Major Divisions and Parts of the Nervous System

1. Central Nervous System (CNS)

• Main processing center for the entire nervous system.

It consists of two main components:

1. Brain

• Located within the skull

• functions as organizer and distributor of information for the body.

It has three main parts:

Cerebrum - large, upper part of the brain that controls activity and thought.

Cerebellum - the part under the cerebrum that controls posture, balance, and coordination.

Brain Stem - the part that connects the brain to the spinal cord and controls automatic functions such as breathing, digestion, heart rate, and blood pressure.

1. Spinal Cord

• Serves as a channel for signals between the brain and the rest of the body.

• Controls simple musculoskeletal reflexes without input from the brain.

2. Peripheral Nervous System (PNS)

• Connects the center nervous system to the organs and limbs

It has two main divisions:

1. Somatic Nervous System

• Associated with the voluntary control of the body movements

Has two main parts:

Spinal Nerves- the nerves that carry motor and sensory signals between the spinal cord and the body.

Cranial Nerves- the nerve fiber that carry information into and out of the brain stem.

1. Autonomic Nervous System

• Associated with the involuntary control of the body movements

Has two subdivisions:

Sympathetic - It is activated when the body is in a dynamic role or stress.

(e g., Increase heart rate and breathing, dilation of people, sweating, etc.)

Parasympathetic - It maintains body functions and restores the body to normal or relaxed mode.

Nerve Cell

• Basic unit of the nervous system

• Are called neurons

• There are billions of neurons in the body; some exist alone. Others are joined together to form or against like the brain and spinal cord.

• There are 12 to 14 billions of neurons in one part of the brain alone.

• A neuron has a cell body containing the nucleus.

Dendrites

• Dendrites carry impulses toward the cell body

• A cell may have as many as 200 dendrites carrying impulses towards the cell body.

• A single dendrite can be over one meter long

Axons

• Carry impulses away from the cell body.

• Axons pass impulses to the dendrites of other neurons or the cell body of muscle cells.

• Axons can group together into cable-like bundles called nerves.

Control of Body Processes through the Nervous System

The Nerve Impulse

• Thoughts, emotions, learning, and many body functions are carried by nerve impulses in the neurons.

• A nerve impulse Is a combination of an electrical charge and a chemical reaction.

• It is not a flow of electricity but an electrochemical signal moving along a neuron.

• A nerve impulse cannot jump from one neuron to another. When a nerve impulse comes to the end of an axon, it produces the chemical called neurotransmitter, to be released.

• The chemical crosses the space between neurons called synapse and stimulates the nerve impulse to start in the next dendrite.

• The nerve impulse is sent by neurotransmitters from neuron to another through a gap called synapse.

Five sense organs: eyes, ears, nose, tongue, and skin.

A stimulus (plural: stimuli) Is any factor in the environment that me trigger and nerve impulse.

A response is a reaction to a stimulus.

A stimulus is received by the body and a response is made.

Reaction time is the length of time between application of a stimulus and detection of a response.

• Endocrine glands secrete chemicals known as hormones

• Send signals all over the body, much like the nervous system, but unlike the instant responses activated by the nervous system, the effects can take a few hours or even weeks.

• The major glands in the body are the pituitary, thyroid, parathyroid, thymus, adrenal, pancreas, ovaries, and testes

Endocrine Glands and Their Hormones

• Hormones act in very small amounts. If the organ and hormones do not produce the regulated amount of chemicals to your body, it may result in an abnormality of your body. This condition is called hormonal imbalance. An increase or decrease in its amount may have a significant effect in the human body.

• Hypothalamus, a structure deep in your brain, acts as your body's smart control coordinating center. Its main function is to keep your body in a stable state called homeostasis. It does its job by directly influencing your autonomic nervous system or by managing hormones.

Exocrine Glands / Duct Glands

• Exocrine glands secrete their substances through ducts onto your body's surfaces.

• secrete their products through well defined ducts like liver, salivary, sweat glands etc.

Endocrine glands / Ductless Glands

• Endocrine glands secrete their substances directly into your bloodstream. They're called ductless glands. Endocrine glands are part of your endocrine system, and they secrete hormones.

Nervous System Working Together with Endocrine System to Maintain Homeostasis

Both the nervous system and endocrine system are important in enabling the body to maintain homeostasis. Homeostasis is the state reached when each part of the body functions in equilibrium with other parts. This is attained through the regulation of the bodily functions by the endocrine and nervous systems.

To achieve homeostasis, the nervous and endocrine systems maintain a normal range of the following variables:

• Body temperature

• Amount of water in the body

• Amount of metabolic wastes in the cell

• Blood calcium level Hormones in the blood

• The system involved in sexual reproduction is called the reproductive system.

The Male Reproductive System

The Female Reproductive System

• The female reproductive system consists of the ovaries, oviducts (Fallopian tubes), uterus, vagina, and external genitalia. The internal reproductive organs of the female are located within the pelvis, between the urinary bladder and rectum. The uterus and the vagina are in the middle part, with an ovary on each side of the uterus.

• Puberty involves the onset of sexual maturity and the ability to reproduce. When a female reaches puberty, egg cells start to develop in her ovaries that produce the sex cells. It is also the time when the body develops the capacity to conceive..

The Role of Hormones in Female and Male Reproductive Systems

• Hormones play an important role in both male and female reproductive systems. The pituitary gland controls the functions of both the testes and the ovaries. These hormones keep the reproductive system properly functioning.

Feedback Mechanisms Involved in Regulating Processes in the Female Reproductive System

The Menstrual Cycle

• We have learned that, on average, an ovary releases only one egg every 28 days. Now, what controls this timing? Hormones control many of the changes in the reproductive system. Remember that hormones are chemicals that affect certain body organs.

• The monthly changes that take place in the female reproductive system are called menstruation.

• This cycle occurs every month from the first onset which could happen when a female is between 10 to 13 years old.

• The monthly cycle continues for about 40 years. Refer to Figure 11 for a clearer explanation of the different events that take place in the cycle. Just follow numbers 1-10 in proper order.

To summarize, the important events during the menstrual cycle are as follows:

1. The pituitary gland controls and starts the cycle.

2. The pituitary gland releases hormones that cause the egg in the ovary to mature. The luteinizing hormone (LH) initiates the maturation of the follicles, converts ruptured follicles into corpus luteum and causes the secretion of progesterone. The follicle stimulating hormone (FSH) assists in the maturation of the follicles and causes the secretion of estrogen from the follicles.

3. Meanwhile, the ovary itself releases a hormone called estrogen, which causes the uterine lining to increase in thickness. The uterine lining becomes thicker so that the fertilized egg can attach to it.

4. The ovary releases an egg on day 14. Assume that no sperm is present.

5. The egg moves through the oviduct and enters the uterus.

6. Meanwhile the uterine lining continues to thicken.

7. The egg has not been fertilized, therefore, it will not attach to the uterus.

8. The thick uterine lining is no longer necessary, so the cells of the thickened uterine lining break off and leave the vagina. The unfertilized egg is lost and some blood is lost too. This loss of cells from the uterine lining, blood, and egg is called menstruation. 9. After menstruation, the cycle starts again.

Did you know that menstrual cramps are the results of the strong contractions of the uterine wall that occur before and during menstruation? The cramps can be caused by excessive secretion of prostaglandins. Shedding of the endometrium of the uterus results in inflammation in the endometrial layer of the uterus and prostaglandins are produced as a consequence of the inflammation.