How a Stimulus Elicits a Reflex Response

REFLEXES

Reacting to Changes

Maintaining constant conditions inside your body. Yet…big changes may be going on outside your body.

Homeostasis

SO…you need to detect a change in the environment (stimulus) and react to the change (response) in a way that maintains homeostasis.

When you do this without thinking, it is called a

REFLEX.

Reacting to Changes

When it gets cold outside (stimulus) you shiver (response) and keep the temperature inside your body from dropping.

When it gets hot outside (stimulus) you perspire (response) and keep the temperature inside your body from rising.

It can get very hot or very cold outside, but the temperature inside your body stays the same. How?

What are Reflexes?

Reflexes are automatic, unconscious to changes, either inside or outside the body.

What do reflexes do?

• Reflexes maintain homeotasis (autonomic reflexes) – heart rate, breathing rate, bp, digestion.

• Reflexes also carry out the automatic actions of swallowing, sneezing, coughing, vomiting.

• Reflexes maintain balance and posture; e.g., spinal reflexes control trunk and limb muscles.

• Brain reflexes involve reflex center in brainstem; e.g., reflexes for eye movement.

What do reflexes do?

Reflex Arc• The reflex arc governs the operation of reflexes. Nerve

impulses follow nerve pathways as they travel through the nervous system. The simplest of these pathways, which include only a few neurons, is called the reflex arc. Reflexes whose arc passes through the spinal cord are called spinal reflexes.

Parts of the Reflex Arc

1. Receptor – detects the stimulus. a) Description: the receptor end of a particular

dendrite or a specialized receptor cell in a sensory organ. b) Function: sensitive to a specific type of internal

or external change.

2. Sensory neuron – conveys the sensory info. to brain or spinal cord.

a. Description: Dendrite, cell body, and axon of a sensory neuron.

b. Function: transmit nerve impulses from the receptor into the brain or spinal cord.

Parts of the Reflex Arc

3. Interneuron: relay neurons. a. Description: dendrite, cell body, and axon of a neuron within the brain or spinal cord. b. Function: serves as processing center, conducts nerve impulses from the sensory neuron to a motor neuron.

Parts of the Reflex Arc

4. Motor neuron: conduct motor output to the periphery.

a. Description: Dendrite, cell body, and axon of a motor neuron.

b. Function: transmits nerve impulse from the brain or spinal cord out to an effecter.

Parts of the Reflex Arc

Parts of the Reflex Arc

5. Effector: a. Description: a muscle or gland. b. Function: Response to stimulation by the motor neuron and produces the reflex or behavioral action.

• Your foot jerks.

The Knee-Jerk Response

• When the stretch receptors are stimulated, they send a message to the muscles of your thigh.

• The muscles in the front of your thigh contract.

• The muscles in the back of your thigh relax.

The Correct Pathway.

• The correct connection between the sensory neuron carrying the message

from the receptor and the motor neuron carrying the message to the effector is

the work of the interneurons of the central nervous system. Making the right

connections is called integration.

Another example…

The correct connection between the sensory neuron carrying the message from the receptor and the motor neuron carrying the message to the effector is the work of the interneurons of the central nervous system. Making the right connections is called integration.

Integration

A Conscious Stimulus-Response

• We react to all stimuli in basically the same way as a reflex. The integration just gets more complex.

• Complex behavior involves complex integration in the brain.

Different TYPES of Reflexes:

MONSYNAPTIC:-Consists of only 2 neurons (sensory and motor)-Brief, direct stimulation results in contraction of muscle

POLYSNAPTIC:-most common, more complex -one of more interneurons connect the sensory and motor signals