the nervous system - buck mountain

Chapter 11

The Nervous System

Chapter 11 The Nervous System

11.1 Structures and Processes of the Nervous System

11.2 The Central Nervous System 11.3 The Peripheral Nervous System


In Section 1, you will learn that:

•  Homeostasis is maintained in the human body by the various parts of the nervous system.

•  Neural transmission occurs along axons, due to an action potential that causes depolarization of the neuron.

•  Electrochemical communication occurs between cells at the synapse.

Divisions of the Nervous System

•  The human nervous system is a complex system composed of many subsystems that all work together to maintain homeostasis in the body.

Divisions of the NS

•  Your brain contains over 100 billion nerve cells!

•  The human NS is divided into 2 parts:

Divisions of the NS

CNS •  The brain and spinal cord

•  Processes info sent from the nerves

PNS •  Carries sensory info to

the CNS and info from the CNS to muscles and glands

•  Is divided into 2 systems:

Divisions of the NS

Somatic System

•  voluntary

Autonomic System

•  Involuntary

•  Also divided into:

–  Sympathetic –  Parasympathetic

Functions of the NS

•  The nervous system gathers and processes information from the external and internal environments and then relays a response to the necessary areas of the body.

The Neuron

•  The neuron is the functional unit of the nervous system.

Glial Cells

•  Neurons are supported by GLIAL CELLS which nourish , remove wastes from and defend against infection

Types of neurons

•  There are three kinds of neurons: sensory neurons, interneurons, and motor neurons.

See p.369

Types of Neurons

•  Sensory neurons receive stimuli and transmit it to the interneurons

•  Interneurons are found in the CNS and act as integrators

•  Motor neurons conduct impulses from the interneurons to effectors (muscles or glands)

The Reflex Arc

•  Neurons allow the nervous system to relay sensory information to the brain and spinal cord for integration, and to produce a response, as needed, by the effectors.

p.370

The Reflex Arc

•  A reflex arc:

–  Is the simplest nerve pathway –  Is involuntary – Occurs w/o brain coordination

Structure of Neurons

Neurons are cells (therefore they contain all of the organelles you have previously learned about – including mitochondria, lysosomes, nucleus, etc) however, they also contain specialized structures not seen in other types of cells that allows them to transmit nerve impulses.

All nerve cells also contain the following structures:


1.  Dendrites -  receive information from sensory receptors or

other neurons and carry it toward the cell body

2.  Cell Body - Sometimes called the soma -  Contains the nucleus -  Processes input from dendrites -  May relay input to axon


3. Axon -  Conducts nerve impulses away from the cell

body -  Range in length from 1mm-over 1m -  Axon terminals (the branching ends) contain

chemicals that allow for an impulse to be transferred to the dendrites of adjacent neurons

-  Some are surrounded by a myelin sheath -  The presence of a myelin sheath speeds up the

rate of impulse transmission


•  Myelinated axons are known as white matter, while unmyelinated axons are known as grey matter

The Nerve Impulse

•  Myelinated neurons are coated in a fatty substance called myelin (this helps prevent the loss of charged particles from the neuron and speeds up the rate of transmission of an impulse since only the Nodes of Ranvier must be depolarized)

•  Myelin is produced by a type of cell called a Schwann cell (which is a type of glial cell)

•  Most neurons in the PNS are myelinated

•  In the CNS, myelinated neurons are referred to as white matter, while unmyelinated neurons are referred to as grey matter.

The Nerve Impulse

•  The transmission of a nerve impulse relies on the movement of ions across the cell membrane of the axon

•  When a neuron is at rest, it maintains its resting membrane potential (remember that “potential” means difference in charge) by:

Maintaining RMP

–  using a membrane protein “pump” which uses ATP to move 3 Na+ toward the outside of the membrane while moving 2 K+ inside

–  This causes a build-up of positive charges on the outside of the membrane

•  The neural cell membrane is more permeable to K+ than Na+

so more positives move to the outside than the inside

•  Larger, negatively charged proteins are generally kept within the cell (they can’t pass through the membrane)

The Nerve Impulse

•  All cells have a membrane potential, but the neuron is unique in that it can change the potential of its membrane to generate an impulse. A nerve impulse consists of a series of action potentials.

Fig.11.15 p.377

The Action Potential

•  A nerve impulse is a series of action potentials

•  In myelinated neurons, APs only occur at the Nodes of Ranvier

•  The threshold potential refers to the minimum stimulus required to generate an AP in a neuron (this can vary amongst different neurons)

•  APs are “all-or-none” meaning that they occur maximally or not at all


•  When the threshold potential is reached (usually around -55mV), special protein channels in the membrane open allowing Na+ to rush inside the axon

•  This causes the inside of the neural membrane to now contain more positives than the outside (thus reversing the normal charge distribution)

•  This reversal of charge is called depolarization


•  As a result of the change in membrane potential, the Na+ gates close and the K+ gates open

•  This moves positive charges back to the outside of the membrane

•  This process is called repolarization

•  Once this is complete, the K+ gates close


•  The Na+/K+ pump then restores the original resting potential

•  There is a short refractory period when the membrane cannot depolarize

The Nerve Impulse

•  In myelinated neurons, depolarization moves from one node of Ranvier to the next

•  This is because the Na+ ions will naturally diffuse down their concentration gradient (and remember the membrane itself is relatively impermeable to Na+) toward the next Node

•  In unmyelinated neurons, the entire length of the neuron must be depolarized – this takes much longer (0.5m/s as opposed to as much as 120 m/s in a myelinated neuron)

•  A disease in which the body’s own immune system is thought to break down the myelin sheath resulting in blurred vision, loss of balance, muscle weakness, fatigue and slurred speech is called multiple sclerosis

The Synapse •  An impulse is transmitted from one neuron to the next

through a synapse.

