What makes your muscles move?
You have learned about reflexes and how messages are sent through the body's nervous system. You also learned about visual and auditory senses. In this section you will explore the relationship between your brain and the muscles in your body.
Imagine that you've burned yourself on a hot grill. Your reflexes pulled your hand back before you felt pain. Sensors in your skin continue to send pain messages to your cortex. Now you look at your hand, remember what to do for burns, and decide to go into the house to treat the injury. How does your brain communicate this decision to the rest of your body? Does your brain send messages to your muscles in the same way your sensors send messages to your brain? First we will learn a few facts about muscles, and then we will study how they are controlled by the brain.
Any movement of your body requires the use of at least one muscle. Most movements, in fact, use many muscles. For example, think about throwing a baseball. As you wind up and raise your arm, muscles in your neck, shoulder, and back work together. Other muscles in your legs and back adjust your posture so you keep your balance. You are not aware of all the adjustments your muscles are making. As you throw, you only think about where you want the ball to go. The rest happens without thinking. How does your body manage such a complex motion, requiring coordination, speed, and strength?
Your body has three types of muscles. Compare the three types in Figure 6.1. Your heart contains cardiac (KAR-dee-ak) muscle that pumps your blood through your body 24 hours a day. Lining your blood vessels and digestive tract are smooth muscles. Muscles called skeletal (SKELL-ih-tuhl) muscles move your bones. Skeletal muscle fibers look striped, or striated (STRI-ayt-ed).
Figure 6.1 You have three types of muscle fibers in your body: cardiac, smooth, and skeletal (striated), each with a different arrangement of muscle fibers.
Your cortex and spinal cord control your voluntary (VAHL-un-tayr-ee) muscles. These are the striated muscles you can move when and how you want to run, dance, play the violin, and talk. Parts of the brain stem, such as the pons and medulla, and the spinal cord control your involuntary (IN-vol-un-tayr-ee) muscles. Examples of involuntary muscles include the smooth muscles that squeeze your intestines and the cardiac muscle that pumps your blood. Involuntary muscles are not under your conscious control. They move automatically to help maintain your body. Although you can't control your involuntary muscles, you can control factors that affect them, such as stress, drugs, and alcohol. You will learn more about some of these factors in the next section.
How Your Brain and Spinal Cord Move Muscles
Think back to the example of the neuron as big as a bedroom in Figure 3.2. Remember that the neurons of your nervous system are divided into three groups. The sensory neurons bring information into the nervous system. Interneurons send messages between different parts of your nervous system. Motor neurons send information from the nervous system to all of the muscles of the body.
This section focuses on how motor neurons work. You may remember that a message moves from the axon of a neuron to the dendrites of another neuron at a synapse. A motor neuron may have hundreds of dendrites and thousands of synapses on its dendrites that bring in messages from other neurons. Some of these neurons carry messages from the cortex and other parts of the brain, others from the spinal cord.
Motor neurons in your spinal cord receive messages from the motor area of your cerebral cortex and cerebellum as well as direct input from sensors and spinal interneurons involved in reflexes. To reach your motor neurons, messages from your cerebral cortex travel down through your brain stem and spinal cord. These messages can tell your muscles to move or to retain just the right amount of tension to keep you from falling over.
You may remember that your cerebrum is divided into two hemispheres, the right and the left. The motor cortex on the left cerebral hemisphere controls the right side of your body. The motor cortex of the right cerebral hemisphere controls the left side of your body. For example, if a neurosurgeon operating on your brain touched the part of your left motor cortex that controls foot movement, you'd move your right foot. Messages coming into and going out of your brain cross over from the left side to the right side in your medulla.
When you made the “Big Brain on a Stick” model in Activity 2-1, you may have noticed that the sensory and motor areas occur where two lobes of the cerebrum meet. The sensory and motor areas lie on either side of a deep crevice. Each area has sections that correspond to body parts. On the “motor” side of the crevice, nerve impulses control movement. On the “sensory” side of this crevice, nerve impulses deliver sensory information from body parts.
Motor neurons reach from the map in your motor cortex all the way down your brain stem and spinal cord. Figure 4.3 shows how one motor neuron from your cortex connects with the motor neurons used in the knee jerk reflex. There are thousands of other motor neurons coming down from the cortex. Also, each muscle is stimulated by many motor neurons from the spinal cord.
