Reflex Response

sensory (afferent) division

receives stimuli from inside and outside the body

motor (efferent) division

sends commands to muscles and glands

The motor (efferent) division is divided into two categories:

• Somatic nervous system

• Autonomic nervous system

Somatic nervous system

controls skeletal muscles

• voluntary and some involuntary (though reflexes)

Autonomic nervous system

controls smooth muscle, cardiac muscle, and glands

• involuntary

The motor (efferent) divisions of the PNS have

reflexes

reflex responses

automatic involuntary motor response to stimuli that does not require complex processing in the brain

Two characteristics of reflex responses

• Fast acting: occur quickly

• Predictable: same response every time

There are two types of reflex responses:

• Somatic reflexes

• Autonomic (visceral) reflexes

Somatic reflexes

involve skeletal muscles

• Ex: withdrawal reflex

Autonomic (visceral) reflexes

involves smooth muscle, cardiac muscles, or glands

• Ex: heart rate regulation

There are two types of somatic reflexes:

• Intrinsic (innate) reflexes

• Learned (acquired reflexes)

Intrinsic (innate) reflexes

automatic and present at birth

Learned (acquired) reflexes

developed through experience and repetition

All reflexes follow a 5-step pathway called the

general reflex arc

5 steps of the general reflex arc:

1. Receptor

2. Sensory Neuron

3. Integration Center

4. Motor neuron

5. Effector

Step 1: Receptor

detects the stimulus

Step 2: Sensory Neuron

transmits afferent impulses to CNS

Step 3: Integration Center

Processes the signal (in CNS)

There are two divisions in step 3: integration center

• Monosynaptic reflex

• Polysynaptic reflex

Monosynaptic reflex

directly connects sensory and motor neurons

• Simpler, faster

Polysynaptic reflex

involves interneurons for more complex processing

• signal relay circuit

Step 4: Motor neuron

sends commands to effector

Step 5: effector

muscle or glands responds

• contracts or secretes

The somatic division controls voluntary movement of skeletal muscles but also controls

some involuntary functions through… reflexes

Important involuntary reflexes controlled by the somatic division

• Withdrawal reflex

• Diaphragm reflex

• Proprioreceptor reflexes

Withdrawal reflex

pulls away from pain

Diaphragm reflex

involuntary breathing

Proprioreceptor reflexes

maintain balance and prevent muscle damage

Propriorecptors

mechanoreceptors in muscles, tendons, and joints that detect position and movement

• part of somatic nervous system

To prevent injury, proprioreceptors monitor

Stretch, strain, and tension

Stretch reflex uses

muscle spindles (sensory receptors inside muscle) that detect excessive stretching

Why is stretch reflex important?

if the muscle stretches too much, the stretch reflex contracts it to prevent injury

Tendon reflex uses

golgi tendon organ (in tendon) that detects excessive contraction

Why is tendon reflex important?

If tension is too high, it triggers tendon reflex which relaxes the muscle to prevent damage

Stretch reflex and tendon reflex are part of the somatic nervous system because

they control muscles

The autonomic division has two divisions

• Parasympathetic

• Sympathetic

Parasympathetic

“rest and digest”

• lowers heart rate, increases digestion

Sympathetic

“fight or flight”

• increases heart rate, decreases digestion

Where do the cell bodies of motor neurons come from in parasympathetic?

brainstem and sacral spinal cord

Where do the cell bodies of motor neurons come from in sympathetic?

lateral gray horn of thoracic and lumbar spinal cord

where do ganglia of motor neurons reside in parasympathetic?

near or inside target organs

where do ganglia of motor neurons reside in sympathetic?

near spinal cord

How long are neurons in parasympathetic?

• Long preganglionic

• Short postganglionic

How long are neurons in sympathetic?

• Short preganglionic

• Long postganglionic

How much of the parasympathetic division does the vagus nerve (X) supply?

90%

The sympathetic division has cell bodies in the

lateral gray horn of the spinal cord

Sympathetic trunk (chain) ganglia

chain of linked ganglia running along the spinal cord that allows widespread, fast sympathetic activation

• Location: near spinal cord

The somatic nervous system uses

plexuses

The sympathetic nervous system bypasses

major plexuses and uses chain ganglia