Introduction to the Nervous System

The Nervous System

• Responsible for maintaining homeostasis.
• Transmits information rapidly.
• Controls voluntary movement, involuntary movement, perception, behavior, and memory.
• Composed of neural tissue, which includes neurons and neuroglia.

Flow of Information

• Sensory information goes to the brain and spinal cord using cranial and spinal nerves (input).
• Sensory information is integrated by analyzing and storing it (processing).
• Information from brain and spinal cord goes to effectors, like muscles/glands (output).

Overview of the Nervous System

• The central nervous system (CNS) consists of the brain and spinal cord.
• The peripheral nervous system consists of cranial and spinal nerves.
• The somatic nervous system controls skeletal muscles.
• The autonomic nervous system controls smooth muscle, cardiac muscle, and glands
• The autonomic nervous system consists of sympathetic & parasympathetic divisions.

The Brain

• The brain is the control center of the body
• It is the center of intelligence, memory, behavior, and emotion
• Coordinates skeletal muscle activity
• Influences activity of visceral organs and glands.
• Composed of 86 billion neurons and even more neuroglia.
• Acts as processing center because interpretation of sensory information depends on path traveled and destination in the brain.
• Anatomical changes happens within the brain when developing new skills.
• Plasticity of the brain develops new synapses, and a pattern of neuronal activity.

Nervous Tissue

• Nervous tissue makes up the nervous system.
• There are two cell types: neurons and neuroglia.
• Neurons are excitable cells.
• Neuroglia support, protect, and nourish neurons.

Neurons

• Neurons are excitable cells
• The cell body and dendrites receive information.
• The axon sends information out
• Axon transmits information to other cells at synapses.
• There are ~200 billion neurons in an adult human body.

Functional Classification of Neurons

• Sensory function (afferent neurons) sends information to the brain and spinal cord via cranial and spinal nerves.
• Integrative function (interneurons) integrates sensory information by analyzing and storing it.
• Motor function (efferent neurons) sends information from the brain and spinal cord to muscles or glands (effectors).

Cranial Meninges

• Connective tissue coverings surround the brain.
• They protect underlying neural tissues
• Also anchors the brain in the cranial cavity

Meninge Layers

• Dura mater has a subdural space.
• Arachnoid mater has a subarachnoid space.
• Pia mater is the third layer.

Dura Mater

• Dura Mater is most superficial layer is fused with periosteum of skull.
• Not fused around the spinal cord.
• Inner layer folds into parts of cranial cavity forming sinuses
• Major dural folds include: falx cerebri, falx cerebelli, tentorium cerebelli.

Arachnoid and Pia Mater

• Arachnoid mater contains spaces for blood vessels.
• The subarachnoid space is filled with cerebrospinal fluid (CSF).
• Pia mater is a thin protective layer of tissue
• Directly covers nerves and neuroglia.

Cerebrospinal Fluid (CSF)

• Filtrate of plasma.
• Contains glucose, oxygen, Na+, Cl-, Mg2+, and other ions
• Functions include cushioning, shock absorption, chemical protection, and exchange of nutrients/wastes.

Ventricles of the Brain

• Fluid-filled chambers within the brain
• Location of CSF formation
• Numbered according to CSF flow
• CSF empties from the 4th ventricle empties it into the central canal of the spinal cord or into the subarachnoid space.

CSF Formation

• CSF is formed through filtration from blood to CSF
• This happens through the choroid plexus.

Recycling CSF

• CSF recycles through arachnoid granulations, collection of arachnoid villi
• CSF is absorbed back into the circulatory system as fast as it is produced, about 500 ml/day.

Hydrocephalus

• Hydrocephalus is the buildup of CSF, or cerebrospinal fluid, in the ventricles.
• Excess fluid increases the size of the ventricles, leads to pressure on surrounding brain tissues.

Blood Brain Barrier (BBB)

• The BBB (blood brain barrier) maintains a stable environment for the brain.
• Protects brain from foreign substances and hormones/neurotransmitters in the blood.

BBB Structure

• Capillaries of the blood brain barrier are the least permeable capillaries
• Astrocytes extend foot processes around capillary endothelial cells
• Endothelial cells make tight junctions between each other

BBB Permeability

• O2, CO2, and small lipid-soluble molecules cross BBB.
• Large lipid-soluble molecules (fatty acids) move slowly.
• Negatively charged molecules do not pass BBB easily.
• Glucose and amino acids are transported across the BBB.