Reflexes

REFLEXES When movements occur without the action of brain, they are called as reflex action or spinal reflexes. It is an involuntary response to a stimuli that depends upon a complete reflex pathway called reflex arc. A REFLEX ARC A Reflex Arc contains two or more types of neurons over which nerve impulses are conducted from a receptor to the brain or spinal cord by sensory neurons and then to effector by way of motor neurons. Basic components of Reflex arc: 1. Receptor 2. Sensory Neuron 3. Center 4. Motor neuron 5. Effector 1. Receptor: The molecules that respond to a specific change in the internal or external environment by initiating a nerve impulse in a sensory neuron by way of receptor potential. 2. Sensory neuron: Passes the nerve impulse from the receptor to its axonal termination in the CNS. 3. Impulse: In the centre the impulse may be inhibited, transmitted or rerouted. 4. Motor neuron centre: a region in the CNS where on incoming sensory Impulse generates an outgoing motor transmits the impulse generated by sensory or association neuron in the centre to the effector organ of the body that will respond, such as a muscle or gland. 5. Effector: the organ of body that responds to the motor nerve impulse. This response is called as reflex action.

REFLEX ARC

REFLEX ACTION

Reflexes can be classified into 2 types: z z

Monosynaptic reflexes Polysynaptic reflexes

Monosynaptic reflex: When a skeletal muscle with intact nerve supply is stretched, it contracts. This response is called the Stretch reflex. Knee jerk is an example.

Muscle spindle: z z z

Muscle spindles are composed of 2- 12 muscle fibers, enclosed in a capsule called intrafusal fibers. The regular contractile units of muscle are called extrafusal fibers. The intrafusal fibers are of 2 types:

Nuclear bag fibres Nuclear chain fibres Number of spindle muscle cells in a muscle depend on the type of muscle. z Small distal muscles contain more spindle cells and are involved in the delicate hand movements. z Large muscles contain lesser number of spindle cell. z

Muscle spindles have both sensory and motor supply: Motor supply to muscle spindle is provided by gamma efferent fibers. They are of 2 types: gamma 1 and 2. Gamma 1 fibers supply ends of nuclear bag fibers. Gamma 2 fibers supply ends of nuclear chain fibers. Sensory supply is provided by alpha and beta fibers. Polysynaptic reflexes Reflexes in which there are many synaptic connections between the afferent and efferent neurons.

Withdrawal reflex: It is a polysynaptic reflex that occurs in response to noxious or painful stimuli. Characteristics of reflexes: 1. Adequate stimulus: The reflex is specific in terms of both stimulus and response. 2. The alpha motor neurons supplying extrafusal fibers in skeletal muscle are a common pathway over which the neural control system coordinates the activity of skeletal muscle fibers. 3. When the stimulus is applied at repeated and frequent intervals, the response declines and disappears called as habituation. The response opposite to this ie. prolonged facilitation of synaptic conduction in a reflex can be produced by noxious stimuli is called sensitization.