Activity 6-1: Connecting Your Brain and Muscles
How does information from your brain connect with your fingers and toes so that you can move them? You know that your brain has to send nerve impulses to muscles in your fingers and toes. In this activity you explore how the brain, nerves, and muscles connect. You work with a group of students to make a full-size map of the nervous system.
- Colored marking pens (black, yellow, green, and brown)
- Butcher paper
- “Big Brain on a Stick” Model (from Activity 2-1)
- Transparent tape
- Colored yarn or string (green and yellow)
- Meter stick
- Activity Report
Step 1 Select one group member as the model to trace.
Step 2 Place the butcher paper on the floor and have the student lie down on the paper with arms and legs extended. It is important that all parts of the body remain on the paper.
Step 3 While the student remains motionless, use the black marking pen to draw an outline of the body on the paper. Be careful not to mark any skin or clothing.
Step 4 Have the student move carefully off of the paper, to avoid tearing it.
Step 5 With the black marking pen, draw in an outline of the brain and spinal cord. You can use your “Big Brain on a Stick” model from Activity 2-1 as a pattern.
Step 6 Use a yellow marking pen to color in the sensory areas of the brain responsible for walking and writing. You can review Section 2 of the text if you need help locating these areas.
Step 7 Use a green pen to color in the motor areas of the brain responsible for walking and writing.
Step 8 Use a piece of yarn to measure the length of the spinal cord of the student. Cut five pieces of yellow yarn and five pieces of green yarn the length of the spinal cord.
Step 9 Position the yarn on the paper in the spinal cord area. Tape the yarn firmly to the paper using transparent tape.
Step 10 Measure the distance from the spinal cord to the tip of a finger on your drawing. Use Figure 1.2 to find out where the nerves enter and leave the spinal cord.
Step 11 Cut two pieces of yarn, one green and one yellow, to represent the nerves connecting the spinal cord with the fingers.
Step 12 Position pieces of yellow and green yarn on the paper, connecting a finger to the spinal cord. Tape the yarn firmly to the paper.
Step 13 Repeat Steps 10-12 to show the nerves connecting the spinal cord and one of the toes.
Step 14 Use a brown marking pen to color the tip of the toe and finger to represent the muscle.
Step 15 Follow your teacher's directions for cleanup and storage or display of your nervous system map.
When you walk, your brain sends messages to direct your steps. When you step forward with your right foot, you turn on the neuron that controls the quadriceps muscle on the front of your thigh. At the same time, you turn off the neuron that controls the biceps muscle on the back of your thigh. These messages let you swing your right foot forward. As you bring your left foot forward and place it on the ground, you turn on the biceps muscle and turn off the quadriceps muscle in your right leg. These messages let you pull your right leg back to get ready for your next step.
Activity 6-2: Moving Muscles
You can use your “Thinking Cap” from Activity 2-2 to show how your brain tells your muscles to move. Work with a lab partner and follow the steps below:
- “Thinking Cap” from Activity 2-2
- Yellow and green yarn
- Activity Report
Step 1 Remember the sensory and motor divisions of your nervous system. Let yellow yarn represent a sensory pathway, and use green yarn to represent a motor pathway. Cut four pieces of yarn of each color: two pieces 1 meter in length and two pieces 1.5 meters in length.
Step 2 Have your lab partner put on the “Thinking Cap.”
Step 3 Tape one long and one short piece of green yarn and one long and one short piece of yellow yarn to the right places on the right and left cerebral hemisphere.
Step 4 Take the ends of the short pieces of green and yellow yarn from each side. Attach them to your partner's arms. Which arms did you attach them to? If you attached the yarn from the right side of the “Thinking Cap” to the left arm, you did so correctly.
Step 5 Repeat Step 4, attaching the longer pieces to your partner's legs.
Step 6 Touch the yellow yarn on one of the arms or legs. Your lab partner should tell you which side of the “Thinking Cap” would receive the information about the touch. On your Activity Report, explain why this happens. Which direction does the nerve impulse travel in the pathways represented by the yellow yarn? The green yarn?
Step 7 How does information get from the sensory pathway to the motor pathway? Describe where synapses occur in the pathways you created with yarn. Write your responses on the Activity Report.
Figure 6.2 With practice, your brain learns to control your body efficiently.
How Do You Learn to Move Efficiently?
