A&P 1 - Chapter 15: Sensory Pathways and the Somatic Nervous System

What are sensory receptors?

specialized cells or cell processes thar monitor specific conditions in the body or the external environment.

Where is somatic sensory information distributed to the brain?

distributed to sensory processing centers – either the primary somatosensory cortex (parietal lobe) or appropriate areas of the cerebellar hemispheres.

Where is visceral sensory information distributed to the brain?

Visceral sensory information: distributed to reflex centers in the brainstem and diencephalon.

Voluntary or involuntary somatic motor commands control which peripheral effector?

Skeletal muscles

What is sensation and perception?

Sensation: the arriving information to the CNS.

Perception: the conscious awareness of a sensation

What is the process of transduction?

the conversion of an arriving stimulus into an action potential by a sensory receptor.

What is receptor specificity?

Receptor specificity: each receptor has a characteristic sensitivity.

[Ex. A touch receptor is very sensitive to pressure but relatively insensitive to chemical stimuli. A taste receptor is sensitive to dissolved chemicals but insensitive to pressure.]

Which type of receptor has the least receptor specificity?

Free nerve endings

Define receptive field.

Receptive field: the area monitored by a single receptor cell

What is the relationship between receptor field size and the ability to localize a stimulus?

The larger the receptive field, the poorer the ability to localize a stimulus.

What is a labeled line?

Labeled line: the link between peripheral receptors and cortical neuron.

Consists of axons carrying information about one modality, or type of stimulus (touch, pressure, light, or sound)

What are tonic receptors?

Tonic receptors: ALWAYS ACTIVE! Action potentials are generated at a frequency that reflects the background level of stimulation. When the stimulus increases or decreases, the rate of action potential generation changes accordingly.

What are phasic receptors?

Phasic receptors: normally INACTIVE! Action potentials are generated only for a short time in response to a change in the conditions they are monitoring.

Define adaptation.

Adaptation: a reduction of receptor sensitivity in the presence of a constant stimulus.

Are tonic receptors fast adapting or slow adapting? Are phasic receptors fast adapting or slow adapting? Note: the way I remember this is that phasic is fast.

Tonic receptors: slow-adapting receptors (tonic water)

Phasic receptors: fast-adapting receptors

Which of the following situations describe a phasic receptor?

jumping into a pool and feeling cold, but then quickly “getting used to” the temperature

Phasic receptors

Which of the following situations describe a tonic receptor?

burning your finger and feeling pain for the entire day

General sense receptors are divided into three types called exteroceptors, proprioceptors, and interoceptors. What does each monitor?

Exteroceptors: provide information about the external environment

Proprioceptors: report the positions and movements of skeletal muscles and joints

Interoceptors: monitor visceral organs and functions.

What general stimulus do nociceptors detect? What does the size of their receptive field indicate?

Nociceptors: pain receptors with large receptive fields making it difficult to determine the exact source of a painful sensation.

Two types of axons, Type A and Type C fibers, carry painful sensations. What is the structural difference between Type A and Type C fibers? Which one carries fast pain (prickling pain) sensations? Which one carries slow pain (burning or aching pain) sensations?

Type A: Myelinated fibers with fast action potentials carrying sensations of fast pain, or prickling pain.

Type C: Unmyelinated fibers with slow action potentials carrying sensations of slow pain or burning and aching pain.

Note: this makes sense, because the presence of myelin causes quicker propagation of action potentials, while non-myelinated fibers have slower action potential propagation.

What is phantom limb syndrome?

Phantom limb syndrome: pain that is still felt in an amputated limb

What is the structure of thermoreceptors? Are they tonic or phasic?

Thermoreceptors: “temperature receptors” are free nerve endings located in the dermis, skeletal muscles, liver, and hypothalamus. They are phasic receptors that allows us to be able to be able to adapt to temperature change quickly when active.

What are mechanoreceptors sensitive to?

sensitive to physical stimuli that distort their plasma membranes.

What are the three classes of mechanoreceptors and what stimuli do each detect?

Tactile receptors: provide sensations of touch, pressure, and vibration.

Baroreceptors: detect pressure changes in the walls of blood vessels and in portions of the digestive, respiratory, and urinary tracts.

Proprioceptors: monitor the positions of joints and skeletal muscles.

Which tactile receptor is the only sensory receptor on the cornea of the eye?

Free nerve ending

Which tactile receptors are most abundant in the eyelids, lips, fingers, nipples, and external genitalia?

Tactile corpuscles

What kind of receptors monitor blood pressure in the carotid and aortic sinuses?

Baroreceptors

What kind of receptors are involved in defecation and urination reflexes?

Stretch receptors

What are the three major groups of proprioceptors?

