Spinal reflexesThe reflex arc

SMS 1084Dr. Mohanad R. Alwan

A few reminders!The central grey matter contains the cell

bodies of relay and motor neurones.The outer white matter contains myelinated

axons, which run up and down the spinal cord to and from the brain.

In the centre of the grey matter is the spinal canal, through which the nutritive cerebrospinal fluid cirulates.

A few reminders!Sensory neurones enter the spinal cord

through the dorsal root and the concentration of their cell bodies forms a swelling called the dorsal root ganglion.

Motor neurones leave the spinal cord via the ventral root.

The Reflex ArcAn automatic, rapid response to an adverse

stimulus.Action is involuntary the brain is not involved

in the event but may be informed of itMany reflex actions are protectiveSome complex actions (swallowing, coughing

and blinking) are coordinated by reflexes.

Example of Neuronal Organization: Reflexes

Reflex arcs – simple neural pathwaysResponsible for reflexes

Rapid, autonomic motor responsesCan be visceral or somatic

Five Essential Components to the Reflex Arc

Receptor – detects the stimulusAfferent (sensory neuron) – transmits

impulses to the CNSIntegration center – consists of one or more

synapses in the CNSEfferent (motor neuron) – conducts impulses

from integration center to an effector Effector – muscle or gland cell

Responds to efferent impulses Contraction or secretion

Example of the Five Components to the Reflex Arc

Reflex Classification

Monosynaptic or polysynapticSpinal or cranialSomatic or autonomicInnate or learned

Types of Reflexes: Number of Classes

Monosynaptic reflex – simplest of all reflexesJust one synapseThe fastest of all reflexesExample – knee-jerk reflex

Polysynaptic reflex – more common type of reflexMost have a single interneuron between the

sensory and motor neuronExample – withdrawal reflexes

Monosynaptic Reflex

Polysynaptic Reflex

Spinal vs Cranial ReflexesSpinal = spinal cord integration center

Ex. Knee-jerk reflexCranial = brain as integration center

Ex. Pupillary light reflex

Somatic vs Autonomic ReflexesSomatic = motor neurons to skeletal muscles

Ex. Knee-jerk reflexAutonomic = autonomic neurons to smooth

muscle and glandsEx. Pupillary light reflex

Innate vs Learned ReflexesInnate = born-with

Knee-jerk reflex, pupillary reflexLearned = develops based on experiences

Pavlov’s dogs salivation in response to bell

A withdrawal reflex

A withdrawal reflex

The stimulus is the heat from the hot object

A withdrawal reflex

The stimulus is the heat

from the hot object

The stimulus is detected by receptors in the dermis of the skin. Since these sensory cells respond to temperature, they are called thermoreceptors.

A withdrawal reflex

The stimulus is the heat

from the hot object

The stimulus is detected by receptors in the dermis of the skin. Since these sensory cells respond to temperature they are called

thermoreceptors.

The thermoreceptors initiate nerve impulses that pass to the spinal cord along a sensory neurone.

A withdrawal reflex

The stimulus is the heat

from the hot object

The stimulus is detected by receptors in the dermis of the skin. Since these sensory cells respond to temperature they are called

thermoreceptors.

The thermoreceptors initiate nerve impulses that pass to the spinal cord along a sensory

neurone.

sensory neurone cell body

A withdrawal reflex

The stimulus is the heat

from the hot object

The stimulus is detected by receptors in the dermis of the skin. Since these sensory cells respond to temperature they are called

thermoreceptors.

The thermoreceptors initiate nerve impulses that pass to the spinal cord along a sensory

neurone.

sensory neurone cell body

grey matter (neurone cell bodies)

A withdrawal reflex

The stimulus is the heat

from the hot object

The stimulus is detected by receptors in the dermis of the skin. Since these sensory cells respond to temperature they are called

thermoreceptors.

The thermoreceptors initiate nerve impulses that pass to the spinal cord along a sensory

neurone.

sensory neurone cell body

grey matter (neurone cell bodies)

spinal cord

A withdrawal reflex

The stimulus is the heat

from the hot object

The stimulus is detected by receptors in the dermis of the skin. Since these sensory cells respond to temperature they are called

thermoreceptors.

The thermoreceptors initiate nerve impulses that pass to the spinal cord along a sensory

neurone.

sensory neurone cell body

grey matter (neurone cell bodies)

spinal cord

white matter (neurone axons)

A withdrawal reflex

The stimulus is the heat

from the hot object

The stimulus is detected by receptors in the dermis of the skin. Since these sensory cells respond to temperature they are called

thermoreceptors.

The thermoreceptors initiate nerve impulses that pass to the spinal cord along a sensory

neurone.

sensory neurone cell body

grey matter (neurone cell bodies)

spinal cord

white matter (neurone axons)

The sensory neurone enters the spinal cord via the dorsal branch of the spinal nerve and forms a synapse with a relay neurone, inside the grey matter

A withdrawal reflex

The stimulus is the heat

from the hot object

The stimulus is detected by receptors in the dermis of the skin. Since these sensory cells respond to temperature they are called

thermoreceptors.

The thermoreceptors initiate nerve impulses that pass to the spinal cord along a sensory

neurone.

sensory neurone cell body

grey matter (neurone cell bodies)

spinal cord

white matter (neurone axons)

The sensory neurone enters the spinal cord via the dorsal branch of the spinal nerve and forms a synapse with a relay neurone, inside the grey matter

The short relay neurone forms a synapse with a motor neurone that leaves the spinal cord via the ventral root of the spinal cord.

A withdrawal reflex

The stimulus is the heat

from the hot object

The stimulus is detected by receptors in the dermis of the skin. Since these sensory cells respond to temperature they are called

thermoreceptors.

The thermoreceptors initiate nerve impulses that pass to the spinal cord along a sensory

neurone.

sensory neurone cell body

grey matter (neurone cell bodies)

spinal cord

white matter (neurone axons)

The sensory neurone enters the spinal cord via the dorsal branch of the spinal nerve and forms a synapse with a relay neurone, inside the grey matter

The short relay neurone forms a synapse with a motor neurone that leaves the spinal cord via the ventral root of the spinal cord.

The motor neurone carries the impulse to an effector, in this case muscles in the arm, which contract to withdraw the hand from the hot object. This action is known as the response.

A withdrawal reflex

The stimulus is the heat

from the hot object

The stimulus is detected by receptors in the dermis of the skin. Since these sensory cells respond to temperature they are

called thermoreceptors.

The thermoreceptors initiate nerve impulses that pass to the spinal cord along a sensory

neurone.

sensory neurone cell body

grey matter (neurone cell bodies)

spinal cord

white matter (neurone axons)

The sensory neurone enters the spinal cord via the dorsal branch of the spinal nerve and forms a synapse with a relay neurone, inside the grey matter

The short relay neurone forms a synapse with a motor neurone that leaves the spinal cord via the ventral root of the spinal cord.

The motor neurone carries the impulse to an effector, in this case muscles in the arm, which contract to withdraw the hand from the hot object. This action is known as the response.

Review of reflex arc.

Other ReflexesStimulus Response

The aroma of your favorite food

Salivation

A nasty odor Nausea

A bright light shining in your eye

Pupils get smaller

An insect flying towards your eye

Blinking