processes contribute to the total latency of the muscle response following electrical stimulation at the elbow?

Answer 45

Initiation and propagation of action potential along the nerve, synaptic transmission at the NMJ, initation and propagation of the action potential along the muscle

Question 46

What is the advantage of having fast conduction velocities in the nerves controlling skeletal muscles?

Answer 46

Allows us to have rapid voluntary control over our skeletal muscles, so that we can exert precise movements, or move out of harms way quickly

Question 47

Muscle twitch

Answer 47

A single contraction in response to a brief stimulation