Chapter 14

What are the main properties of a reflex?

-A stimulus is required to initiate a response to sensory input. -A rapid response requires that few neurons are involved and synaptic delay is minimal. -A pre-programmed response occurs the same way every time. -An involuntary response requires no conscious intent or pre-awareness of the reflex activity.

What is the difference between an ipsilateral and a contralateral reflex?

-Ipsilateral Reflex- Both the receptor and effector organs of the reflex are on the same side of the spinal cord. Example: muscles in left arm contract to pull left hand from hot object. -Contralateral Reflex- The sensory impulses from a receptor organ cross over through the spinal cord to activate effector organs in the opposite limb. Example: step on sharp object with left foot and then muscles in right leg contract to maintain balance as you withdraw left leg from the damaging object.

3 characteristics common to most nervous system pathways

-Most pathways decussate, or cross over, from one side of the body to the other side at some point in its travels. Meaning that the left side of the brain processes information from the right side of the body, and vice versa. -All pathways are compose of paired tracts. A pathway on the left side of the CNS has a matching tract on the right side of the CNS. -Most pathways are composed of a series of two or three neurons that work together.

What are the five steps involved in activation of a reflex?

1. A stimulus activates a receptor. 2. A nerve signal travels through the sensory neuron to the CNS. 3. Information from the nerve signal is processed in the integration center by interneurons. -The simplest reflexes don't involve interneurons. 4. The motor neuron transmits a nerve signal to an effector. 5. The effector responds to the nerve signal from the motor neuron. (Response counteracts original stimulus.)

What are the locations and functions of the following neurons in a sensory pathway chain: 1. Primary Neuron 2. Secondary Neuron 3. Tertiary Neuron

1. Primary Neuron- Located in posterior root ganglia of spinal nerves. The dendrites of this sensory neuron are part of the receptor that detects a specific stimulus. 2. Secondary Neuron- Interneuron located in the posterior horn of the spinal cord or brainstem nucleus. The axon projects to the thalamus for conscious sensations or to the cerebellum for unconscious proprioception. 3. Tertiary Neuron- Interneuron located in the thalamus. The axon projects to the primary somatosensory cortex of the parietal lobe.

What are the locations and functions of the following neurons in a motor pathway chain: 1. Upper Motor Neuron 2. Lower Motor Neuron

1. Upper Motor Neuron- Found in the cerebral cortex or brainstem nuclei. The axons synapse either directly on lower motor neurons or on interneurons that synapse directly on lower motor neurons. Either excites or inhibits the activity of the lower motor neuron. 2. Lower Motor Neuron- Located in the anterior horn of the spinal cord or brainstem CN nucleus. Always excitatory and exit the CNS & project to skeletal muscle.

Stretch Reflex

A monosynaptic reflex that monitors and regulates skeletal muscle length. The stretching of a muscle causes reflexive muscle contraction. Stretch in a muscle is monitored by a stretch receptor called a muscle spindle, which is composed of intrafusal muscle fibers surrounded by C.T.

Withdrawal Reflex

A polysynaptic reflex initiated by a painful stimulus, such as touching something hot or painful.

Crossed-Extensor Reflex

A polysynaptic reflex often in conjunction with the withdrawal reflex, usually in the lower limbs. When the withdrawal reflex is occurring in one limb, the crossed-extensor reflex occurs in the other limb.

Golgi Tendon Reflex

A polysynaptic reflex that prevents muscles from contracting excessively and results in muscle lengthening and relaxing. It occurs in response to increased tension at the Golgi tendon organ.

Dermatome

A specific segment of skin supplied by a single spinal nerve. All spinal nerves except for C1 innervate a segment of skin, so each of these nerves is associated with a dermatome. The skin is divided into segments called the dermatome map. Dermatomes are clinically important because they can indicate potential damage to one or ore spinal nerves.

Reciprocal Inhibition

All reflexes require this process to work properly. It inhibits antagonistic muscle contraction. Example: It inhibits the triceps brachii muscle from contracting when the biceps brachii is stimulated, so the biceps movement will not be countered by the triceps.

Describe the characteristics of a sensory pathway.

Ascending pathways that conduct information about limb proprioception and the sensations of touch, temperature, pressure, and pain. Sensory pathways use a series of two or three neurons to transmit nerve signals from the body to the brain: primary neuron, secondary neuron, and tertiary neuron.

Describe the characteristics of a motor pathway.

Descending pathways in the brain and spinal cord that control effectors. May be formed from the cerebral cortex, cerebral nuclei, cerebellum, descending projection tracts, or motor neurons. Use at least two motor neurons: upper motor neuron and lower motor neuron.

What is a polysynaptic reflex?

Has more complex neural pathways that exhibit a number of synapses involving interneurons within the reflex arc. More components causes a longer synaptic delay. Example: Withdrawal reflex initiated by a painful stimulus.

Explain the indication of a hypoactive reflex versus those of a hyperactive reflex?

Hypoactive reflex- Reflex response is diminished or absent which may indicate spinal cord damage, muscle disease, or damage to NMJ. Hyperactive reflex- Abnormally strong reflex response which may indicate damage to the brain or spinal cord.

Reflex Arc

Neural "wiring" of a single reflex (activation of a reflex). It always begins at a receptor in the PNS, communicates with the CNS, and ends at a peripheral effector.

Discuss how sensory information and motor commands are relayed to/from the spinal cord by spinal nerves.

Sensory pathways ascend through the spinal cord to the brain. Motor pathways descend from the brain through the spinal cord to muscles or glands. These pathways travel through the white matter of the spinal cord as they connect various CNS regions with spinal nerves.

What is a monosynaptic reflex?

Simplest. No interneurons; sensory axons directly synapse on motor neurons; minor synaptic delay. Example: Patellar reflex that physicians use.

contralateral

Used to indicate the relationship to the opposite side of the body.

ipsilateral

Used to indicate the relationship to the same side of the body.