Overview of the Nervous System

Our nervous system is our control center and body's communication. It is made up of our brain, spinal cord, nerves and ganglia.

Role of Action Potentials:

(Action potentials are electrical signals which are transmitted along specialized nervous tissue called neurons)

• Collect information

  • Specialized nervous system structures called receptors monitor changes in the internal and external environment (stimuli). Ex. If the skin is touched, sensory information is relayed along neurons to the spinal cord and brain.

• Process and evaluate information

  • After the information is processed, the brain and spinal cord determine the necessary response if any.

• Initiate a response to the information

  • Motor information is relayed along neurons to effectors. Effectors include all three types of muscle tissue (cardiac, smooth, and skeletal) and glands. Some responses can include a change in gland secretion, muscle contractions, or muscle relaxations.

Anatomical Divisions of the Nervous System:

• Central Nervous System (CNS)

  • Includes the brain and spinal cord

  • Brain is protected and enclosed by the skull

  • The spinal cord is housed and protected within the vertebral canal

• Peripheral Nervous System (PNS)

  • Includes nerves, which are bundles of axons of neurons and ganglia (clusters of neuron cell bodies located along nerves)

Functional Divisions of Nervous System:

• Sensory Nervous System - directs stimuli and transmits information from receptors to the Central Nervous System (CNS).

  • Somatic sensory - sensory input from the receptors of the five senses and proprioceptors.

  • Visceral sensory - sensory input from receptors of internal organs and blood vessels.

• Motor Nervous System - indicates and transmits information from the Central Nervous System to effectors.

  • Somatic motor - motor output to skeletal muscle.

  • Autonomic motor - motor output to cardiac muscle, smooth muscle, and glands.

Nerves:

• Nerve- An organ that is a part of the PNS and is made up of a cable-like bundle of axons, connective tissue layers, and blood vessels.

• Three successive connective tissue wrappings:

  • Epineurium - thick layer of dense irregular connective tissue that encloses the nerve. This covers the entire nerve to protect and support it.

  • Perineurium - a layer of dense irregular connective tissue that wraps each fascicle. They provide protection and support to each axon bundle and support blood vessels.

  • Endoneurium - a delicate layer of areolar connective tissue that surrounds each axon. The function of these coverings is to separate and electrically insulate each axon.

Classification of Nerves:

• Nerves can be classified both structurally and functionally.

• Structural (based upon the CNS component from which the nerve extends):

  • Cranial nerves - extend from the brain.

  • Spinal nerves - extend from the spinal cord.

• Functional (based upon the functional type of neuron a nerve contains):

  • Sensory nerves - contain only neurons that relay information toward the CNS

  • Motor nerves - contain neurons that relay information away from the CNS

  • Mixed nerves - contain both sensory and motor neurons; most named nerves including all spinal and most cranial nerves are mixed nerves. Individual sensory or motor neurons still only transmit one type of information.

Features of the Nervous System

• Communication Method

  • A nerve signal causes neurotransmitter release from a neuron into a synaptic cleft.

• Target of Stimulation

  • Other neurons, muscle cells, and gland cells.

• Response Time

  • Rapid reaction time: Typically milliseconds or seconds.

• Range of Effect

  • Typically has localized, specific effects in the body.

• Duration of Response

  • Short-term: Milliseconds; terminates with removal of stimulus.