Nervous System and Anatomy

Anatomy and Physiology

• The nervous system consists of two main parts: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS)
• The CNS is made up of the brain and spinal cord, which are protected by the skull and vertebral column, respectively
• The PNS is made up of all nerves that are not in the brain or spinal cord, including cranial nerves and spinal nerves
• The PNS can be divided into two divisions: sensory (afferent) and motor (efferent)

Nervous Tissue

• Nervous tissue is responsible for communication between cells of the body by forming a system of electrical impulses
• Yellow indicates nervous tissue

Central Nervous System

• The CNS is made up of the brain and spinal cord
• The CNS sends and receives impulses from the peripheral nervous system
• Nuclei are collections of cell bodies inside the CNS
• Tracts are collections of nerve axons inside the CNS

Peripheral Nervous System

• The PNS is made up of all nerves that are not in the brain or spinal cord
• The PNS can be divided into two divisions: sensory (afferent) and motor (efferent)
• Sensory neurons receive information from sensory organs throughout the body and send it to the CNS
• Motor neurons relay signals from the CNS to muscles and glands

Neurons

• Neurons are nerve cells that conduct electrical impulses and relay information throughout the body
• Neurons are made up of three basic parts: dendrite, cell body, and axon
• Dendrites receive information from other neurons and conduct impulses to the cell body
• Cell bodies synthesize all nerve cell products and receive information from dendrites
• Axons conduct nerve impulses away from the cell body and terminate in axon terminals
• Axon terminals store neurotransmitters inside secretory vesicles

Neurotransmitters

• Neurotransmitters are chemical messengers that communicate between neurons across a synapse
• Neurotransmitters are released from axon terminals and bind to receptors on adjacent neurons

Neuroglial Cells

• Neuroglial cells are support cells for neurons
• Schwann cells are a type of neuroglial cell that provides insulation for axons in the PNS
• Oligodendrocytes provide insulation for axons in the CNS
• Astrocytes control the chemical environment of neurons and wrap around blood capillaries
• Microglial cells protect the CNS and engage in phagocytosis

Action Potentials

• Resting potential is the state of a neuron at rest, with a negative charge of around -70mV
• Action potential is a rapid change in polarity across the membrane, resulting in depolarization and repolarization
• Sodium and potassium gates are special protein-lined channels with gates in the membrane that allow sodium or potassium to pass through
• Depolarization occurs when the membrane potential becomes more positive, and repolarization occurs when the potential returns to normal

Chemical Transmission of an Action Potential

• Synapse is a minute fluid-filled space between the axon terminal of the presynaptic neuron and dendrite of postsynaptic neuron
• Neurotransmitters are released from axon terminals and bind to receptors on adjacent neurons
• Chemical transmission occurs when the vesicles filled with neurotransmitters fuse with the plasma membrane and release the neurotransmitters

Reflexes

• Reflexes are instantaneous, automatic, and involuntary motor responses within the nervous system
• Reflexes start from stimuli occurring inside or outside the body and always involve the CNS
• Examples of reflexes include subconscious reflexes, such as regulation of blood sugar, and conscious reflexes, such as withdrawing a hand from a hot object
• Reflex arc is the neural pathway that a nerve impulse travels
• Spinal cord anatomy includes dorsal roots, ventral roots, dorsal root ganglion, gray matter, and white matter
• Reflexes provide immediate responses to dangerous stimuli and are faster than conscious responses### Reflexes
• Patellar reflex, also known as knee-jerk reflex, prevents overstretching of quadriceps femoris
• Flexor withdrawal reflex allows removal of hand from hot or dangerous objects, protecting skin and underlying tissue

Anatomy and Physiology of the Nervous System

• Anatomy: structure of the body
• Physiology: how body parts function together
• Nervous system: integrates and controls other body systems
• 2 parts: central nervous system (CNS) and peripheral nervous system (PNS)

