Introduction to the Nervous System

Why Study the Nervous System?

• The nervous system controls seeing, hearing, smelling, tasting, feeling, thinking, remembering, and doing.
• Understanding the nervous system is important for allied health professionals before studying other body systems.
• A good understanding of the nervous system is required to understand physical/psychological disorders that arise when it fails to function properly.

Functions of the Nervous System

• The nervous system maintains homeostasis via communication by performing three main functions.
• Receptors detect sensory input then send it to the control center during the sensory function.
• The integrative function involves analyzing and interpreting sensory input to then determine appropriate responses.
• The integrative function generates a motor output that causes a response.
• The motor function issues a motor output to activate an effector.

Sensory Input

• General and special sensory receptors detect sensory input.
• General sensory receptors are located in the skin, skeletal muscles, tendons, joints, and visceral organs.
• Thermoreceptors detect changes in temperature.
• Nociceptors detect painful stimuli.
• Mechanoreceptors include tactile receptors, baroreceptors, and proprioceptors.
• Tactile receptors detect touch, pressure, and vibration stimuli.
• Baroreceptors detect changes in blood pressure.
• Proprioceptors detect changes in body position, also known as proprioception.
• Special sensory receptors are located in the eyes, ears, mouth, and nose.
• Photoreceptors detect light, enabling vision.
• Chemoreceptors detect chemicals in solution, facilitating taste and smell.
• Mechanoreceptors called hair cells enable the detection of hearing and balance stimuli.

Motor Output

• Motor output activates a specific muscle to contract or a gland to secrete, causing a response.

Divisions of the Nervous System

• The nervous system has two major anatomical divisions which include the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).
• The CNS consists of the brain and spinal cord, performing the function of integration.
• Key functions of the CNS include controlling emotions, behaviors, personality, intellectual (cognitive) functions and memory storage.
• The PNS consists of sensory receptors, cranial nerves, spinal nerves, and peripheral nerves.
• The PNS links all parts of the body to the CNS.
• Cranial nerves and their branches primarily innervate structures of the head and neck.
• Spinal nerves branch to form the peripheral nerves that innervate all parts of the body below the head.
• Sensory and Motor Divisions are two functional divisions of the nervous system.
• The Sensory Division (afferent division) conveys sensory input from receptors to the CNS.
• The Motor Division (efferent division) conveys motor output from the CNS to a muscle or gland.
• The motor division has two functional systems, the Autonomic Nervous System, and the Somatic Nervous System.
• The Somatic Nervous System conveys "somatic" motor output from the CNS to the body's skeletal muscles.
• Somatic motor output controls voluntary and involuntary (somatic reflexes) skeletal muscle movements.
• The Autonomic Nervous System conveys "autonomic" motor output from the CNS to the body's glands, cardiac, and smooth muscles.
• Autonomic motor output controls involuntary (automatic) activities, e.g. heart rate, respiration, blood vessel and pupil diameter, digestion of food, urination & defecation, perspiration & salivation.
  • The Two functional divisions of the Autonomic Nervous System are Sympathetic and Parasympathetic.
• The sympathetic division controls "fight-or-flight" activities by activating body functions that support physical activity and inhibits those that do not.
• The sympathetic division increases heart rate, respiratory airflow, blood flow to skeletal muscles and sweat gland activity.
• It also dilates pupils, inhibits digestive functions, inhibits urination, and inhibits defecation.
• The parasympathetic division controls "rest and digest" activities by activating body functions that conserve and restore body energy.
• The parasympathetic nervous system stimulates digestive functions, urination & defecation, constricts pupils, decreases heart rate, and decreases respiratory airflow.

