Human Body Systems: Nervous, Locomotor, and Endocrine Functions
The Nervous System: Cells and Structures
The nervous system comprises two primary cell types:
Neurons: Structure and Function
Neurons are responsible for conducting nerve impulses. They consist of the following components:
- Neuronal Cell Body (Soma): Contains the nucleus, neurofilaments, groups of rough endoplasmic reticulum (Nissl bodies), and mitochondria.
- Dendrites: Tree-like ramifications extending from the cell body. They receive nerve impulses and send them to the cell body.
- Axon: An extension of the cell body, terminating in fine branches that end in small terminal buttons (axon terminals). Axons transmit nerve impulses away from the neuron, often to other neurons or target cells.
Neuron Classification by Function
Neurons can be classified according to their function:
- Sensory Neurons: Responsible for transmitting stimuli from receptors to the nerve centers.
- Interneurons (Association Neurons): Located in the central nervous system, they connect sensory neurons with motor neurons.
- Motor Neurons: Transmit responses from the nerve centers to target organs.
Glial Cells: Support and Protection
Glial cells perform crucial functions including support, nutrition, and protection of nervous tissue. The main glial cells are astrocytes, oligodendrocytes, and Schwann cells. Schwann cells and oligodendrocytes are notable for producing myelin, a lipid-rich plasma membrane that insulates axons.
Nerve Fibers: Myelinated and Unmyelinated
Nerve fibers consist of axonal extensions of neurons, often associated with glial cells. These fibers are classified into two types: unmyelinated and myelinated.
Nerves: Peripheral Nervous System Bundles
Nerves are bundles of several nerve fibers, often accompanied by blood vessels, forming the main components of the peripheral nervous system. The fibers are grouped into bundles that are wrapped by a connective tissue called perineurium. The entire nerve is then encased by the epineurium.
Ganglia: Neuronal Cell Body Clusters
Ganglia are clusters of neuronal cell bodies located in the peripheral nervous system. In the central nervous system, similar clusters form the gray matter.
The Locomotor System
Types of Skeleton
The locomotor system relies on various skeletal structures.
The Muscular System and Locomotion
Of the different types of muscle, skeletal muscles are primarily involved in locomotion. The locomotion of all animals with a backbone is based on the principle of the lever. Muscles attach to bones by tendons, and their actions are antagonistic.
Vertebrate Skeletal Muscle Structure and Contraction
Skeletal muscles are composed of millions of myofibrils arranged longitudinally. Each myofibril is composed of two types of protein myofilaments: the thin actin and the thick myosin. Muscle contraction implies a muscle shortening that is not due to the actin and myosin filaments themselves decreasing in length, but because the sarcomeres shorten as the actin and myosin filaments slide over each other, increasing their overlap. This sliding filament model is similar to what happens in an extensible ladder.
Neuromuscular Synapse: Muscle Activation
The junction between a motor neuron and a muscle fiber is called the neuromuscular synapse, and the specific structure where this occurs is called the motor endplate. When a motor neuron is stimulated, it releases a neurotransmitter, acetylcholine.
Key Hormones and Their Functions
- Thyroxine (Thyroid)
- Activates cellular metabolism and influences growth and development.
- Insulin (Pancreas)
- Reduces blood sugar concentration and stimulates the formation of glycogen.
- Glucagon (Pancreas)
- Raises blood sugar concentration, counteracting insulin’s effects.
- Cortisol (Adrenal Glands)
- Controls the metabolism of carbohydrates, fats, and proteins.
- Estrogen (Ovaries)
- Responsible for developing and maintaining female sexual characteristics.