Components of the Nervous System

• Nerve cells (neurons)
• Dendrites(in)

Dendrites are the primary targets for synaptic input from the axon of other neurons. - Axons(out)

Axons are the synaptic output channel from neurons. However, it is important to note that not all neurons have dendrites and axons, but atleast on of them! This can be seen in the retinal amacrine cell, which has dendrites but no axon. - Synapse

The neuron sending information(the synapse) is presynaptic, the one receiving is the postsynaptic one. In general, there is no physical continuity between these two elements. Instead, the pre- and postsynaptic components communicate via exchaging molecules. These molecules travel a nonempty space between the two componets called the synaptic cleft. The synapse itself is a self-generating wave of electrical activity, an action potential. The all-or-nothing(either 0 or 1) change in the voltage across the nerve cell membrane conveys information from one point to another. Most synapses in the nervous system are chemical synapses, a less frequent one is the electical synapse.

• Glial cells (glia)

Glial cells are quite different from neurons, but are vital to the NS. They are the repair-men and maintainers, and has nothing to do with exchanging information, like neurons. There are three kinds of glial cells.

• Astrocytes

Restricted to the CNS. Primary function is to maintain an appropriate environment for neuronal signaling.

• Oligodendrocytes

Restricted to the CNS. Lays down myelin around some axons. This speeds up the transmissing of signals along the axon.

• Microglial cells

Shares many properties with macrophages. They remove cellular debris from cell turnover or injury.

• Glial stem cells

Found throughout the adult brain, and has the ability to proliferate and generate new glial cells, or neurons.

Neural Circuits

Neurons never function by themselves. They are organized into networks called neural circuits(or neural networks). The NN architecture varies greatly according to which functions it serves:

• Information flow
  • Nerve cells that carry information towards the brain are afferent neurons
  • Nerve cells that carry infomration away from the brain are efferent neurons
  • Inter-network nerve cells are called interneurons. These only operate in a small area of the network. A good example of this is the knee-jerk reaction and the neuron in the spinal cord.

Organization of the NS

Neural networks that process similar types of information can be grouped into systems that server a broader purpopse.

• Sensory systems

Acquire information from the envirionment. Ex visual system.

• Motor systems

Respond to information by doing movements and other behavious.

• Associational systems

  These are neurons and networks that lie between the other well defined systems.

The NS is also grouped by physical location.

• Central Nervous System(CNS) Includes the brain and the spinal cord. Nerve cells are arranged in two ways:

• Nucliei Local accumulations of neurons that have roughly the same connections and functions.

• Cortex Sheet-like arrays of nerve cells. Especially found in the Cerebelum.

Axons in the CNS are grouped into tracts, analogous to nerves in the PNS. Tracts that cross the midline of the brain are commissures.

• Peripheral Nervous System(PNS)

Consists of the nerves and ganglia outside of the brain and spinal cord. The main function of the PNS is to act as a communication channel between th CNS and the limbs and organs. Peripheral axons are gathered into bundles called nerves. Many of these are enveloped by glial cells.