The Nervous System

1. The Nervous System Brain, spinal cord, and nerves

2. Functions of the Nervous System • 1. Communication and coordination system in the body • 2. Seat of intellect and reasoning • The goal is to monitor changes in the environment inside and outside the body, interpret the changes, and initiate a response in an effort to maintain homeostasis.

3. Electrochemical messages called Nerve impulses race through your body every moment, traveling along special routes, or Nerves, at high speeds. • This is how communication happens

4. Divisions of the Nervous system • Two main groups • Central Nervous System (CNS) which includes the brain and spinal cord • Peripheral Nervous System (PNS) which includes the nerves and sensory receptors • The PNS is divided into the Autonomic Nervous System (ANS) which controls unconscious activities and the Somatic Nervous System (SNS) which oversees conscious activities

5. Nervous tissue • The functional cells of nervous tissue are called neurons, which receive support from nearby neuroglial cells (connective part) • Each neuron consists of a cell body and branches. The cell body contains the nucleus and most of the cytoplasm, and the branches include many dendrites which carry impulses toward the cell body, and a single axon which carries impulses away.

6. Nervous tissue

7. Nervous tissue • In many neurons, the axon is covered with numerous neuroglial cells known as Schwann cells, which provide a white-colored protective sheath that is mostly fat. • This fat layer is called the myelin sheath and it insulates and protects the axon (some axons are nearly one meter – about 3 feet – long).

8. Types of neurons • Sensory (afferent) neurons carry nerve impulses from body parts into the brain or spinal cord. They have receptors at the end of their dendrites • Motor (efferent) neurons carry nerve impulses out of the brain or spinal cord to effectors • Interneurons (mixed nerves) lie within the brain or spinal cord and link other neurons

9. Cell membrane potential • The surface of a cell membrane is usually electrically charged, or polarized, with respect to the outside • This polarization is due to an unequal distribution of positive and negative ions between sides of the membrane • Important in the conduction of muscle and nerve impulses

10. Because of the active transport of sodium and potassium ions, cells throughout the body have a relatively greater concentration of sodium ions (Na+) outside and a relatively greater concentration of potassium ions (K+) inside • The cytoplasm of these cells has many large negatively charged particles that cannot diffuse across the cell membranes.

11. Sodium and potassium ions follow the laws of diffusion and show mvmt from high to low concentration as permeability permits • The difference in electrical charge between two regions is called a potential difference and in a resting nerve cell this is called resting potential • When permeability changes in the region of cell membrane that is being stimulated, channels open and allow sodium to diffuse inward

12. This mvmt is aided by the negative electrical condition on the inside of the membrane, which attracts the positively charged sodium ions • Now the membrane loses its negative charge and becomes depolarized • At almost the same time, membrane channels open and allow potassium ions to pass through and diffuse outward so the inside again becomes negatively charged and repolarized

13. This rapid sequence of depolarization and repolarization takes about one-thousandth of a second and is called Action Potential • www.youtube.com/watch?v=yQ-wQsEK21E • A wave of action potentials moves down the fiber to the end. This delivers a nerve impulse

14. The Synapse • Neurons have the ability to conduct nerve impulses very quickly, but how does one cell communicate with another cell? • Adjacent neurons communicate by releasing chemicals across tiny gaps that separate them, called synapses (synaptic cleft) • The chemicals, known as neurotransmitters, are released by a neuron when a nerve impulse reaches its distal end

15. The neurotransmitters then diffuse across the synaptic cleft to contact the adjacent cell • Contact with the next neuron may stimulate it to trigger a nerve impulse, or it may inhibit it (pain meds)

16. The Reflex Act • The simplest type of nervous response is the reflex act, which is unconscious and involuntary (examples) • Every reflex act is preceded by a stimulus (any change in the environment) • Special structures called receptors pick up these stimuli • Afferent neurons take the message to the interneuron

17. The interneuron interprets the info and decides the action • The efferent neuron takes the message to the responding organ • Reaction to the stimulus is called the response and if there is mvmt the muscles are called the effectors

18. The Reflex Arc

19. A simple reflex is one in which there is only a sensory nerve and a motor nerve involved • Classic example is the knee-jerk reflex • Also the withdrawal reflex

20. Central Nervous System • The “central station” for incoming and outgoing nerve impulses • Includes the brain and spinal cord • Both are protected by bones (cranium and vertebral column) and a thick set of membranes called the meninges (located between the soft nervous tissue and the hard bones, and is several layers thick)

21. Meninges • Outer layer is tough, white fibrous connective tissue and cantains many nerves and bv - dura mater • As it continues down the spinal cord it does not attach directly to the vertebrae but is separated by an epidural space (filled with adipose and connective tissue to pad around the spinal cord) • Middle layer is thin weblikearachnoid (lacks nerves and bv) • Inner layer is thin and plastered to the nervous tissue pia mater (contains many nerves and bv)

22. Between middle and inner is subarachnoid space that is filled with a slightly yellowish fluid called cerebrospinal fluid (CSF) • CSF is also found in the ventricles of the brain (small spaces within the brain’s center)

23. The Spinal Cord