Chapter 33 Notes, The Nervous System

Nervous System A neuron is a cell of the nervous system that carries nerve impulses through the body. There are three kinds of neurons in the body. Interneurons are neurons found in the brain and the spinal cord. Interneurons process information received by the sensory neurons, and transmit that information to motor neurons. Sensory neurons are mostly in the skin and motor neurons control muscles and glands.

Neurons

The Nervous System The nervous system consists of two divisions. The central nervous system (CNS) includes the interneurons of the brain and spinal cord. The peripheral nervous system consists of the sensory neurons and motor neurons that carry information to and from the CNS. Sometimes an impulse triggers an involuntary reflex that does not involve the brain. The sensory neurons send information to the spinal cord, and the spinal cord sends information to a motor neuron. This is known as a reflex arc.

The Central and Peripheral Nervous Systems

Reflex Arc

The Brain There are over 100 billion neurons in the brain. The largest part of the brain is called the cerebrum. It is divided into two halves called hemispheres. The cerebrum is responsible for learning, memory, language, speech, voluntary body movements and sensory perception. The many folds and grooves on the surface of the cerebrum increase the surface area and allow more complicated thought processes.

The Cerebrum

The Brain The next largest part of the brain, located near the back of the brain, is called the cerebellum. The cerebellum is responsible for balance, posture, and coordination. The brain stem connects the brain to the spinal cord. The brain stem is made up of two parts; the medulla oblongata and the pons. The medulla oblongata relays signals between the brain and the spinal cord, and it controls the breathing rate, heart rate, and blood pressure.

Cerebellum, Medulla Oblongata, and the Pons

The Brain The pons relays information between the cerebrum and the cerebellum. Located between the brain stem and the cerebrum is a small region of the brain called the hypothalamus. The hypothalamus is responsible for maintaining homeostasis The hypothalamus maintains body temperature, thirst, appetite, and water balance. Additionally, the hypothalamus helps regulate blood pressure, sleep, aggression, and fear.

The Hypothalamus

The Nervous System Most of the nerves in the body that are under voluntary control, that use the skeletal muscles are part of the somatic nervous system. The somatic nervous system relays information from the external sensory receptors to the CNS. Somatic motor nerves relay information from the CNS to the skeletal muscles. A reflex arc is also an involuntary response of the somatic nervous system.

The Nervous System The part of the peripheral nervous system that transmits impulses from the CNS to internal organs, that are under involuntary control is called the autonomic nervous system. If you have ever woken up from a nightmare and your heart was pounding, this is a response caused by the autonomic nervous system. If you ever came close to running into a moose or were involved in a car accident, you probably felt your body's fight-or-flight response.

Organization of the Nervous System

The Nervous System The fight-or-flight response is a response created by the autonomic nervous system. The autonomic nervous system has two branches; they are called the sympathetic and parasympathetic nervous system. The sympathetic nervous system is most active in times of emergency or stress. The heart rate and breathing rate is elevated, pupils dilated. The parasympathetic nervous system is most active when the body is relaxed.

Sympathetic vs. Parasympathetic Nervous System

The Sensory Organs A region of the brain that has special sensory neurons responsible for the sense of smell is called the olfactory bulb. The nerves of the olfactory bulb are called olfactory nerves. Areas of chemical receptors on the tongue responsible for taste are called taste buds. Taste buds are connected to sensory neurons. The olfactory nerves and taste buds communicate with one another to enhance the flavor of food. Without smell, food loses flavor.

Taste and Smell

The Sensory Organs When light enters the eye it first travels through a transparent layer of cells called the cornea. The amount of light entering the eye is controlled by muscles of the iris, which is the part of the eye that is colored. Behind the iris is the lens. The lens inverts the image and projects it onto the retina. The retina contains receptor cells called rods and cones. Rods help to see in low-light. Cones function in bright light and detect color.

Anatomy of the Eye

The Sensory Organs The two main functions that the ear is responsible for are hearing and balance. Sound waves enter the auditory or ear canal. Sound waves cause the eardrum or tympanum to vibrate. These vibrations cause three bones called the malleus (hammer), incus (anvil), and stapes (stirrup) to vibrate. Vibrations from these bones cause fluid inside the cochlea to move tiny hairs. These tiny hairs send nerve impulses to the brain that interpret sounds generated by sound waves.

The Sensory Organs Inside the inner ear, located near the cochlea are three canals called semicircular canals. The semicircular canals are also filled with fluid and tiny hairs. Only these hairs are not responsible for hearing, but for balance. As you move, the fluid in the canals causes the hairs to bend in response to gravity. The way the hairs bend sends signals to the brain that allows us to achieve balance and equilibrium.

Anatomy of the Ear