The nervous system and sense organs

The nervous system and sense organs

The nervous system

Divisions of the nervous system

- The first way to divide the nervous system is into two basic
anatomic areas:
o The central nervous system (CNS) which consists of the brain
and spinal cord and their associated structures.
 acts as a central processing unit for the body.
o The peripheral nervous system (PNS) which comprises the
nerves traveling from the brain or spinal cord to the target
organs and back.
 acts as the messenger, carrying signals generated by the
central nervous system to the target organs, or carrying
signals from sensory nerves back to the central nervous
system.
- The second way to describe the nervous system is to divide it
into two basic functional groupings:
o The voluntary, or somatic, nervous system carries out conscious
activities, such as walking and eating, and activities that are to
some degree unconscious, like posture.
o The involuntary, or autonomic, nervous system performs
functions that don’t require conscious thought, such as breathing
and regulating blood pressure and heart rate.
- The third way to categorize the nervous system is in terms of
the following two impulses
o Afferent impulses
 considered the sensory nerve fibers, as they conduct
sensations from the sensory receptors of the body to the
CNS.
o Efferent impulses
 considered the motor nerve fibers, as they conduct
impulses from the CNS to the organs and muscles of the
body.

Neuron structure and cell location

- basic structural and functional unit of the nervous system is
the neuron, or nerve cell

neuron structure

, - Two important types of neurons are sensory neurons, which carry
signals of touch, taste, and sensation to the brain, and motor
neurons, which carry instructions from the brain to the muscles,
causing them to move.
- There are two types of motor neurons:
o Lower motor neurons are in the peripheral nervous system.
o Upper motor neurons carry signals that initiate body
movement from nuclei in the brain
- Two types of cell extensions project from the cell body:
o Dendrites: The highly branched cell receptors
 receptors that either sense the environment or receive
signals from other neurons.
o Axons: A cylindrical projection extending from the cell body,
usually in the direction opposite of the dendrites
 transmit signals from the cell body to other neurons or to
target organs such as muscles. Only one axon is present in
each neuron.

Cell locations

- Most neuron cell bodies are located within the brain and spinal cord.

Physiology of a neuron

- Transmission of an electrical signal is dependent on the difference
between the electrical charges on either side of the neuron cell
membrane. This difference is known as the membrane potential.
- The membrane potential of a nerve cell is established in two ways. The
first is by the diffusion of ions across the cell membrane. The
membrane is selectively permeable to ion diffusion (some ions can
diffuse while others can’t). The second is by active transport of
selected ions across the cell membrane. Active transport is achieved
by molecules embedded in the cell membrane. These molecules are
called the sodium-potassium pump
- Depolarization is the change in membrane potential that occurs
when a nerve is stimulated.
- Repolarization is the reestablishment of normal resting membrane
potential.
- The process of depolarization and repolarization is called the action
potential
- Some of the more important neurotransmitters include the
following: