lesson 6.4 reflexes...

Lesson 6.4

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REFLEXES AND PROPRIOCEPTION

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TOPICS COVERED IN THIS LESSON

• (a) The Reflex Arc

• (b) Proprioception and Proprioceptors

What Are Reflexes?

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Reflexes are an important part of all

physical movement. They are an

automatic, rapid, and unconscious

response to a particular stimulus.

•If the command centre, or

control, for the reflex is located in

the brain, it is called a cerebral

reflex.

•If the control is located in the

spinal cord, it is called a spinal

reflex.

Classification of Reflexes

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Reflex contraction of the skeletal muscle is not

dependent on conscious intervention by higher

centres of the brain but are a way in which the

body responds to an unexpected stimulus.

• Autonomic reflexes are mediated by the

autonomic division of the nervous system and

usually involve the activation of smooth muscle,

cardiac muscle, and glands. These reflexes

regulate such bodily functions as digestion,

elimination, blood pressure, salivation, and

sweating.

• Somatic reflexes involve stimulation of skeletal

muscles by the somatic division of the nervous

system, and include such reflexes as the stretch

reflex and the withdrawal reflex.

What Is the “Reflex Arc”?

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Neurons in our bodies transmit information

to each other through a series of neural

connections that form a pathway, or circuit.

A reflex arc is a simple neural pathway along

which an initial sensory stimulus and a

corresponding message travel.

• The stimulus from sensory neurons is sent to the

central nervous system (CNS), but there is little

or no interpretation of the signal. Few, if any,

interneurons are involved.

• The signal is transmitted to motor neurons, which

elicit a response, e.g., a knee jerk.

The Five Parts to a Reflex Arc

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• The receptor, which receives the initial stimulus

(e.g., a pinprick to the skin or a loud noise)

• The sensory (or afferent) nerve, which carries

the impulse to the spinal column or brain

• The intermediate nerve fibre (the adjustor

or interneuron), which interprets the signal and

issues an appropriate response

• The motor (or efferent) nerve, which then

carries the response message from the spinal cord

to the muscle or organ, and

• the effector organ itself (e.g., a skeletal muscle),

which carries out the response (such as removing

the hand or leg away from danger)

The Reflex Arc:

The Body’s Response to the Threat of Pain

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Proprioception and Proprioceptors

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Proprioception is a person’s ability to sense the

position, orientation, and movement of the body.

• Proprioceptors are sensory receptors found in

muscles, tendons, joints, and the inner ear that can

detect the motion or position of the entire body or

a limb by responding to stimuli from within the

organism.

• Muscle spindles and tendon organs are two

proprioceptors that continuously monitor muscle

actions and are essential components of the

neuromuscular system. They “tell” the nervous

system about the state of muscle contraction and

allow the nervous system to respond accordingly.

Muscle Spindles and the

Stretch Reflex

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Muscle spindles are sensory receptors (i.e.,

proprioceptors) within a muscle fibre that

help to maintain muscle tension and that are

sensitive to changes in muscle length (rather

than tension).

• Muscle spindles are involved in the reflex

contraction of muscles (the so-called stretch

reflex).

• The usual example of the stretch reflex action is

the knee-jerk reflex.

A Muscle Spindle

(Stretch Detector)

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The Knee-Jerk Reflex

(The Patella Reflex)

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The Crossed-Extensor Reflex:

A More Complex Reflex Action

[CATCH: FIGURE 6.17 ON PAGE 187—THE

CROSSED-EXTENSOR REFLEX]

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Golgi Tendon Organs and

the Tension Reflex

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Golgi tendon organs are found at the end of

muscle fibres that merge into the tendon itself.

GTOs detect changes in muscle tension. They

help protect the muscle from excessive tension

that might damage the muscle, joint, or both.

• A Golgi tendon organ projects to the motor

neurons located within the spinal cord. When the

change in tension is detected, an impulse is sent

along afferent neurons to the central nervous

system (CNS), where they synapse with motor

neurons of the same muscle.

• The efferent neurons instantly transmit an

impulse, causing the muscle to relax, thereby

preventing injury.

A Golgi Tendon Organ

(Tension Detector)

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