The nervous system C3.1. 4 - C3.1.10

• What is the brain?

  The brain is the central integrating organ in the body. It receives information, processes it, stores it and sends instructions to all parts of the body to coordinate life processes. The information received by the brain comes from sensory receptors, both in specialised sense organs such as the eye and also from receptor cells in other organs (e.g. pressure receptors in blood vessels).

  The brain's capacity to store information is called memory, which is essential for learning. Processing of information is decision-making, which results in signals sent to the muscles or glands, causing them to carry out a response.

  It consists of two classes of cells: neurons (transmit electrical impulses) and glial cells (support the structure and function of neurons).
• What are some examples of different forms of brains?

  Jellyfish have a nerve net

  Flatworms have a primitive brain with nerve chords

  Earthworms have a primitive brain and ganglia along a nerve chord

  Fish have a small forebrain

  Geese have a large forebrain

  Humans have a very large forebrain, dominating the brain.
• What is the function of the brain?

  The brain centralises control of the body, allowing for a rapid and coordinated response to multiple inputs about changing body conditions. These inputs come from sensory receptors in other organs.

  Some basic responses, like reflexes, can be mediated by the spinal chord. However, control of complex behaviour, like learning and memory, requires the information integration abilities of a centralised brain.
• What is the CNS?

  The nervous system is made up of the CNS (Central nervous system) and nerves that connect the CNS to other organs.

  The CNS consists of the brain and the spinal chord.
• What is the spinal chord?

  The spinal chord is located in the vertebral column, and is widest at its junction with the brain, tapering as it goes down towards the pelvis. Spinal nerves branch from it, with humans having 31 pairs, each serving a different body region.

  It connects the brain to the rest of the body, and is essentially a column of nerves.

  It consists of two main tissues:

  White matter

  Grey matter
• What is white matter?

  Contains myelinated axons and other nerve fibres

  Conveys signals from sensory receptors to the brain and organs
• What is grey matter?

  Contains the cell bodies of motor neurons and interneurons

  Has many synapses between its neurons
• What are synapses?


  Synapses are used to process information and to make decisions, so the grey matter of the spinal chord means that the spinal chord is an integrating centre (like the brain).
• What is the function of the spinal chord?

  The spinal chord only coordinates unconscious process, especially reflexes, and can do this faster than if signals were conveyed to and from the brain.
• What are unconscious actions?

  Performed when awake or asleep

  Performed involuntarily

  Coordinated by brain and spinal chord

  e.g. secretion from glands and contraction of smooth muscle

  e.g. swallowing food after it has entered the oesophagus and vomiting when stomach contents are regurgitated
• What are conscious actions?

  Performed when awake

  Performed voluntarily

  Coordinated by cerebral hemispheres of the brain

  e.g. contraction of striated muscle

  e.g. initiation of swallowing when food is pushed from the mouth cavity to the pharynx.
• What are non-binary actions?

  Many actions are non-binary, meaning that one choses (consciously) to carry it out, but its processing is unconscious.

  One example is striated muscle control - standing up is conscious, while postural reflexes are unconscious.
• What is the nervous system?

  The nervous system is a complex network that controls and coordinates the body's actions and sensory information. It consists of the CNS and the peripheral nervous system:

  CNS- The brain and the spinal chord

  Peripheral nervous system- The nerves outside of the brain and spinal chord
• What are sensory neurons?

  Sensory neurons carry information towards the CNS. They consist of:

  Cell body - contains nucleus and other organelles

  Dendrites- Finger like projections that act as the primary site for receiving sensory information from the environment. These may have specialised receptors to detect specific stimuli

  Schwann cells- Glial cells that wrap around the axon and form the myelin sheath

  Myelin sheath- Layers of lipid that act as an insulator, allowing for faster signal transmission along the axon through a process called saltatory conduction

  Axon terminal - The part of the neuron that releases neurotransmitters into the synapse to transmit signals to other neurons or cells

  Axon - A long, slender extension that carries the electrical signal. Most sensory neurons have a single axon that branches into two parts, one extending to the periphery and the other to the spinal chord.
• Where are sensory neurons located, and what receptors do they have?

  They are located in the skin and sense organs, and contain receptors stimulated by Light (photoreceptors), chemicals (chemoreceptors), physical forces (mechanoreceptors) and temperature (thermoreceptors).
• What is the function of sensory neurons?

  Sensory neurons respond to stimuli about environmental conditions and transmit that information as electrical impulses to the CNS for processing and interpretation.

  The spinal chord receives sensory information about Pian, temperature, touch and body position.

  The brain receives sensory input from the eyes, ears nose and tongue, so interprets sensory information of the five senses. There are specialised areas of the brain cerebral hemispheres that receive and process different sensory information.
• What is the cerebrum?

  The cerebrum is the part of the brain that controls muscle functions and also controls speech, thought, emotions, reading, writing and learning.

  The right cerebral hemisphere controls the muscles on the left hand side of the body, while the left cerebral hemisphere controls the muscles on the right hand side of the body.
• What is the primary motor cortex?