Central Nervous System

• Composed of brain and spinal cord
• Protected by skull and spinal vertebrae
• Collections of cell bodies: nuclei
• Collections of nerve axons: tracts

Peripheral Nervous System

• Includes cranial and spinal nerves that project from CNS
• Collections of cell bodies: ganglia
• Collections of nerve axons: nerves
• 2 divisions: motor (efferent) and sensory (afferent)
• Motor division: sends signals to muscles and glands
• Sensory division: receives impulses from sensory organs

Divisions of the Peripheral Nervous System

• Motor division: further divided into somatic and autonomic
• Somatic nerves: under voluntary control, found in skeletal muscles, skin, and joints
• Autonomic nerves: not under conscious control, found in glands and smooth muscles of organs
• Autonomic system: divided into sympathetic and parasympathetic systems
• Sympathetic system: activates and prepares body for activity, stress, and emergency situations
• Parasympathetic system: operates during normal situations, permits digestion and conserves energy

Neurons

• Composed of: axon, cell body, dendrites, and axon terminals
• 3 unusual characteristics: do not undergo mitosis, require enormous amounts of energy, and can only survive minutes without oxygen
• Cell body: receives signal, processes it, and manufactures neurotransmitters
• Dendrites: receive information from other neurons and send signals to cell body
• Axon: carries signal from cell body to axon terminals
• Axon terminals: store signals in secretory vesicles, released during electrochemical transmission in the synapse

Synapse

• Liquid-filled space between axon terminal of presynaptic neuron and dendrites of postsynaptic neuron
• Postsynaptic neuron: receives signals from presynaptic neuron
• Neurotransmitters: synthesized in cell body, stored in axon terminals, and released into synapse

Types of Neurons

• Multipolar neurons: 3 or more extensions from cell body, 1 axon, and many dendrites
• Bipolar neurons: central cell body with 2 extensions
• Unipolar neurons: 1 extension off cell body, branches into 2 processes
• Sensory neurons: unipolar, carry information from peripheral to central nervous system
• Interneurons: also known as association neurons
• Motor neurons: send messages from central nervous system to peripheral

Neuroglial Cells

• Support cells for neurons
• 4 types in CNS: ependymal cells, oligodendrocytes, astrocytes, and microglial cells
• PNS: Schwann cells and satellite cells

Action Potentials

• Technical term: nerve impulse
• Caused by movement of unequally distributed ions on either side of plasma membrane
• Resting potential: -70mV, maintained by pumping 3 sodium ions out and 2 potassium ions in
• 4 steps: resting state, depolarization, repolarization, and afterpolarization

Intensity of Sensation

• Determined by number of neurons and/or frequency of stimulation

Electrochemical Nature of Neuron Signals

• Impulse uses both chemical and electrical measures to transmit signal across synapse### Spinal Cord Structure and Function

• Sensory neurons synapse on cells in the posterior horn of the spinal cord.

• Motor neuron cell bodies are in the anterior horn of the spinal cord.

• Motor neurons (axons) leave the spinal cord via the ventral roots.

Reflex Arc

• A reflex arc consists of five components:
  • The receptor at the end of a sensory neuron reacts to a stimulus.
  • The sensory neuron conducts nerve impulses along an afferent pathway towards the CNS.
  • The integration center consists of one or more synapses in the CNS.
  • A motor (efferent) neuron conducts a nerve impulse along an efferent pathway from the integration center to an effector.
  • An effector responds to the efferent impulses by contracting (a muscle) or secreting a product (a gland).

Stretch Reflex

• The stretch reflex utilizes muscle spindles to detect over-stretch.
• The purpose of the stretch reflex is to protect the muscle against increases in length that may tear or damage muscle fibers.

Spinal Reflexes

• Spinal reflexes are faster because they require fewer neurons and the signal does not have to travel all the way to the brain and back.
• The signal is sent to the spinal cord and back, which is a shorter distance to travel.