The Brain The Nervous System

The two sub-systems of the nervous system are:

1. The central nervous system

2. The peripheral nervous system

The brain and the spinal cord.

All of the body’s nerves.

The peripheral nervous system is further sub-divided into the somatic and the autonomic nervous systems.

The somatic nervous system is the sub-system of the PNS responsible for the voluntary control of muscle functioning and the detection of stimuli through the body’s sensory receptors.

The autonomic nervous system is the sub-system of the PNS responsible for subconscious, involuntary, visceral functions that maintain the body at a homeostatic state.

The autonomic nervous system coordinates activities such as digestion, heart rate, respiratory rate, salivation, excretion of urine, secretion of hormones, and sexual arousal.

The sympathetic nervous system (SNS) is a sub-system of the autonomic nervous system that acts to prepare the body for physical or mental activity, particularly in the fight-or-flight response.

The SNS responds to major stressors by increasing heart rate, respiratory rate, blood pressure and perspiration, dilating pupils and bronchi, and decreasing digestion.

The parasympathetic nervous system (PNS) is a sub-system of the autonomic nervous system responsible for “rest and digest”, or the overall slow-down of the body’s functions in order to conserve energy.

The only body functions that the PNS augments are digestion and sexual arousal.

neurons and glial cells

A neuron is the electrically excitable cell that comprises the nervous system and functions to process and transmit impulses through the body.

A neuron contains a cell body, dentrites, which gather and carry impulses to the cell body, and an axon, which carries information away from the cell body and towards other cells.

A glial cell functions to support the neuronal cells within the nervous system by providing nourishment, protection, and physical support.

There are many different types of glial cells, all with specific functions and locations.

Neurons could not function, or survive, without the support of glial cells.

A synapse is the location where the axon of one neuron connects to the dendrite of the next.

The gap between neurons is called the synaptic gap, or cleft.

Synapses can either be chemical or electrical in nature.

An electrical synapse is a synapse in which the neurons are directly touching and are connected by small junctions that allow nerve impulses to pass directly from one neuron to the other.

Electrical synapses are not as common as chemical synapses.

A chemical synapse is a synapse in which the neurons do not actually touch, and instead communicate by sending chemical messengers (neurotransmitters) that bridge the gap between them.

Chemical synapses are more common than electrical synapses.

A neurotransmitter is a chemical messenger that allows for the communication between neurons in a chemical synapse.

Neurotransmitters are released from the axon of the presynaptic neuron, diffuse across the synaptic gap, and attach to specific receptors on the dendrites of the postsynaptic neuron.

An action potential is the technical term used to describe a nerve impulse.

Action potentials consist of a brief, reversible polarization of the neuron. The polarization then travels along the axon, allowing for the propagation of the impulse.

Myelin is a fat-like insulating sheath, made by glial cells, that surrounds the axons of neurons and dramatically increases the speed of the action potential propagation.

The human brain has four pairs of external brain lobes. They include the following:

1. Frontal lobe

2. Parietal lobe

3. Temporal lobe

4. Occipital lobe

There is a fifth lobe, called the insular lobe, but it cannot be seen from the surface.

The frontal lobe is involved with reasoning, planning, regulating emotions, forming personality, and initiating voluntary movements.

A small part of the left frontal lobe, called Brocca’s area, is responsible for converting thoughts into words.

The parietal lobe manages sensory perceptions, gives meaning to visual and auditory signals by associating them with past memories, and allows for the understanding of written and spoken language.

The temporal lobe is responsible for distinguishing between sounds, interpreting and understanding spoken language, and forming and recalling memories.

While the left temporal lobe is extensively involved with verbal memory, the right temporal lobe is more involved with visual memory.

The occipital lobe, also known as the visual cortex, is responsible for visually recognizing and identifying objects, and analyzing visual information.

The insular lobe is believed to be partly responsible for emotion, states of consciousness, and regulation of the body’s homeostatic states.

The insular lobe has functions as diverse as assisting with motor control, perception, interpersonal experience, cognitive functioning, and self-awareness.

The thalamus functions as a relay station for all information entering and leaving the brain.

The thalamus passes on sensory information to the cortex, determines which pieces of information should reach consciousness, and exchanges motor information from the cortex to lower parts of the brain.

The hypothalamus is responsible for many of the body’s important homeostatic functions such as body temperature, hunger, thirst, and circadian rhythms.

The hypothalamus also connects the nervous system with the endocrine system via the pituitary gland, which sits directly below the hypothalamus.

The cortex contains all the external lobes of the brain, as well as the insular lobe, and therefore is responsible for all of their functions, including sensory and motor control, reasoning, planning, personality, language, memory, emotions, and consciousness.

The basal ganglia are responsible for controlling sensations, and initiating and regulating movements.

The amygdala is involved with learning, memory, and the regulation of emotions, especially fear and aggressive reactions.

The hippocampus is the relay station for memories, assisting in encoding information for long-term memory storage, and retrieving these memories for recall.

The hippocampus is connected to all parts of the cortex, enabling memories to maintain contextual relevance involving more than one sense.

1. The medulla oblongata

2. The pons

3. The midbrain

The medulla oblongata regulates basics life functions, such as heart rate, blood pressure, and respiratory rate, as well as controls reflexes such as swallowing, sneezing, and vomiting.

The pons is the major relay station between the forebrain and the cerebellum, and controls body functions, including respiration and sleep.

The reticular formation is a structure of the pons that is responsible for regulating the sleep-wake cycle, attentiveness, and concentration.

The reticular formation serves as a filter for incoming stimuli in which it removes irrelevent background stimuli, allowing for better concentration in the midst of distractions.

The midbrain is responsible for many different functions including motor control, vision, hearing, temperature regulation, and alertness.

The spinal cord functions to relay sensory information from the body up to the brain, and transmit motor information from the brain to the muscles of the body.

The spinal cord also contains reflex pathways that are independent of the brain, such as the knee-jerk reflex.

The cerebellum is responsible for balance, muscle tone, and posture, as well as the ability to make muscle movements smooth and precise.

The corpus callosum is the central connection point between the two hemispheres of the brain and facilitates inter-hemispheric communication.

The cingulate gyrus deals with the emotional reactions that influence the functions of the autonomic nervous system, including adjusting blood pressure, heart rate, and respiratory rate.

The mammillary bodies are involved with recognition memory, or the memory of events, people, and objects. They also seem to play a role in associating smells with memory.

The limbic system is involved in learning, memory, emotions, and olfaction.

The limbic system is composed of many structures including, but not limited to, the amygdala, the hippocampus, the cingulate gyrus, and the mammillary bodies.

The meninges are three thin, yet strong, membranes that surround and protect the brain.

The three layers of meninges, from closest to the brain to farthest, respectively, are the pia mater, the arachnoid mater, and the dura mater.

The cerebrospinal fluid (CSF) is a circulating fluid that cushions and protects the brain. The CSF circulates through the ventricles and between layers of meninges, allowing the brain to "float" and reduce the pressure on the base of the brain.

CSF is made by the choroid plexus of the ventricles and is released at the base of the brain.