· Muscle spindles

· Golgi Tendon Organs

· Receptor in Joint Capsules

What are chemoreceptors and what do they detect?

specialized nerve cells that detect small changes in the concentration of specific chemicals or compounds.

Which type of receptor monitors pH, and carbon dioxide and oxygen levels in arterial blood?

Chemoreceptors.

Describe the function of a first-order neuron

First-order neuron: a sensory neuron that delivers sensations directly to the CNS.

What is a second-order neuron?

Second-order neuron: the axon of a first-order sensory neuron with an interneuron

In what part of the brain would you find a third-order neuron?

Third-order neuron: For us to become aware of the sensation, the second-order neuron in turn must pass on the signal to the third-order neuron in the thalamus.

Note: You can think of the passing of information from neuron to neuron like passing a baton from runner to runner in a relay race.

What is the process called when a second-order neuron crosses over to the opposite side of the CNS?

Decussation

Due to the decussation of second-order neurons, the right side of the thalamus receives information from which side of the body?

The right side of the thalamus receives information from the left side of the body; the left side of the thalamus receives information from the right side of the body.

Where does the spinothalamic pathway begin and end?

Spinothalamic pathway: starts at the spinal cord and ends in the thalamus.

Note: Somatic Sensory Pathways: Sensory pathway spinal tract names often give clues to their function. For example, if the name of a tract begins with spino-, the tract starts in the spinal cord, ascends upward then ends in the brain. The rest of the name indicates the tract’s destination. Since the information is traveling up to higher centers in the brain, it must be an ascending tract that carries sensory information. For example: The spinocerebellar pathways start at the spinal cord and end in the cerebellum.

List the three main somatic sensory pathways.

1. Spinothalamic pathway

2. Posterior column pathway

3. Spinocerebellar pathway

What sensations are carried by the spinothalamic pathway?

Spinothalamic pathway: carries sensations of crude touch, pressure, pain, and temperature.

Which sensory pathway is associated with phantom limb pain?

Phantom limb pain: caused by activity in the sensory neurons or interneurons along the spinothalamic pathway.

Which sensory pathway is associated with? What is a familiar example of this that the book states?

Referred pain: feeling pain in an uninjured part of the body when the pain actually originates at another location. Associated with the spinothalamic pathway.

Example: pain of a heart attack.

Which tracts in the spinothalamic pathway carry sensations of crude touch and pressure sensations?

Anterior spinothalamic tract

Which tracts in the spinothalamic pathway carry pain and temperature sensations?

Lateral spinothalamic tract

What sensations are carried by the posterior column pathway?

carries sensations of fine touch, vibration, pressure, and proprioception.

Which two spinal tracts are part of the posterior column pathway?

Gracile & Cuneate fasciculus

What is a sensory homunculus?

Sensory homunculus: functional somatopic map of the primary motor cortex that shows the relative size of the cortex devoted to a particular body region.

What information is conveyed by the spinocerebellar pathway?

Spinocerebellar pathway: conveys information about muscles, tendon, and joint positions from the spine to the cerebellum.

Which tracts of the spinocerebellar pathway contain axons that do not cross over to the opposite side of the spinal cord?

Posterior spinocerebellar tracts

tracts of the spinocerebellar pathway are dominated by axons that have crossed over to the opposite side of the spinal cord.

Anterior spinocerebellar tracts

Where is visceral sensory information primarily collected from? (ie: which body cavities)

collected by interoceptors monitoring visceral tissues and organs, primarily within the thoracic and abdominopelvic cavities.

Which nuclei function as major processing and sorting centers for visceral sensory information?

nucleus of the solitary tract: a large nucleus on each side of the medulla oblongata that has extensive connections with various cardiovascular and respiratory centers that allow the nuclei to process and sort the information.

Somatic motor pathways always involve at least two motor neurons. What are they called?

Upper motor neurons: whose cell body lies in a CNS processing center

Lower motor neurons: whose cell body lies in a nucleus of the brainstem or spinal cord

Conscious and subconscious motor commands control skeletal muscles by traveling over three integrated motor pathways. Name the motor pathways.

· Corticospinal pathway

· Medial pathway

· Lateral pathway

Note: Because the name of the corticospinal tract ends in -spinal, this indicates that the tract ends in the spinal cord.

Which motor pathway carries motor command that provides voluntary control over skeletal muscles?

Corticospinal pathway

Name the three pairs of descending tracts of the corticospinal pathway. (You can remember these because all three contain the root word “cortico- “)

· Corticobulbar tracts

· Lateral corticospinal tracts

· Anterior corticospinal tracts

What is a motor homunculus? What do larger areas of the homunculus represent?

