Lecture on the Peripheral Nervous System (PNS) pt2

These flashcards cover key concepts and terminologies related to the Peripheral Nervous System as discussed in the lecture, focusing on muscle function, reflexes, and nervous system pathways.

Action potential (AP)

A brief electrical impulse that occurs when a neuron sends information down the axon.

Levels of Motor Control:

• Cerebellum and basal

nuclei are the ultimate

planners and coordinators

of motor activities

• Complex motor behavior

depends on complex

patterns of control

Reflexes: 

• classified functionally as:

  • Intrinsic vs. learned

  • Somatic reflexes

  • Activate skeletal muscle

  • Autonomic (visceral) reflexes

  • Activate visceral effectors (smooth or cardiac muscle or glands)

Components of a reflec arc (neural path):

1. Receptor: site of stimulus action

2. Sensory neuron: transmits afferent impulses to CNS

3. Integration center: either monosynaptic or polysynaptic region within CNS

4. Motor neuron: conducts efferent impulses from integration center to

effector organ

5. Effector: muscle fiber or gland cell that responds to efferent impulses by

contracting or secreting

Spinal Reflexes:

• occur without direct involvement of higher brain centers

  • Brain is still advised of spinal reflex activity and may have an effect on the

  reflex

  • Testing of somatic reflexes is important clinically to assess condition of

  nervous system

  • If abnormal, may indicate degeneration or pathology of specific nervous

  system regions

  • Most commonly assessed reflexes are stretch, flexor, and superficial reflexes

Stretch and Tendon Reflexes:

• • In order to smoothly coordinate and protect skeletal muscle,

  nervous system must receive proprioceptor input regarding:

  • Length of muscle

  • Information sent from muscle spindles

  • Amount of tension in muscle

  • Information sent from tendon organs

Function Anatomy of Muscle Spindles:

• • Composed of 3–10

  modified skeletal muscle

  fibers called intrafusal

  muscle fibers that are

  enclosed in a connective

  tissue capsule

  • Regular effector fibers of

  muscle referred to as

  extrafusal muscle fibers

Stretch Reflex:

• monosynaptic and ipsilateral (motor activity is on

  same side of body)

Tendon Reflex:

• Involves polysynaptic reflexes

  • Helps prevent damage due to excessive stretch

  • Produces muscle relaxation (lengthening) in response to tension

  • Contracting muscle relaxes; antagonist contracts (reciprocal activation)

Flexor (withdrawl) reflex:

• initiated by painful stimulus

  • Causes automatic withdrawal of threatened body part

  • Ipsilateral and polysynaptic

  • Brain can override

  • Knowing a finger stick for blood test is coming, brain overrides pulling arm

  away

Crosses extensor reflex:

• occurs in weight-bearing limbs to

  maintain balance

  • Consists of ipsilateral withdrawal reflex plus contralateral extensor

  reflex

  • Stepping barefoot on broken glass causes damaged leg to withdraw and

  opposite leg to extend to support weight shift

Neuromuscular junction

The synapse between a motor neuron and a muscle fiber.

Acetylcholine (ACh)

A neurotransmitter released at the neuromuscular junction that binds to receptors on the sarcolemma.

End plate potential

A transient depolarization of the postsynaptic membrane potential of the neuromuscular junction.

Acetylcholinesterase

An enzyme that breaks down acetylcholine in the synaptic cleft.

Reflex Arc

The neural pathway involved in reflex actions, typically including a receptor, sensory neuron, integration center, motor neuron, and effector.

Intrinsic reflexes

Reflexes that are genetically hardwired and occur automatically.

Learned reflexes

Reflexes that are acquired through experience.

Spinal reflex

A reflex that occurs without the direct involvement of the brain.

Muscle spindles

Proprioceptors that detect changes in muscle length and the rate of that change.

Extrafusal muscle fibers

Regular muscle fibers that are responsible for muscle contraction.

Intrafusal muscle fibers

Modified skeletal muscle fibers found within muscle spindles that help monitor muscle stretch.

Stretch reflex

A monosynaptic reflex that prevents muscle from excessive stretching.

Tendon reflex

A polysynaptic reflex that causes muscle relaxation in response to tension.

Flexor (withdrawal) reflex

A reflex that results in the automatic withdrawal of a body part from a painful stimulus.

Plantar reflex

A superficial reflex testing the spinal cord integrity from L4 to S2, causing downward flexion of the toes.

Babinski's sign

An abnormal reflex response characterized by the upward flexion of the big toe when the lateral aspect of the sole is stroked.

Superficial Reflexes:

• elicited by gentle cutaneous

  stimulation of area

Plantar reflex:

• Tests integrity of cord from L4 to S2

  • Stimulus: stroke lateral aspect of sole of foot

  • Response: downward flexion of toes

  • Damage to motor cortex or corticospinal tracts

  causes abnormal response known as Babinski’s sign

Abdominal reflexes:

• ests integrity of cord from T8 to T12

  • Stimulus: stroking skin of lateral abdomen above, below, or to side of

  umbilicus

  • Response: contraction of abdominal muscles and movement of umbilicus

  toward stimulus

  • Absent when corticospinal tract lesions are present