8. Nerve centers. Types and properties. Cerebral blood flow. Cerebrospinal fluid.

Overview

Nerve centres are groups of neurons that together control a function, and they show typical properties such as convergence, divergence, summation and inhibition. The topic also covers the blood supply of the brain and the cerebrospinal fluid that surrounds it.

A reflex is a predictable, involuntary response to a specific stimulus. Reflexes occur without conscious thought and are essential for maintaining posture, protecting from harm, and regulating bodily functions. They involve a pathway called a reflex arc, which typically includes:

1. Sensory Receptor: Detects the stimulus.
2. Sensory (Afferent) Neuron: Transmits information from the receptor to the CNS. The cell body of this neuron is located in the dorsal root or cranial nerve ganglia.
3. Interneurons (optional): Located within the CNS, interneurons process and modify the sensory information.
4. Motor (Efferent) Neuron: Carries signals from the CNS to the effector organ.
5. Effector Organ: Muscle or gland that produces the response.

Reflex arc and types of reflexes

Types of Reflexes

1. (1) Based on the mechanism of formation:-

• Innate (Unconditioned) Reflexes: Genetically determined, present at birth.
• Monosynaptic Reflexes: Only one synapse between sensory and motor neurons (e.g., the stretch reflex). For e.g. biceps reflex test is when a tendon hammer is used on the biceps brachii tendon in the cubital fossa. It activates stretch receptors sending a signal to the dorsal horn, which in monosynaptic reflex, is directly connected to the anterior horn alfa motoneuron and the biceps contracts jerking the forearm.

• Polysynaptic Reflexes: Involve multiple synapses and interneurons (e.g., the flexor-withdrawal reflex).

• For e.g. when you touch a hot object and the pain receptors are activated, they send a signal to the dorsal horn and there the signal is passed to an interneuron, then the interneuron synapses with the anterior horn alfa motor neuron and an efferent signal is sent to the flexor muscles to flex and remove the hand.
• Reciprocal inhibition: is the coordinated inhibition of a muscle(s) that is antagonist to the muscle(s) that is stimulated to contract. For .e.g. if biceps brachii is stimulated to contract then triceps brachii is inhibited so it doesn’t interrupt the flexion. The reflexes are via spinal reflex pathways

• Acquired (Conditioned) Reflexes: Learned through experience (e.g., Pavlov's dog).