Have you tried to learn to play a musical instrument or ride a bike? Have you watched a baby learning to walk? You know that moving muscles gently, smoothly, and in rhythm takes practice. What happens when you practice moving?
You have some muscles that contract more slowly, but tire less easily. These muscles are called slow-twitch muscles. You also have fast-twitch muscles. They work more quickly, but tire more easily. Endurance sports, such as swimming, develop slow-twitch muscles. Track sprinters develop their fast-twitch muscles. Individuals better at one kind of sport than another may naturally have more fast-twitch or slow-twitch muscles.
Why does practice help you move better? What happens in your cerebellum and your cerebral cortex? Your brain learns to inhibit or stop unwanted movements. It turns on neurons that inhibit unwanted movements, so that only the wanted movements happen. It takes time and practice for your cerebral cortex and brain stem to learn to move your body efficiently. Learning new things makes the neurons in your brain establish new connections with other neurons. As you learn more, the connections between your neurons form a complex web.
What Part of the Brain Coordinates Your Movements?
Have you ever seen a drunken person moving? People who are drunk stumble, fall, and slur their words. What's happening in their brains? Alcohol can pass through the blood-brain barrier between the blood vessels and the neurons in the brain. Alcohol slows down all neurons, and therefore impairs thinking, decision making, and judgment. The effect of alcohol on the cerebellum is particularly noticeable, however.
The cerebellum helps the brain coordinate muscle movements. The cerebellum is sort of like the conductor of an orchestra. The conductor sees the music as it is written and hears the music as it is played. When there is a mismatch, the conductor tries to fix it by getting some of the musicians to play louder or softer or faster or slower. The cerebellum gets information from the motor cortex and from sensors in the joints and muscles. It can compare these two kinds of information and modify the commands to the muscles. When the cerebellum isn't working right, such as when a person is drunk, the person can't coordinate muscles. As a result, the person stumbles, staggers, and cannot walk or drive in a straight line.
How would drinking alcohol affect your performance in the following activities? What are some things that might happen if you drank alcohol and tried to do these activities?
- taking a math test
- riding a bike
- playing baseball
How Does Muscle Control Differ in Your Feet and Hands?
Many motor neurons control each of your muscles. Small muscles in the fingers and eyes have finer control than leg and foot muscles. Each motor neuron connected to your finger muscles controls just a small part of each finger muscle. It would be much harder to learn to write with your feet because each motor neuron connected to a foot muscle controls a larger part of that muscle. Therefore, control of movement in your feet is not as fine as the control of movement in your hands.
Many things can affect your nervous system and your movements. Some examples include drugs, alcohol, injury, illness, or even what you eat.
Activity 6-3: The Nervous System and Muscles Working Together
What is the role of the nervous system when you use your muscles for walking or writing? Can you write better with your hand or your foot? In this activity you explore how your nervous system is responsible for muscle action.
- Writing paper
- Marking pens
- Activity Report
Part A. Walking Muscles
Step 1 Walk forward slowly, one step at a time. Think about the thigh muscles you are using.
Step 2 Now, take a step with your right foot. Bend over to feel the quadriceps and biceps muscles in your thigh as you take a step. Describe your observations to your lab partner.
Step 3 With your weight on your right foot, feel the quadriceps and biceps muscles in your left thigh as you move it forward to Take a step. Describe your observations to your lab partner.
Step 4 What is the difference between the muscles of a walking leg that is taking a step, and a walking leg that is just finishing a step?
Step 5 Complete section A in the Activity Report.
Part B. Writing Muscles
Step 1 Tape a piece of paper onto the wall.
Step 2 Hold a marking pen in your hand and write your name on the paper on the wall.
Step 3 Remove your shoe. Secure a marking pen between your toes.
Step 4 Lie on your back or sit in a chair near the wall, so you can reach the paper with your “writing foot.” Write your name on the paper on the wall.
Step 5 Complete section B of the Activity Report.
What activities are you really good at? Are you good because you have natural talent or because you've worked hard to make the connections between neurons that you need to be successful?
- Sample answers to these questions will be provided upon request. Please send an email to firstname.lastname@example.org to request sample answers.
- What causes muscles to contract?
- How do we learn new movements, such as dance steps or a new piece on the piano?
- What role does the cerebellum play in moving muscles?
- Do all muscles have the same number of motor neurons? Explain.