Motor homunculus: A map showing that the areas of primary motor cortex devoted to a specific body region is proportional to the density of motor units involved in the region’s control. Large areas represent areas that are capable of varied and complex movements, such as the hands, face, and tongue.

Which motor pathway is associated with the control of muscle tone and gross movements of the neck, trunk, and proximal limb muscles?

Medial pathway

Name four tracts that are part of the medial motor pathway.

· Medial and lateral reticulospinal tracts

· Tectospinal tract

· Vestibulospinal tract

Which motor pathway is associated with the control of muscle tone and more precise movements of the distal parts of the limbs?

Lateral pathway

The rubrospinal tract is part of which motor pathway?

descending tracts of the lateral pathway that carry involuntary motor commands issued by the red nucleus of the midbrain.

What part of the brain contains the basal nuclei? What information do the basal nuclei provide?

Basal nuclei of the cerebrum provide the background patterns of movement involved in voluntary motor activities.

What is the function of the cerebellum?

monitors proprioceptive (position) sensations, visual information from the eyes, and vestibular (balance) sensations from the internal ear and adjusts the activities of the upper motor neurons involved to inhibit the number of motor commands used for that movement accordingly.

HOMEWORK QUESTIONS:

The sensory receptors, neurons and their pathways make up the ________ division of the nervous system.

afferent

The conversion of a stimulus into an action potential is

transduction

Which of the following best explains why the body "remembers" painful injuries long after they have been healed?

Pain receptors are slow-adapting receptors, so they adapt very slowly to changes in the PNS, such as the healing of an injury.

Your brain is not constantly bombarded with signals telling it that you are wearing socks. This is because the touch receptors around your ankle are

phasic receptors and fast-adapting

In order for a sensation to become a perception

it must reach the somatosensory cortex

Central adaptation refers to

inhibition of nuclei along a sensory pathway

Peripheral adaptation ________ the number of action potentials that reach the CNS

decreases

Sensory information from all parts of the body is routed to

the thalamus

A ________ potential is a depolarization of a sensory dendrite that can lead to an afferent nerve impulse

generator

If a nerve impulse was transmitted to the central nervous system (CNS) on a C fiber,__________.

it would lead to a slow sensation of pain

Which of the following is the best illustration for Golgi tendon organ activation?

a male gymnast performing an iron cross on the rings apparatus

You cannot tell when your small intestines shift position as food enters because you do not have any ________ there.

proprioceptors

Receptors in your stomach that determine when food enters because the pH changes are

chemoreceptors

Two types of receptors that are least likely to adapt are

proprioceptors and nociceptors

Chemoreceptors of the general senses do not send information to the primary sensory cortex but instead send information to the

brain stem

Endorphins can reduce the perception of sensations initiated by

nociceptors

Mechanoreceptors that respond to changes in blood pressure are

baroreceptors

A fast-adapting tactile receptor that is composed of a single dendrite enclosed by concentric layers of collagen is a

lamellar (Pacinian) corpuscle

For the last few weeks, you have had an odd sensation on your upper thigh. When you touch the naked skin, it feels very strange, like you are touching your skin through layers of cloth. The surrounding area feels normal. Your doctor thinks that this may be related to spinal nerve damage due to a herniated disc. The doctor calls this change in sensation

paresthesia

The tract that carries sensations from muscle spindles to the CNS is the __________.

spinocerebellar

What is the minimum number of synapses that must be crossed before an afferent nerve impulse becomes a perception?

three

The spinal tract that relays information concerning pain and temperature to the CNS is the

lateral spinothalamic

Your uncle was just diagnosed with a heart attack. One of his major symptoms is left arm pain. This phenomenon is ________ pain.

referred

Each of the following is an ascending tract in the spinal cord except the

reticulospinal tract

The afferent neuron that carries the sensation and enters the CNS is a ________

neuron.

first-order

We can localize sensations that originate in different areas of the body because

sensory neurons from specific body regions project to specific cortical regions.

The motor pathway that decussates in the medulla oblongata is the __________

tract.

lateral corticospinal

The corticospinal pathway begins in which area of the brain?

primary motor cortex

The larger the body structure is on the motor homunculus the ________ they have.

more motor units

The rubrospinal tracts originate in the

red nuclei

The neurons of the primary motor cortex are ________ cells.

pyramidal

The spinal tract that plays a role in the subconscious regulation of the muscles of the arms is the ________ tract.

rubrospinal

The spinal tract that unconsciously maintains balance and muscle tone is the ________ tract.

vestibulospinal

The medial pathway that controls involuntary movements of head, neck, and arm position in response to sudden visual and auditory stimuli is the ________

tract.

tectospinal

Some neurons within the basal nuclei are known to

inhibit neurons with GABA and stimulate neurons with acetylcholine.