Nervous System: Introduction and Functions

• The nervous system, essentially the body's electrical wiring, comprises nerves and neurons that transmit signals between body parts.
• Vertebrates have central and peripheral nervous systems.

Introduction to the Nervous System

• The nervous system facilitates communication and reaction to the environment, overseeing life activities.
• It functions as a master controller and communicator, is a physically connected network of cells, tissues, and organs.
• It has two main divisions being the Central Nervous System (CNS) and Peripheral Nervous System (PNS).

Primary Functions

• The four primary functions of the nervous system are sensing the world, transmitting information, processing information, and producing a response

Organization of the Nervous System

• The Central Nervous System (CNS) includes the brain and spinal cord.
• The CNS integrates, controls, receives interprets and sends signals to the PNS
• The Peripheral Nervous System (PNS) consists of nerves (31 pairs of spinal nerves, 12 pairs of cranial nerves)
• The PNS provides communication lines between the CNS and the rest of the body.
• There are Two Divisions of the PNS called the Sensory (afferent) Division and Motor (efferent) Division.
• The Sensory (afferent) Division contains sensory receptors which communicate with the CNS.
• The Motor (efferent) Division contains the CNS which communicates with effectors (muscles & glands).

Motor Division (of PNS)

• The Somatic nervous system (voluntary) controls skeletal muscles (1 motor neuron)
• The Autonomic nervous system (ANS) (involuntary) regulates smooth muscles, cardiac muscles, and glands (2-neuron system)
• The autonomic nervous system (ANS) has subdivisions called sympathetic and parasympathetic.
• Sympathetic division prepares the body for stressful or energetic the activity; "fight or flight"
• Parasympathetic division dominates during times of "rest and rumination"; directs maintenance activities

The Complex Brain

• The mammalian brain is complex, containing regions with specific functions.
• The brain is divided into the hindbrain, midbrain, and forebrain.

Hindbrain

• The medulla controls autonomic functions.
• The pons control sleep stages.
• The cerebellum coordinates movement, stores some motor memory, helps maintain posture, muscle control, and balance.

Midbrain

• The midbrain is located between the hindbrain and forebrain.
• All sensory and motor information that travels between the forebrain and the spinal cord passes through the midbrain.
• The midbrain serves as a relay station for the central nervous system.
• The midbrain functions as the "traffic cops" of the brain.
• The midbrain filters sensory input, allowing us to concentrate; this filtering can be affected by higher thoughts.

Forebrain

• The thalamus is a relay station channeling sensory information.
• The limbic system is involved in basic emotions, drives, and behaviors.
• the cortex controls higher thought.

Limbic System

• The limbic System "controls: emotions and memories”
• The hypothalamus is the master controller of the endocrine system
• The amygdala controls sensations of pleasure or fear, recognition of fear in others.
• The hippocampus is responsible for the formation of memories.
• Damage to these areas can lead to amnesia or emotional disturbances

Cortex

• The cortex has various areas that control sensory processing, motor control, thought, and memory.
• The cortex is the top layer of the brain and is critical for storing experiences and learning.

Spinal Cord

• The grey matter is mostly made up of cell bodies of neuron
• The white matter is composed of nerve fibers (ascending and descending tracts) embedded in neuroglial cells

Bundles of Nerve Fibers

• Ganglia are collections of cell bodies.
• Bundles of nerve fibers are called tracts (CNS) or nerves (PNS).
• White matter consists of dense collections of myelinated fibers.
• Gray matter consists of unmyelinated fibers & cell bodies including nerve cell bodies, dendrites, axon terminals, bundles of unmyelinated axons and neuroglia (gray color).

Nervous Tissue

• Neurons (nerve cells) serve as the functional unit of the nervous system and functions to transmit messages.

Anatomy of a Neuron

• Cell body - contains nucleus; metabolic center
• Dendrite - fiber that conveys messages toward cell body
• Axon - conducts nerve impulses away from the cell body
• Axon terminals – end of axon; contain neurotransmitters & release them
• Synaptic cleft/synapse - gap between neurons

Supporting Cells (Neuroglia) in the CNS

• Astrocytes, microglia, ependymal cells, oligodendrocytes serve several functions, including to barrier between capillaries and neurons and protecting neurons.
• Other functions include immune/defense, lining the brain and spinal cord cavities for wrapping of nerve fibers, and producing myelin sheaths (covering)

PNS Supporting Cells

• Schwann cells and satellite cells surround large neurons and protect & cushion.

Myelin

• Myelin is a whitish, fatty material that covers nerve fibers to speed up nerve impulses.
• Schwann cells surround axons and form myelin sheath.
• Myelin sheath is a tight coil of wrapped membranes.
• Nodes of Ranvier are gaps between schwann cells.

The Neuron Function

• Dendrites receive signals from other neurons.
• The cell body integrates signals and coordinates metabolic activities.
• The axon transmits the action potential.
• Myelin insulates the axon and speeds conduction.
• Synaptic terminals transmit signals to other neurons.

Neuron Functions

• Irritability is the ability to respond to a stimulus & convert to a nerve impulse.
• Conductivity is the ability to transmit impulse to other neurons, muscles, or glands.

Classification of Neurons

• Neurons are classified based on functional and structural characteristics.

Functional Classes of Neurons

• Sensory neurons carry impulses from sensory receptors to CNS
• Motor neurons carry impulses from CNS to muscles & glands
• Interneurons connect sensory & motor neurons

Structural Classification

• Multipolar: 1 axon, several dendrites (Most common (99%))
• Bipolar: 1 axon, 1 dendrite (Rare)
• Unipolar: 1 process (Short with 2 branches (sensory, CNS))

Reflexes

• A reflex is an action that is performed without conscious thought as a response to a stimulus.
• Reflexes are rapid, predictable, involuntary responses to stimuli.
• Somatic Reflexes stimulate skeletal muscles i.e. jerking away hand from hot object.
• Autonomic Reflexes: regulate smooth muscles, heart, glands i.e. salivation, digestion, blood pressure, sweating.
• The reflex arc is the nerve pathway involved in a reflex action, including a sensory and motor nerve with a synapse between them.

Voluntary Reactions

• More neurons and synapses are involved, resulting in longer response times.