  The primary motor cortex is a region of the cerebrum that controls voluntary movement via motor neurons to skeletal muscles in the body. Skeletal muscles are responsible for locomotion and posture, by pulling bones in different directions.
• What are motor neurons?

  A motor neuron is a neuron whose cell body is located in the CNS and whose axon extends to connect to a muscle or a gland. They carry information away from the CNS towards a muscle or a gland.

  They contain:

  Neuromuscular junction- The point of contact between a motor neuron and a muscle fibre, where the nerve transmits an electrical impulse to the muscle to cause it to contract.

  Cell body - contains nucleus and other organelles

  Dendrites- Finger like projections that act as the primary site for receiving sensory information from the environment. These may have specialised receptors to detect specific stimuli

  Myelin sheath- Layers of lipid that act as an insulator, allowing for faster signal transmission along the axon through a process called saltatory conduction

  Axon - A long, slender extension that carries the electrical signal. Most sensory neurons have a single axon that branches into two parts, one extending to the periphery and the other to the spinal chord.

  Motor neurons can form a two-neuron circit. One motor neuron will originate in the cerebral cortex and travel down the brainstem or the spinal chord. There it forms a synapse with a second motor neuron that extends to innervate muscles and glands throughout the body. When a nerve impulse reaches the end of a motor neuron's axon, it stimulates the muscle fibres to contract and gland cells to secrete. Axons of motor neurons can also bundle together with sensory neurons, within a nerve.
• What are nerves?


  Nerves are a part of the peripheral nervous system. They are a bundle of neurons (nerve cells) and supportive tissues that are surrounded by a protective sheath.

  The entire nerve is encased in a connective tissue sheath- the epineurium.

  The neuron axons are sub-grouped into bundles known as fascicles, each of which is also encased in a connective tissue sheath, known as the perineurium. One fascicle contains hundreds of neurons.

  Some of these axons are surrounded by the myelin sheath (myelinated axons). The solvents used in slide preparation dissolve myelin, leaving a white place where the myelin was, making it easily identifiable.
• What are neurons?


  A neuron is a nerve cell that processes and transmits information through electrical and chemical signals in the nervous system. Nerves can contain different types of neurons.

  Sensory neurons carry signals to the CNS.

  Motor neurons carry signals from the CNS to the muscles or glands

  All neurons contain:

  Cell body - contains nucleus and organelles

  Dendrites- receives signals

  Axon- sends signals Axons can be myelinated or unmyelinated. The myelin sheath acts as an insulator, allowing for faster signal transmission along the axon through a process known as saltatory conduction.
• What is a reflex?

  A reflex is an automatic, involuntary response to a stimulus that occurs through rapid neural pathways in the nervous system. It protects the body from potential harm rapidly, through triggering immediate reactions without conscious thought. Reflexes are simplest type of coordination by the nervous system, as the signals pass through the smallest number of neurons, which means that they can occur rapidly.

  Examples: Coughing (response to blockage in the airways), blinking (response to obstruction in the eye)
• What is a reflex arc?

  1- Receptor cells sense a change in conditions, which is known as a stimulus. Each receptor cell only detects one type of stimulus. Sometimes the nerve endings of a sensory neuron contain sensory receptors (used for pain and heat!), so a separate receptor cell is unnecessary.

  2- Sensory neurons receive signals (from receptor cells or their nerve endings) and pass this to the neurons in the CNS. They do this via their long axons, that end at synapses with interneurons in the grey matter of the spinal chord or the brain. Grey matter is the tissue that contains many cell bodies of interneurons and motor neurons.

  3- Interneurons are located in the CNS, and they process signals from sensory neurons, making decisions about appropriate responses by combining impulses from multiple inputs and then passing impulses to specific sensory neurons. This decision-making is simple, as there is usually only one interneuron that connects a specific sensory neuron to the motor neuron that causes the appropriate response.

  4- Motor neurons receive signals via synapses with interneurons. If a motor neuron's threshold potential is reached, then an impulse is passed along the axon which leads out of the CNS to an effector. The axon does not change its position of connections, so the impulse always travels to the same effector cell or group of effector cells.

  5- Effectors carry out the response to a stimulus when they receive the signal from a motor neuron. There are two types, muscles respond via contraction and glands respond via secretion.

  Summary:

  1- Receptors

  2- Sensory neurons

  3- Interneurons

  4- Motor neurons

  5- Effectors
• What is the cerebellum?

  The cerebellum is a part of the brain that is located in the back of the head, between the brainstem and the cerebrum.

  The brain does not initiate movements in the cerebellum- the cerebellum modifies the commands of the cerebral hemispheres (symmetrical halves of the cerebrum) to fine tune them.
• What are the functions of the cerebellum?

  Maintenance of balance and posture

  Coordination of voluntary movements

  Facilitation of motor memory (the ability to learn and remember how to coordinate muscles to perform a specific function e.g. walking)