Nerve cells are called neurons and are divided anatomically into the cell body, dendrites, and axon. Axons are covered by schwann cells, with gaps between these cells called nodes of ranvier. Neurons are specialized cells that respond to physical or chemical changes in their environment. At rest, nerve cells are negatively charged in the anterior when compared to the electrical charge outside the cell. This difference in ele ctrical charge is called the resting membrane potential. A neuron fires due to a stimulus changing the permeability of the membrane, allowing sodium to enter at a high rate, depolarizing the cell. When the depolarization reaches threshold, an action potential or nerve impulse is initiated.
Repolarization occurs immediately following depolarization due to an increase in membrane permeability to potassium, and a decreased permeability to sodium. Neurona communicate with other neurons at junctions called synapsis. Synaptic transmission occurs when sufficient amounts of a specific neurotransmitter are released from the presynaptic neuron. Upon release, the neurotransmitter binds to a receptor on the post synaptic on the postsynaptic membrane. An excitatory transmitter increases neuronal permeability to sodium and results in excitatory postsynaptic potentials. However, some transmitters are inhibitory and cause the neuron to become more negative or hyperpolarized. This hyperpolarization of the membrane is called an inhibitory postsynaptic potential.
Propriosceptors are position receptors located in joint capsules, ligaments, and muscles. The three most abundant joint and ligament receptors are free nerve endings, golgi-type receptors, and pacinian corpuscles. These receptors provide the body with a conscious means of recognition of the orientation of body parts as well as feedback relative to the rates of limb movement.
Reflexes provide the body with a rapid unconscious means of reacting to some stimuli. The vestibular apparatus is responsible for maintaining general equilibrium and is located in the inner ear. Specifically, these receptors provide information about linear and angular acceleration.
The spinal cord plays an important role in voluntary movement due to groups of neurons capable of controlling certain aspects of motor activity. The spinal mechanism by which a voluntary movement is translated into appropriate muscle action is termed spinal tuning.
The brain can be divided into three parts: the brain stem, the cerebrum, and the cerebellum. The motor cortex controls motor activity with the aid of input from subcortical areas. The cerebellum receives feedback from proprioceptors after movement has begun and sends information to the cortex concerning possible corrections of that particular movement pattern.
The basal ganglia are neurons involved in organizing complex movements and the initiation of slow movements. The premotor cortex operates in conjunction with the motor cortex to refine complex motor actions and may be important in acquisition of motor skills.
The autonomic nervous sytem is responsible for maintaining the constancy of the body’s internal environment and can be separated into two divisions: the sympathetic division and the parasympathetic division. In general, the sympathetic portion tends to excite an organ, while the parasympathetic portion tends to inhibit the same organ.