Components of the Nervous System

• Neural tissue consists mostly of two cell types, neuroglia and neurons (nerve cells).
• Neuroglia ("nerve glue") support neuron development and function.
• The six different types of neuroglia cells collectively nourish, protect, insulate, and structurally support neurons.
• Neurons are specialized cells that perform the function of communication.
• When stimulated, neurons generate electrical signals called graded potentials and action potentials.
• These electrical signals conduct sensory and motor information from one part of the body to another part.
• Neurons require oxygen and glucose for survival.
• Neurons are unable to divide and replace themselves if destroyed.
• Neurons vary in size, shape, and length, but share four common parts which are dendrites, cell body, axon (fiber), and axon terminals.
• Dendrites are short processes that are the main receptive (or input) region of a neuron.
• Dendrites act as sensory receptors to detect stimuli and receive information from other neurons.
• Dendrites convert received information into a graded potential, which conveys the information towards the cell body.
• The cell body contains a nucleus and organelles, e.g. ribosomes, to synthesize chemical neurotransmitters.
• The cell body receives information from other neurons and converts this information into a graded potential.
• The cell body integrates information (graded potentials) and conveys information towards the initial segment (or first part) of the axon.
• The axon (a.k.a. fiber) is a single process that connects to the cell body at the axon hillock.
• The axon is the conducting region of a neuron that generates and conducts action potentials.
• Action potentials convey information from the initial segment to the axon terminals.
• The axon is covered with a segmented myelin sheath produced by Schwann cells and oligodendrocytes.
• Myelin increases the speed of signal conduction, and segments are separated by gaps called nodes of Ranvier known as (internodes).
• Destruction of myelin (oligodendrocytes) in the CNS causes multiple sclerosis.
• Axon terminals form a synapse with another cell, i.e., a neuron, muscle, or gland.
• Axon terminals are the secretory region of a neuron, containing synaptic vesicles that store and release neurotransmitters.
• Neurotransmitters are chemicals that carry the information from one neuron to another or to a muscle cell or gland.
• In the CNS and PNS, neuron parts are usually grouped together.
• Neuron cell bodies are organized into clusters called nuclei (nucleus) in the CNS and ganglia (ganglion) in the PNS.
• Neuron axons are bundled together to form tracts in the CNS and nerves in the PNS.

Classification of Neurons

• Neurons are classified according to their structure and function, using structural and functional classifications.
• Multipolar neurons are common in the CNS & PNS.
• Bipolar neurons are rare and found in special sense organs.
• Unipolar neurons are located in the PNS.
• Sensory neurons conduct sensory input from receptors to the CNS and are unipolar in structure.
• Interneurons conduct information within the CNS and are multipolar in structure.
• Motor neurons conduct motor output away from the CNS to a muscle or gland and are multipolar in structure.
• Lower motor neurons conduct somatic motor output while preganglionic & postganglionic neurons conduct autonomic motor output

Electrical Signals

• For neurons to generate an electrical signal, their plasma membrane must exhibit a resting membrane potential of -70mV.
• Neurons plasma membranes must also contain protein channels which allow specific ions to diffuse down their concentration gradient.
• Ions flow across the plasma membrane of a neuron which changes the membrane potential (voltage) generating electrical signals.
• The two main types of ion channels are leakage channels and gated channels.
• Leakage channels are always open while gated channels open and close in response to a specific stimulus.
• A larger number of ions move across the plasma membrane when a stimulus opens gated ion-channels.
• Gated channels include chemically-gated channels, mechanically-gated channels and voltage-gated channels.
• Chemically-gated channels open in response to a chemical stimulus, e.g. neurotransmitters, and are located along the plasma membrane of the dendrites & cell body.
• Mechanically-gated channels open in response to mechanical stimulation, e.g. touch, vibration, and pressure.
• Mechanically-gated channels are located along the plasma membrane of the dendrites.
• Voltage-gated channels open and close in response to voltage changes, (i.e. changes in membrane potential).
• Voltage-gated channels are located along the plasma membrane of the axon and axon terminals.

Gated Channels & Membrane Potential

• Stimuli open gated channels which allows Na+ or K+ ions to flow across the plasma membrane.
• Ion flow across the plasma membrane leads to membrane potential changes to generate electrical signals.

Depolarisation & Hyperpolarisation

• Changes in membrane potential (voltage) occur when a stimulus opens Na+ or K+ gated channels which are relative to the resting membrane potential of -70 mV.
• These changes are described by the terms depolarization and hyperpolarization.
• Depolarization occurs when the membrane potential becomes less negative, often caused by an influx of Na+ ions into the intracellular fluid (ICF).
• The influx of Na+ ions increases the concentration of positive ions within the cell's interior which leads to the membrane potential becoming less negative.
• Hyperpolarization occurs when the membrane potential becomes more negative, which often happens when K+ gated channels open which allows efflux of K+ ions out of the ICF.
• The efflux of K+ ions decreases the concentration of positive ions in the cell resulting in a membrane potential that is said to be more negative.

