Pre-Topic 12: The Spinal Cord, Somatic Reflexes, and Sensory Reception

How many muscle fibers are generally found within a muscle spindle?

7 to 8

What is the cauda equina?

A bundle of nerve roots occupying the vertebral canal from L2 to S5

Which of the following describes a sensory nerve?

A nerve containing only fibers carrying incoming nerve signals to the CNS

Which describes a motor nerve?

A nerve containing only fibers carrying outgoing nerve signals from the CNS

What is a muscle spindle?

A type of stretch receptor embedded in skeletal muscle

During a crossed extension reflex, the ... nerve fiber of the reflex arc crosses to the contralateral side.

Afferent

During a crossed extension reflex, which nerve fibers cross to the contralateral side?

Afferent

Which fiber type is indicated by the arrow labeled A?

Alpha motor neuron

Which horns of the spinal cord contain the cell bodies of outgoing somatic motor neurons?

Anterior

Within the spinal cord, which tracts carry information up to the brain?

Ascending

Where do gamma motor neurons innervate a muscle spindle?

At the contractile end of each intrafusal fiber

In the spinal cord, what is the space within the gray commissure that contains CSF?

Central canal

What does the central canal of the spinal cord contain?

Cerebrospinal fluid

Match each type of receptor to the stimulus it detects. Chemoreceptor Thermoreceptor Photoreceptor Nociceptor Mechanoreceptor

Chemoreceptor Chemicals Thermoreceptor Changes in temperature Photoreceptor Light Nociceptor Tissue damage (or potentially damaging stimuli) Mechanoreceptor Touch, pressure, stretch, vibration

In response to stretching, a muscle reflexively does which of the following?

Contracts

Which tract will have its origin and destination of opposite sides of the body?

Contralateral

The nerve fibers of which tract form ridges called pyramids on the anterior surface of the medulla?

Corticospinal

In which reflex does a contralateral extensor muscle contract?

Crossed extension

Which reflex is contralateral?

Crossed extension reflex

What factor accounts for the fact that the right side of the brain communicates with the left side of the body, and vice versa?

Decussation

Within the spinal cord, which tracts carry motor signals down from the brain?

Descending

... fibers are working muscle fibers outside a muscle spindle.

Extrafusal

True or false: A reflex involving one or more interneurons is a monosynaptic reflex.

False

True or false: Sensory receptors are composed of sensory organs.

False

In which reflex is there a quick contraction of flexor muscles in response to a painful stimulus?

Flexor

Which nerve fiber is indicated by the arrow labeled A?

Gamma motor neuron

Which innervates the contractile ends of intrafusal fibers within a muscle spindle?

Gamma motor neurons

Which spinal cord areas are composed of gray matter?

Gray commissure Posterior horn Anterior horn

Which stimulus causes a muscle to reflexively contract?

Increased stretch

In which reflex arc do the input and output occur at different levels of the spinal cord?

Intersegmental

muscle fibers are modified muscle fibers within a muscle spindle.

Intrafusal

Name the fiber type that has sarcomeres at the ends of the cell, but lack sarcomeres at the middle.

Intrafusal fibers

Within a muscle, how are intrafusal fibers structurally different from extrafusal fibers?

Intrafusal fibers have sarcomeres at their ends; the middle lacks sarcomeres.

Which describes a muscle spindle?

It contains 7 or 8 modified muscle fibers and a few nerve fibers enclosed in a fibrous capsule.

Within a muscle spindle, gamma motor fibers contract the ends of the intrafusal fibers. As a result, what happens to the sensitivity of the muscle spindle to stretch?

It is increased.

How does the neural pathway of the flexor reflex compare with that of the tendon reflex?

It is more complex.

What are the three funiculi of white matter found in the spinal cord?

Lateral funiculus Ventral funiculus Posterior funiculus

In descending tracts, which motor neuron innervates the target organ?

Lower

Which type of receptor detects touch, pressure, stretch, or vibration?

Mechanoreceptor

What are intrafusal fibers?

Modified muscle fibers within a muscle spindle

Which type of reflex arc consists of only an afferent neuron and an efferent neuron?

Monosynaptic

Within the spinal cord, what type of information do the descending tracts carry?

Motor

Proprioceptors respond to which type of stimuli?

Movements or changes in body position

Define extrafusal fibers.

Muscle fibers outside of a muscle spindle

What do muscle spindles monitor?

Muscle length and body movements

Which is a type of stretch receptor embedded in skeletal muscle?

Muscle spindle

Muscle spindles would be more abundant in which of the following?

Muscles that require fine control

Which type of receptor is involved in the sensation of pain?

