The Nervous System

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NERVOUS SYSTEM

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Peripheral Nervous System --The peripheral nervous system is divided into two major parts: the somatic nervous system and the autonomic nervous system.

Central Nervous System

In the Peripheral Nervous System, neurons can be functionally divided in 3 ways:

1 Sensory (afferent) - carry information INTO the central nervous system from sense organs.

OR

Motor (efferent) - carry information away from the central nervous system (for muscle control).

2 Cranial - connects the brain with the periphery.OR

Spinal - connects the spinal cord with the periphery.

3 Somatic - connects the skin or muscle with the central nervous system.

OR

Visceral - connects the internal organs with the central nervous system.

Notice that the somatic nervous system has only one neuron between the central nervous system and the target organ while the autonomic nervous system uses two neurons.

1. Somatic Nervous System

The somatic nervous system consists of peripheral nerve fibers that send sensory information to the central nervous system AND motor nerve fibers that project to skeletal muscle.

2. Autonomic Nervous System

The autonomic nervous system is divided into three parts: the sympathetic nervous system, the parasympathetic nervous system and the enteric nervous system. The autonomic nervous system controls smooth muscle of the viscera (internal organs) and glands.

sympathetic nervous system

the parasympathetic nervous system

The enteric nervous system is a third division of the autonomic nervous system that you do not hear much about.
The enteric nervous system is a meshwork of nerve fibers that innervate the viscera (gastrointestinal tract, pancreas, gall bladder).

1. Brain

The brain develops from three swellings at the anterior end of the neural canal of the embryo. From front to back these develop into the

forebrain (also known as the prosencephalon)
midbrain (mesencephalon)
hindbrain (rhombencephalon)

The brain receives nerve impulses from
the spinal cord and 12 pairs of cranial nerves
Some of the cranial nerves are "mixed", containing both sensory and motor axons
Some, e.g., the optic and olfactory nerves (numbers I and II) contain sensory axons only
Some, e.g. number III that controls eyeball muscles, contain motor axons only.

2. Spinal Cord

Conducts sensory information from the peripheral nervous system (both somatic and autonomic) to the brain
conducts motor information from the brain to our various effectors
o skeletal muscles
o cardiac muscle
o smooth muscle
o glands
serves as a minor reflex center

31 pairs of spinal nerves arise along the spinal cord. These are "mixed" nerves because each contain both sensory and motor axons. However, within the spinal column, all the sensory axons pass into the dorsal root ganglion where their cell bodies are located and then on into the spinal cord itself.
all the motor axons pass into the ventral roots before uniting with the sensory axons to form the mixed nerves.

The spinal cord carries out two main functions:

It connects a large part of the peripheral nervous system to the brain. Information (nerve impulses) reaching the spinal cord through sensory neurons are transmitted up into the brain. Signals arising in the motor areas of the brain travel back down the cord and leave in the motor neurons.

The spinal cord also acts as a minor coordinating center responsible for some simple reflexes like the withdrawal reflex.

The interneurons carrying impulses to and from specific receptors and effectors are grouped together in spinal tracts.

The Brain in more detail

The brain develops from three swellings at the anterior end of the neural canal of the embryo. From front to back these develop into the:

forebrain (also known as the prosencephalon)
midbrain (mesencephalon)
hindbrain (rhombencephalon)

The brain receives nerve impulses from the spinal cord and 12 pairs of cranial nerves

Forebrain

Midbrain

The midbrain occupies only a small region in humans (it is relatively much larger in "lower" vertebrates).

Hindbrain

Telencephalon

a pair of large cerebral hemispheres

Frontal/Parietal/Occipital/Temporal
lobes

Hidden beneath these regions of cerebral cortex are the olfactory bulbs; they receive input from the olfactory epithelia. Link to discussion of olfaction.striatum; it receives input from the frontal lobes and also from the limbic system (below). At its base is the nucleus accumbens (NA). The pleasurable (and addictive) effects of amphetamines, cocaine, and perhapsother psychoactive drugs seem to depend on their producing increasing levels of dopamine at the synapses in the nucleus accumbens (as well as the VTA).

Diencephalon

a group of unpaired structures located deep within the cerebrum,

Thalamus

All sensory input (except for olfaction) passes through it on the way up to the somatic-sensory regions of the cerebral cortex and then returns to it from there. signals from the cerebellum pass through it on the way to the motor areas of the cerebral cortex.

Posterior lobe of the pituitary.

Receives antidiuretic hormone (ADH) and oxytocin from the hypothalamus and releases them into the blood.

Hypothalamus.

The seat of the autonomic nervous system. Damage to the hypothalamus is quickly fatal as the normal homeostasis of body temperature, blood chemistry, etc. goes out of control. The source of 8 hormones, two of which pass into the posterior lobe of the pituitary gland.

(LGN).

All signals entering the brain from the optic nerves enter the LGN and undergo some processing before moving on the various visual areas of the cerebral cortex.

The Reticular Formation: collects input from higher brain centers and passes it on to motor neurons.

The Substantia Nigra: helps "smooth" out body movements;

The Ventral Tegmental Area (VTA): packed with dopamine-releasing neurons that synapse deep within the forebrain. The VTA seems to be involved in pleasure: amphetamines and cocaine bind to the same receptors that it activates and this may account at least in part for their addictive qualities.

Click here for The Brain's Stress Response