Types of Electrical Signals

• The nervous system generates two types of electrical signals: graded potentials and action potentials (nerve impulse).
• Graded potentials are small changes in the membrane potential (i.e., a small depolarization or hyperpolarization) that originate in the dendrites or cell body of a neuron when a stimulus opens chemically-gated or mechanically-gated channels.
• Graded potentials are short-distance signals in which the distance traveled is proportional to stimulus strength, and a stronger stimulus results in a bigger change in membrane potential, leading to a further signal travel.
• Graded potentials in response to a stimulus can initiate an action potential by traveling to the initial segment of an axon.
• When a graded potential depolarizes the initial segment to -55 mV (threshold) and stimulates voltage-gated Na+ channels to open it generates an AP.
• Action potentials (AP) are long distance signals that originate at the initial segment of an axon and involve voltage-gated channels.
• Action potentials are self-propagating and the process involves the first AP triggering the second AP in the adjacent region of the membrane.
• This will result in the 2nd AP triggering the 3rd AP, etc propagating down to the axon terminals.
• Action potentials are generated by three consecutive changes in membrane potential.
• Changes in membrane potential are always the same regardless of stimulus strength.

The processes of Action Potentials

• Depolarisation, the membrane potential becomes less negative,
• Repolarisation, the membrane returns towards resting membrane potential
• Hyperpolarisation, the membrane potential becomes more negative than
• membrane potential briefly dips below the resting level.
• At threshold (-55 mV), voltage-gated Na+ channels open, allowing Na+ ions to enter the cell.
• The membrane potential inside the cell becomes less negative which shifts from -55 mV to +30 mV.
• At +30 mV, voltage-gated Na+ channels close and voltage-gated K+ channels open, allowing K+ ions to leave the intracellular fluid (ICF).
• Membrane potential returns towards resting state, which shifts from +30 mV to -70 mV.
• As the membrane potential approaches -70 mV, voltage-gated K+ channels close slowly, but excess K+ ions still leave the intracellular fluid creating a more negative shift.
• Membrane potential becomes more negative than the resting, shifting from -70 mV to -90 mV.
• All voltage-gated channels are now closed, and the resting membrane potential of -70 mV is restored.

Graded Potentials vs Action Potentials

• Graded potentials originate in dendrites or the cell body; action potentials originate in the axon's initial segment.
• Graded potentials involve Chemically-gated or Mechanically-gated channels; action Potentials involve Voltage-gated channels.
• Graded potentials have a small depolarisation or a small hyperpolarisation and has an change in membrane potential that is proportional to the strength of the stimulus.
• Action potentials involve the processes that contribute to generating an signal which include:
  • Depolarisation
  • Repolarisation
  • Hyperpolarisation
• There is a change in membrane potential of Action Potentials that is independent of the stimulus strength. Graded potentials travel short distances while action potentials travel long distances (self-propagating).

Propagation of Action Potentials

• Continuous conduction occurs in unmyelinated axons, where action potentials are generated at the voltage-gated channels along the length of the axon.
• Continuous conduction speeds are ≤ 2 m/s.
• Saltatory Conduction occurs in myelinated axons, where action potentials are generated at the Nodes of Ranvier, yielding conduction speeds of >100 m/s.
• Local anaesthetics block voltage-gated Na+ channels, resulting in no action potentials, which prevents the conduction of pain signals to the brain, and leads to no sensation of pain.
• Cold and pressure reduce pain sensations by impairing signal conduction.

Chemical Synapse

• A chemical synapse is a junction that mediates the transfer of information and will be present at a chemical synapse between two neurons.
• The neuron sending the information is the presynaptic neuron.
• The neuron receiving the information is the postsynaptic neuron.
• Presynaptic and postsynaptic membranes are separated by the synaptic cleft.
• Signal transmission involves chemical neurotransmitters.

Information Transfer

• An action potential arrives at and depolarises the axon terminal in a presynaptic neuron
• A voltage- gated calcium channels then open
• Influx of calcium triggers synaptic vesicles to release neurotransmitter into the synaptic cleft.
• The released synaptic neurotransmitters will bind to chemically-gated channels on the postsynaptic neuron's dendrites or cell body, and these actions will have a variety of effect on neuron electrical propeties.
• This will cause the chemically-gated ion channels to open triggering subsequent actions such as Na+ ions entering ICF plasma membrane of postsynaptic resulting in neuron depolarisation.
• All of the previously mentioned actions generate an excitatory postsynaptic potential (EPSP) that depolarises the initial segment of postsynaptic and then triggers action potential generation to get information sucessfully transmitted.
• In summary a postsynaptic neuron’ s neurotransmitter diffuses away from synaptic enzymatic cleft and is degraded by enzymes that destroy it or its re-enters neuron.

Termination of Synaptic Transmission

• The neurotransmitter diffuses away from the synaptic cleft.
• The neurotransmitter is degraded by enzymes present in the synaptic cleft.
• The neurotransmitter re-enters the axon terminal and destroyed by enzymes or reused through a process is known as reuptake.