The Nervous System

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Every second, millions of signals make their way to your brain and inform it about what your body is doing and feeling. Your nervous system interprets these messages and decides how to respond. Responding to stimuli requires the actions of specialized cells called neurons. Neurons are capable of carrying electrical and chemical messages back and forth between your brain and other parts of the body. These electrochemical message-carrying cells are often bundled together, producing structures called nerves.

Neurons and nerves are extensively networked throughout your body. They are found in your brain, spinal cord, and sense organs such as your eyes and ears. Nerves connect various organs to each other and link the nervous system with other organ systems (Figure 13.1).

Figure 13.1 The nervous system. The nervous system includes the brain, spinal cord, sense organs, and the nerves that interconnect those organs.

Figure 13.1 The nervous system. The nervous system includes the brain, spinal cord, sense organs, and the nerves that interconnect those organs.

How Cure Nervous Systems

To carry information between parts of the body, the cells of the nervous system pass electrical and chemical signals to each other. Signals are transmitted from one end of a nerve cell to the other end, between nerve cells, and from nerve cells to the cells of tissues, organs, or glands that respond to nerve signals called effectors. Information is carried along nerves by electrical changes called nerve impulses and transmitted between nerve cells and from nerve cells to the cells they act on by chemical stimuli, called neurotransmitters, released from nerve cells.

Neurons can be grouped into three general categories: (1) sensory neurons carry information toward the brain; (2) motor neurons carry information away from the brain toward effector organs and glands; and (3) interneurons, located between sensory and motor neurons within the brain or spinal cord. Regardless of category, their job is to integrate information from various sensory inputs. Most actions involve input from all three sources, with sensory input followed by integration and motor output (Figure 13.2).

Sensory input is detected by sensory receptors. Receptors are usually neurons or other cells that communicate with sensory neurons. They detect changes in conditions inside or outside the body. When receptors are stimulated, signals are generated and carried to the brain.

The general senses are temperature, pain, touch, pressure, and body position, or proprioception. The sensory receptors for the general senses are scattered throughout the body. The special senses are smell, taste, equilibrium, hearing, and vision. The sensory receptors for these five special senses are found in complex sense organs (Table 13.1).

As sensory information is passed to and from your brain, it travels through the main nerve pathway: the spinal cord. Your spine, which protects your spinal cord from injury, is made up of 33 separate bones called vertebrae.

Juice Cure Neveus Sistem
Figure 13.2 Neurons. The nervous system's three main functions are sensory input, integration, and motor output. Most actions, such as being tempted to eat cookies, involve input from sensory neurons, followed by integration, via inter-neurons, and motor output.

Sense and its location Temperature, pain, pressure, touch

Sense and its location Temperature, pain, pressure, touch

Sensory Receptor Types

The eyes contain sensory receptors that permit sight. Neuronal receptors in the retina relay sensory information to the brain.

Description

Sensory nerve endings in the skin are sensitive to temperature, pain, pressure, and touch. These receptors are specialized to respond to different levels of sensation (for example, slight cold versus freezing cold).

Body position, or proprioception, is monitored by neurons that sense the position of joints, the level of tension in joints and ligaments, and the state of muscular contraction. Information gathered by these neurons is relayed to the brain, where it is integrated with balance information from the inner ear.

The sense of smell, or olfaction, is provided by the paired olfactory organs located in the nasal cavity. Receptors within the olfactory organs are specialized neurons. Chemicals bind to, and stimulate, receptor molecules on hairlike projections, called cilia, that line the nasal cavity. From the olfactory organs, nerve impulses are relayed to the brain.

Taste receptors are found along the surface of the tongue, clustered together into taste buds. When food or drink contacts the taste buds, a nerve impulse is sent to the brain. There are four primary taste sensations: sweet, salty, sour, and bitter. Each taste bud has a sensitivity to one of these tastes.

The eyes contain sensory receptors that permit sight. Neuronal receptors in the retina relay sensory information to the brain.

The senses of equilibrium and hearing are provided by the inner ear. Equilibrium determines the position of the body in space by monitoring gravity, acceleration, and rotation. Hearing enables us to detect and interpret sound waves. The receptors for both of these senses are ciliated cells. Movement of the cilia causes the generation of nerve impulses.

Table 13.1 The senses. There are two groups of senses: the general senses and the special senses. Both groups are illustrated here, along with their locations and functions.

Interneurons to and from brain (within spinal cord)

Spinal cord

Interneurons to and from brain (within spinal cord)

Spinal cord

What Branches Out From The Spinal Cord

Figure 13.3 Spinal nerves and vertebrae. Spinal nerves branch out between the vertebrae and go to all parts of the body.

Spinal nerves

Vertebrae Cartilage

Figure 13.3 Spinal nerves and vertebrae. Spinal nerves branch out between the vertebrae and go to all parts of the body.

Spinal nerves branch out between the vertebrae and go to every part of the body (Figure 13.3).

In addition to transmitting messages to and from the brain, the spinal cord also serves as a reflex center. Reflexes are automatic responses to a stimulus. They are prewired in a circuit of neurons, called a reflex arc, which often consists of a sensory neuron that receives information from a sensory receptor, an interneuron that passes the information along, and a motor neuron that sends a message to the muscle that needs to respond.

Reflexes make a person react quickly to dangerous stimuli; for instance, the withdrawal reflex occurs when you encounter a dangerous stimulus such as touching something hot. When you touch something hot, sensory neurons from touch receptors send the message to your spinal cord. Within the spinal cord, interneurons send the message to motor neurons to withdraw your hand from the hot surface (Figure 13.4).

Figure 13.4 A reflex arc. A reflex arc can consist of a sensory receptor, a sensory neuron, an interneuron, a motor neuron, and an effector. Touching a hot baking sheet evokes the withdrawal reflex.

Figure 13.4 A reflex arc. A reflex arc can consist of a sensory receptor, a sensory neuron, an interneuron, a motor neuron, and an effector. Touching a hot baking sheet evokes the withdrawal reflex.

Something Hot Message

While the spinal reflexes are removing your hand from the source of the heat, pain messages are also being sent through your spinal cord to your brain. This takes a little longer because the distance to the brain is longer than the distance to the spinal cord. Therefore, by the time the pain message reaches your brain, your hand has already been removed from the hot surface.

The reflex arc illustrates how the brain, spinal cord, and nerves work together to evoke a response. Even though these parts of the nervous system work together, they are separated into two different anatomical subdivisions. The central nervous system (CNS), consists of your brain and spinal cord and is responsible for interpreting and acting upon information received by the senses. The CNS is the seat of functions such as intelligence, learning, memory, and emotion. The second subdivision of the nervous system, the peripheral nervous system (PNS), includes the network of nerves that radiates out from your brain and spinal cord (Figure 13.5).

Media Activity 13.1A Brain and Neuron Structure

The brain is the region of the body where decisions are reached and where bodily activities are directed and coordinated. About the size of a small cantaloupe, the brain is housed inside the skull, where it sits in a liquid bath, called cerebrospinal fluid, that protects and cushions it.

In addition to housing neurons, the brain is composed of other cells called glial cells. There are 10 times as many glial cells in the brain as there are neurons.

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