AJ-4-The-human-body

SMOOTH MUSCLES:
Most of our internal organs are made up of smooth muscles. They are found in the urinary bladder, gallbladder, arteries, and veins. Also the digestive tract is made up of smooth muscle as well. The smooth muscles are controlled by the autonomic nervous system and hormones. We cannot consciously control the smooth muscles and that is why they are often called involuntary muscles.

CARDIAC MUSCLE:
The cardiac muscle is a type of an involuntary striated muscle found exclusively within the heart. Its function is to "pump" blood through the circulatory system by contracting. Unlike skeletal muscle, which contracts in response to nerve stimulation, cardiac muscle’s function is based on self-excitable stimulating contraction without an electrical impulse coming from the central nervous system.

Muscles generally work in pairs to produce movement: when one muscle flexes (or contracts) the other relaxes, a process known as antagonism.

An extensor muscle is any skeletal muscle that opens a joint increasing the angle between components of a limb, such as straightening the knee or elbow and bending the wrist or spine. With the exception of the knee joint the movement is directed backward. This action is known as extension.

A flexor muscle is a skeletal muscle whose contraction bends a joint, decreasing the angle between components of a limb, such as bending the knee or elbow. This action is known as flexion.

An abductor muscle is any of the muscles that cause movement of an extremity (limb) away from the midline of the body or away from a neighbouring part or limb.

An adductor muscle is any of the muscles that draw a part of the body toward its median line or toward the axis of an extremity.

NERVOUS SYSTEM

Structurally, the nervous system is composed of two main parts:

1. The central nervous system which consists of the brain and spinal cord.

2. The peripheral nervous system, the spinal and cranial nerves.

There are also two functional subdivisions of the peripheral nervous system:

• somatic nervous system associated with the voluntary control of body movements through the action of skeletal muscles, and also reception of external stimuli.

• autonomic nervous system is the involuntary nervous system associated with regulation of the activities of visceral organs and glands. It has two divisions: sympathetic and parasympathetic system.

Neurons may be classified according to their function as:

• motor neurons (motoneurons); these neurons are efferent are carry motor impulses from the brain or spinal cord to the muscles, or organs to initiate activity,

• sensory neurons; these neurons are afferent and carry sensory impulses from a body part to the brain or spinal cord,

• connecting neurons, which transmit impulses from one part of the brain to another.

• In all skeletal muscles, contraction is stimulated by electrical impulses transmitted by the nerves, and motor neurons(efferent neurons, motoneurons) in particular. Neurons are basic nerve cells consisting of three parts: cell body, dendrite and axon. Dendrites are extensions from the cell body which conduct impulses to the cell body, axons are extensions carrying the impulses away from the cell body. Neurons communicate with one another via synapses. A synapse is a microscopic space between an axon and a dendrite. Chemicals which help an impulse cross the synapse (such as acetylcholine and catecholamine) are called neurotransmitters.

• Motor neurons innervate or activate muscles groups to perform. A single motoneuron may synapse with one or more muscle fibers. One motoneuron and all of the muscle fibers to which it connects is a motor unit. Groups of motor units often work together to coordinate contractions of a single muscle; the number of muscle fibers within each unit can vary. Thigh muscles can have a thousand fibers in each unit, eye muscles might have only ten. In general, the number of muscle fibers innervated by a motor unit is a function of a muscle's need for refined motion. Muscles requiring more refined motion are innervated by motor units that synapse with fewer muscle fibers.