The Nervous System Overview

The nervous system is a complex communication system.
It contains over 100 billion nerve cells, primarily in the brain.
It's divided into two main parts: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

Endocrine system control takes time.
The hormone must be detected, the appropriate gland stimulated, and then travel through the bloodstream to reach target cells.
The nervous system can respond quickly (seconds or fractions of seconds) to environmental changes.

Central Nervous System (CNS): Includes the brain and spinal cord. It coordinates all incoming and outgoing information.
Peripheral Nervous System (PNS): Composed of nerves. It carries information between all body areas and the CNS.

The somatic system controls skeletal muscles, bones, and skin, relaying external environment information to the brain.
It also controls muscle movement in response to brain signals.
It mediates awareness and voluntary actions like reacting to heat, light, touch, taste, smell, laughing, smiling, and moving.

The autonomic portion of the PNS controls internal organs.
Examples include digestion and movement of digested food through intestines.
It's involuntary, meaning we don't consciously control it.
It's maintained by sympathetic and parasympathetic systems that balance each other.

These divisions are always active with opposing actions on organs.
Sympathetic: Active during stress, danger, excitement, or intense physical activity. It increases heart rate, blood pressure, and dilates pupils.
Parasympathetic: Active during quiet, low-stress situations. It reduces sympathetic effects and promotes maintenance activities like digestion.

Neurons are the fundamental units of the nervous system.
Three main types:
- Sensory (afferent): Sense and relay information from the environment to the CNS (central nervous system). Located in ganglia outside the spinal cord.
- Interneurons (association): Connect neurons within the CNS, primarily in the brain and spinal cord.
- Motor (efferent): Relay information from the brain to effectors (muscles, organs, glands). They help counteract environmental changes.

Contains standard cell organelles.
Carries out metabolic functions (necessary processes).
Forms the axon, the extension of the cytoplasm that transmits signals away from the cell body.

Carries signals away from the cell body.
Many axons are coated with myelin sheath, a fatty protein created by glial cells (Schwann cells).
Myelin speeds up signal transmission along axons.
Nodes of Ranvier are gaps in the myelin that also aid in quick signal transmission.

Resting Potential: When a neuron is not transmitting information, there's a charge difference across its membrane due to unequal concentrations of positive ions inside and outside. This difference is -70 mV.
Sodium-Potassium Pump: Actively pumps sodium ions out and potassium ions in to maintain resting potential.
Depolarization: When a neuron is excited, its membrane becomes more permeable to sodium ions, causing a change in internal charge to a positive value of +40 mV (action potential).
Repolarization: After action potential, the membrane returns to its resting state.
Refractory Period: A short time period after an action potential when a neuron cannot send another one.

A stimulus must reach a certain intensity (threshold) to trigger an action potential.
The intensity of the stimulus doesn't change the size of the action potential; it is an all-or-none response.
The brain interprets signal intensity by the frequency of action potentials, not their strength.

The brain interprets signal intensity based on the rate or frequency of nerve impulses from the stimulus. A strong stimulus generates frequent impulses.

When an impulse reaches the end of a neuron, it needs to be transferred to another neuron/muscle/gland.
The synapse, the space between neurons, is where this transfer occurs.

Chemical neurotransmitters transmit the signal across the synapse by diffusing into the neighboring cell/nerve/gland.
Neurotransmitters bind with receptor sites on the postsynaptic cell.
This typically results in producing an action potential
The process of transporting neurotransmitters usually involves vesicles containing neurotransmitters at the end of the neurons, transferring into the synapse/synaptic cleft.

Chemical messengers that transmit signals across the synapse.
Acetylcholine is one example of a neurotransmitter that triggers the opening of sodium ion channels in the post-synaptic neuron.
In synaptic transmission, the enzyme cholinesterase breaks down acetylcholine after the signal has been transmitted.

Clostridium botulinum bacteria produce a toxin interfering with neurotransmission.
The toxin inhibits the release of acetylcholine, paralyzing muscles.