Fig.11.18 p.379

The Synapse

•  A synaptic cleft is the gap (or space) between 2 neurons or between a neuron and a muscle/ gland

•  In order for an impulse to be transferred to the next neuron, chemicals called neurotransmitters are released from the “synaptic terminal” (found at the end of the axon)

The Synapse

•  These neurotransmitter chemicals are stored in vesicles which then fuse with the neural membrane and are released by exocytosis into the synaptic cleft

•  These chemicals then diffuse across the synaptic cleft and attach to receptors on the postsynaptic membrane

•  This can result in either depolarization or hyperpolarization of the postsynaptic membrane (meaning an AP would be harder to achieve)

The Synapse

•  One example of a common neurotransmitter is acetylcholine

•  An enzyme called cholinesterase is released by the postsynaptic neuron to break down Ach (that way the postsynaptic neuron (or muscle) would not be in a constant state of depolarization (or contraction)

•  Other examples of neurotransmitters include: dopamine, serotonin, endorphins and norepinephrine

•  Many drugs interfere with the action of neurotransmitters

11.1 Structures and Processes of the Nervous System

•  Many substances, such as drugs, painkillers, chemicals, and neurotoxins, can interfere with the functions of synapses and neurotransmitters.



•  The central nervous system is the body’s control centre. It consists of the brain and spinal cord.

•  The brain includes centres that control involuntary responses and voluntary responses.

•  The cerebrum is the largest part of the brain. It contains four pairs of lobes, each of which is associated with particular functions.

11.2 The Central Nervous System

•  The regulation centre for the nervous system is the central nervous system, which consists of the brain and spinal cord.


•  The brain and spinal cord are protected by the cerebrospinal fluid, the meninges, and the skull and spinal column (vertebrae).

The meninges are layers of tissue that protect the brain and spinal

cord.


•  The brain and spinal cord themselves are composed of myelinated neurons (white matter) and unmyelinated neurons (grey matter).

Brain cross section Spinal cord cross section

grey matter

white matter


•  The hindbrain is composed of the cerebellum (involved in controlling body movements), medulla oblongata (controls many involuntary responses), and pons (relay station between different parts of brain).


•  The midbrain is a part of the brain stem.


•  The forebrain includes the thalamus and hypothalamus, involved in sensing the external and internal environment, as well as the cerebrum.

•  The outer layer of the cerebrum, called the cerebral cortex, is composed of grey matter, and is thought to be the source of human intellect.


•  The right and left halves of the cerebral cortex are made of four pairs of lobes, each of which is associated with particular functions: frontal lobes (conscious thought and movements, speech), parietal lobes (touch, taste), temporal lobes (hearing and speech), and occipital lobes (vision).



•  The peripheral nervous system is comprised of the somatic (voluntary) system and the autonomic (involuntary) system.

•  The autonomic system is divided into the sympathetic and parasympathetic nervous systems.


•  MRI and PET scans are non-invasive tools that can be used to map human brain function and screen for diseases.

MRI PET

11.3 The Peripheral Nervous System

•  The peripheral nervous system contains components that gather sensory information and then relay this information to the muscles and glands for a voluntary (somatic) or involuntary (autonomic) response.

PNS (sensory and

motor neurons)

Somatic System

Autonomic System

The PNS

•  Remember that sensory neurons carry info from all parts of the body to the CNS and motor neurons carry info from the CNS to the effectors

The Somatic System

•  Mainly voluntary

•  Its motor neurons relay to muscles

•  Includes 12 pairs of cranial nerves and 31 pairs of spinal nerves (all of which are myelinated)

The Autonomic System

•  Involuntary

•  The nerves control glands, cardiac and smooth muscle

•  Controlled by the hypothalamus and medulla

•  Has 2 divisions: sympathetic and parasympathetic

The Autonomic System

Sympathetic •  Activated in stressful

situations •  Epinephrine/

norepinephrine released from the adrenal glands

•  Polygraphs measure some of these responses

•  causes responses such as increased heart rate and breathing rate ; slower digestion

Parasympathetic

•  Activated when the body is at rest

•  Acts to restore and conserve energy

•  Neurotransmitter used is acetylcholine

•  Responses are in opposition to those of the sympathetic system (i.e.decreased heart rate, etc)

11.3 The Peripheral Nervous System

•  Homeostasis is maintained in the body by the often-antagonistic actions of the sympathetic and parasympathetic nervous systems.

•  In general, the sympathetic nervous system prepares the body for fight-or-flight, while the parasympathetic system returns the organs to a resting state.

Sympathetic Nervous System

Parasympathetic Nervous System

inhibits tears stimulates tears

dilates pupils constricts pupils

dilates air passages constricts bronchioles

speeds heart slows heart

stimulates liver to release glucose

stimulates gallbladder to release bile

inhibits activity of kidneys, stomach,

pancreas

increases activity of stomach and

pancreas decreases intestinal

activity increases intestinal

activity

inhibits urination stimulates urination

the nervous system - buck mountain

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