What Are Epsps and Ipsps, and How Are They Produced?

Assignment 2

Weight: 5%
Minimum Pass Grade: 50%

Each question is worth 10 marks.

1. Name the type of joint, and list the movements permitted at the shoulder joint. Under each movement’s name, list the names of the muscles responsible for each of these movements along with descriptions of their bone insertion.

The shoulder is a ball and socket joint formed by the head of the humerus and the glenoid cavity of the scapula; it contains a synovial cavity united by dense irregular connective tissue of an articular capsule and by accessory ligaments such as the coracohumeral ligamemt, glenohumeral ligaments, transverse humeral ligaments, glenoid labrum and bursae. The accessory ligaments strengthen the shoulder to some extent, but most of the strength of the shoulder comes from the muscles that surround the joint, specially the rotator cuff muscles (supraspinatus, infraspinatus, teres minor and subscapularis) which anchor the humerus to the scapula. The shoulder joint is classified as diarthrosis (freely moveable).

Movements;

Flexion

Deltoid (anterior fibers) – inserts on the deltoid tuberosity of humerus

Coracobrachialis – inserts on the medial surface of shaft of humerus

Biceps brachii – inserts on radial tuberosity of radius biciptal aponeurosis

Extension;

Latissimus dorsi – inserts on the intertubercular sulcus of humerus

Deltoid (posterior fibers) – inserts on the deltoid tuberosity of humerus

Teres major – inserts on the medial lip of intertubercular culcus of humerus

Teres minor – inserts on the greater tubercle of humerus

Triceps brachii – inserts on the olecranon of ulna

Hyperextension

Latissimus dorsi – inserts on the intertubercular sulcus of humerus

Deltoid (posterior fibers) – inserts on the deltoid tuberosity of humerus

Abduction

Deltoid (lateral fibers) – insert on the deltoid tuberosity of humerus

Supraspinatus – inserts on the greater tubercle of humerus.

Adduction

Pectoralis major – inserts on the greater tubercle and lateral lip of intertubercular sulcus of humerus

Latissimus dorsi – inserts on the intertubercular sulcus of humerus

Teres major – inserts on the medial lip of intertubercular sulcus of humerus medial rotation

Medial Rotation

Pectoralis major – inserts on the greater tubercle and lateral lip of intertubercular sulcus of humerus

Latissimus dorsi – inserts on the intertubercular sulcus of humerus

Subscapularis – inserts on the lesser tubercle of humerus

Teres major – inserts on the medial lip of intertubercular sulcus of humerus

Lateral Rotation

Detoid (posterior fibers) – inserts on the deltoid tuberosity of humerus

Infraspinatus – inserts on the greater tubercle of humerus

Teres minor – inserts on the greater tubercle of humerus

Circumflexion

Pectoralis major – inserts on the greater tubercle and lateral lip of intertubercular sulcus of humerus

Deltoid (anterior, lateral, and posterior fibers) – inserts on the deltoid tuberosity of humerus

Supraspinatus – inserts on the lesser tubercle of humerus

Triceps (long head) – inserts on olecranon of ulna

Biceps Brachii – inserts on radial tiberosity of radius and bicipital aponeurosis.

1. What are EPSPs and IPSPs, and how are they produced? Explain how these electrical currents are used in spatial and temporal summation to initiate or inhibit the generation of an action potential.

• A neurotransmitter that causes depolarization of the postsynaptic membrane is excitatory because it brings the membrane closer to threshold. A depolarizing postsynaptic potential is called an excitatory postsynaptic potential (EPSP). Although a single EPSP does not initiate a nerve impulse, the postsynaptic cell does become more excitable. Because it is partially depolarized, it is more likely to reach threshold when the EPSP occurs.

• A neurotransmitter that causes hyperpolarization of the postsynaptic membrane is inhibitory. During hyperpolarization, generation of an action potential is more difficult than usual because the membrane potential becomes inside more negative and even farther from threshold than in its resting state. A hyperpolarizing postsynaptic potential is termed an inhibitory postsynaptic potential (IPSP)

• How are EPSP and IPSP produced? Neurotransmitters released from a presynaptic neuron bind to neurotransmitter receptors in the plasma membrane of a postsynaptic cell. Each type of neurotransmitter binds. When a neurotransmitter binds to the correct neurotransmitter receptor, an ion channel opens and a postsynaptic potential (either EPSP or IPSP) forms in the membrane of the postsynaptic cell. Neurotransmitter receptors are classified as either ionotropic receptors or metabotropic receptors based on whether the neurotransmitter binding sites and the ion channel are components of the same protein or are components of different proteins.

• EPSP and IPSP use in spatial and temporal summation of postsynaptic potential. A typical neuron in the CNs receives input from 1000 to 10,000 synapses. Integration of these inputs involves summation of the postsynaptic potentials that form in the postsynaptic neuron. Summation is the process by which graded potentials add together. The greater the summation of EPSPs, the greater the chance that threshold will be reached. At threshold, one or more nerve impulses (action potentials) arise. There are two types of summation: spatial and temporal summation.

• Spatial summation is summation of postsynaptic potentials in response to stimuli that occur at different locations in the membrane of a postsynaptic cell at the same time. For example, spatial summation results from the buildup of neurotransmitter released simultaneously by several presynaptic end bulbs.

• Temporal summation on the other hand, is summation of postsynaptic potentials in response to stimuli that occur at the same location in the membrane of the postsynaptic cell but at different time. For example, temporal summation results from buildup of neurotransmitter released by a single presynaptic end bulb two or more times in rapid succession. Because a typical EPSP lasts about 15msec, the second and subsequent release of neurotransmitter must occur soon after the first one if temporal summation is to occur.

• A single postsynaptic neuron receives input from many presynaptic neurons, some of which release excitatory neurotransmitters and some release inhibitory neurotransmitters.

• EPSP. If the total excitatory effects are regular than the total inhibitory effects but less than the threshold level of stimulation, the result is an EPSP that does not reach threshold. Following an EPSP, subsequent stimuli can more easily generate a nerve impulse through summation because the neuron is partially depolarized.
• Nerve impulses. If the total excitatory effects are greater than the inhibitory effects and threshold is reached, one or more nerve impulses (action potentials) will be triggered. Impulses continue to be generated as long as the EPSP is at or above the threshold level.
• IPSP. If the total inhibitory effects are greater than the excitatory effects, the membrane hyperpolarizes (IPSP). The result is inhibition of the postsynaptic neuron and an inability to generate a nerve impulses (Tortora & Derrickson pages 427-431)  

1. Match the items in column A with the descriptions in column B to create the BEST MATCHES.

Items in column A can be used only once when making matches to column B. There is only one correct answer for each blank space.