Nociceptor

Which type of neural circuit is used in a flexor reflex?

Parallel after-discharge

What is a reflex involving one or more interneurons called?

Polysynaptic

What are the characteristics of a phasic receptor?

Produces a quick burst of action potentials Adapts quickly to a stimulus

What are the characteristics of a tonic receptor?

Produces a steady, prolonged response Adapts slowly to a stimulus

Muscle spindles are classified as what type of receptor?

Proprioceptor

Which muscles are the effectors in a patellar reflex?

Quadriceps

Which term refers to any structure specialized to detect a stimulus?

Receptor

During a reflex response, what is it called when antagonist muscles are inhibited so that they do not oppose the action of the reflex?

Reciprocal inhibition

Which term refers to sensory receptors accompanied by accessory tissues that facilitate transduction of a stimulus?

Sense organ

The anterior horns of the spinal cord contain the somas of which neurons?

Somatic motor

Which is responsible for the inhibition of antagonistic contraction during a reflex response?

Spinal cord interneurons inhibit alpha neurons to the antagonists.

Which reflexes are important in stabilizing joints?

Stretch

reflexes are important in stabilizing joints because they balance the tension between extensors and flexors.

Stretch

In the spinal cord, where is the white matter in relation to the gray matter?

Superficial

Where is the white matter in the spinal cord?

Superficial to the gray matter

Which sensory modalities depend on the activation of chemoreceptors?

Taste Smell

What does contralateral mean?

That two points are on opposite sides of the body.

Which neurons would need to be inhibited in order for reciprocal inhibition to occur?

The alpha motor neurons to the antagonistic muscle

Stretch reflexes are mediated primarily by which of the following?

The brain

Which type of receptor is specialized to respond to hot and cold stimuli?

Thermoreceptor

What is the function of a muscle spindle?

To detect muscle length and body movements

True or false: Muscle spindles sense changes in muscle length and are therefore proprioceptors.

True

True or false: Muscle spindles would be more abundant in muscles requiring fine control.

True

True or false: The flexor reflex uses an ipsilateral reflex arc.

True

True or false: The neural pathway of a flexor reflex is more complex than that for the tendon reflex.

True

In descending pathways, which motor neurons have their somas in the cerebral cortex or brainstem?

Upper

A(n) ______ fiber is a nerve fiber carrying nerve signals toward the central nervous system.

afferent

The ... horns of the spinal cord contain the cell bodies of outgoing somatic motor neurons.

anterior

The bundle of nerves inferior to the medullary cone of the spinal cord is called the

cauda equina

In the spinal cord, the ... canal is lined with ependymal cells and contains cerebrospinal fluid.

central

The nerve fibers of the ... tract form ridges called pyramids on the anterior surface of the medulla.

corticospinal

A reflex in which a contralateral extensor muscle is contracted is called a ... extension reflex.

crossed

Because the nerve tracts of the spinal cord undergo ... , the right side of the brain communicates with the left side of the body, and vice versa.

decussation

An ______ fiber is a nerve fiber carrying nerve signals away from the central nervous system.

efferent

The crossed ... reflex is contralateral.

extensor

A quick contraction of flexor muscles in response to a painful stimulus is called a ... reflex.

flexor

Which reflex is ipsilateral?

flexor

A flexor reflex involves contraction of ______ muscles and reciprocal inhibition of ______ muscles.

flexor; extensor

Within a muscle spindle, ... motor fibers contract the ends of the intrafusal fibers.

gamma

An ... reflex arc is a reflex arc in which the input and output occur at different levels of the spinal cord.

intersegmental

In descending pathways, the upper motor neurons synapse with ______ motor neurons in the brainstem or spinal cord.

lower

The ______ motor neurons (which are the motor neurons connecting the brainstem and spinal cord to muscle fibers) of the descending pathway innervate skeletal muscle.

lower

A ... reflex arc consists of only one synapse between two neurons.

monosynaptic

The anterior horns of the spinal cord contain somas of somatic ______ neurons.

motor

A flexor reflex uses a ... after-discharge neural circuit.

parallel

Which type of receptor responds to light?

photoreceptor

The position and movements of body parts is detected by receptors called ______.

proprioceptors

In a patellar reflex, the effector is the ______ muscle group of the thigh.

quadriceps

In some reflexes, ... inhibition is used to inhibit the action of antagonists so they don't oppose the reflex response.

reciprocal

A ______ nerve is a peripheral nerve carrying nerve signals to the CNS.

sensory

Which reflex is mediated by the brain?

to stretch

Consider the two motor neurons that form descending pathways. The neurons called the ... motor neurons have their somas in the cerebral cortex or brainstem.

upper